Test 4: Digestive & Reproductive System Flashcards
Digestive System Functions
Digestive system—organ system that processes food, extracts nutrients, and eliminates residue
Five stages of digestion
- Ingestion: selective intake of food
- Digestion: mechanical and chemical breakdown of food into a form usable by the body
- Absorption: uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph
- Compaction: absorbing water and consolidating the indigestible residue into feces
- Defecation: elimination of feces
Digestive Types
- Mechanical digestion—the physical breakdown of food into smaller particles
2.Chemical Digestion: a series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues)
Mechanical Digestion
the physical breakdown of food into smaller particles
- Cutting and grinding action of the teeth
- Churning action of stomach and small intestines
- Exposes more food surface to digestive enzymes
Chemical Digestion
a series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues)
•Carried out by digestive enzymes produced by salivary glands, stomach, pancreas, and small intestine
Polysaccharides into monosaccharides
Proteins into amino acids
Fats into monoglycerides and fatty acids
Nucleic acids into nucleotides
Some nutrients are present in a usable form in ingested food and can be directly absorbed
•Vitamins, amino acids, minerals, cholesterol, and water
Digestive System: Subdivisions
- digestive tract
- accessory organs
Digestive Tract
Aka the alimentary canal
- 30 ft long muscular tube extending from mouth to anus
- Mouth, pharynx, esophagus, stomach, small intestine, and large intestine
- Gastrointestinal (GI) tract is the stomach and intestines
Accessory Organs
•Teeth, tongue, salivary glands, liver, gallbladder, and pancreas
Digestive Tract Structure & Layers
Most of digestive tract follows a basic structural plan with the digestive tract wall consisting of layers:
- Mucosa: epithelium, lamina propria, muscularis mucosae
- Submucosa
- Muscularis externa: inner circular layer, outer longitudinal layer
- Serosa: areolar tissue, mesothelium
Mucosa (Mucous Membrane) & Layers
lines the lumen and consists of:
1.Inner epithelium
- Simple columnar in most of digestive tract
- Stratified squamous from mouth through esophagus, and in lower anal canal
- Lamina propria: loose connective tissue layer
- Muscularis mucosa: thin layer of smooth muscle
- Tenses mucosa creating grooves and ridges that enhance surface area and contact with food
- Improves efficiency of digestion and nutrient absorption
4.Mucosa-associated lymphatic tissue (MALT): the mucosa exhibits an abundance of lymphocytes and lymphatic nodules
Submucosa: Structure
thicker layer of loose connective tissue
- Contains blood vessels, lymphatic vessels, a nerve plexus, and in some places mucus-secreting glands that dump lubricating mucus into the lumen
- MALT extends into the submucosa in some parts of the GI tract
Muscularis Externa: Structure & Layers
consists of usually two layers of muscle near the outer surface
1.Inner circular layer
•In some places, this layer thickens to form valves (sphincters) that regulate the passage of material through the tract
2.Outer longitudinal layer
•Responsible for the motility that propels food and residue through the tract
Enteric Nervous System: Structure & Function
nervous network in esophagus, stomach, and intestines that regulates digestive tract motility, secretion, and blood flow
- Thought to have over 100 million neurons
- Can function independently of central nervous system
- But CNS usually exerts influence on its action
- Often considered part of autonomic nervous system
Serosa: Structure
composed of a thin layer of areolar tissue topped by simple squamous mesothelium
- Begins in the lower 3 to 4 cm of the esophagus
- Ends just before the rectum
- Adventitia: fibrous connective tissue layer that binds and blends the pharynx, most of the esophagus, and the rectum into adjacent connective tissue of other organs
Enteric Nervous System: Two Networks of Neurons
1.Submucosal plexus: in submucosa
- Controls glandular secretions of mucosa
- Controls movements of muscularis mucosae
2.Myenteric plexus: parasympathetic ganglia and nerve fibers between the two layers of the muscularis externa
Controls peristalsis and other contractions of muscularis externa
Mesenteries: Structure & Function
connective tissue sheets that suspend stomach and intestines from abdominal wall
Functions
- Looseness allows stomach and intestines to undergo strenuous contractions with freedom of movement in the abdominal cavity
- Hold abdominal viscera in proper relationship to each other
- Prevent intestines from becoming twisted and tangled by changes in body position and by its own contractions
- Provide passage of blood vessels and nerves that supply digestive tract
- Contain many lymph nodes and lymphatic vessels
Parietal Peritoneum: Structure
a serous membrane that lines the wall of the abdominal cavity
- Turns inward along posterior midline
- Forms posterior mesentery: a translucent two-layered membrane extending to the digestive tract
- The two layers of the mesentery separate and pass around opposite sides of the organ forming the serosa
- Come together on the far side of the organ and continue as another sheet of tissue, called the anterior mesentery
- May hang freely in the abdominal cavity
May attach to the anterior abdominal wall or other organs
Lesser Omentum
a ventral mesentery that extends from the lesser curvature of the stomach to the liver
Greater Omentum
hangs from the greater curvature of the stomach (its left inferior margin)
- Covers small intestine like an apron
- The inferior margin turns back on itself and passes upward
- Forming a deep pouch between its deep and superficial layers
- Inner superior margin forms serous membranes around the spleen and transverse colon—mesocolon
- Part of the body’s first line of defense against toxins and infections
Intraperitoneal
when an organ is enclosed by mesentery on both sides
- Considered within the peritoneal cavity
- Stomach, liver, and parts of small and large intestine
Retroperitoneal
when an organ lies against the posterior body wall and is covered by peritoneum on its anterior side only
- Considered to be outside the peritoneal cavity
- Duodenum, pancreas, and parts of the large intestine
Regulation of the digestive tract
Motility and secretion of the digestive tract are controlled by:
- neural
- hormonal
- paracrine mechanisms
Digestive Tract Regulation: Neural Control
- Short (myenteric) reflexes: stretch or chemical stimulation acts through myenteric plexus
- Stimulates peristaltic contractions of swallowing
- Long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion
Digestive Tract Regulation: Hormones
- Chemical messengers secreted into bloodstream that stimulate distant parts of the digestive tract
- Gastrin and secretin
Digestive Tract Regulation: Paracrine Secretions
•Chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells
The Mouth: Structure & Functions
The mouth is known as the oral, or buccal cavity
Functions:
- Ingestion (food intake)
- Taste and other sensory responses to food
- Chewing and chemical digestion
- Swallowing, speech, and respiration
Mouth enclosed by cheeks, lips, palate, and tongue
The Mouth: Oral Fissure
anterior opening between lips
The Mouth: Fauces
posterior opening to the throat
Mouth Tissue Type
Stratified squamous epithelium lines mouth
- Keratinized in areas subject to food abrasion: gums and hard palate
- Nonkeratinized in other areas: floor of mouth, soft palate, and insides of cheeks and lips
The Cheeks & Lips: Structure & Functions
Structure:
- Fleshiness due to subcutaneous fat, buccinator muscle of the cheek, and orbicularis oris of the lips
- Labial frenulum: median fold that attaches each lip to the gum between the anterior incisors
- Vestibule: space between cheeks or lips and the teeth
Function:
- Retain food and push it between the teeth
- Essential for speech
- Essential for sucking and blowing actions, including suckling by infants
The Tongue: Structure & Function
Lingual glands: serous and mucous glands amid the extrinsic muscles
•Secrete a portion of the saliva
Lingual tonsils: contained in the root
The Palate: Structure & Function
•separates oral cavity from nasal cavity; makes it possible to breath while chewing food
•Hard (bony) palate
•Hard (bony) palate—anterior portion that is supported by the palatine processes of the maxillae and the palatine bones
Soft Palate
•posterior to hard palate with more spongy texture
Structure/ Function:
- Composed of skeletal muscle and glandular tissue
- No bone
- Uvula: conical medial projection visible at the rear of the mouth
- Helps retain food in the mouth until one is ready to swallow
•Palatine rugae:
transverse ridges that help the tongue hold and manipulate food
Pairs Muscular Arches on each side of oral cavity
- Palatoglossal arch: anterior arch
- Palatopharyngeal arch: posterior arch
- Palatine tonsils are located on the wall between the arches
The Teeth: Structure & Function
Structure:
32 adult teeth
16 in mandible (lower jaw)
16 in maxilla (upper jaw)
From midline to the rear of each jaw
2 incisors—chisel-like cutting teeth used to bite off a piece of food
1 canine—pointed and act to puncture and shred food
2 premolars—broad surface for crushing, shredding, and grinding
3 molars—even broader surface for crushing, shredding, and grinding
Function:
Masticate (chew) food into smaller pieces
- Makes food easier to swallow
- Exposes more surface area for action of digestive enzymes, speeding chemical digestion
The Teeth: Alveolus
tooth socket in bone
•Gomphosis joint formed between tooth and bone
The Teeth: Periodontal Ligament
—modified periosteum whose collagen fibers penetrate into the bone on one side and into the tooth on the other
- Anchors tooth firmly in alveolus
- Allows slight movement under pressure of chewing
Gingiva (gum)
covers the alveolar bone
Regions of a tooth
- Crown: portion above the gum
- Root: the portion below the gum, embedded in alveolar bone
- Neck: the point where crown, root, and gum meet
- Gingival sulcus: space between the tooth and the gum
•Hygiene in the sulcus is important to dental health
The Teeth: Dentin
hard yellowish tissue that makes up most of the tooth
The Teeth: Enamel
covers crown and neck
•A noncellular secretion that cannot regenerate
The Teeth: Cement
—covers root
Cementum and dentin are living tissue and can regenerate
The Teeth: Root Canal
—space in a root leading to pulp cavity in the crown
- Nerves and blood vessels
- Apical foramen: pore at the basal end of each root canal
The Teeth: Occlusion
meeting of the teeth with the mouth closed
The Teeth Development
20 deciduous teeth (milk teeth or baby teeth)
Teeth develop beneath gums and erupt in a predictable order
- Erupt from 6 to 30 months
- Beginning with incisors
- Between 6 and 25 years of age, are replaced by 32 permanent teeth
Third molars (wisdom teeth) erupt from age 17 to 25 years
•May be impacted: crowded against neighboring teeth and bone so they cannot erupt
Teeth & Gum Disease
The human mouth is home to more than 700 species of microorganisms, especially bacteria
1.Plaque—sticky residue on the teeth made up of bacteria and sugars
- Calculus: calcified plaque
- Bacteria metabolize sugars and release acids that dissolve the minerals of enamel and dentin to form dental caries (cavities)
- Root canal therapy is necessary if cavity reaches pulp
- Calculus in the gingival sulcus wedges the tooth and gum apart
- Allows bacterial invasion of the sulcus
- Gingivitis: inflammation of the gums
- Periodontal disease: destruction of the supporting bone around the teeth which may result in tooth loss
Saliva & the Salivary Glands: Function
Saliva
- Moistens mouth
- Begins starch and fat digestion
- Cleanses teeth
- Inhibits bacterial growth
- Dissolves molecules so they can stimulate the taste buds
- Moistens food and binds it together into bolus to aid in swallowing
Saliva Structure
Saliva is a hypotonic solution of 97.0% to 99.5% water and the following solutes:
- Salivary amylase: enzyme that begins starch digestion in the mouth
- Lingual lipase: enzyme that is activated by stomach acid and digests fat after food is swallowed
- Mucus: binds and lubricates a mass of food and aids in swallowing
- Lysozyme: enzyme that kills bacteria
- Immunoglobulin A (IgA): an antibody that inhibits bacterial growth
- Electrolytes: Na^+, K^+, Cl^-, phosphate, and bicarbonate
•pH: 6.8 to 7.0
Saliva: Mucus
•binds and lubricates a mass of food and aids in swallowing
Saliva: Salivary Amylase
•enzyme that begins starch digestion in the mouth
Saliva: Lingual Lipase
enzyme that is activated by stomach acid and digests fat after food is swallowed
Saliva: Lysozyme
enzyme that kills bacteria
Saliva: Immunoglobulin A
•an antibody that inhibits bacterial growth
Saliva Electrolytes
Na^+, K^+, Cl^-, phosphate, and bicarbonate
Saliva pH
6.8 to 7.0
Salivary Glands Structure
Compound tubuloacinar glands
•Branched ducts ending in acini
Mucous cells secrete mucus
Serous cells secrete thin fluid rich in enzymes and electrolytes
Mixed acinus has both mucous and serous cells
The Pharynx: Structure/ Function
muscular funnel connecting oral cavity to esophagus and nasal cavity to larynx
- Digestive and respiratory tracts intersect
- Has deep layer of longitudinal skeletal muscle
- Has superficial layer of circular skeletal muscles that form pharyngeal constrictors (superior, middle, and inferior) that force food downward during swallowing
- When not swallowing, the inferior constrictor (upper esophageal sphincter) remains contracted to exclude air from the esophagus
- Disappears at the time of death when the muscles relax, so it is a physiological sphincter, not an anatomical structure
The Esophogus: Structure & Function
Esophagus—straight muscular tube 25–30 cm long
- Begins at level between C6 and the cricoid cartilage
- Extends from pharynx to cardial orifice of stomach passing through esophageal hiatus in diaphragm
- Lower esophageal sphincter: food pauses here because of constriction
- Prevents stomach contents from regurgitating into the esophagus
- Protects esophageal mucosa from erosive stomach acid
- Heartburn—burning sensation produced by acid reflux into the esophagus
The Esophogus: Tissue
Nonkeratinized stratified squamous epithelium
Esophageal glands in submucosa secrete mucus
Deeply folded into longitudinal ridges when empty
Skeletal muscle in upper one-third, mix of muscle types in middle one-third, and only smooth muscle in bottom one-third
Meets stomach at level of T7
Covered with adventitia
Swallowing: Function
a complex action involving over 22 muscles in the mouth, pharynx, and esophagus
- Swallowing center: pair of nuclei in medulla oblongata that coordinates swallowing
- Communicates with muscles of the pharynx and esophagus by way of trigeminal, facial, glossopharyngeal, and hypoglossal nerves
Swallowing Phases: Oral Phase
- Oral Phase; The tongue forms a food blus and pushes it into the laryngopharynx
Swallowing: Pharyngeal Phase
The palate, tongue,vocal cords, and the epiglottis block the oral and nasal cavities and airway while pharyngeal constrictors push the bolus into the esophogus
Swallowing: Esophogeal Phase
Peristalsis drives the bols downward and relaxation of the lower esophogeal sphincter admits it into the stomach
The Stomach: Structure & Function
Stomach—a muscular sac in upper left abdominal cavity immediately inferior to the diaphragm
- Primarily functions as a food storage organ
- volume of about 50 mL when empty
- 1.0 to 1.5 L after a typical meal
- Up to 4 L when extremely full—can extend nearly as far as the pelvis
The Stomach: General Mechanics
Mechanically breaks up food, liquefies it, and begins chemical digestion of protein and fat
•Chyme: soupy or pasty mixture of semidigested food in the stomach
Most digestion occurs after the chyme passes on to the small intestine
Chyme
•soupy or pasty mixture of semidigested food in the stomach
Stomach Anatomy: 4 Regions
J-shaped; relatively vertical in tall people, horizontal in short people
•Divided into four regions:
- Cardial part (cardia)—small area within about 3 cm of the cardial orifice
- Fundus—dome-shaped portion superior to esophageal attachment
- Body—makes up the greatest part of stomach
- Pyloric part—narrower pouch at the inferior end
- Subdivided into the funnel-like antrum
- Narrower pyloric canal that terminates at pylorus
- Pylorus—narrow passage to duodenum
- Pyloric sphincter—regulates the passage of chyme into the duodenum
Stomach Anatomy: Microscopic
Stomach has a simple columnar epithelium covered by mucous
- Apical regions of its surface cells are filled with mucin
- Mucin swells with water and becomes mucus after it is secreted
Mucosa and submucosa are flat when stomach is full but form longitudinal wrinkles called gastric rugae when empty
Muscularis externa has three layers (instead of the two seen elsewhere)
•Outer longitudinal, middle circular, and inner oblique layers
Stomach: Gastric Pits
Gastric pits—depressions in gastric mucosa
- Lined with simple columnar epithelium
- Two or three tubular glands open into the bottom of each gastric pit
- Cardiac glands in cardial part
- Pyloric glands in pyloric parts
Gastric glands in the rest of the stomach
Stomach: Mucus Cells
secrete mucus
- Predominate in cardiac and pyloric glands
- In gastric glands, called mucous neck cells since they are concentrated at the neck of the gland
Stomach: Regenerative (Stem Cells)
found in base of pit and in neck of gland
•Divide rapidly and produce continual supply of new cells to replace cells that die
Stomach: Parietal Cells
found mostly in the upper half of the gland
•Secrete hydrochloric acid (HCl), intrinsic factor, and a hunger hormone called ghrelin
Hydrochloric Acid
HCl activates pepsin and lingual lipase
Breaks up connective tissues and plant cell walls
•Helps liquefy food to form chyme
Converts ingested ferric ions (Fe^(3+) ) to ferrous ions (Fe^(2+) )
•Fe^(2+) absorbed and used for hemoglobin synthesis
Contributes to nonspecific disease resistance by destroying most ingested pathogens
Most ulcers are caused by acid-resistant bacteria Helicobacter pylori, which can be treated with antibiotics and Pepto-Bismol
Somtach: Chief Cells
most numerous
- Secrete gastric lipase and pepsinogen
- Dominate lower half of gastric glands
- Absent from pyloric and cardiac glands
Enteroendocrine Cells
concentrated in lower end of gland
•Secrete hormones and paracrine messengers that regulate digestion
Pepsin
Zymogens—digestive enzymes secreted as inactive proteins
•Converted to active enzymes by removing some of their amino acids
Pepsinogen—zymogen secreted by chief cells
- Hydrochloric acid removes some of its amino acids and forms pepsin that digests proteins
- Autocatalytic effect—as some pepsin is formed, it converts more pepsinogen into more pepsin
Pepsin digests dietary proteins into shorter peptides
Protein digestion is completed in the small intestine
Gastric Lipase
produced by chief cells
Gastric lipase and lingual lipase play a minor role in digesting dietary fats
- Digests 10% to 15% of dietary fats in the stomach
- Rest digested in the small intestine
Intrinsic Factor
a glycoprotein secreted by parietal cells
Essential to absorption of vitamin B_12 by the small intestine
•Binds vitamin B_12 and then intestinal cells absorb this complex by receptor-mediated endocytosis
Vitamin B_12 is needed to synthesize hemoglobin
•Deficiency causes anemia
Secretion of intrinsic factor is the only indispensable function of the stomach
•Digestion can continue if stomach is removed (gastrectomy), but B_12 supplements will be needed
Chemical Messengers
Gastric and pyloric glands have a variety of cells that produce a variety of chemical messengers
- Most are hormones that enter blood and stimulate distant cells
- Some are paracrine secretions that stimulate neighboring cells
- Several are peptides produced in both the digestive tract and the central nervous system: gut–brain peptides
- Substance P, vasoactive intestinal peptide (VIP), secretin, gastric inhibitory peptide (GIP), cholecystokinin, and neuropeptide Y (NPY)
Gastric Motility: Process
Swallowing center of medulla oblongata signals stomach to relax
Vagus nerve relays message from medulla and activates a receptive-relaxation response in stomach
•Resists stretching briefly, but relaxes to hold more food
Soon stomach shows a rhythm of peristaltic contractions controlled by enteric pacemaker cells in longitudinal layer of muscularis externa
- A ring of constriction every 20 seconds
- Becomes stronger contraction at pyloric part
Thick muscularis of antrum acts as a strong pump that breaks up semidigested food and prepares it for intestine
Antral contractions come in waves that churn and break up the chyme into small particles
Only about 3 mL of chyme is squirted into the duodenum at a time; this small amount allows duodenum to:
- Neutralize the stomach acid
- Digest nutrients little by little
If duodenum is overfilled, it inhibits gastric motility
Typical meal emptied from stomach in 4 hours
•Less time if the meal is more liquid
As long as 6 hours for a high-fat meal
- After 30 minutes or so these contractions become quite strong
- They churn the food, mix it with gastric juice, and promote its physical breakup and chemical digestion
Gastric Motility
Vomiting
forceful ejection of stomach and intestinal contents (chyme) from the mouth
Vomiting & Emetic Center
Emetic center in the medulla oblongata integrates multiple muscle actions
Vomiting induced by
- Overstretching of the stomach or duodenum
- Chemical irritants such as alcohol and bacterial toxins
- Visceral trauma
- Intense pain or psychological and sensory stimuli
Chronic Vomiting
- Results in dangerous fluid, electrolyte, and acid–base imbalances
- Bulimia: eating disorder; hydrochloric acid in vomit causes tooth enamel to erode
Digestion & Absorbtion
Stomach does not absorb any significant amount of nutrients
•Does absorb aspirin and some lipid-soluble drugs
Alcohol is absorbed mainly by small intestine
•Intoxicating effects depend partly on how rapidly the stomach is emptied
Regulation of Gastric Function: Steps
- Cephalic Phase: Vagus nerve stimulates gastric selection even before food is swallowed
- Gastric Phase: Food stretches the stomach and activate myentric and vagovagal reflexes. These reflexes stimulate gastric secretion. Histamine and gastrin also stimualate acid and enzyme secretion.
- Intestinal Phase: Intestinal gastrin briefly stimulates the stomach but then secretin, CCK, and the enterogastric reflex inhibit gastric secretion and motility where the duodenum processes the chume already in it. Symphathetic nerve fibers supress gastric activity, while vagal (parasymphatetic) stimulation of the stomach is now inhibited.
Regulation of Gastric Function: Cephalic Phase
- Stomach responds to sight, smell, taste, or thought of food
- Sensory and mental inputs converge on hypothalamus
- Hypothalamus relays signals to medulla oblongata
- Vagus nerve fibers from medulla stimulate the enteric nervous system of stomach, stimulating gastric secretion
- 40% of stomach’s acid secretion occurs in cephalic phase
Regulation of Gastric Function: Gastric Phase
- Period in which swallowed food and semidigested protein activate gastric activity
- Two-thirds of gastric secretion and one-half of acid secretion occur in this phase
- Ingested food stimulates gastric activity in two ways
- By stretching the stomach
- Activates short reflex mediated through myenteric plexus; activates long reflex mediated through the vagus nerves and the brainstem
By increasing the pH of its contents
Gastric secretion is stimulated by three chemicals
- Acetylcholine (ACh)—secreted by parasympathetic nerve fibers of both reflexes
- Histamine—a paracrine secretion from enteroendocrine cells in the gastric glands
- Gastrin—a hormone produced by the enteroendocrine G cells in pyloric glands
Regulation of Gastric Function: Intestinal Phase
- Duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes
- Duodenum initially enhances gastric secretion but soon inhibits it
- Stretching of duodenum accentuates vagovagal reflex that stimulates stomach
- Peptides and amino acids in chyme stimulate G cells of duodenum to secrete more gastrin, further stimulating stomach
- Soon acids and fats trigger enterogastric reflex—duodenum sends inhibitory signals to stomach by way of enteric nervous system
- Duodenum also signals medulla to inhibit vagal nuclei (reducing vagal stimulation of stomach) and stimulate sympathetic neurons (sending inhibitory signals to the stomach)
- Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin
- They stimulate the pancreas and gallbladder
- Also suppress gastric secretion
- Gastrin secretion declines and pyloric sphincter contracts tightly to limit chyme entering duodenum
- Gives duodenum time to work on chyme
- Enteroendocrine cells also secrete glucose-dependent insulinotropic peptide (GIP) originally called gastrin-inhibiting peptide
- Stimulates insulin secretion in preparation for processing nutrients about to be absorbed by small intestine
The liver, gallbladder, and pancreas
Small intestine receives chyme from stomach and secretions from liver and pancreas
- These secretions enter digestive tract near the junction of stomach and small intestine
- These secretions are important to the digestive process of the small intestine
The Liver: Anatomy
Liver—reddish brown gland located immediately inferior to the diaphragm
The body’s largest gland
•Weighs about 1.4 kg (3 lbs)
Variety of functions
Secretes bile which contributes to digestion
Four lobes—right, left, quadrate, and caudate
- Falciform ligament separates left and right lobes
- Sheet of mesentery that suspends the liver from the diaphragm
- Round ligament (ligamentum teres)—fibrous remnant of umbilical vein
- Carries blood from umbilical cord to liver of the fetus
Hilum—irregular opening between quadrate and caudate lobes
- Point of entry for hepatic portal vein and proper hepatic artery
- Point of exit for the bile passages
- All travel in lesser omentum
Gallbladder—adheres to a depression on the inferior surface of the liver, between right and quadrate lobes
Bare area on superior surface where it attaches to diaphragm
The Liver: Microscopic Anatomy
Hepatic lobules—tiny cylinders that fill the interior of the liver
- About 2 mm long and 1 mm in diameter
- Central vein: passes down the core
- Hepatocytes: cuboidal cells surrounding central vein in radiating sheets or plates
Each plate of hepatocytes is an epithelium one or two cells thick
Hepatic sinusoids: blood-filled channels that fill spaces between the plates
- Lined by a fenestrated endothelium that separates hepatocytes from blood cells
- Allows plasma into the space between the hepatocytes and endothelium
- Hepatocytes have brush border of microvilli that project into this space
- Blood filtered through the sinusoids comes directly from the stomach and intestines, enters through hepatic portal vein and leaves to drain into vena cava
Stellate macrophages: phagocytic cells in the sinusoids that remove bacteria and debris from the blood
The Liver: Microscopic Anatomy- Hepatocytes
Hepatocytes
- After a meal, hepatocytes absorb from the blood: glucose, amino acids, iron, vitamins, and other nutrients for metabolism or storage
- Between meals, hepatocytes break down stored glycogen and release glucose into the blood
- Remove and degrade: hormones, toxins, bile pigments, and drugs
Secrete into the blood: albumin, lipoproteins, clotting factors, angiotensinogen, and other products
The Liver: Microscopic Anatomy- Bile Canaliculi
Bile canaliculi—narrow channels into which the liver secretes bile
Bile passes into bile ductules of the triads
Ultimately into the right and left hepatic ducts
The Liver: Microscopic Anatomy- Common Hepatic Duct
formed from convergence of right and left hepatic ducts on inferior side of the liver.
The Liver: Microscopic Anatomy- Cystic & Bile Duct
Cystic duct coming from gallbladder joins common hepatic duct
Bile duct: formed from union of cystic and common hepatic ducts
•Descends through lesser omentum toward the duodenum
Gallbladder
a pear-shaped sac on underside of liver
•Serves to store and concentrate bile by absorbing water and electrolytes
Bile
Bile—yellow-green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments, and bile acids
Bilirubin: principal pigment derived from the decomposition of hemoglobin
Bacteria in large intestine metabolize bilirubin to urobilinogen
Stercobilin responsible for the brown color of feces
Urobilin responsible for yellow color of urine
Bile acids (bile salts): steroids synthesized from cholesterol
Bile acids and lecithin, a phospholipid, aid in fat digestion and absorption
Gallstones may form if bile becomes excessively concentrated with wastes
Gallstones
hard masses in either the gallbladder or bile ducts
•Composed of cholesterol, calcium carbonate, and bilirubin
Cholelithiasis
presence of gallstones
•Most common in obese women over 40—excess cholesterol
Painful obstruction of ducts
•Result in jaundice (yellowing of skin), poor fat digestion, and impaired absorption of fat-soluble vitamins
Lithotripsy
use of ultrasonic vibration to pulverize stones without surgery
The Pancreas
spongy retroperitoneal gland posterior to greater curvature of stomach
- Measures 12 to 15 cm long, and 2.5 cm thick
- Has a head encircled by duodenum, a body (midportion), and a tail on the left
Pancreas Glands
- Endocrine portion—pancreatic islets that secrete insulin and glucagon; concentrated in the tail of the gland
- Exocrine portion—99% of pancreas that secretes 1,200 to 1,500 mL of pancreatic juice per day
- Secretory acini release their secretion into small ducts that converge on the main pancreatic duct
Pancreatic Ducts
1.Pancreatic duct runs lengthwise through middle of the gland
- Joins the bile duct at the hepatopancreatic ampulla
- Hepatopancreatic sphincter controls release of both bile and pancreatic juice into the duodenum
2.Accessory pancreatic duct: smaller duct that branches from the main pancreatic duct
- Opens independently into the duodenum
- Bypasses the sphincter and allows pancreatic juice (alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes ) to be released into duodenum even when bile is not
Pancreatic zymogens: 3 Types
1.Trypsinogen
- Secreted into intestinal lumen
- Converted to trypsin by enteropeptidase that is secreted by mucosa of small intestine
Trypsin is autocatalytic—converts trypsinogen into still more trypsin
- Chymotrypsinogen: converted to trypsinogen by trypsin
- Procarboxypeptidase: converted to carboxypeptidase by trypsin
Pancreatic Enzymes
- Panncreatic Amylase:digests starch
- Pancreatic Lipase:digests fat
- Ribonuclease and deoxyribonuclease: digest RNA and DNA, respectively
Regulation of Secretion in Pancreas: Stimuli Types
Three stimuli are chiefly responsible for the release of pancreatic juice and bile: acetylcholine, cholecystokinin, and secretin
Regulation of secretion: Acetylcholine
- from vagus and enteric nerves
- Stimulates acini to secrete enzymes during cephalic phase of gastric control even before food is swallowed
- Enzymes remain in acini and ducts until chyme enters the duodenum
Regulation of secretion: Cholecystokinin
(CCK): secreted by mucosa of duodenum in response to arrival of fats in small intestine
- Stimulates pancreatic acini to secrete enzymes
- Strongly stimulates gallbladder
- Induces contractions of gallbladder and relaxation of hepatopancreatic sphincter to discharge bile into duodenum
Secretin: released from duodenum in response to acidic chyme arriving from the stomach
•Stimulates ducts of both liver and pancreas to secrete more sodium bicarbonate
Raises pH to the level required for activity of the pancreatic and intestinal digestive enzymes
Regulation of secretion: Secretin
Secretin: released from duodenum in response to acidic chyme arriving from the stomach
•Stimulates ducts of both liver and pancreas to secrete more sodium bicarbonate
Raises pH to the level required for activity of the pancreatic and intestinal digestive enzymes
The Small Intestine: general structure & function
Function:
Nearly all chemical digestion and nutrient absorption occurs in the small intestine
Structure:
The longest part of the digestive tract
- About 5 m long in a living person
- Up to 8 m long in a cadaver—no muscle tone
“Small” intestine refers to the diameter—not length
•Diameter is about 2.5 cm (1 in.)
Small Intestine: Gross Anatomy/ Structure & Location
coiled tube filling most of the abdominal cavity inferior to stomach and liver
Small Intestine: Gross Anatomy/ Three Divisions
- Duodenum: first 25 cm (10 in.)
- Jejunum:first 40% of small intestine beyond duodenum
- Ileum: forms last 60% of the postduodenal small intestine
Small Intestine: Gross Anatomy/ Duodenum
Structure:
- Begins at pyloric valve
- Major and minor duodenal papillae distal to pyloric valve
- Receives major and minor pancreatic ducts, respectively
- Arches around head of the pancreas
Ends at a sharp bend called the duodenojejunal flexure
•Most is retroperitoneal
Function:
- Receives stomach contents, pancreatic juice, and bile
- Stomach acid is neutralized here
- Fats are physically broken up (emulsified) by bile acids
- Pepsin is inactivated by increased pH
- Pancreatic enzymes perform chemical digestion
Small Intestine: Gross Anatomy/ Jejunum
Structure:
first 40% of small intestine beyond duodenum
- Roughly 1.0 to 1.7 m in a living person
- Has large, tall, closely spaced circular folds
- Its wall is relatively thick and muscular
- Especially rich blood supply which gives it a red color
Function
•Most digestion and nutrient absorption occurs here
Small Intestine: Gross Anatomy/ Ileum
Structure: forms last 60% of the postduodenal small intestine
- About 1.6 to 2.7 m, thinner, less muscular, less vascular, and paler pink color
- Aggregated lymphoid nodules—prominent lymphatic nodules in clusters on the side opposite the mesenteric attachment; visible to naked eye; become larger near large intestine
- Ileocecal junction—end of the small intestine; where the ileum joins the cecum of the large intestine
- Ileal papilla—formed by the thickened muscularis of the ileum; protrudes into the cecum; regulates passage of food residue into the large intestine
- Both jejunum and ileum are intraperitoneal and covered with serosa
Small Intestine: Circulation
The small intestine receives nearly all of its blood supply from the superior mesenteric artery
Fans out through the mesentery to give rise to 12 to 15 jejunal and ileal arteries
Superior mesenteric vein
•This joins the splenic vein and then flows into the hepatic portal system, headed for the liver with its load of nutrients
Small Intestine: Microscopic Anatomy Structure/ Function
Small intestine tissues designed for nutrient digestion and absorption
- Lumen lined with simple columnar epithelium
- Muscularis externa is noted for a thick inner circular layer and a thinner outer longitudinal layer
- Large internal surface area—great length and three types of internal folds or projections
- Circular folds (plicae circulares)—increase surface area by a factor of 2 to 3
- Villi—increase surface area by a factor of 10
- Microvilli—increase the surface area by a factor of 20
Small Intestine: Microscopic Anatomy: Circular Folds
Circular folds (plicae circulares)—largest folds of intestinal wall
- Up to 10 mm high
- Involve only mucosa and submucosa
- Occur from duodenum to middle of ileum
- Relatively small and sparse in ileum; not found in distal half, as most nutrient absorption is completed by this point
- Cause chyme flow in spiral path causing more contact with mucosa
- Promote more thorough mixing and nutrient absorption
Small Intestine: Microscopic Anatomy- Villi
Villi—finger-like projections 0.5 to 1 mm tall
- Make mucosa look fuzzy
- Villus covered with two types of epithelial cells
- Absorptive cells (enterocytes)
- Goblet cells—secrete mucus
- Epithelia joined by tight junctions that prevent digestive enzymes from seeping between them
- Core of villus filled with areolar tissue of lamina propria
- Contains arteriole, capillaries, venule, and lymphatic capillary called a lacteal
Small Intestine: Microscopic Anatomy- Microvilli
Microvilli: form a fuzzy brush border on apical surface of each absorptive cell
- Brush border enzymes—contained in plasma membrane of microvilli
- Carry out some of the final stages of enzymatic digestion
- Not released into the lumen
- Contact digestion: chyme must contact the brush border for digestion to occur
- Intestinal churning of chyme ensures contact with the mucosa
Small Intestine: Microscopic Anatomy- Intestinal Crypts
Intestinal crypts—numerous pores that open into tubular glands on the floor of the small intestine between bases of the villi
- Similar to gastric glands
- In upper half, have enterocytes and goblet cells like the villi
- In lower half, dominated by dividing stem cells
- Life span of 3 to 6 days; new epithelial cells migrate up the crypt to the tip of the villus where they are sloughed off and digested
- A few Paneth cells are clustered at the base of each crypt
- Secrete lysozyme, phospholipase, and defensins—defensive proteins that resist bacterial invasion of the mucosa
Small Intestine: Microscopic Anatomy- Duodneal Glands
in submucosa of duodenum
- Secrete an abundance of bicarbonate-rich mucus
- Neutralize stomach acid and shield the mucosa from its erosive effects
Small Intestine: Microscopic Anatomy- Lymphocytes
Large population of lymphocytes throughout lamina propria and submucosa of small intestine
•Intercept pathogens before they can invade bloodstream
Aggregated into lymphatic nodules in ileum
Intestinal Motility: Small Intestine Contraction Functions: 3 Reasons
1.To mix chyme with intestinal juice, bile, and pancreatic juice
- To neutralize acid
- Digest nutrients more effectively
- To churn chyme and bring it in contact with the mucosa for contact digestion and nutrient absorption
- To move residue toward large intestine
Intestinal Motility: Segmentation
movement in which stationary ring-like constrictions appear in several places along the intestine
- They relax and new constrictions form elsewhere
- Most common kind of intestinal contraction
- Enteric pacemaker cells in muscularis externa set rhythm of segmentation
- Contractions about 12 times per minute in the duodenum
- 8 to 9 times per minute in the ileum
- When most nutrients have been absorbed and little remains but undigested residue, segmentation declines and peristalsis begins
Ileal papilla usually closed
- Food in stomach triggers gastroileal reflex that enhances segmentation in the ileum and relaxes the valve
- As cecum fills with residue, pressure pinches the valve shut
Prevents reflux of cecal contents into the ileum
Carbohydrate Digestion: Starch
Starch—most digestible dietary carbohydrate
- Cellulose is indigestible
- Starch is first digested to oligosaccharides (up to eight glucose residues long)
- Oligosaccharides then digested to the disaccharide maltose
- Maltose finally digested to glucose which is absorbed by the small intestine
Carbohydrate Digestion: Process (Mouth)
Process begins in the mouth
- Salivary amylase hydrolyzes starch into oligosaccharides
- Amylase works best at pH of 6.8 to 7.0 of oral cavity
- Amylase quickly denatured on contact with stomach acid and digested by pepsin
- About 50% of dietary starch is digested before it reaches small intestine
- Pancreatic amylase resumes starch digestion in intestine
Carbohydrate Digestion: Process (small Intestine)
When reaching small intestine, pancreatic amylase quickly converts starch to oligosaccharides and maltose
Brush border enzymes continue carbohydrate digestion:
- Dextrinase and glucoamylase hydrolyze oligosaccharides
- Maltase hydrolyzes maltose (a disaccharide)
- Sucrase and lactase hydrolyze the disaccharides sucrose and lactose
- In most people, lactase production stops in childhood
- Monosaccharides produced by disaccharide hydrolysis (such as glucose) are immediately absorbed
Carbohydrate Digestion: Process (Monosaccharides/ Glucose)
Plasma membrane of absorptive cells has transport proteins that absorb monosaccharides as soon as brush border enzymes release them
80% of absorbed sugar is glucose
- Taken up by sodium–glucose transport (SGLT) proteins
- Glucose is transported out the base of absorptive cell into ECF by facilitated diffusion
- Sugar entering ECF increases its osmolarity
- Draws water osmotically from lumen of intestine, through leaky tight junctions between epithelial cells
- Water carries more glucose and other nutrients with it by solvent drag
Carbohydrate Digestion: Process (Galactose, Fructose, etc)
SGLT also absorbs galactose
Fructose is absorbed by facilitated diffusion (by a different carrier protein) and converted to glucose
Glucose, galactose, and any remaining fructose are transported out of the base of the cell by facilitated diffusion
Absorbed by blood capillaries in the villus
Hepatic portal system delivers them to the liver
Lactose Intolerance
In people without lactase, lactose passes undigested into large intestine
- Increases osmolarity of intestinal contents
- Causes water retention in the colon and diarrhea
- Gas production by bacterial fermentation of the lactose
Occurs in many people
•15% of American Whites, 90% of American Blacks, 70% of Mediterraneans; and nearly all of Asian descent
Can consume yogurt and cheese since bacteria have broken down the lactose
Protein Digestion- Sources
Amino acids absorbed by the small intestine come from three sources
- Dietary proteins
- Digestive enzymes digested by each other
- Sloughed epithelial cells digested by enzymes
Endogenous amino acids from last two sources total about 30 g/day
Exogenous amino acids from our diet total about 44 to 60 g/day
Protein Digestion: Proteases
Proteases (peptidases)—enzymes that digest proteins
- Begin their work in stomach in optimum pH of 1.5 to 3.5
- Pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine
- Pepsin digests 10% to 15% of dietary protein into shorter peptides and some free amino acids
Protein Digestion: Small Intestine- Pepsin
Protein digestion continues in small intestine
- Pepsin inactivated when it passes into the duodenum and mixes with alkaline pancreatic juice (pH 8)
- Pancreatic enzymes trypsin and chymotrypsin take over the process
Hydrolyze polypeptides into even shorter oligopeptides
Protein Digestion: Carboxypeptidase, Aminopeptidase, & Dipeptidase
1.Carboxypeptidase—removes amino acids from –COOH end of the chain
Carboxypeptidase is a pancreatic secretion
- Aminopeptidase (Brush border enzyme)—removes amino acids from –NH2 end
- Dipeptidase (brush border enzyme)—splits dipeptides in the middle and release two free amino acids
Protein Absorbstion
Brush border enzymes finish task, producing free amino acids that are absorbed into intestinal epithelial cells
- Sodium-dependent amino acid cotransporters move amino acids into epithelial cells
- Facilitated diffusion moves amino acids out into bloodstream
Infants absorb proteins by pinocytosis (maternal IgA) and release them into the blood by exocytosis
Lipids Absorption
Hydrophobicity of lipids makes their digestion and absorption complicated
Lipases—fat-digesting enzymes
- Lingual lipase secreted by intrinsic salivary glands of the tongue
- Active in mouth, but more active in stomach along with gastric lipase
- 10% to 15% of lipids digested before reaching duodenum
- Before digestion in duodenum, vigorous pumping in stomach’s antrum emulsifies the fat (breaks up globs)
Emulsification droplets are passed to small intestine
Lipid Digestion: Emulification Droplets Breakdown
Emulsification droplets are broken down further by bile, lecithin, and agitation produced by intestinal segmentation
•Exposes more fat surface to enzymatic action
There is enough pancreatic lipase in the small intestine after a meal to digest the average daily fat intake in as little as 1 to 2 minutes
Lipase acts on triglycerides
•Removes first and third fatty acids from glycerol backbone, but leaves the middle one
The product of lipase action are two free fatty acids (FFAs) and a monoglyceride
Lipid Digestion: Micelles
Absorption of free fatty acids, monoglycerides, and other lipids depends on minute droplets in the bile called micelles
- Made in the liver
- Consist of 20 to 40 bile acid molecules aggregated with their hydrophilic side groups facing outward and their hydrophobic steroid rings facing inward
Bile phospholipids and cholesterol diffuse into the center of the micelle to form its core
Lipid Digestion: Micelles Function
Micelles pass down the bile duct into the duodenum
•There they absorb fat-soluble vitamins, cholesterol, and the FFAs and monoglycerides produced by fat digestion
They transport lipids to the surface of the intestinal absorptive cells
Lipids leave the micelles and diffuse through the plasma membrane into the cells
Micelles are reused, picking up another cargo of lipid,
transporting them to the absorptive cells
Lipid Digestion: free fatty acids and monoglycerides
Within the intestinal cell, free fatty acids and monoglycerides are transported to the smooth ER
Resynthesized into triglycerides
Golgi complex coats these with phospholipids and protein to form chylomicrons
- Packaged into secretory vesicles that migrate to basal surface of cell
- Release their contents into core of villus
- Taken up by lacteal into lymph
- White, fatty intestinal lymph (chyle) flows into larger and larger lymphatic vessels until it enters the bloodstream
Nucleic Acid Digestion
Nucleic acid
- Nucleases (deoxyribonuclease and ribonuclease) of pancreatic juice hydrolyze DNA and RNA to nucleotides
- Nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar, and nitrogenous bases
Membrane carriers allow absorption
Vitmain Digestion
- Absorbed unchanged
- Fat-soluble vitamins: A, D, E, and K absorbed with other lipids
- If ingested without fat-containing food, they are not absorbed at all but are passed in the feces and wasted
- Water-soluble vitamins, B complex and C, absorbed by simple diffusion and B_12 if bound to intrinsic factor from the stomach
Mineral Digestion (Electrolytes)
Minerals (electrolytes)
- Absorbed all along small intestine
- Na+ cotransported with sugars and amino acids
- Cl− exchanged for bicarbonate reversing chloride–bicarbonate exchange that occurs in the stomach
- K+ absorbed by simple diffusion
Mineral Digestion (Iron & Calcium)
Iron and calcium absorbed as needed
- Iron absorption is inhibited by liver hormone hepcidin to prevent overload
- Absorptive cells bind ferrous ions (Fe2+) and internalize them by active transport
- Unable to absorb ferric ions (Fe3+) but stomach acid reduces ferric ions to absorbable ferrous ions
- Transferrin (extracellular protein) transports iron in blood to bone marrow, muscle, and liver
Mineral Digestion: Calcium Absorbtion Mechanisms
1.Transcellular absorption in the duodenum
- Enters through calcium channels in apical cell membrane
- Binds to calbindin protein so concentration gradient will continue to favor calcium influx
- Actively transported out of base of cell into bloodstream by calcium–ATPase and Na+–Ca2+ antiport
2.Diffusion between epithelial cells in jejunum and ileum
- Most absorbed calcium is from meat and dairy
- Dietary fat retards calcium absorption
Mineral Digestion: Calcium PTH
Parathyroid hormone—secreted in response to a drop in blood calcium levels
- Stimulates kidney to synthesize vitamin D from precursors made by epidermis and liver
- Vitamin D affects absorptive cells of the duodenum in three ways
- Increases number of calcium channels in apical membrane
- Increases the amount of calbindin in cytoplasm
- Increases the number of calcium–ATPase pumps at basal membrane
- Parathyroid hormone increases the level of calcium in the blood
Water Digestion
Digestive system is one of several systems involved in fluid balance
Digestive tract receives about 9 L of water/day
- 0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions
- 8 L is absorbed by small intestine and 0.8 L by large intestine
0.2 L voided in daily fecal output
Water Digestion: Mechanisms
Water is absorbed by osmosis following the absorption of salts and organic nutrients
Diarrhea—occurs when large intestine absorbs too little water
- Feces pass through too quickly if intestine is irritated
- Feces contain high concentrations of a solute (such as lactose)
Constipation—occurs when fecal movement is slow, too much water gets reabsorbed, and feces become hardened
Large Intestine: Gross Anatomy
Large intestine
- Measures 1.5 m (5 ft.) long and 6.5 cm (2.5 in.) in diameter in cadaver
- Begins as cecum inferior to ileal papilla
- Appendix attached to lower end of cecum
- Densely populated with lymphocytes—a significant source of immune cells
- Ascending colon, right colic (hepatic) flexure, transverse colon, left colic (splenic) flexure, and descending colon frame the small intestine
- Sigmoid colon is S-shaped portion leading down into pelvic cavity
Large Intestine: Gross Anatomy- Rectume
Rectum: portion ending at anal canal
•Has three curves and three infoldings, called the transverse rectal folds (rectal valves)
Large Intestine: Gross Anatomy- Anal Canal
Anal canal: final 3 cm of the large intestine
- Passes through levator ani muscle and pelvic floor, terminates at the anus
- Anal columns and sinuses—exude mucus and lubricant into anal canal during defecation
- Large hemorrhoidal veins for superficial plexus in anal columns and around orifice
- Hemorrhoids—permanently distended veins that protrude into anal canal or bulge outside the anus
- Obesity and pregnancy
Large Intestine: Muscularis Externa Features
- Taenia coli—longitudinal fibers concentrated in three thickened, ribbon-like strips
- Haustra—pouches in the colon caused by the muscle tone of the taeniae coli
- Internal anal sphincter—smooth muscle of muscularis externa
- External anal sphincter—skeletal muscle of pelvic diaphragm
Omental appendices
club-like, fatty pouches of peritoneum adhering to the colon; unknown function
Large Intestine Blood Supply
Supplied by mesenteric arteries.
The superior and inferior mesenteric veins drain the same parts of the large intestine as the correspondingly named arteries and drain into the hepatic portal system
Large Intestine: Microscopic Anatomy- Mucosa
simple columnar epithelium
•Anal canal has nonkeratinized stratified squamous epithelium in its lower half for abrasion resistance
Large Intestine: Microscopic Anatomy- Circular Folds/ Villi
None
Large Intestine: Microscopic Anatomy- Intestinal Crypts
Intestinal crypts—glands sunken deep into lamina propria with a high density of mucus-secreting goblet cells
Large Intestine: Microscopic Anatomy- Lamina Propia & Submucosal Layer
Lamina propria and submucosal have a lot of lymphatic tissue
•Provides protection from large population of bacteria in large intestine
Intestineal Microbes
Gut microbiome—about 800 species of bacteria that populate the large intestine
- Bacteria digest cellulose, pectin, and other carbohydrates for which our cells lack enzymes
- Help in synthesis of vitamins B and K
Flatus
Flatus—intestinal gas
- Average person produces 500 mL of flatus per day
- Most gas in large intestine is reabsorbed instead
- Much of flatus is swallowed air, but bacteria add to it
- Hydrogen sulfide, indole, and skatole produce odor
- Hydrogen gas may explode during electrical cauterization used in surgery
Large Intestine: Absorbtion & Motility
Large intestine takes about 36 to 48 hours to reduce residue of a meal to feces
- Most time in transverse colon
- Does not chemically change the residue
- Reabsorbs water and electrolytes
Feces consist of about 75% water and 25% solids
•Solids: 30% bacteria, 30% undigested fiber, 10% to 20% fat, small amount of mucus, proteins, salts, digestive secretions, and sloughed epithelial cells
Large Intestine: Absorbtion & Motility/ Contraction & Movement
Haustral contractions occur every 30 minutes
- Distension of a haustrum stimulates it to contract
- Churns and mixes residue promoting water and salt absorption
Mass movements—stronger contractions that occur one to three times a day
- Triggered by gastrocolic and duodenocolic reflexes
- Filling of the stomach and duodenum stimulates motility of the colon
- Move residue several centimeters
Neural Control of Defecation
- Feces stretch the rectum and stimulate stretch receptors, which transit signal to the spinal cord.
- A spinal reflex stimulates a contraction of the rectum
- The spinal reflex also relaxes the internal anal sphincter
- Impulses from the brain prevent untimely defecation by keeping the extermal anal sphinchter contracted
***Defecation only occurs only if this sphincter also relaxes
of Chromosome & Types
•Our cells contain 23 pairs of chromosomes
–22 pairs of autosomes
–1 pair of sex chromosomes (XY males; XX females)
- Males produce half Y-carrying sperm and half X-carrying sperm
- All eggs carry the X chromosome
Chromosomal Sex Determination:
•Sex of child determined by type of sperm that fertilizes
mother’s egg
–X-carrying sperm fertilizes the egg: female
–Y-carrying sperm fertilizes the egg: male
Prenatal Hormones and Sexual Differentiation
- Initially, a fetus is sexually undifferentiated
- Gonads begin to develop at 5 or 6 weeks as gonadal ridges
- Two sets of ducts adjacent to each gonadal ridge
–In males, mesonephric ducts develop into reproductive tract; paramesonephric ducts degenerate
–In females, paramesonephric ducts develop into reproductive tract; mesonephric ducts degenerate
Sexual Differentiation:Paramesoephric Ducts
- The two sexes indistinguishable for first 8 to 10 weeks of development
- Female reproductive tract develops from the paramesonephric ducts
–Not because of the positive action of any hormone
–Because of the absence of testosterone and müllerian-inhibiting factor (MIF)
Sexual Differentiation:Prenatal Hormones/ SRY Gene
•SRY gene (sex-determining region of Y chromosome) found only in males
–SRY codes for a protein, testes-determining factor (TDF), that initiates development of testes
–Testes begin to secrete testosterone at 8 to 9 weeks
•Stimulates mesonephric ducts to develop into male tracts
–At same time, the testes secrete müllerian-inhibiting factor causing degeneration of the paramesonephric ducts
Sexual Differentiation: Estrogen
•Estrogen levels are always high in pregnancy
–If estrogen was the hormone that directed female development, all fetuses would be feminized from mothers hormone
•Female development of a fetus occurs whenever there is an absence of androgen hormones
–Not because estrogen is present
Descent of gonads
•Gonads initially develop high in the abdominal cavity and then migrate into the pelvic cavity (ovaries) or scrotum (testes)
–Gubernaculum—embryonic connective tissue cord extending from gonad to pelvic cavity floor
•In the male, it passes between the internal and external abdominal oblique muscles into the scrotal swelling
–Vaginal process: fold of peritoneum that extends into the scrotum
–Inguinal canal: pathway of low resistance through the groin created by gubernaculum and vaginal process
- Most common site of hernia in males
- Descent of the testes begins as early as 6 weeks
–In seventh month, testes pass through the inguinal canal into the scrotum guided by the gubernaculum
–Testes accompanied by elongating testicular arteries and veins, lymphatic vessels, nerves, spermatic ducts, and extensions of internal abdominal oblique muscle
Cryporchidism
•undescended testes
–Occurs in about 3% of male births
–In most cases the testes descend during the first year of infancy
–If not, testosterone injection or simple surgery can draw testes into the scrotum
Uncorrected cases lead to sterility or testicular cancer
The Testes: Anatomy
- combined endocrine and exocrine glands that produce sex hormones and sperm
- Each testis: oval and slightly flattened, 4 cm long x 2.5 cm in diameter
–Covered anteriorly and laterally by tunica vaginalis
- Tunica albuginea: white fibrous capsule on testes
- Connective tissue septa divides testes into 250 to 300 wedge-shaped lobules
The Testes: Semineferous Tubules
•ducts where sperm are produced
–One to three in each lobule
–Each tubule lined with a thick germinal epithelium of germ cells (becoming sperm) and nurse cells
–Nurse cells in between germ cells
- Protect the germ cells, and promote their development
- Germ cells depend on them for nutrients, waste removal, growth factors, and other needs
The Testes: Interstitial Endocrine Cells
•Interstitial endocrine cells between tubules produce testosterone
