Digestion FAQ, Defecation reflex, etc.

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Defecation reflex:

Fecal Material and Distention:

• Fecal material pushed from the sigmoid colon to rectum.

• Resulting distention at rectal wall stimulates stretch receptors that initiate defecation reflex that empties the rectum.

Stretch Receptors:

• Stretch receptors send nerve impulses to the sacral spinal cord.

• Motor impulses from the cord travel long parasympathetic nerves back to the descending colon, sigmoid colon, rectum and anus.

Longitudinal Muscles:

• Longitudinal muscles of the rectum contract, shortening the rectum, thus, increasing the pressure within it.

• These pressures, along with voluntary contractions of diaphragm and abdominal muscles, plus parasympathetic stimulation, open the internal anal sphincter.

External Anal Sphincter:

• The external anal sphincter is under voluntary control, and if opened, feces are expelled through the anus.

Pelvic Nerve:

• Pelvic nerve controls the internal anal sphincter.

Pudendal Nerve:

• Pudendal nerve controls the external anal sphincter.

Gastroenterology Frequently Asked Questions:

1. Describe the following passive processes and give an example of each- simple diffusion, osmosis, bulk flow, and facilitated diffusion.

Simple Diffusion:

• Net movement of particles in solution down their concentration gradient. For example, when a crystal dye is placed in a vat of water, the dye diffuses from a region of higher concentration to a region of lower concentration.


• This is the net movement of solvent through a selectively permeable membrane. In living systems, the solvent is water moving from a region of higher concentration to a region of lower concentration.

Bulk Flow:

• This is the passive movement of a large numbers of ions, molecules, or particles in a fluid. The substances move together and in the same direction, and move faster that in diffusion. Bluk flow is more important for regulation of the relative volumes of blood and interstitial fluid.

2. Describe the absorption of glucose and galactose. How does glucose absorption influence the absorption of water?

Glucose is absorbed in the small intestine only by co-transport with sodium.

Glucose > SGUT-1 > Intestinal Lumen > Epithelium > Blood

The glucose/galactose transporter is initially facing the lumen. It binds sodium only and not glucose. When sodium binds, it induces a conformational change that opens the glucose binding pocket. Then, the transporter binds glucose and the pockets holding sodium and glucose are moved inside.

When glucose is absorbed, salt is also absorbed. When salt is retained, water is also retained.

3. Describe lipid digestion, where it occurs and how it occurs. What is a triglyceride? What is a micelle?

Triglyceride aggregates are insoluble in aqueous solutions, and must be broken down physically and held in suspension - a process called emulsification. Triglyceride molecules are enzymatically digested to yield monoglycerides and fatty acids, which are absorbed into enterocytes. These processes are carried out by bile salts and pancreatic lipase. Bile salts surround triglycerides and break them down into smaller and more manageable fat droplets. When monoglycerides and fats are released from triglycerides, they aggregate to form structures called micelles.

4. What 2 organs of the digestive system secrete the most fluid?

Liver and pancreas.

5. Describe the absorption of sodium.

See #2

6. Define Diarrhea:

Frequent defecation of liquid feces caused by increased mobility of the intestines.

7. What is the effect of increased dietary fiber on digestive function? What are the potential benefits of increased dietary fiber?

Dietary fiber is contained in indigestible plant carbohydrates. Dietary fiber passes through the intestines largely unchanged, but speeds up the passage of material through the tract. People who eat a diet rich in dietary fiber have a lower risk of developing obesity, diabetes, atherosclerosis, gallstones, hemorrhoids, etc.

8. Define body fluid. List the principal body fluid compartments and describe how they are separated.

Body fluid is the liquid content within all the compartments of the body. The main fluid compartments of the body are extracellular fluid (intravenous and interstitial fluid), and intracellular fluid.

9. Describe some of the causes and some of the physiologic effects of the following: hyponatremia, hypernatremia, hypochloremia, hyperkalemia and hypokalemia.


• Deficiency of Na ions in blood caused due to excessive sweating. Effect: collapse, convulsions, and sometimes even death.


• Excessive sodium in blood caused due to inadequate intake of water. Effect: lethargy, weakness, irritability, and edema.


• Deficiency of Cl in blood caused due to excessive vomiting, diarrhea, etc. Effect: collapse, convulsions, and sometimes even death.


• Higher than normal levels of K in blood stream caused by disorders that reduce the kidneys' ability to excrete potassium (kidney failure). Effect: No fatal symptoms; there may be nausea or irregular heartbeat.


• Low levels of K in blood caused due to excessive urination. Effect: confusion, disorientation, weakness, and discomfort of muscles.

10. Indicate the function and regulation of each of the following ions: sodium, chloride and potassium.


Na: Sodium ions are the most abundant ions in extracellular fluid. Sodium is used to generate and conduct action potentials in neurons and muscle fibers. Sodium is controlled by hormones like aldosterone, ADH and ANP.


Cl: Cl- anions are the most prevalent anions in extracellular fluid. Chloride ions help balance the levels of anions in different fluid compartments. Cl is regulated by ADH.


K: Potassium ions are the most abundant ions in the intracellular fluid. K is used in maintaining resting membrane potential and in the repolarization phase of action potentials. Blood levels are controlled by aldosterone.

Additional Reading:

Basic Gastroenterology

1. Basic Gastrointestinal Physiology
2. Digestion FAQ, Defecation reflex, etc.
3. Digestion
4. Notes on Functions of the Liver
5. Notes on Jaundice
6. Types of Jaundice
7. Diagram of Gastric Blood Supply
8. FAQ on Gastric Digestion
9. Usage of the D-xylose Absorption Test

Gastroenterology Videos

1. Video of Abdominal Examination in a Clinical Setting

Related Topics

1. Gastrointestinal Disorders
2. Hepatobiliary and Pancreatic Disorders
3. Histology of the Digestive Tract I: Oral Cavity
4. Histology of the Digestive Tract II: Esophagus through Intestines
5. Histology of the Liver, Pancreas, and Gall Bladder
6. Abdominal Examination for Internal Medicine

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