Chapter 66

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What are almost all carbohydrates of diet?

Polysaccharides or disaccharides (which are combinations of monosaccharides)

In general, the carbohydrates are converted into what before passing beyond duodenum or upper jejunum?

Carbohydrates are converted into maltoe and/or other mall glucose polymers

What enzymes do enterocytes contain?

Lactase, surrase, maltase and alpha-dextrinase

What do gastric glands secrete?

A large quantity of hydrochloic acid (HCl)

Which are one of the important features of pepsin digestion?

Its ability to digest protein collagen (a type of protein that is affected little by other digestive enzymes)

Where does most protein digestion occur?

IN upper small intestine, in duodenum and jejunum, under the influence of proteolytic enzymes from pancreatic secretion

Which are the pancreatic enzymes?

Trypsin, chymotrypsin, carboxypolypeptidase and elastase

Describe enzymes activity?

Trypsin and chymotrypsin split protein molecules into small polypeptides, carboxypolypeptidase then cleaves amino acids, proelastase is converted into elastase, which then digests elastin fibers that partially hold meats together

What do enterocytes have?

They have a brush border that consists of microvilli projecting from the surface(where there are peptidases, aminopolypeptidase and dipeptidases)

What do aminopolypeptidase and dipeptidases do?

They split the remaining polypeptides into tripeptides and dipeptides and a few into amino acids

What do triglycerides composed of?

Glycerol nucleus and three fatty acid side chains

What properties (characteristics) do cholesterol serve?

Cholesterol does exhibit some of the physical and chemical characteristics of fats

How is a small amount of triglycerides digested?

In the stomach by lingual lipase secreted by lingual glands in mouth and swallowed with saliva

What is the first 1st step in fat digestion?

It is the emulsification by bile acids and lecithin (it is the emulsification of fat) occuring in duodenum due to bile, which contain bile salts and lecithin, for emulsification

What about the polar part of bile salts and lecithin molecules?

They are highly soluble in water, wheras the remaining portions are soluble in fat

What about the lipase enzymes?

They are water-soluble compounds and can attack fat globules only on their surfaces

What are triglycerides digested by?

Pancreatic lipase (which is present in enormous quantities in pancreatic juice) enough to digest all triglycerides that it can reach

What unneeded lipase do enterocytes of small intestine contain?

Enteric lipase

What are most of triglycerides of diet split by pancreatic lipase into?

Into free fatty acids and 2-monoglycerides

What happens when bile salts are of a high enough concentration in water, what do they have the propensity to form?

To form micelles, which are spherical, cylindrical globules

What is most cholesterol in diet, in what form?

In form of cholesterol esters (which are combinations of free cholesterol and one molecule of fatty acid)

What are both the cholesterol esters and phospholipids hydrolyzed by?

The enzyme cholesterol ester hydrolase and phospholipase A2

What is the total quantitiy of fluid that must be absorbed each day by intestines?

It is equal to ingested fluid (1,5 liters) plus that secreted in various gastrointesitnal secretions (about 7 liters)

What is the stomach?

It is a poor absorptive area of gastrointesitnal tract because it lacks typical villus type of absorptive membrane

What about the absorptive surface of small intestinal mucosa?

It has many folds called valvulae conniventes (folds of Kerckring) increasing the surface area of absorptive mucosa about threefold

What are located on epithelial surface of small intestine all the way down to ileocecal valve?

Millions of small villi, projecting 1 millimeter from surface of mucosa

What does the presence of villi on mucosal surface do?

It enhances the total absorptive area another 10-fold

Combination of fold of Kerckring, villi and microvilli do what?

They increase the total absorptive area of mucosa 1000-fold, making a tremendous total area of 250 or more square meters for entire small intestine

How is the villus organized?

Arrangement of central lacteal lymph vessel for absorption into lymph, pinocyti vesicels which are pinched off portions of infolded enterocyte membrane forming vesicels of absorbed fluids that have been tentrapped (pinocytosis something)

What are extending from the epithelial cell body into each microvillus of brush border?

Multiple actin filaments thata contract rhythmicall to cause continual movement of microvilli

Absorption from small intestine each day consists of what?

Several hundred grams of carbohydrates, 100 or more grams of fat, 50 to 100 grams of amino acids, 50 to 100 gramas of ions, and 7 to 8 liters of water (large intestine can absorb still more water and ions, although it can absorb very few nutrients)

How is water transported through intestinal membrane?

By diffusion entirely which obeys the laws of osmosis (throguh intestinal mucosa into blood, and from plasma into chyme)

Twenty to 30 grams of sodium are secreted in intestinal secretions each day, and average person eats 5 to 8 grams of sodium each day, therefore to prevent net loss of sodium into feces what is needed?

Intestines must absorb 25 to 35 grams of sodium each day

What is the basic mechanism of sodium absorption from the intestine, chloride, glucose and amino acids through intestinal epithelium, which are co-transporters and etc?


What does aldosterone greatly enhance?

Sodium absorption

What happens when a person becomes dehydrated?

Large amounts of aldosterone are secreted by cortices of adrenal glands, causing activation of enzyme and transport mechanisms for all sodium absorption causing increases in absorption of chloride ions, water

Where is aldosterone mostly important?

In the colon, because it allows virtually no loss of sodium chloride in the feces and also little water loss

How does absorption of chloride ions in small intestine occur?

In upper part of small intestine, chloride ion absorption is rapid and occurs by diffusion, chloride ions then move along the sodium ions, chloride exits teh cell on basolateral membrane through chloride channels

how does absorption of bicarbonate ions in duodenum and jejunum occur?

Large quantities of bicarbonate ions are reabsroebd from upper small intestine, sodium ions are absorbed, hydrogen ions are secreetd into lumen of gut in exchange for some sodium, hydrogen ions combine with bicarbonate ions to form carbonic acid (H2CO3) dissociating to wwater and carbon dioxide

What about secretion of bicarbonate and abasorption of chloride ions in ileum and large intestine?

Epithelial cells on surfaces of villi in ileum and large intestine can secrete bicarbonate ions in exchange for absorption of chloride ions, causing alkaline bicarbonate ions neutralizing acid products formed by bacteria in large intestine

How are calcium ions absorbed?

Into the blood, form duodenum

What is one important factor controlling calcium absorption?

Parathyroid hormone secreted by parathyroid glands and another is vvitamin D, (parathyroid hormone activates vitamin D and activated vitmain D enhances calcium absorption)

What are also actively absorbed from small intestine?

Iron ions

What ions are also actively absorbed through intestinal mucosa?

Potassium, magnesium, phosphate and other ions

How are carbohydrates mainly absorbed as?

As monosaccharides

What is by far the most abundant of the absorbed monosaccharides?

Glucose, whih usually accounts for more than 80% of carbohydrate calories absorbed, (the remaining 20% of absorbed monosaccharides is galactose and frutose, galactose derived from milk and frutose digested from cane sugar)

What happens in the absence of sodium transport through intestinal memrbane?

There is virtually no glucsoe that can be absorbed because glucose absorption occurs in a cotransport mode with active transport of sodium

How does the absorption of monosaccharides (other) look like?

Galactose is transported by the same mechanis mas glucose, fructose transprot does not occur by sodium-cotransport mechanism, instead fructose is transported by factiltiated diffusion all the way throguh intestinal epithelium

How does absorption of fats occur?

When fats are digested to form monoglycerides and free fatty acids, both of these digestive end products first become dissolved in central lipid portions of bile micelles

Small quantities of short- and medium-chain fatty acids such as butterfat are absorbed how?

Absorbed directly into portal blood rather than being converted into triglcyerides and absorbed by way of lymphatics

About how much chyme pass through ileocecal valve into large intestine each day?

About 1500 milliliters of chyme (most of t he water and electrolytes in this chyme are absorbedi n colon, usually leaving less than 100 milliliters of fluid to be excreted in feces)

Where does most of absorption in large intestine occur?

In proximal one half of colon (absorbing colon) whereas distal colon functiosn for feces torage (storage colon)

What about the tight junctions between epithelial cells of large intessitnal epithelium?

They are much tigheter than those of small intestine (preveniing significant amounts of back-diffusion of ions through thee junctions, allowing large intestinal mucsoa to absorb sodium ions far more completely)

What does the mucsoa of large intestine secrete?

Bicarbonate ions, while it simultaneously absorbs an equal number of chloride ions in an exchange transport process

When does diarrhea occur?

When the toal quantity fluid and electrolytes entering large intestine through ileocecal valve exceeds the amount of 5 to 8 liters of fluid (which the large intestine can absorb9

What is the composition of feces?

The feces normally are about three-fourths water and one-fourth solid matter that is compsoed of about 30 percent dead bacteria, 10 to 20 percent fat, 10 to 20 percent inorganic matter,

What is causing the brown color of feecs?

Stercobilin and urobilin, derivatives of bilirubin

What is causing the odor?

It is caused by products of bacterial action, varying from one person to another, including odoriferous products such as indole, skatole, mercaptans and hydrogen sulfide