Digestive System Flashcards
Intracellular digestion
As part of metabolism, involves the oxidation of glucose and fatty acids for energy
Alimentary canal
Runs from the mouth to the anus and is sectioned off by sphincters, or circular smooth muscles around the canal that can contract to allow compartmentalization of function
Mechanical digestion
The physical breakdown of large food particles into smaller food particles, but does not involve breaking chemical bonds
Chemical digestion
The enzymatic cleavage of chemical bonds, such as the peptide bonds of proteins or the glycosidic bonds of starches
Absorption
Involves the transport of products of digestion from the digestive tract into the circulatory system for distribution to the body’s tissues and cells.
Pathway of food in the digestive tract
Oral cavity/mouth –> pharynx –> esophagus –> stomach –> small intestine –> large intestine –> rectum
Enteric nervous system
A collection of one hundred million neurons that govern the function of the gastrointestinal system –> trigger peristalsis
Mastication
Chewing: helps to increase surface area-to-volume ratio of food, creates more surface area for enzymatic digestion as it passes through the gut tube
Salivary amylase
Capable of hydrolyzing starch into smaller sugars (maltose and dextrins)
Lipase
Catalyzes the hydrolysis of lipids
Pharynx
Cavity that leads from the mouth and posterior nasal cavity to the esophagus
3 parts of the pharynx
Nasopharynx (behind the nasal cavity), the oropharynx (at the back of the mouth), and the laryngopharynx (above the vocal cords)
Epiglottis
A cartilaginous structure that folds down to cover the laryngeal inlet –> prevents food from entering the larynx
Esophagus
Muscular tube that connects the pharynx to the stomach
Muscular composition of esophagus
Top third of the esophagus is composed of skeletal muscle, bottom third is composed of smooth muscle, and middle third is a mix of both
Emesis
vomiting
Upper esophageal sphincter
Where swallowing is inititaed; in the orpharynx
Lower esophageal sphincter (cardiac sphincter)
Relaxes and opens to allow the passage of food
What are the 3 main energy sources for the body
Carbohydrates, fats, and proteins
Stomach
Highly muscular organ with a capacity of approximately 2 liters. In humans, the stomach is located in the upper left quadrant of the abdominal cavity, underneath the diaphragm.
The 4 main anatomical divisions of the stomach
The fundus and body, which contain mostly gastric glands,
antrum and pylorus, which contain mostly pyloric glands.
Lesser curvature
The internal curvature of the stomach
Greater curvature
External curvature of the stomach
Rugae
Lining of the stomach is thrown into folds
Gastric glands
Respond to signals from the vagus nerve of the parasympathetic nervous system, which is activated by the brain in response to the sight, taste, and smell of food.
What are the 3 different cell types in gastric glands
Mucous cells, chief cells, and parietal cells
Mucous cells
Produce the bicarbonate-rich mucus that protects the muscular wall from the harshly acidic (pH = 2) and proteolytic environment of the stomach
Gastric juice
A combination of secretions from the other 2 cell types in the gastric glands: chief cells and parietal cells
Chief cells
Secretes pepsinogen: the inactive, zymogen form of pepsin, a proteolytic enzyme.
Parietal cells
Secrete hydrogen ions in the stomach as hydrochloric acid, which cleave pepsinogen to pepsin. Also secretes intrinsic factor.
Pepsin
Digests proteins by cleaving peptide bonds near aromatic amino acids, resulting in short peptide fragments
Intrinsic factor
A glycoprotein involved in the proper absorption of vitamin B12.
Pyloric glands
Contain G-cells that secrete gastrin, a peptide hormone –> Induces the parietal cells in the stomach to secrete more HCL and signals the stomach to contract, mixing its contents.
Chyme
Acidic, semifluid mixture from digestion of solid food in the stomach
What are the 3 segments of the small intestine
The duodenum, the jejunum, and the ileum
Where does the majority of chemical digestion in the small intestine occur
Duodenum
Where does the majority of absorption in the small intestine occur
The jejenum and the ileum
Pyloric sphincter
Sphincter between the stomach and the duodenum
Brush-border enzymes
Present on the luminal surface of cells lining the duodenum and break down dimers and trimers of biomolecules into absorbable monomers –> released by presence of chyme
Disaccharidases
Digest disaccharides: maltase digests maltose, isomaltase digests isomaltose, lactase digests lactose, and sucrase digests sucrose.
Peptidases
Break down proteins (or peptides)
Aminopeptidase
A peptidase secreted by glands in the duodenum that removes the N-terminal amino acid from a peptide
Dipeptidases
Cleave the peptide bonds of dipeptides to release free amino acids.
Enteropeptidase
An enzyme critical for the activation of trypsinogen, a pancreatic protease, to trypsin. It also activates procarboxypeptidases A and B to their active forms
Secretin
A peptide hormone that causes pancreatic enzymes to be released into the duodenum. Also regulates the pH of the digestive tract by reducing HCl secretion from parietal cells and increasing bicarbonate secretion from the pancreas.
Enterogastrone
Includes secretin; A hormone that slow motility through the digestive tract –> increases time for digestive enzymes to act on chyme
Cholecystokinin (CCK)
A peptide hormone secreted in response to the entry of chyme into the duodenum –> stimulates the release of both bile and pancreatic juices and also acts in the brain, where it promotes satiety
Bile
A complex fluid composed of bile salts, pigments, and cholesterol
Bile salts
Derived from cholesterol: not enzymes but they emulsify fats and cholesterol into micelles. These micelles increase the surface area of the fats and prevent them from spontaneously separating out, allowing lipase to act.
Pancreatic juices
A complex mixture of several enzymes in a bicarbonate-rich alkaline solution. The bicarbonate helps neutralize acidic chyme and provides ideal pH for digestive enzymes. Contains enzymes that digest carbs, fats, and proteins.
Accessory organs of digestion
Pancreas, liver, and gallbladder. They all originate as outgrowths of endoderm from the gut tube during development.
Acinar cells
Make up the bulk of the pancreas: exocrine cells that produce pancreatic juices.
Pancreatic amylase
Breaks down large polysaccharides into small disaccharides and is therefore responsible for carb digestion
Pancreatic peptidases (trypsinogen, chymotrypsinogen, and carboxypeptidases A and B)
Released in their zymogen form, but once activated are responsible for protein digestion.
Pancreatic lipase
Breaks down fats into free fatty acids and glycerol
Major and minor duodenal papillae
Ducts that empty into the duodenum and through which acinar cells secrete their products
Liver’s location
Located in the upper right quadrant of the abdomen
Liver’s functions
Include processing and synthesis of nutrients (glycogenesis and glycogenolysis, storage and mobilization of fats, gluconeogenesis), production of urea, detoxification of chemicals, production of bile, and synthesis of albumim and clotting factors
Bile ducts
Connect the liver with both the gallbladder and small intestine: bile is produced in liver and travels down bile ducts where it may be stored in the gallbladder or secreted into the duodenum
Hepatic portal vein
Vein through which the liver receives all blood draining from the abdominal portion of the digestive tract.
Nutrient-rich blood can be processed by the liver before draining into the inferior vena cava on its way to the right side of the heart.
Bilirubin
Major pigment in bile, a byproduct of the breakdown of hemoglobin.
Gallbladder
Located just beneath the liver and both stores and concentrates bile. Upon release of CCK, the gallbladder contracts and pushes bile out into the biliary tree
Villi
Small, fingerlike projections from the epithelial lining of the small intestine
Microvilli
Many are present on each villus –> drastically increases the surface area available for absorption
Concentration gradient between blood and epithelial cells
Blood always has a lower concentration of monosaccharides and amino acids than inside the epithelial cells. Thus, simple carbs and amino acids diffuse from the epithelial cells into the capillaries –> absorbed molecules then go to the liver via the hepatic portal circulation.
Chylomicrons
Packages for triglycerides and esterified cholesterol molecules
How do chylomicrons enter the lymphatic circulation
through lacteals, which converge and enter the venous circulation at the thoracic duct in the base of the neck, which empties into the left subclavian vein.
Where are vitamins absorbed
Small intestine
What are the 4 fat-soluble vitamins
A, D, E, and K
How are fat-soluble vitamins absorbed
They dissolve directly into chylomicrons to enter the lymphatic circulation
How are water-soluble vitamins absorbed
They are taken up, along with water, amino acids, and carbohydrates, across the endothelial cells of the small intestine, passing directly into the plasma.
How is water absorbed into blood
Transcellularly (across the cell membrane) and paracellularly (squeezing between cells)
What is the primary function of the large intestine
Primarily involved in water absorption
What are the 3 major sections of the large intestine
The cecum, the colon, and the rectum.
Cecum
Simply an outpocketing that accepts fluid exiting the small intestine through the ileocecal valve and is the site of attachment of the appendix.
Parts of the colon
Ascending, transverse, descending, and sigmoid colons
Main function of colon
To absorb water and salts (such as sodium chloride) from the undigested material left over from the small intestine. –> material concentrated to form feces
Purpose of rectum
Serves as a storage site for feces, which consists of indigestible material, water, bacteria, and certain digestive secretions that are not reabsorbed (enzymes and some bile).
What are the 2 sphincters of the anus
The internal and external anal sphincters. External sphincter is under voluntary control (somatic), but the internal sphincter is under involuntary control (autonomic).
