Unit VI- Colon and Urinary Excretion

Question 1

Micturition

Answer 1

• the process by which urinary bladder empties when full
• progressive filling until the tension in the walls rises above the threshold
• triggering of a reflex that empties the bladder

Question 2

Urine production

Answer 2

• urine produced in the kidneys continually passes to the bladder via the ureters (1-2 L/day)
• smooth muscles in the ureter walls contract regularly to draw urine into the bladder
• normally the urinary bladder is completely emptied during voiding and urine is sterile until it reaches the urethra

Question 3

Bladder

Answer 3

• smooth muscle chamber composed of the rough/folded body where the urine collects and the neck/posterieror urethra ( a funnel shaped extension of the body that connects to the urethra and includes the internal sphincter)
• the smooth trigone area lies on the posterior wall of the bladder between the orifices of the ureters
• the body of the bladder is composed of smooth muscle called the detrusor. Contraction of the detrusor muscle is a major step in bladder emptying
• the trigone and internal sphincter are also composed of smooth muscle which relaxes during bladder emptying

Question 4

Voluntary control of micturition

Answer 4

• beyound the posterior urethra, the urethra passes through the urogenital diaphragm containg a layer of skeletal muscle that is under voluntary control, called the external sphincter
• stretch signals from the posterior urethra are particularly strong and primarily involved in triggering the emptying reflexes.
• however, this response is normally inhibited and control of this voluntary reflex is learned during childhood

Question 5

Pontine micturition center

Answer 5

• controls the detrusor muscle and urinary sphincters

- in turn, suprapontine centers control the pontine micturition center providing voluntary control

Question 6

Involnutary control of micturition

Answer 6

• the pelvic nerves connect with the spinal cord through the sacral plexus. These nerves contain both sensory (afferent stretch) and motor (efferent parasympathetic) fibers
• PNS fibers terminate on ganglion cells in the bladder wall. Short posterganglionic nerves innervate the detrusor muscle to stimulate contraction + internal sphincter relaxation during voiding. The presynaptic PNS neurons are inhibted by efferent impulses in the brain (learned reflex)
• skeletal motor fibers in the pudendal nerve innervate the external sphincter. Voiding begins with voluntary relaxation of the external urinary sphincter followed by the internal sphincter
• when a small amount of urine reaches proximal urethra afferents signal the cortex that voiding is imminent. The micturition reflex continues to pontine centers no longer inhibit the parasympathetic preganglionic neurons that innervate the detrusor muscle. As a result the bladder contracts, expelling urine
• SNS fibers inhibit contraction of the detrusor (B adrenergic response) and stimulate contraction of the internal sphincter muscles (alpha adrenergic). They also regulate blood vessels in the bladder

Question 7

Abnormalities of micturition

Answer 7

1. atonic bladder and incontinence caused by destruction of sensory nerve fibers (crush injuries)
2. automatic bladder caused by spinal cord damage above the sacral region
3. uninhibited neurogenic bladder caused by a lack of inhibitory signals from the brain

Question 8

Primary functions of colon

Answer 8

• move material through the large intestine to the rectum
• eliminate fecal material through the anus
• extract water, electrolytes and some nurients (some B vitamins and K

Question 9

Ilocecal valve

Answer 9

• sphincter that separates the small and large intestines
• when open/relaxed liquid moves from small intestine into the cecum (note that carnivores have little or no cecum whereas herbivores have a large cecum)
• the gastroileal reflex intensified peristalsis in the ileum, and emptying of ileal contents into the cecum
• prevent backflow of cecal contents from colon to small intestine
• 2 liters of chyme empty into cecum
• distension/pressure or irritation of the cecum inhibits ileal peristalsis and excites sphincter contraction, delaying emptying
• these reflexes are mediated locally by the myenteric plexus, and extrinsically by the SNS

Question 10

Ascending colon

Answer 10

• travels up the right side of the body and is involved in the extraction of water and electrolytes
• dwell time of chyme is comparatively short

Question 11

Transverse colon

Answer 11

• runs across the body from right to left and functions primarily to remove electrolytes and water
• dwell time is long (24 hours)

Question 12

Descending colon

Answer 12

• runs down the left side of the body

- it functions mainly to store stool

Question 13

Sigmoid colon

Answer 13

-moves stools between the descending segment and the rectum

Question 14

Rectum

Answer 14

-last 8 inches of the colon and stores stools until defecation occurs

Question 15

Anus

Answer 15

-the externior opening and is controlled by internal and external sphincters

Question 16

Constipation

Answer 16

• results from poor motility which leads to greater absorption of liquids and hard feces
• in most cases, dietary fiber promotes normal colonic function
• there is a direct correlation among increased dietary fiber, increased colonic intraluminal bulk and enhanced transit through colon

Question 17

Diarrhea

Answer 17

• rapid movement of fecal matter through the large intestine
• in a healthy individual diarrhea short term is not problematic, but it can be fatal in infants- need oral rehydration solutions
• osmotic form- from non-absorbable solutes in the lumen (lactase defiency/lactose not broken down to glucose and galactose)
• the secretory form- from excessive secretion of fluids by crypt cells due to bacterial overgrowth (cholera toxin)

Question 18

Common causes of diarrhea

Answer 18

• bacterial infections
• viral infections
• food intolerances
• parasites
• intestinal diseases
• reactions to medications
• functional bowel disorders
• psychogenic factors
• long distance running

Question 19

Appendicitis

Answer 19

• a medical emergency acutely presenting as severe gastric pain followed by vomiting, then fever
• it is typically due to an obstruction of the appendix lumen by calcified fecal matter
• often the appendix is surgically removed
• left untreated, ischemia and tissue necrosis may lead to peritonitis, septicemia and death

Question 20

Colon epithelium

Answer 20

• the human colon epithelium performs both absorptive and secretory functions
• the colon lacks the numerous villi and folds that characterize the small intestine
• it contains numerous crypts or invaginations of the epithelium
• it contains solitary lymphatic nodules but few or no Peyer’s patches
• the surface epithelium is columnar with many mucus-secreting goblet cells and columnar absorptive cells, which together comprise -95% of cells
• the alkaline mucus serves protective functions and also provides an adherent medium for binding feces
• colonic epithelial cells are polarized and contain various ion channels, carriers, and pumps, lcoated on the luminal or basolateral membrane
• these proteins regulate highly efficent transport of large amounts of electrolytes and water

Question 21

Colon pumps and balances

Answer 21

• the colon absorbs Na+, Cl-, and H2O and secretes K+ and HCO3-
• the major exchanges occur in the ascending and transverse colon
• fluid balance in GI tract
• the large intestine removes approximately 1.9 liters/day leaving approximately 0.1 liter/day in the feces

Question 22

Exchangers for Na+ absorption and HCo3- secretion in colon

Answer 22

• Na+0H+ and Cl- -HCO3- exchange is coupled by a change in intracellular pH that results in electroneutral NaCl absorption which is the primary mechanism for interdigestive Na+ absorption
• absorption of Na+ and Cl- creates an osmotic gradient across the intestinal mucosa, promoting absorption of water
• the colon is a net secretor of K. There is passive K+ secretion through tight junctions which occurs throughout the colon
• the driving force is a lumen negative transpithelial voltage
• these is also active K+ secretion throughout in the colon (stimulated by aldosterone and cAMP)
• stimulation of K+ secretion by cholera toxin (which increases cAMP) accounts for significant fecal K+ loss in diarrhea
• the tight junctions in the colon offer greter resistance to flow (they are tighter) through the paracellular pathway than the small intestine. This prevents back-diffusion of ions and more complete absorption of Na+ ions, compared to the small intestine. This is enhanced when aldosterone is present

Question 23

Motility in the colon

Answer 23

• similar to small intestine, the colon contains ICCs that provide spontaneous electrical activity, and other factors (PNS, SNS, chemical, stretch) influence the triggering of spikes/action potentials at the peaks of these slow waves
• similar to the small intestines, slow waves are no observed in the absence of ICCs
• motility in the large intestine is characterized by slow segmental propulsion, segmental mixing, and mass movements (motility is sluggish compared to small intestine)

Question 24

Haustrations

Answer 24

• specialized for slow segmental propulsion and mixing, alllowing time for electrolyte and fluid absorption, thus solidifying chyme
• they are formed by large circular muscle contrictions and also by contriction of 3 flat bands of longitudinal muscle called taenia coli
• each haustration reaches peak intensity in 30 sec and disappears during the next min. The tern haustration derives from the outward bulging of the baglike sacs in the unstimulated portion of the colon

Question 25

Mass movemetns

Answer 25

• 1-3 times a day (often following breakfast) mass movements create peristaltic movements that force the chyme/feces toward the rectum
• haustrations disappear during mass movements which persist for 10-30 minutes
• typically signal the urge to defecate
• following a meal, an increased incidence of mass movements occurs via the gastrocolic reflex
• factors that increase motility of the colon can cause diarrhea, by liming haustrae formation and increasing mass movements

Question 26

Autonomic Innervation of the Colon

Answer 26

• the sm intestine and colon are innervated by both symp and parasym nerves
• symp fibers from the spinal cord lead to the celiac, superior mesenteric ganglia and inferior mesenteric ganglia
• parasym fibers arise from the vagus (innervate through transverse colon) and pelvic (descending colon, sigmoid, and recum) nerves
• sympathetic stimulation reduces motility and secretions, whereas para stimulates increases motility secretions and relaxation of the internal anal sphincter
• when extrinsic autonomic nerves to the colon are removed, the gastrocolic and duodenocolic reflexes (which facilitate appearance of mass movements after a meal in response to distension of the stomach and duodenum) are weak or absent
• individuals suffering from ulcerative colitis have persistent mass movements due to irritation in the colon

Question 27

Hirschsprung’s Disease (Megacolon)

Answer 27

• is congenital/present at birth
• results from lack of ENS in the distal part of the GI tract (aganglionosis). The aganglionic segment is tonically contracted due to lack of inhibitory motor neuron function. As a consequence the proximal segment becomes distended with fecal matter that cannot be passed
• the affected segment extends cranially from the anus and encompasses a variable portion of the gut
• the lack of propulsive movements may lead either to an early obstructive syndrome in infants (life-threatening) or to severe constipation
• in mild cases, Hirschsprung’s disease may not be detected until later in life
• Hirshsprung’s disease is treated with surgery to remove the affected portion of the colon
• after surgery, most children pass stool normally

Question 28

Flatus

Answer 28

• derived from swallowed air, bacterial action and diffusion from blood
• each day 7-10 liters enter colon: 0.6 liters is expelled through anus and the remainder is absorbed into the blood and exhaled or combined with fecal matter and excreted
• composed of nitrogen, hydrogen, carbon dioxide, oxygen and methane (varies with diet eg ingestion of baked beans increases carbon dioxide and methane)
• the composition of colonic microflora is highly individual and impacts the composition and net production of flatus (methanogenic and sulfate-reducing bacteria differ)
• flatulence is an important signal of bowel activity, and is often documented by nursing staff following surgical or other procedure
• symptoms of excessive flatulence can indicate irritable bowel syndrome, lactose intolerance, celiac disease or other organic disease

Question 29

Feces

Answer 29

• average weight is 100-150 g (70% water)
• contains organic materials: mucus, desquamated cells, enzyme secretions, undigested food residues- stercobilin is responsible for color
• electrolyte content extremely variable (typically higher in K+ and lower in Na+ compared to chyme)
• bacteria comprise -10% of dry weight

Question 30

Irritable bowel syndrome

Answer 30

• common GI disorder that leads to crampy pain, bloating, gassiness, and altered bowel habits (diarrhea-predominant, constipation-predominant, alternating between both)
• no evidence of organic disorder
• observed in 10-20% of population, more commonly in females (majority 20-40 years of age)
• often associated with stress and/or high anxiety
• symptoms treated with opioids and serotonin receptor antagonists in diarrhea-predominant cases
• symptoms treated with selective serotonin receptor agonists and/or soluble fiber in constipation -predominant cases
• cramping pain symptoms treated with tricyclic antidepressants or antispasmodics
• probiotics to alter intestinal microflora are under investigation (gut flora may lead to mucosal cell inflammation)
• no cure, but not life threatening (eg not associated with increased risk of cancer or damage to intestine

Question 31

Reflexive defecation

Answer 31

• the rectum is usually empty
• this results partly from a weak functional sphincter -20 cm from the anus at the juncture between the sigmoid colon and the rectum
• there is also a sharp angulation here that contributes additional resistance to filling of the rectum
• tension in the wall of the rectum typically signals the urge to defecate
• movement of stool from sigmoid colon to rectum causes increased pressure due to passive distension
• in turn, this triggers active contraction of rectal smooth muscles in conjunction with relaxation of internal anal sphincter

Question 32

Fecal incontinence

Answer 32

• involuntary defacation
• the pathophysiology is often related to trauma, injury to the pelvic floor such as during childbirth or surgery or prolapsed rectum
• the rectophincteric reflex are typically normal, but the pathology is with the external anal sphincter
• treatments include bulking agents, surgery, and strengthening the pelvic floor or spincter muscles

Question 33

Events of defecation

Answer 33

• the defecation or rectosphincteric reflex is triggered by distension of the rectum by feces
• this is sensed by the mechanoreceptors which activate the myenteric nerves and impulses are transduced to the spinal cord and back to relax the internal anal sphincter
• the spinal cord conveys signals to the brains to stimulate the urge to defecate
• these reflex action cause the voluntary contration of the external anal sphincter until defecation occurs
• during defecation, the EAS relaxes, the person increases intra-abdominal pressure, and feces are eliminated from the rectum

Question 34

Cortex connections to defecation

Answer 34

-defecation is initiatd by defecation reflexes
-one of these is an intrinsic reflex mediated by the local ENS in the rectal wall (a myenteric reflex)
-this can be descrived as follows: when feces enters the rectum, distension of the rectal wal initiates afferent signals that spread through the myenteric plexsus to inititiate peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus.-
as the peristaltic wave approaches the anus, the internal anal sphincter is relaxed by inhibitory signals from the myenteric plexus; if the external anal sphincter is also consciously, voluntarily relaxed at the same time defcation occurs

Question 35

Intrinsic myenteric defecation reflex

Answer 35

• functioning by itself normally is relatively weak
• to be effective in causing defecation, it is fortified by another type of decationf reflex, a parasympathetic defecation reflex that involves the sacral segments of the spinal cord. When the nerve endings in the rectum are stimulated, signals are transmitted first into the spinal cord and then reflexly back to the descending colon, sigmoid, rectum and anus via PNS fibers in the pelvic nerves
• these ssignals greatly intesify the peristaltic waves and relax the interal anal sphincter, thus converting the intrinsic myenteric defecation reflex from a weak effort into a powerful process of defecation that is sometimes effective in emptying the large bowel all the way from the splenic flexure of the colon to the anus

Question 36

Other effects of defecation signals

Answer 36

-taking a deep breath, closure of the glottis, and contraction of the abdominal wall muscles to force the fecal contents of the colon downward and at the same time cause the pelvic floor to relax downward and pull outward on the anal ring to evaginate the feces

Question 37

Destruction of pathways

Answer 37

• in the spinal cord and cerebral cortex results in loss of voluntary control of defecation
• destruction of the nerves leading to the anorectal region can cause fecal retention