THE HUMAN DIGESTIVE SYSTEM CONSISTS OF AN ALIMENTARY CANAL AND ACCESSORY GLANDS

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1 /1 st lecture for Final/Tuesday DIGESTION Chapter 21 Ingestion, the act of eating, is only the first of four main stages of food processing. Digestion, the second stage, is the breaking down of food into molecules small enough for the body to absorb. Most of the organic matter in food consists of proteins, fats, carbohydrates all large polymers. In the third, absorption, the cells lining the digestive tract take up (absorb) small nutrient molecules. From here the molecules travel in the blood to other body cells, where they are incorporated into the cells or broken down further to provide energy. In the fourth and last stage of food processing, elimination, undigested material passes out the digestive tract. THE HUMAN DIGESTIVE SYSTEM CONSISTS OF AN ALIMENTARY CANAL AND ACCESSORY GLANDS Food is how we get our energy. We eat proteins, which is made out of amino acids. We have to eat the essential 9 out of 20 amino acids. NINE AMINO ACIDS ARE ESSENTIAL NUTRIENTS Adult humans cannot make nine out of the 20 kinds of amino acids needed to synthesize proteins. These nine known as the essential amino acids must be obtained from the diet. Complex sugars get broken down to mono or simple sugars. Polysaccharides change to saccharides The digestive glands, the salivary glands, pancreas, and liver, secrete digestive juices that enter the alimentary canal through ducts. Secretions from the liver are stored in the gall bladder before they are released into the intestine. Once food is swallowed, muscles propel it through the alimentary canal by peristalsis, rhythmic waves of contraction of smooth muscle. A muscular ring, called the pyloric sphincter, regulates the passage of food out of the stomach and into the small intestine. The final step in digestion and nutrient absorption occur in the small intestine, where water is taken into the body from the remains of the food and digestive juices and feces are formed. DIGESTION BEGINS IN THE ORAL CAVITY Saliva contains several substances important in food processing. A slippery glycoprotein protects the soft lining of the mouth and lubricates the solid food for easier swallowing. Saliva also contains a digestive enzyme called salivary amylase that begins hydrolyzing starch. Chewing grinds the food, making it easier to swallow and exposing more food surface area to digestive enzymes. In swallowing, the tongue pushes the bolus to the back of the oral cavity and into the pharynx. 1

2 THE FOOD AND BREATHING PASSAGES BOTH OPEN INTO THE PHARYNX Openings into both our esophagus and our trachea (windpipe) are in the pharynx. Most of the time, the esophageal opening is closed off by a sphincter and the trachea is open for breathing. When the bolus enters the pharynx, the swallowing reflex makes the esophageal sphincter relax and allows the bolus to enter the esophagus. At the same time, the larynx moves upward and tips the epiglottis over the tracheal opening. In this position, the epiglottis prevents food from passing into the windpipe. THE ESOPHAGUS SQUEEZES FOOD ALONG TO THE STOMACH The esophagus is a muscular tube that conveys food bolus from the pharynx to the stomach. The esophageal muscles are arranged in two layers. A circular muscle layer runs around the esophagus, a longitudinal layer runs the length of the esophagus. Wavelike contractions, peristalsis, of the circular and longitudinal muscles squeeze a bolus toward the stomach. 2

3 THE STOMACH STORES FOOD & BREAKS IT DOWN WITH ACID AND ENZYMES Our stomach is highly elastic and can stretch to accommodate about 2 liters of food and drink, usually enough to satisfy our body s needs for many hours. Some chemical digestion occurs in the stomach. The stomach secretes gastric juice, which is made up of mucus, enzymes and strong acid. The interior surface of the stomach wall is highly folded and it is dotted with pits leading down into tubular gastric glands. The gastric glands secrete different components of the gastric juice. Mucous cells secrete mucus, which lubricates and protects the cells lining the stomach. Parietal cells secrete HCL. And chief cells secrete pepsinogen, an inactive form of the digestive enzyme pepsin. 1. Pepsinogen and HCL are secreted into the interior of the gastric gland. 2. Next, the HCL converts pepsinogen to PEPSIN. 3. Pepsin itself then activates more pepsinogen, starting a chain reaction. Pepsin begins the chemical digestion of proteins. It splits the polypeptide chains of proteins into smaller polypeptides. Why doesn t this strong acid eat at our stomach lining? Because pepsinogen helps protect the stomach lining from both pepsin and acid. Mitosis must generate enough new cells to replace the stomach lining completely about three days. Contraction of the walls aids chemical digestion. The active stomach churns the food with the gastric juice, forming a mixture called acid chyme. Most of the time, the stomach is closed off at both ends. The opening between the esophagus and the stomach is closed except when a bolus driven downward by peristalsis arrives there. Between the stomach and the small intestine, the pyloric sphincter helps regulate passage of acid chyme leaving the stomach only a squirt at a time. The stomach takes about 2 6 hours to empty a meal. 3

4 BACTERIAL INFECTIONS CAN CAUSE ULCERS. The chemicals/acids that digest food can dissolve steel. A coat of mucus normally protects the stomach wall from the corrosive effect of digestive juice. But when it does, open sores called gastric ulcers can develop in the stomach wall. THE SMALL INTESTINE IS THE MAJOR ORGAN OF CHEMICAL DIGESTION & NUTRIENT ABSORPTION The food has been reduced to a soup. Virtually all chemical digestion occurs in the small intestine. Nutrients are also absorbed into the blood from the small intestine. The pancreas produces digestive enzymes and an alkaline solution rich in bicarbonate. The alkaline solution neutralizes acid chyme as it enters the small intestine. The liver performs a wide variety of functions, including the production of bile. Bile contains no digestive enzymes, but bile salts dissolved in it makes fats more susceptible to enzyme attack. The gallbladder stores bile until it is needed in the small intestine. The small intestine also completes the digestion of proteins begun in the stomach. The pancreas and the duodenum secrete hydrolytic enzymes that completely dismantle polypeptides into amino acids. In contrast to starch & proteins, nearly all the fat in food remains completely undigested until it reaches your duodenum. Hydrolysis of fats is a special problem because fats are insoluble in water. First bile salts from the gallbladder coat tiny fat droplets and keep them separate from one another, a process called emulsification. When there are too many droplets, a large surface area of fat is exposed to lipase, an enzyme that breaks fat molecules down into fatty acids and glycerol. By the time peristalsis has moved the chyme mixture through the duodenum, chemical digestion of our meal is almost complete. The remaining regions of the small intestine are adapted for the absorption of nutrients. 4

5 The small intestine is well suited for the task of absorbing nutrients. Its lining has a huge surface area roughly 300m 2, about the size of a tennis court. Around the wall of the small intestine are large circular folds with numerous small, fingerlike projections called VILLI. If we look at the epithelial cells of a villus with a microscope we see many tiny surface projections called MICROVILLI. The microvilli extend into the interior of the intestine. A small lymph vessel and a network of capillaries penetrate the core of each villus. Nutrients pass first across the intestinal epithelium and then the thin walls of the capillaries or lymph vessel. Some nutrients simply diffuse from the digested food into the epithelial cells and then into the blood or lymph. The capillaries that drain nutrients away from the villi converge into larger blood vessels and eventually a main vessel that leads directly to the liver. The liver thus gets first access to nutrients absorbed from a meal. The liver converts many of the nutrients into new substances that the body needs. One of its main functions is to remove excess glucose from the blood and convert it to glycogen, which is stored in liver cells. From the liver, blood travels to the heart, which pumps the blood and the nutrients it contains to all parts of the body. Your pizza and soft drink are now on their way to being incorporated into your body. 5

6 THE LARGE INTESTINE RECLAIMS WATER The large intestine or colon is about 1.5 meters long and 5cm in diameter. It joins the small intestine at a T shaped junction where a sphincter controls the passage of unabsorbed food material out of the small intestine. One arm of the T is a blind pouch called the cecum. The appendix, a small fingerlike extension of the cecum, contains a mass of white blood cells that make a minor contribution to immunity. The colon s main function is to absorb water from the alimentary canal. As the water is absorbed, the remains of the digested food become more solid as they are conveyed along the colon by peristalsis. These waste products of digestion, the feces, consists mainly of indigestible plant fibers and bacteria that normally live in the colon. 6

7 /2nd lecture for Final/Thursday & /3 rd lecture for Final/Tuesday Circulation of blood by the Heart Chapter 23 & Lymphatic system Chapter 24 The Lymphatic System The lymphatic system consists of lymph; a network of transporting structures called lymph vessels & lymph nodes. The primary function of the lymphatic system is to drain fluid from the tissue spaces and return it to the blood. Other functions include transporting materials to body cells and carrying waste products from body tissues back to the blood stream. It also aids in the control of infection by providing lymphocytes and monocytes to defend against infections caused by microorganisms. Lymph originates from blood plasma, as whole blood circulates through the capillaries, some of the plasma seeps out of these thin walled vessels. This fluid, now called interstitial fluid resembles plasma with less protein. Interstitial fluid nourishes and cleanses the body tissues. It collects cellular debris, bacteria and particulate matter. Eventually, interstitial fluid enters into blind ended vessels called LYMPH CAPILLARIES. Once it enters a capillary. It is called LYMPH. Lymph passes from capillaries to larger vessels and finally to nodes, which serve as depositories for cellular debris. The lymph vessels are not connected to the heart. The NODES have plenty of WBC here. Bacteria are phagocytized by macrophages that line the nodes. At times, the number of bacteria entering a node is so great that the node enlarges and becomes tender. Lymph vessels from the right chest and arm join the right lymphatic duct. This duct drains into the right subclavian vein. Lymph from all other parts of the body enters the thoracic duct, which drains into the left subclavian vein. Veins have no blood pressure. In this way, lymph is redeposited into the circulating blood in order to begin the cycle, once again, throughout the body. 60 billion capillaries in the human body. 80% of the fluids return to the capillary. 7

8 5/13/99 Thursday/Chapter 27 LAST LECTURE FOR THE FINAL TEST! REPRODUCTION & STD s FEMALES A woman s ovaries are each about an inch long, each consisting of a single developing egg cell surrounded by one or more layers of follicle cells that nourish and protect the developing egg. A woman is born with between 40,000 and 400,000 follicles, but only several hundred will release egg cells during her reproductive years. Starting at puberty and continuing until menopause, one follicle matures and releases its egg about every 28 days. An egg cell is ejected from the follicle in the process called ovulation. After ovulation, the remaining follicular tissue grows within the ovary to form a solid mass called corpus luteum. The uterus has a thick muscular wall and blood vessels. The embryo implants in the endometrium and development is completed there. The term embryo is used for the stage in development from the first division of the zygote until body structures begin to appear, about the ninth in humans. From the 9 th week on, we are called fetuses. The UTERUS is the normal site of pregnancy. However, in an ectopic pregnancy the embryo implants itself in the oviduct tubal pregnancy. Then they rupture and cause severe bleeding and even death. MALES The male gonads, the testes, are both housed outside in a sac called the scrotum. Sperm cannot develop at human body temperature. From the testes, sperm passes through a coiled tube called the EPIDIDYMIS, which stores sperm while they develop motility and fertilizing ability. Sperm leave the epididymis during ejaculation, the expulsion of sperm containing fluid from the penis. At that time, muscular contractions propel the sperm from the epididymis through another duct, the vas deferens. The vas deferens joins a short duct from a gland, the seminal vesicle. Union of these two ducts forms a short ejaculatory duct, which joins its counterpart conveying sperm from the other testis. 8

9 In addition to the testes and ducts, the male reproductive system contains three sets of glands: 1. The two SEMINAL VESICLES secrete a thick fluid that lubricates and nourishes the sperm. 2. The PROSTATE GLAND secretes a milky, alkaline fluid that balances the acidity of any traces of urine in the urethra and helps protect the sperm from the natural acidity of the vagina. 3. The two BULBOURETHAL GLANDS secrete only a few drops of fluid into the urethra, during sexual arousal. The fluid may help lubricate the urethra, helping sperm move through it. Together the sperm and the secretions of the glands make up SEMEN, the fluid that is discharged from the penis. HORMONAL CONTROL Hormones control production by the testes. Influenced by signals from other parts of the brain, the hypothalamus secretes a releasing hormone that regulates release of FOLLICLE STIMULATING HORMONE and LUTEINIZING HORMONE by the anterior pituitary gland. FSH increases sperm production by the testes, while LH promotes the secretion of androgens, mainly testosterone. Androgens stimulate sperm production. In addition, androgens carried in the blood help maintain homeostasis by a negative feedback mechanism, inhibiting secretion of both the releasing hormone and LH. 9

10 THE FORMATION OF SPERM & OVA REQUIRES MEIOSIS. Ovum = Egg Both sperm and ova are HAPLOID CELLS, developed by meiosis from diploid cells in the gonads. Sperm develop in the testes in coiled tubes called SEMINIFEROUS TUBULES. A female s ovary at birth contains all the follicles she will ever have. OOGENESIS actually begins prior to birth, when a diploid cell in each developing follicle begins meiosis. At birth, each follicle contains a dormant primary oocyte, a diploid cell that is resting in prophase of meiosis I. After puberty, about every 28 days, FSH from the pituitary stimulates one of the dormant follicles to develop. The follicles enlarges, and the primary oocyte completes meiosis I. In the female, the division of the cytoplasm in meiosis I is unequal, with a secondary oocyte receiving almost all of it. The smaller of the two daughter cells, called the polar body receives no cytoplasm. The secondary oocyte is the stage released by the ovary during ovulation. It enters the oviduct and if the sperm penetrates it, the secondary oocyte undergoes meiosis II. Meiosis II yields a second polar body and the actual ovum. The haploid nucleus of the ovum can then fuse with the haploid nucleus of the sperm cell, producing a zygote. Polar body formation enables the ovum to acquire nearly all the cytoplasm and thus the bulk of the nutrients contained in the original diploid cell. Meiosis I occurs as the follicle matures. About the time the secondary oocyte forms, the pituitary hormone LH triggers the ovulation, the expulsion of the secondary oocyte from the follicle. Oogenesis & Spermatogenesis are alike in that they produce haploid gametes. However they differ in 3 ways: 1. Only one ovum results from each diploid parent cell that undergoes oogenesis, whereas 4 sperm cell result from each parent cell that undergoes spermatogenesis. 2. An ovary at birth contains all the primary oocytes it will ever have 3. Oogenesis is not completed without stimulation from a sperm cell. Whereas spermatogenesis produces mature sperm in an uninterrupted sequence. 10

11 Sexually Transmitted Diseases. 1/3 to 40% of all 14 to 25 year olds will have an STD. Herpes Simplex II is a Virus we cannot cure a virus of this type. Oral herpes is type I. Herpes I is not harmful to a fetus but II is. But both viruses can end up on both places through oral sex. I can be on the genitals. Starts out with vesicles, cold sores, which are painful and sometimes hidden. Females can have it out of sight. Will develop flu like symptoms. You break out during stressful situations like a delivery of a child. 10 to 20 million new cases each year. Wow! So a C section is done to prevent this, but the female must first notify the doctor. Gonorrhea - >10,000,000. Burning in males during urination. We can cure it because it is a bacterial infection. Can cause blindness in newborns unless special eyedrops are put in their eyes. Chlamydia fastest growing STD. >10,000,000. The number is in the tens of millions. Another bacterial infection. Asymptomatic no symptoms shown. Can also cause blindness in newborns. STD s in women cause pelvic inflammatory disease. Diseases that cause infertility in women. A scar develops in the fallopian tube which blocks the zygote from passing into the uterus. Causing an ectopic pregnancy. Trichomonas protozoa. Syphilis Bacterial infection. A chancre forms and a rash develops. The microorganisms gain entrance to vital organs such as the heart, liver and brain & spinal cord. Genital Warts >10,000,000 cases. It is an incurable virus. Burn this off liquid nitrogen and cauterize them with fire. No danger to baby. Papilloma virus. 60 different types. Warts on the hand are different from genital warts. HIV/AIDS going up in women and people in their 60 s. 11