First Aid Cases for the USMLE Step 1 3rd edition

First Aid Cases for the USMLE Step 1 3rd edition

First Aid Cases for the USMLE Step 1

Edition : 3rd Edition | | ISBN : 0071743979

Author Name : Tao Le; James Yeh;


Before we going to discuss the description we need to know some basic core topics in medicine.

Discuss about colon



The main function of the colon is the reabsorption of electrolytes and
water, as well as short-term storage of undigested material (feces), followed by
its excretion. The colon is larger in diameter than the small intestine and is inhabited by normal bacterial flora.
The colon is divided into several anatomic segments: the cecum and ascending, transverse, descending, and sigmoid colon. Overall length averages 1.5-1 .8 m.
β€’ In contrast to the small intestine, the colon has no villi and only a few folds. It contains goblet, endocrine, and absorptive cells.
β€’ The colon is the most efficient absorber of water via actively transported sodium. It can also absorb chloride, potassium, and short-chain fatty acids.
β€’ Its muscle layer has three longitudinal bands of smooth muscle (tenia coli) and typical haustrations (sacculated wall.

  • The colonic bacteria synthesize vitamin K, B-complex vitamins, folic acid,
    and short-chain fatty acids (a preferred source of nutrition by colonocytes)
    and metabolize nitrogen from urea to NH3 .
    Peristaltic waves open the ileocecal valve and force chyme into the cecum.

The colon has slow waves that slowly mix the feces, allowing fluid reabsorption in the ascending colon with haustrations (segmental contractions), which take about 8-15 hours and cause a net movement of feces to the transverse colon, where transit speed tends to increase.

Large contractions of the colon (mass movements) occur one to three times per day and prepare stool to be eliminated by moving feces large distances toward the rectum. Distention of the rectum produces the urge to defecate and initiates the rectosphincteric reflex (both intrinsic and cord reflexes that relax the internal anal sphincter).

Contractions increase with parasympathetic stimulation of the vagus and sacral nerves and decrease with sympathetic stimulation from the mesenteric ganglion.

  • A full stomach can cause a parasympathetic reflex known as the gastrocolic reflex, which increases the frequency of mass movements.
    The colon is capable of secreting potassium and bicarbonate.

Feces consist of water, bacteria, undigested plant products, and inorganic matter

First Aid Cases for the USMLE Step 1 3rd edition