Lecture 6: Digestive and Excretory System
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Digestion is the break down of ingested foods before they are absorbed into the body.
-The major reaction involved in the digestion of all macromolecules is hydrolysis.
The basic anatomy is: mouth, esophagus, stomach, small intestine, large intestine, rectum, anus
Mouth and Esophagus
Digestion begins in the mouth with a-amylase contained in saliva. Starch is the major carbohydrate in the human diet. A-amylase begins breaking down the long straight chains of starch into polysaccharides. Chewed food forms a clump in the mouth called a bolus. The bolus is pushed into the esophagus by swallowing, and then moved down the esophagus via peristaltic action.
-Peristaltic action is a wave motion, similar to squeezing a tube of toothpaste at the bottom and sliding your fingers toward the top to expel the toothpaste. This movement is performed by smooth muscle.
-No digestion is performed in the esophagus
The stomach is a very flexible pouch that both mixes and stores food, reducing it to a semifluid mass called chyme. The stomach contains exocrine glands.
Another function of the stomach is to begin protein digestion with the enzyme pepsin. The low pH of the stomach assists this process by denaturing the proteins. A full stomach has a pH of 2. This low pH helps to kill ingested bacteria.
There are 4 major cell types in the stomach:1) mucous cells 2) chief (peptic) cells 3) parietal (oxyntic) cells 4) G cells
There are different types of mucous cells, but all of them perform the same function, secreting mucous. The mucous cells line the stomach wall and the necks of the exocrine glands. Mucus lubricates the stomach wall so that food can slide along its surface without causing damage, and mucus protects the epithelial lining from the acidic environment of the stomach.
Chief cells are found deep in the exocrine glands. They secrete pepsinogen, the zymogen precursor to pepsin. Pepsinogen is activated to pepsin by the low pH in the stomach. Once active, pepsin begins protein digestion.
Parietal Cells are found in the exocrine glands of the stomach. Parietal cells secrete HCL which diffuses to the lumen. The net result is to lower the pH of the stomach and raise the pH of the blood.
G Cells secrete gastrin into the interstitium. The gastrin, a large peptide hormone, is absorbed into the blood and stimulates parietal cells to secrete HCL.
-No absorption occurs in the stomach
The small intestine
-About 90% of digestion and absorption occurs in the small intestine.
-The outermost layer of the small intestine cotains villi. The villi increase the surface area of the intestinal wall allowing for greater digestion and absorption. Within each villus are a capillary network and a lymph vessel, called a lacteal. Nutrients absorbed through the wall of the small intestine pass into the capillary network and the lacteal.
On the surface of the viili cells are smaller finger-like projections called microvilli. The microvilli increase the surface area of the intestinal wall even further. Under a light microscope the microvilli appear as a fuzzy covering. This fuzzy covering is called the brush border. The brush border contains membrane bound digestive enzymes.
Some of the epithelial cells are goblet cells that secrete mucus to lubricate the intestine and help protect the brush border from mechanical and chemical damage.
The semifluid chyme is squeezed out of the stomach into the duodenum. The fluid inside the duodenum has a pH of 6 due mainly to bicarbonate ion secreted by the pancreas.
The major enzymes released by the pancreas are trypsin, chymotrypsin, pancreatic amylase, lipase, ribonuclease, and deoxyribonuclease.
Trypsin and chymotrypsin degrade proteins into small polypeptides. Most proteins reach the brush border as small polypeptides. Most proteins reach the brush border as small polypeptides. Here they are reduced to amino acids, dipeptides, and tripeptides before they are absorbed into the enterocytes.
Pancreatc amylase hydrolyzes polysaccharides to disaccharides and trisacchardies; however pancreatic amylase is much more powerful then salivary.
Lipase degrades fat, specifically triglycerides.
Bile is produced by the liver and is stored in the gall bladder. Bile emulsifies the fat, which means it breaks it up into small particles without changing it chemically. This increases the surface area of the fat, allowing the lipase to degrade it into mainly fatty acids and monoglycerides.
Chyme is moved through the intestines by peristalsis.
The Large Intestine
The major functions of the lagre intestines are water absorption and electrolyte absorption. When this function fails, diarrhea results. The large intestine also contains the bacteria E. Coli. The bacteria produce vitamin K, B12, thiamin, and riboflavin.
Absorption and Storage
Once absorbed into the enterocytes, nutrients are processed and carried to the individual cells for use.
Carbohydrates (not as important)
By far, the major carbohydrates in a human diet are sucrose, lactose, and starch. Typically 80 percent of the end product of carbohydrate digestion is glucose.
One of the jobs of the liver is to maintain a fairly constant blood glucose level. The liver absorbs the carbohydrates and converts nearly all the galactose and fructose into glucose, and then into glycogen for storage. The formation of glycogen is called glycogenesis. When the blood glucose level decreases, glycogenolysis takes place in the liver, and glucose is returned to the blood.
Glucose is transported from high concentration to low concentration via facilitated diffusion. Only muscle cells and especially liver cells store large amount of glycogen.
The conversion of glucose to fat takes place in the liver and fat cells and is stored in the fat cells. The role of the liver is processing carbohydrates.
Proteins (not as important)
Protein digestion results in amino acids, dipeptides, and tripeptides.
Nearly all polypeptides absorbed into an enterocyte are hydrolyzed to their amino acid constituents by enzymes within the enterocytes. Transport into the cells may be facilitated or active, but is never passive, since amino acids are too large and polar to diffuse through the membrane.
Ammonia, a nitrogen containing compound, is a by product of gluconeogenesis from protein. Nearly all ammonia is converted to urea by the liver and then excreted in the urine by the kidney.
Most of dietary fat consists of triglycerides. From adipose tissue, most fatty acids are transported in the form of a free fatty acid, which combines immediately in the blood with albumin.
Fat = efficient long-term energy storage; lots of calories (energy) with little weight.
When the liver mobilizes fat or protein for energy, the blood acidity increases.
Functions: Blood storage, blood filtration, carbohyrate metabolism, fat metabolism, protein metabolism, detoxification, erthrocyte destruction, vitamin storage.
The function of the kidney is:
1) to excrete waste products, such as urea, uric acid, ammonia, and phosphate
2) to maintain homeostasis of the body fluid volume and solute composition
3) help control plasma pH
Each kidney is a fist-sized organ made up of an outer cortex and an inner medulla. Urine is created by the kidney and emptied into the renal pelvis. The renal pelvis is emptied by the ureter which carries urine to the bladder. The bladder is drained by the urethra.
The functional unit of the kidney is the nephron. Blood flows into the first capillary bed of the nephron called the glomerulus. Together, Bowman's capsule and the glomerlus make up the renal corpuscle. Hydrostatic pressure forces some plasma through fenestrations of the glomerular endothelium and into Bowman's capsule.
The proximal tube is where most reabsorption takes place.
Drugs, toxins, and other solutes are secreted into the filtrate by the cells of the proximal tube. Hydrogen ions are secreted through an antiport system with sodium, which is driven by the sodium concentration gradient.
The net result of the proximal tubule is to reduce the amount of filtrate in the nephron while changing the solute composition without changing the osmolarity.
The filtrate flows into the loop of Henle. Its function is to increase the solute concentration, and thus the osmotic pressure, of the medulla. The descending loop of Hendle has low permeability to salt. The ascending loop of Hendle is nearly impermeable to water.
The distal tubule reabsorbs Na+ and Ca2+ while secreting K+, H+, and HCO3-.
ADH acts to increase the permeability of the cells to water. Therefore, in the presence of ADH, water flows from the tubule, concentrating the filtrate.
The juxtaglomerular apparatus monitors filtrate pressure in the distal tubule.
Filtration occurs in the renal corpuscle.
Secretion occrus in the proximal tubule.
The function of the kidney is homeostasis.
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