A. Human Digestion
1. The human digestive tract is complete and complex. (Fig. 43.5) [transp. 236]
2. The functions of the human digestive system include the following:
a. ingestion---taking food and drink into the mouth (i.e., eating and drinking).
b. digestion or breakdown of food by both mechanical and chemical processes.
c. absorption of nutrients from alimentary canal into the cardiovascular and lymphatic systems, and
d. elimination of indigestible substances and wastes from the digestive tract.
3. Each part of the digestive system has a specific function. (Table 43.1)
4. Digestion of food in humans is an extracellular process; digestive enzymes are secreted into the lumen of the digestive tract by cells in its walls or by adjacent accessory glands (which never contain food themselves).
5. Digestion requires a cooperative effort achieved by production of hormones and actions of nervous system.
B. Mouth Receives Food
1. Human dentition has a variety of specializations because humans are omnivores. (Fig.43.4c)
2. When food is chewed in the mouth, it is mixed with saliva.
a. Three pairs of salivary glands secrete saliva by way of ducts into the mouth.
b. Salivary amylase is the enzyme found in saliva that begins the process of starch digestion.
starch + H
salivary
starch + H
amylase
starch + H2O
maltose
c. Food in the mouth is manipulated by a muscular tongue with touch and pressure receptors.
d. Taste buds are located primarily on the tongue, but also on the surface of the mouth and the wall of the pharynx; contain chemical receptors that are stimulated by the chemical composition of food.
e. If food is chewed and mixed with saliva, tongue pushes the food back to pharynx in process of swallowing.
C. Esophagus Conducts Food
1. Digestive and respiratory passages come together in the pharynx, then separate. (Fig. 43.6)
a. During swallowing, the path of air to the lungs could be blocked if food entered the trachea.
b. The epiglottis covers opening into trachea as muscles move the bolus through pharynx into the esophagus.
c. The esophagus is a muscular tube that moves swallowed food to the stomach by peristalsis. (Fig. 43.7)
D. Stomach Stores Food
1. The stomach stores up to two liters of partially digested food, permitting time free from eating.
2. Pioneer work by Dr. William Beaumont revealed much of the stomach's functions in the mid-1800s.
a. French-Canadian fur trapper had an opening or fistula through which Dr. Beaumont could observe.
b. He collected gastric juice, a fluid produced by cells of gastric glands. (Fig. 43.8)
[micro. slide 95]
c. The walls of the stomach contract vigorously and mix food with juices secreted when food enters.
d. He found that gastric juice contains hydrochloric acid and another substance active in digestion (pepsin).
e. He found that gastric juices are produced independently of the protective mucous secretions.
f. His careful work pioneered the study of the physiology of digestion.
3. Hydrochloric acid (HCl) lowers the pH of the gastric contents to about 2.
a. This kills most bacteria and other microorganisms.
b. The low pH also stops the activity of salivary amylase and promotes activity of pepsin.
4. Pepsin is a hydrolytic enzyme that acts on proteins to produce peptides.
protein + H pepsin
protein + H2O
peptides
5. The stomach contents have a thick, soupy mixture and are called chyme.
6. At the base of the stomach is a narrow opening controlled by a sphincter.
a. Whenever the sphincter relaxes, chyme enters the duodenum and sets off a neural reflex that causes the muscles of the sphincter to contract vigorously, so as to close off the opening temporarily.
b. The duodenum is the first part of the small intestine. [transp. 236]
c. Then the sphincter relaxes again and allows more chyme to enter the duodenum.
d. This slow, rhythmic manner in which chyme enters the small intestine allows for thorough digestion.
7. A thick layer of mucus protects the wall of the stomach and first part of duodenum from the HCl and pepsin.
8. If gastric juice does penetrate the mucus, pepsin begins digesting the lining, resulting in an ulcer.
9. Originally, all ulcers were attributed to over-secretion of gastric juice due to nervous stimulation; now there is evidence a bacterial infection is often the cause.
E. Small Intestine Absorbs Nutrients
1. The human small intestine is a coiled muscular tube about three meters long.
2. Mucous membrane lining has ridges and furrows with surfaces that are covered by villi. (Fig. 43.9) [transp. 237]
a. The villi are fingerlike projections whose surface cells are covered by microvilli.
b. Microvilli are minute projections of the plasma membrane of surface cells of intestinal villi.
c. The ridges, furrows, villi, and microvilli greatly increase the effective surface area of the small intestine; if the intestine were a smooth tube, it would have to be 500-600 meters long to have a comparable surface area.
3. As chyme enters duodenum, proteins and carbohydrates are partly digested, but no fat digestion has occurred.
4. Additional digestion is aided by secretions from the liver and pancreas.
a. Bile is a secretion of liver that is temporarily stored in gallbladder before being released into duodenum.
b. Bile emulsifies fat; bile is a green byproduct of the breakdown of hemoglobin.
c. Bile contains salts that help in the emulsification of fat.
1) Emulsification breaks fat globules into microscopic droplets.
2) This increases fat digestion by increasing the surface area of fat globules exposed to enzymes.
d. Pancreatic juice is secreted by the pancreas; it contains the following:
1) sodium bicarbonate [NaCO3] that neutralizes acidity of chyme; pH of small intestine is slightly basic;
2) pancreatic amylase that digests starch to maltose; (Table 43.2)
starch + H
pancreatic
starch + H
amylase
starch + H2O
maltose
3) trypsin and other enzymes that digest protein to peptides; and
protein + H trypsin
protein + H2O
peptides
3) lipase that digests fat droplets to glycerol and fatty acids. (Table 43.2)
fat droplets + H lipase
fat droplets + H2O
glycerol + fatty acids
e. Epithelial cells of villi produce intestinal enzymes that are attached to plasma membrane of microvilli.
f. Intestinal secretions complete digestion of peptides and sugars; peptides are digested by peptidases to amino acids;
peptides + H peptidases
peptides + H2O
amino acids
and maltose from the first step in starch digestion is converted by maltase to glucose.
maltose + H maltase
maltose + H2O
glucose
5. Large molecules of carbohydrates, proteins and fats are broken into small molecules that are absorbed by villi.
F. Into the Villi
1. Small intestine is specialized for absorption by the huge number of villi that line the intestinal wall.
2. Each villus contains blood vessels and a lymphatic lacteal. (Fig. 43.9) [transp. 237] [micro. slide 96]
3. A lacteal is a lymphatic vessel in an intestinal villus that aids in the absorption of fats.
4. Sugars, amino acids, and water enter the villi cells and then are absorbed into the bloodstream.
5. Glycerol and fatty acids enter villi cells and are reassembled into fat molecules, which move into the lacteals.
6. Absorption involves active transport and requires expenditure of cellular energy.
G. Control of Digestive Juices (Fig. 43A)
1. Gastrin is produced by cells in the gastric glands of the stomach wall; stimulates gastric glands and increases gastric motility; its secretion is stimulated by a meal that is particularly rich in protein.
2. Secretin is produced by cells in the duodenal wall; stimulates the pancreas to secrete a fluid rich in NaCO3 into the duodenum; its secretion is stimulated by acid chyme.
3. Cholecystokinin (CCK) produced by duodenal wall stimulates pancreas to increase pancreatic juice and liver to increase output of bile; causes gallbladder to release bile; secretion is stimulated by fats.
4. Gastric inhibitory peptide (GIP) from duodenal wall inhibits gastric gland secretion and stomach motility.
H. Two Accessory Organs Help Out (Fig. 43.5) [transp. 236]
1. Pancreas Secretes Juices
a. Pancreas lies deep within abdominal cavity, just below stomach, and rests on posterior abdominal wall.
b. It is an elongated and somewhat flattened organ.
c. As an endocrine gland, it secretes glucogon and the hormone insulin into the bloodstream.
d. As an exocrine gland, the pancreas secretes pancreatic juice, which contains digestive enzymes and buffers, into the duodenum by way of ducts. (Table 43.2)
2. The liver is a large glandular organ that fills the top of the abdominal cavity, just below the diaphragm.
a. Blood vessels from large and small intestines form hepatic portal vein leading to the liver.
(Fig. 43.10)
b. The liver has numerous functions, including the following.
1) It detoxifies the plasma by removing and metabolizing poisonous substances.
2) The liver makes the plasma proteins.
3) Liver destroys old red blood cells; converts hemoglobin to bilirubin and biliverdin in bile.
4) It produces bile, which is stored in the gallbladder before entering the duodenum to emulsify fats.
5) Liver stores glucose as glycogen; breaks down glycogen to glucose to maintain constant glucose concentration in blood.
6) The liver produces urea from amino groups and ammonia.
c. Liver maintains glucose concentration in blood at about 0.1% by removing excess glucose from hepatic portal vein and storing it as glycogen; if needed, glycogen is broken down and glucose enters hepatic vein.
d. Before amino acids can be converted to glucose, deamination or removal of amino groups must take place.
e. Urea is most common human nitrogenous waste product; it is transported by bloodstream to kidneys.
3. Liver Disorders
a. Jaundice has a symptomatic yellowish skin due to a large amount of bilirubin in the blood.
b. In hemolytic jaundice, RBCs are broken down in abnormally large amounts.
c. In obstructive jaundice, there is blockage of the bile ducts or damage to liver cells.
d. Viral hepatitis is liver infection; hepatitis A results from eating contaminated food; hepatitis B and C are spread by blood transfusions, kidney dialysis, and unsterile needle use; all three can be caused from sexual contact.
e. Cirrhosis is a chronic liver disease where liver tissue is replaced by fatty and then scar tissue; it is a problem for alcoholics who provide excessive amounts of alcohol for the liver to break down.
I. Large Intestine Absorbs Water
1. The large intestine is the region following the small intestine. (Fig. 43.5) [transp. 236]
2. It has four parts: cecum, colon, rectum, and anal canal.
3. About 1.5 liters of water enter the digestive tract daily from drinking; another 8.5 liters enter from various secretions.
a. About 95% of this total liquid is reabsorbed by the small intestine and cells of the colon.
b. If water is not reabsorbed, it causes diarrhea which can cause serious dehydration and ion loss.
4. Large intestine functions in ion regulation, absorbing salts plus vitamin K produced by intestinal bacteria.
5. The last 20 cm of the large intestine is the rectum that terminates at the anus, an external opening. (Fig. 43.5)
6. Feces consists of 75% water and 25% solid matter; one-third of the solid matter is intestinal bacteria and the remainder is undigested wastes, fats, organic material, mucus, and dead cells from the intestinal lining.
7. Two serious medical conditions are associated with the large intestine.
a. Appendicitis is an infection of the appendix.
1) Appendix is fingerlike projection that extends at the cecum, a blind sac at the junction of small and large intestines. (Fig. 43.5)
2) If an infected appendix bursts before removed, it results in general abdominal infection or peritonitis.
b. Intestinal polyps are small growths arising from epithelial lining.
1) Whether they are benign or cancerous, they can be removed surgically.
2) A low-fat, high-fiber diet promotes regularity; recommended as protection against mutagenic agents.