Unit 9 - Digestive System Flashcards
Digestive system role
The digestive system plays a critical role in breaking down and absorbing the nutrients, electrolytes, and water the body needs in order to maintain homeostasis.
Basic Functions of the Digestive Tract
- Motility
- Digestion
- Secretion
- Absorption
- Storage & Elimination of Wastes
- Immune Functions
Motility
food is ingested and moved through the tube of the GI tract using the following processes:
a. Ingestion
b. Deglutition
c. Peristalsis
d. Mass movements
Ingestion
taking food into the mouth
Deglutition
swallowing of food.
Peristalsis
rhythmic wavelike contractions of smooth muscle that move food through GI tract (like squeezing toothpaste out of a tube starting from the closed end and squeezing toward the open end).
Mass movements
a strong wave of smooth muscle contraction over a long segment of the tract (usually intestines). Usually involved in defecation reflex.
Digestion
a. Mechanical
i. Mastication
ii. Churning
iii. Segmentation
b. Chemical
i. Acidic pH
ii. Digestive enzymes
Mechanical Digestion
the physical breakdown of large pieces of food into smaller pieces. Involves:
i. Mastication
ii. Churning
iii. Segmentation
Mastication
chewing of food and mixing it with secretions from the salivary glands.
Churning
mixing of food in stomach due to contraction of 3 layers of smooth muscle.
Segmentation
alternating contractions of smooth muscle between adjacent areas of the stomach / intestines – mixes material being digested/absorbed.
Chemical Digestion
the chemical breakdown of large nutrient molecules (macromolecules/macronutrients) in small molecules (micronutrients) that are capable of being absorbed. Involves:
i. Acidic pH
ii. Digestive enzymes
Acidic pH
important for the chemical digestion of proteins and fats.
Digestive enzymes
act on macromolecules to break them down into smaller ones. For example, PANCREATIC AMYLASE digests complex carbohydrates like starch and glycogen into disaccharides and oligosaccharides in the small intestine.
Secretion
various substances are secreted either by cells lining the tract or cells of accessory organs.
a. Exocrine secretions
b. Endocrine secretions
Exocrine secretions
into lumen of GI tract. Includes:
i. Water (note: water is also reabsorbed)
ii. Hydrochloric acid (HCl)
iii. Bicarbonate (HCO3-)
iv. Enzymes
v. Bile
vi. Mucus
Endocrine secretions
into blood. Includes hormones such as gastrin, cholecystokinin, secretin, etc. that help to regulate digestion.
Absorption
transport of digested molecules from the lumen of (primarily) the small intestine into enterocytes (cells of the intestinal epithelium) followed by transport into the interstitial fluid and then into the blood or lymph.
Storage and Elimination of Wastes
- The large intestine (colon) stores waste and any undigested material until they can be eliminated.
- Excretion of waste by the digestive tract = DEFECATION
Excretion of waste by the digestive tract =
DEFECATION
Immune Functions
- Acts as a PHYSICAL BARRIER to foreign pathogens. TIGHT JUNCTIONS between columnar cells of the gastrointestinal epithelium stop pathogens and their toxins from entering into the body.
- Gut microbiome outcompetes harmful microbes and promotes production of antimicrobial peptides on the surface of intestinal cells (recall Unit 4: the Immune System).
- M cells (microfold cells) in small intestine sample gut contents and endocytose any antigens. Antigens are transported to Peyer’s patches (in contact with basolateral membrane of M cells) and are presented to dendritic cells, macrophages and B and T lymphocytes that will initiate the immune response.
M cells (microfold cells) in small intestine
sample gut contents and endocytose any antigens. Antigens are transported to Peyer’s patches (in contact with basolateral membrane of M cells) and are presented to dendritic cells, macrophages and B and T lymphocytes that will initiate the immune response.
The gastrointestinal tract is essentially a
long tube, divided into various regions that are specialized to carry out the basic functions related to digestion. Accessory organs including the salivary glands, pancreas, liver, and gall bladder, produce and/or release secretions into the tube (or blood) that assist the processes of digestion and absorption.
Accessory organs including the…
salivary glands, pancreas, liver, and gall bladder, produce and/or release secretions into the tube (or blood) that assist the processes of digestion and absorption.
GI Tract structures, listed in order, include:
- Mouth/Oral Cavity
- Pharynx
- Esophagus
- Upper Esophageal Sphincter
- Lower Esophageal Sphincter - Stomach (passage time ~2.5-5 hours)
- Small Intestine – digestion and absorption of nutrients. (passage time ~2.5-6 hours)
a. Duodenum – main site of chemical digestion b. Jejunum
c. Ileum
- Ileocecal valve - Caecum
- Large intestine (colon)
- Rectum
- Anus
- Internal Anal Sphincter
- External Anal Sphincter
Upper Esophageal Sphincter
skeletal muscle – opens so that food bolus can enter esophagus; closes to prevent backflow from esophagus into pharynx.
Lower Esophageal Sphincter
closes to prevent backflow (reflux) of food and gastric juices into the esophagus (i.e. heartburn).
Stomach
(passage time ~2.5-5 hours)
- Connected to duodenum via pyloric sphincter/valve. This valve controls the release of chyme (mix of partially digested food material and gastric juices) into the duodenum.
Small Intestine
digestion and absorption of nutrients. (passage time ~2.5-6 hours)
a. Duodenum – main site of chemical digestion b. Jejunum
c. Ileum
- Ileocecal valve
Duodenum
main site of chemical digestion
Ileocecal valve
controls release of chyme from ileum into caecum. Prevents entry of bacteria from colon into small intestine
Caecum
blind pouch that includes appendix, leads into ascending colon. Contains bacteria (probiotics).
Large intestine (colon)
absorbs water, vitamins (passage time ~30-40 hours).
- Parts: Ascending, Transverse, Descending, Sigmoid colon.
Anus
two sphincters involved in the defecation reflex.
- Internal Anal Sphincter – smooth muscle, involuntary control.
- External Anal Sphincter – skeletal muscle, voluntary control.
Internal Anal Sphincter
smooth muscle, involuntary control.
External Anal Sphincter
skeletal muscle, voluntary control.
Accessory structures to the GI tract include:
- Teeth
- Tongue
- Salivary glands
a. Parotid
b. Sublingual
c. Submandibular - Liver
- Gall Bladder
- Pancreas
Teeth
chop and tear food into smaller pieces.
Tongue
manipulates food and plays a role in taste and swallowing.
Salivary glands
secrete saliva which helps to moisten food and contains digestive enzymes to begin chemical digestion.
a. Parotid (just anterior to the ears)
b. Sublingual (under the tongue)
c. Submandibular (under the lower jaw bone)
Parotid Salivary Glands
just anterior to the ears
Sublingual Salivary Glands
under the tongue
Submandibular Salivary Glands
under the lower jaw bone
Liver
produces bile and filters and detoxifies absorbed nutrients.
Gall Bladder
stores, concentrates and releases bile, which acts to emulsify fats and increase the surface area for the action of pancreatic lipase.
Pancreas
secretes digestive enzymes and alkaline fluid into duodenum.
Tissue Layers of the GI tract wall:
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
Mucosa Includes
a. Mucosal Epithelium
b. Lamina propria
c. Muscularis mucosae
Mucosal Epithelium
lines inside surface of the tract
- Apical surface is in contact with lumen of tract, basolateral surface in contact with basement membrane.
- Esophagus and rectum – have stratified squamous epithelium (provides protection from abrasiveness of swallowed foods and solid wastes respectively).
- Stomach and Intestines – have simple columnar epithelium (involved in absorption and secretion)
Lamina propria
holds the epithelium in place
- Loose connective tissue layer
- Contains small blood vessels, lymph LACTEALS (i.e. lymphatic capillaries, which will play an important role in fat absorption).
Muscularis mucosae
Thin layer of smooth muscle separating mucosa and submucosa. Contraction can help to dislodge food from the folds/crypts of the intestine.
Submucosa
- Thick layer of connective tissue containing glands, nerves and blood vessels.
- Contains the SUBMUCOSAL PLEXUS, the first nerve network of the enteric nervous system.
Muscularis externa
Two layers of smooth muscle:
a. Inner layer = circular muscle; allows for constriction/dilation of the tract.
b. Outer layer = longitudinal muscle that shortens and lengthens tract.
- Stomach has a third layer that helps to churn stomach contents.
a. oblique layer = muscle located between the submucosal plexus and the
the circular layer. - Contains the MYENTERIC PLEXUS (between the inner circular and outer longitudinal), the second nerve network of the enteric nervous system.
- While most of the tract consists of these 2-3 layers, of smooth muscle, the middle 1/3 of the esophagus contains both skeletal muscle and smooth muscle to allow for some voluntary control of swallowing.
Serosa
- Part of peritoneal membrane (peritoneum) lining the abdominal cavity.
- Composed of connective tissue and simple squamous epithelium.
- Forms sheets of connective tissue that help to hold organs in place.
- Also provides support for blood vessels and nerves going to/from abdominal organs.
Motility of the gastrointestinal tract is
critical not only for moving material along the tube, but also for controlling the conditions and amount of time available for adequate digestion and absorption of nutrients.
Contraction of smooth muscle in the GI Tract is of 3 types:
- Peristalsis
- Segmental contractions
- Migrating Motor Complexes
Peristalsis
- small, weak waves of contraction in which circular smooth muscle contracts upstream from the bolus of food, while muscle downstream (in front of bolus) relaxes. Result is bolus is pushed further along the tube.
- In small intestine. – occurs as part of a reflex response triggered by distension of intestinal wall
- The frequency of waves is efficient enough to move the contents of the lumen.
- Transit time along the small intestine is ~3-9 hours
- Primarily controlled by MYENTERIC PLEXUS
Segmental contractions
mixes
- Alternating contraction and relaxation of circular muscle in adjacent sections of the intestine.
- Mixes up and churns the ingested material, which ensures that it all gets exposed to the absorptive surface (material from the middle of the lumen gets moved to be in direct contact with the mucosa). And also ensures exposure to digestive enzymes (mixing distributes enzymes amongst the food particles for digestion).
- Contributes very little to propulsion of the material through the tube, since material is being moved both backward and forward along the tube.
Migrating Motor Complexes
(mass movements)–
- Similar to peristalsis but involves a larger/stronger wave of
contraction that propels the substance over a longer distance (up to the entire length of the small and large intestines).
- These movements are a cleansing contraction that propel undigested material as well as pathogens and bacteria out of the small intestine and into the large intestine.
- Typically occur during or right after a meal (cleans out the tube to get it ready to process the incoming food).
- Part of the gastrocolic reflex (a long reflex arc, also known as the defecation reflex) which is initiated by distension of the stomach or duodenum when consuming a meal. And is the reason that people may feel the urge to defecate right after (or even during) a meal, particularly breakfast. Any physiological response after eating a meal is known as a POSTPRANDIAL RESPONSE.
Accelerated motility of the gut results in
diarrhea, which is associated with increased frequency and strength of migrating motor complexes.
Decreased motility results in
constipation.
Constipation may lead to perforation of the large intestine (a hole in the bowel). This causes the release of bacteria and feces into the abdominal cavity, and the resulting infection (sepsis) is often life threatening (similar to a burst appendix)
Regulation of GI Tract Motility & Secretion
Involves
Involves both intrinsic control by the enteric nervous system and local signaling molecules (paracrines) and extrinsic control by the parasympathetic and sympathetic branches of the nervous system and hormones of the endocrine system.
Involves reflex responses that are integrated in both the enteric nervous system (short reflexes) and the central nervous system (long reflexes). This includes any reflex involving the brain (cephalic reflexes).
Regulation of GI Tract Motility & Secretion
- Neural Control
a. Enteric NS
b. The Autonomic Nervous System
c. Cognitive and Emotional Control - Chemical Signaling
Neural Control
a. Enteric NS – “brain” of the gut; contains as many neurons as the spinal cord and provides local intrinsic control of motility and secretion. Involved in short reflexes.
i. Submucosal plexus
ii. Myenteric plexus
b. The Autonomic Nervous System
i. Parasympathetic Nervous System (PSNS)
ii. Sympathetic
c. Cognitive and Emotional Control
Enteric NS
“brain” of the gut; contains as many neurons as the spinal cord and provides local intrinsic control of motility and secretion. Involved in short reflexes.
Submucosal plexus
- Located within the submucosal layer of the GI tract wall
- Controls functions associated with the mucosa including: sensing the environment of the GI tract lumen; regulating local blood flow; and controlling epithelial cells (including secretions from glands).
Myenteric plexus
- Located in the muscularis externa between the circular and longitudinal layers.
- Controls GI motility by:
- Increasing smooth muscle tone
- Increasing the intensity of rhythmic contractions
- Increasing the rate of rhythmic contractions
- Increasing the velocity of peristaltic waves
- Decreasing sphincter muscle tone
The Autonomic Nervous System
i. Parasympathetic Nervous System (PSNS)
- Involved in long reflexes.
- Activity of the PSNS increases most GI functions.
- The proximal gut is innervated by the vagus nerve (controls the esophagus, stomach, pancreas, and upper large intestine)
- As a result this nerve plays an important role in deglutition (swallowing), vomiting gastric emptying, and pancreatic secretion).
- The distal gut is innervated by the pelvic nerves which control the remainder of the large intestine and rectum and so help to control defecation.
- There is very little innervation of the small intestine, as the activities carried out there are primarily regulated by the enteric nervous system as well as hormones.
The Autonomic Nervous System
ii. Sympathetic
- Involved in long reflexes.
- Activity generally decreases most GI functions.
- Involves spinal nerves from T5 to L2 that carry preganglionic neurons to the autonomic sympathetic ganglia. Postganglionic neurons then carry signals from the ganglia to all parts of the GI tract
- The release of norepinephrine (noradrenaline) inhibits the neurons of both the myenteric plexus and submucosal plexus.
Cognitive and Emotional Control
- There are neurological pathways connecting the senses and brain with the stomach – cephalic reflexes (long reflexes).
- Feedforward reflexes in response to the sight, smell, thought of food, increases acid production, motility and blood flow in the gut.