PCAT Biology: Digestive System
What is cholecystokinin (CCK)?
A hormone that is produced and stored in the I cells of the duodenal and jejunal mucosa. It is involved in stimulation of pancreatic enzyme and somatostatin secretion as well as gallbladder contraction. Oh and also its a hunger suppressant
What is the stomach?
A large muscular organ located in the upper abdomen that stores and partially digests food. Its walls are lined by gastric mucosa
What is the gastric mucosa?
A mucous membrane that contains the gastric glands
What is the lower esophageol sphincter?
A muscular structure that closes off the esophagus from the stomach by contracting
What causes saliva to be secreted?
A nervous reflex triggered by the presence of food in the oral cavity
What does the large intestines do?
Absorbs salts and any water not already absorbed by the small intestine
What is the small intestines highly adapted to?
Absorption
What is salivary amylase?
An enzyme in the saliva that hydrolyzes starch to maltose
What is digestion?
Degradation of large molecules into smaller molecules that can be absorbed into the bloodstream and used directly by cells
What is gastrin?
Hormone produced in the G cells of the duodenum that function to stimulate histamine and pepsinogen secretion as well as increase gastric blood flow. It also stimulates the parietal cells to produce HCl which denatures proteins and activates digestive enzymes
What is secretin?
Hormone synthesized and stored in the S cells of the upper intestine. It stimulates the secretion of bicarbonate-containing substances from the pancreas and inhibits gastric emptying and gastric acid production
Where does most digestion in the small intestions occur?
In the duodenum
How does food enter the body?
Ingestion
What happens to food after ingestion?
It begins digestion
Describe the area the esophagus is located:
It is within the thoracic cavity which is negatively pressured relative to the environment on inhalation. The abdominal cavity has a relative positive pressure so without normal defense mechanisms, the pressure gradient will favor a continual reflux of gastric materials into the esophagus resulting in gastroesophageal reflux disease
Does bile have enzymes?
No
What are villi?
Numerous finger-like projections that extend out of the intestinal wall. They contain capillaries and lacteals
What are heterotrophs?
Organisms unable to synthesize all of their own nutrients and must use food to provide raw energy materials for energy, repair, and growth of tissues
What does the liver do?
Produces bile that is stored in the gallblader before release into the small intestine. It stores glycogen, converts ammonia to urea, does protein synthesis, detoxification, and cholesterol metabolism
What does the pancreas do?
Produces enzymes such as amylase, trypsin, and lipase
What does the rectum do?
Provides storage for transient storage of feces before elimination through the anus
What is chemical digestion?
Refers to the enzymatic breakdown of macromolecuels into smaller molecules and begins int he mouth when the salivary glands secrete saliva.
What accessory organs also play a role in the digestive tract?
Salivary glands, pancreas, liver, and gallbladder
What does the intestinal mucosa do?
Secretes lipases (for fat), aminopeptidases (for polypeptide digestion), and disaccharidases (for maltose, lactose, and sucrose digestion).
What happens in the duodenum?
Secretions of the intestinal glands, pancreas, liver, and gallbladder mix together with the acidic chyme entering from the stomach
What regulates the activity of the digestive system?
Several hormones which allow for optimum flow of materials through the digestive tract and help regulate the hunger and satiation mechanisms
Where is the chemical digestion completed?
Small intestine
Are all nutrients in the intestines actively absorbed?
Some are like glucose and amino acids but others can be passively absorbed
What do mucous cells in the gastric pits along the membrane do?
Ssecrete mucous to protect the stomach lining from the harshly acidic juices present in the stomach
What do the chief cells in th egastric glands do?
Synthesize pepsinogen that converts to pepsin when in contact with stomach acid and breaks down proteins
What is mechanical digestion?
The breakdown of large food particles into smaller particles through physical actions such as biting and chewing action of teeth, or the churning motion of the stomach
What do parietal cells do?
They are present within the gastric glands and they synthesize and release HCl (which alters the pH of the stomach and kills bacteria) and intrinsic factor (which is necessary for the absorption of vitamine B12)
What happens to large fatty acids and glycerol that pass into the lacteals?
They are reconverted into fats
Describe the movement of amino acids and monosaccharides in the intestines:
They pass through the villi walls into the capillary system
Why is the intestine extremely long and highly coiled?
To maximize the surfae area available for digestion and absorption
Why does the pancreas secrete a bicarbonate-rich juice?
To neutralize the acidic chyme arriving from the stomach in the duodenum because the pancreatic enzymes work better at a high pH
What are lacteals?
Vessels of the lymphatic system
What hormones regulate the digestive system?
Gastrin, intrinsic factor, cholecystokinin, and secretin
What is the path food must take in the digestive tract?
Begins in the oral cavity then leads to the pharnyx, esophagus, stomach, small intestine, large intestine, and anus
What does disaccharidase lactase do? Why is this important?
Breaks down lactose which is milk sugar. This is present in infants but as adults many dont have it making them lactose intollerant. The lactose in their small intestines cannot be digested and thuse are metabolized by bacteria which produces intestinal discomfort
What is extracellular digestion?
Digestion that occurs outside of the cell, within a lumen or tract
What does the enzyme amylase do?
Digests carbohydrates
What does lipase do?
Digests fat
What does the enzyme trypsin do?
Digests protein
What is intracellular digestion?
Disgestion that occurs inside the cell usually in membrane bound vesicles
What does bile do?
Emulsifies fat, breaking down large globules into small droplets
What happens after the pancreas releases chymotrypsin and enterokinase?
Enterokinase cleaves trypsinogen to trypsin. The trypsin then cleaves and activates the other zymogens.
What does the emulsification of fat do?
Exposes a greater surface area of the fat to the action of pancreatic lipase
What is bolus?
Food that has been sufficiently moistened and broken down to be swallowed down the esophagus
What does saliva do?
Lubricates food to facilitate swallowing and provides a solvent for food particles
What happens in the oral cavity?
Mechanical and chemical digestion of food begins
Where does blood from the digestive tract enter? Why?
The hepatic portal system of the liver where it is detoxified and stripped of some of its nutrients
What is the oral cavity?
The mouth
What is the esophagus?
The muscular tube leading from the mouth to the stomach
What is peristalsis?
The rhythmic wave of involuntary muscular contractions that moves food down the esophagus
What is the intrinsic factor?
The secretion of the parietal cells that facilitates the absorption of vitamin B12 across the intestinal lining
What are the three divisions of the small intestines?
duodenum, jejunum, ileum
