Human Physiology - Exam 5

Digestion

The process by which food is broken down into smaller molecules that can be absorbed and metabolized by cells.

Peristalsis

wavelike muscular movement that moves contents through the gastrointestinal tract

Cephalic Phase

Phase of gastrointestinal control initiated by sensory receptors in the head, involving sight, smell, taste, and chewing.

Gastric Phase

Phase of gastrointestinal control initiated by the distension, acidity, and presence of certain nutrients in the stomach.

Intestinal Phase

Phase of gastrointestinal control initiated by stimuli in the small intestine, such as distension and the presence of digestive products.

Mucosa

The innermost layer of the gastrointestinal tract wall where most digestion and absorption occurs.

Enteric Nervous System

The system of nerves in the walls of the intestines, partially controlling gastrointestinal function independently from the central nervous system.

Chyme

The mixture of ingested food particles and gastric secretions in the stomach and small intestine.

Bile

A digestive fluid produced by the liver, necessary for fat digestion and the absorption of fat-soluble vitamins.

Pancreatic Enzymes

Digestive enzymes secreted by the pancreas which break down carbohydrates, proteins, and fats in the small intestine.

Gastrin

A hormone produced in the stomach that stimulates the secretion of hydrochloric acid and gastric motility.

Glucose-dependent insulinotropic peptide (GIP)

A hormone produced in the small intestine that stimulates insulin secretion from the pancreas in response to glucose and fat.

• also called gastric inhibitory peptide

Amylase

An enzyme that begins the digestion of carbohydrates by breaking down polysaccharides into simpler sugars.

Lipase

An enzyme that breaks down triglycerides into free fatty acids and monoglycerides.

Villi

Fingerlike projections in the small intestine that increase surface area for nutrient absorption.

Lacteals

Lymphatic capillaries in the villi of the small intestine that absorb fats into the lymphatic system.

Absorptive State

The metabolic state following a meal when nutrients are being absorbed into the blood.

Insulin

A hormone produced by the pancreas that regulates glucose uptake and is crucial for metabolism.

Glucagon

A hormone that raises blood glucose levels by promoting the release of glucose from liver stores.

Somatostatin

A hormone that inhibits gastric acid secretion and regulates the digestive system.

What is the primary function of the digestive system?

the intake, processing, and absorption of ingested nutrients and water needed for ATP production and bodily homeostasis.

What are the main hormones involved in the gastrointestinal system?

secretin, cholecystokinin (CCK), gastrin, and glucose-dependent insulinotropic peptide (GIP), which regulate various digestive functions

What is the role of pancreatic lipase in digestion?

it is the main digestive enzyme that splits triglycerides into free fatty acids and a monoglyceride, vital for fat digestion.

What occurs during the absorptive state?

during this state, ingested nutrients enter the blood from the GI tract, occurring after a meal and lasting about four hours, where some nutrients are used for energy and others are stored.

What is pepsin and how is it activated?

this is an enzyme that digests proteins, secreted in inactive form (pepsinogen) by chief cells and activated in the acidic environment of the stomach.

What are micelles and their function in fat digestion?

they are structures formed by bile salts and lipids that facilitate the absorption of fats in the small intestine by keeping fat droplets dispersed.

Hydrochloric Acid (HCl) Secretion

Secreted by parietal cells in the stomach, lowers pH to aid in digestion and activates pepsinogen to pepsin.

Pepsinogen

Inactive enzyme secreted by chief cells, activated to pepsin by acidic environment in the stomach.

Emulsification

Process of breaking down large fat droplets into smaller ones, increasing surface area for lipase action.

Microvilli

Small projections on epithelial cells of the small intestine, forming a brush border that aids in absorption.

Sodium-Glucose Transporter (SGLT)

Transporter that uses sodium ion gradients to facilitate glucose and galactose absorption.

Glucose Transporters (GLUT)

Facilitated diffusion transporters for monosaccharides like glucose and fructose across cell membranes.

Trypsin

Pancreatic enzyme that breaks down proteins into smaller peptide fragments, activated from trypsinogen.

Cholecystokinin (CCK)

Hormone that stimulates pancreatic enzyme secretion and gallbladder contraction in response to fatty acids.

Secretin

Hormone released in response to acid in the small intestine, stimulates bicarbonate secretion from the pancreas.

Gastrin

Hormone that stimulates HCl production and gastric motility, produced in the stomach's antrum.

Amino Acids Transport

Free amino acids from protein digestion enter epithelial cells through secondary active transport coupled to Na+.

Cyclic AMP (cAMP)

Secondary messenger involved in signaling pathways for gastric acid secretion stimulation by gastrin.

Lacteals

Lymphatic capillaries in the small intestine that absorb dietary fats in the form of chylomicrons.

Glycogenolysis

Process of breaking down glycogen into glucose when the body needs energy, primarily in the liver.

Acinar Cells

Cells in the pancreas responsible for producing and secreting digestive enzymes into the small intestine.

Bicarbonate Secretion

Neutralizes stomach acid entering the small intestine, secreted by the pancreas via epithelial cells.

Incretins

Hormones that enhance insulin secretion in response to nutrient ingestion, including GLP-1 and GIP.

Carbonic Anhydrase

Enzyme that catalyzes the formation of bicarbonate and protons from carbon dioxide and water in parietal cells.

Postabsorptive State

Metabolic phase when no nutrients are in the GI tract, body taps into energy stores for energy needs.

Peptide Chains

Chains of amino acids that are broken down into smaller fragments by enzymes like trypsin and chymotrypsin.

Feedback Mechanisms in Digestion

Regulatory processes where hormones and neural reflexes adjust digestive activity based on luminal stimuli.

Pancreatic Enzymes

Include amylase, lipase, and proteases that digest carbs, fats, and proteins, secreted into the duodenum.

Functions of Bile

Helps emulsify fats, neutralizes stomach acid, and facilitates the absorption of fat-soluble vitamins.

Mechanisms of Gastrointestinal Motility

Involves contraction of smooth muscles for mixing and propelling contents through the gastrointestinal tract.

Short Neural Reflexes

• Local reflexes within the gastrointestinal tract mediated by enteric neurons, responding to changes in the environment.

• originate from receptors in the GI tract and terminate via the two nerve plexuses on effector cells within the GI tract wall

Long Neural Reflexes

• Reflexes that involve the central nervous system; process information received from the GI tract.

• originate from receptors within the GI tract, send signals via afferent nerves to the CNS, and then return signals to the GI tract via autonomic nerves

Feedback Control of Gastric Acid

Regulated by a negative feedback loop where increased acidity limits gastrin secretion.

Chylomicrons

Lipoprotein particles formed from triglycerides within intestinal cells and transported via the lymphatic system.

Mechanoreceptors in GI Tract

Sensory receptors that respond to distension in the wall of the gastrointestinal tract.

Osmoreceptors in GI Tract

Detect changes in osmolarity of the chyme to help regulate digestive processes.

Pancreatic ducts

Transport digestive enzymes and bicarbonate from the pancreas to the duodenum.

Intrinsic Factor

Protein secreted by parietal cells that is essential for vitamin B12 absorption in the intestines.

Gastric Emptying Regulation

Controlled by the composition of gastric contents and feedback from the small intestine.

Which hormone is stimulated by the presence of acid in the small intestine and stimulates HCO3- release from the pancreas and bile ducts?

Secretin → goal is to neutralize the acid and protect the small intestine

Which hormone is stimulated by glucose and fat in the small intestine and increases insulin and amplifies the insulin response to glucose?

GIP → sends signals to the pancreatic islet cells that the products of food digestion are on their way to the blood, which results in an augmented insulin response to a meal

Which hormone is inhibited by acid in the stomach and stimulates acid secretion from the stomach?

Gastrin → inhibited when stomach becomes very acidic to prevent continued acid production

Which hormone is stimulated by amino acids and fatty acids in the small intestine and stimulates pancreatic enzyme secretion?

CCK → primary signal from the small intestine to the pancreas to increase digestive enzyme release

How do micelles increase the absorption of fat?

They keep the insoluble products of fat digestion in small aggregates.

• because fat is insoluble in an aqueous environment, they keep fat droplets from reaggregating and allow gradual absorption of fatty acids and other small lipids

What can inhibit gastric HCl secretion during a meal?

Distension of the duodenum — signals the stomach that the meal has moved on and continued acid secretion in the stomach is not necessary until the next meal

If the salivary glands were unable to secrete amylase, what effect would this have on starch digestion?

it would not be significantly affected, since the undigested starch that reaches the small intestine will still be digested by the amylase secreted by the pancreas

Can fat be digested and absorbed in the absence of bile salts?

Yes, but in greatly decreased amounts.

• without adequate emulsification of fat by bile salts and phospholipids, only the fat at the surface of large lipid droplets is available to pancreatic lipase → rate of fat digestion is very slow

• without formation of micelles with the aid of bile salts, the products of fat digestion become dissolved in the large lipid droplets, where they are not readily available for diffusion into the epithelial cells → only about half of the fat is digested and absorbed

What is the order of the alimentary canal?

Mouth → pharynx → esophagus → stomach → small intestine → large intestine → rectum → anus

What are the accessory organs?

Gallbladder, liver, salivary glands, pancreas

What are the four layers of the GI tract wall?

1. Mucosa — innermost layer (epithelium)

2. Submucosa — blood vessels and nerve endings

3. Muscularis externa — sooth muscle and neurons

4. Serosa — outermost layer (connective tissue)

Luminal stimuli that initiate gastrointestinal reflexes

• distension of the wall by the volume of the luminal contents

• chyme osmolarity (total solute concentration)

• chyme acidity

• chyme concentrations of specific digestion products like monosaccharides, fatty acids, peptides, and amino acids

Why doesn’t the high concentration of H+ in the stomach lumen destroy the lining of the stomach wall?

Mucus secreted by cells in the gastric gland creates a protective coating and traps HCO3-. This gastric mucosal barrier protects the stomach from luminal activity.

Enteroendocrine cells

• one surface of each endocrine cell is exposed to the lumen of the GI tract

• at this surface, various chemical substances in the chyme stimulate the cell to secrete its hormones from the opposite side of the cell into the blood

What are the four neurohumoral inputs to parietal cells that regulate acid secretion by generating second messengers?

1. Gastrin → increase H+

2. Histamine → increase H+

3. Acetylcholine → increase H+

4. Somatostatin → decrease H+