CK-12 Biology I - Honors

by CK-12 Foundation

  Colitis is inflammation of the colon. It has many possible causes, ranging from bacterial infections to immune reactions against the body’s own tissues. Symptoms of colitis include pain and tenderness in the abdomen. Treatment of colitis may include medication, surgery, and changes in diet.

  Appendicitis is inflammation of the appendix. It is most common in children and teens. The appendix is a small, fingerlike pouch that extends from the cecum (see Figure above). Inflammation of the appendix is usually caused by a bacterial infection. Symptoms include abdominal pain, loss of appetite, fever, and vomiting. Appendicitis is most often treated with surgery to remove the infected appendix. Without treatment, an infected appendix can be fatal.

  Diseases of the Accessory Organs

  Accessory organs of digestion can also be affected by disease, and this may interfere with normal digestion. A disease that affects the pancreas is cystic fibrosis. Cystic fibrosis (CF) is an inherited disease in which the body produces abnormally thick and sticky mucous. In the pancreas, the mucus blocks the duct to the duodenum, preventing pancreatic enzymes from reaching it. As a result, proteins and lipids cannot be digested properly. People with CF may take digestive enzymes by mouth to improve their digestion. However, the disease has no known cure. (For more information on CF, see chapter titled Human Genetics.)

  Hepatitis is inflammation of the liver. It is usually caused by a viral infection. Several different viruses can cause hepatitis. Some of the viruses spread through contaminated foods or beverages, others through sexual contact. Symptoms of hepatitis include fever, headache, vomiting, and abdominal pain. Another symptom is jaundice, which is yellowing of the skin and eyes. If the symptoms are mild, the disease may clear up without treatment. If the symptoms are more severe, the disease may damage the liver so it can no longer produce bile. This interferes with the digestion of lipids. Medications are available to treat hepatitis. Some types of hepatitis can also be prevented with vaccines.

  Gall bladder problems occur mainly in adults. They are often caused by gall stones (Figure below). Gall stones are crystals that form in the bile in the gall bladder. There are many possible reasons why gall stones form, including abnormal body chemistry and too much fat in the diet. Gall stones start out as small as a grain of sand but may grow to the size of a golf ball. There may be one large stone or many small ones. If gall stones block the duct that carries bile to the duodenum, they may cause inflammation of the gall bladder and severe abdominal pain. Generally, the only way to treat these problems is to surgically remove the gall stones or the entire gall bladder.

  Figure 23.8

  Gall stones.

  Lesson Summary

  The digestive system includes the gastrointestinal tract and accessory organs such as the pancreas. The major functions of the digestive system are to digest food, absorb nutrients, and eliminate solid waste.

  Both mechanical and chemical digestion of food start in the mouth. The esophagus carries the food to the stomach, and the stomach continues mechanical and chemical digestion.

  Most chemical digestion takes place in the small intestine with the help of several digestive enzymes. Virtually all absorption of nutrients also takes place in the small intestine.

  The large intestine removes excess water from waste and eliminates waste from the body. It also provides a home for helpful intestinal bacteria.

  Many diseases affect the digestive system and may interfere with digestion. They include food allergies, infections, and inherited conditions.

  Review Questions

  Name, in sequence, the digestive organs that food passes through in the gastrointestinal tract, from the mouth to the anus.

  Describe two ways that the mouth helps digest food.

  How do villi and microvilli help the small intestine absorb nutrients?

  What are two functions of helpful bacteria in the large intestine?

  Describe what happens to carbohydrates as they pass through the organs of the GI tract.

  Antibiotics are medications that destroy bacteria. Explain how antibiotics might help treat stomach ulcers.

  Why is it important for digestive system functions that mucous membranes can secrete and absorb substances?

  Compare and contrast two digestive enzymes that work in the duodenum.

  Further Reading / Supplemental Links

  Brynie, Faith Hickman, 101 Questions about Food and Digestion that Have Been Eating at You Until Now. Lerner Publishing Group, 2002.

  Dowshen, Stephen, M.D., “Digestive System.” Nemours Foundation, 2007. Available online at: www.kidshealth.org/teen/your_body/body_basics/digestive_system.html.

  Johnson, Rebecca L., The Digestive System. Lerner Publications, 2004.

  Minocha, Anil and Adamec, Christine A., The Encyclopedia of the Digestive System and Digestive Disorders. Facts on File, 2004.

  Nagel, Rob, Body by Design: From the Digestive System to the Skeleton. UXL, 2000.

  http://en.wikibooks.org/wiki/Human_Physiology/The_gastrointestinal_system

  http://library.thinkquest.org/12090/

  http://www.biology-online.org/7/6_food.htm

  http://www.vivo.colostate.edu/hbooks/pathphys/digestion/

  http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/

  http://en.wikipedia.org

  Vocabulary

  absorption

  The process in which substances pass into the blood stream, where they can circulate throughout the body; occurs mainly in the small intestine.

  amylase

  The major salivary enzyme is amylase; begins the chemical digestion of carbohydrates in the food; helps break down complex starch molecules into simpler sugar molecules.

  cecum

  The first part of the large intestine, where waste enters from the small intestine.

  celiac disease

  An immune reaction to a food protein called gluten, which is found in grains.

  chemical digestion

  The chemical breakdown of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by the blood; takes place mainly in the small intestine.

  colon

  The second part of the large intestine, where excess water is absorbed. After the excess water is absorbed, the remaining solid waste is called feces.

  duodenum

  The first part of the small intestine; site where most chemical digestion occurs.

  esophagus

  A narrow tube - begins at the pharynx, passes through the chest, and ends at the opening to the stomach. The function of the esophagus is to pass food from the mouth to the stomach.

  gall bladder

  A small, pear-shaped structure below the liver; stores substances from the liver until they are needed by the small intestine.

  gastritis

  Inflammation of the lining of the stomach.

  gastrointestinal (GI) tract

  Organ of the digestive system; a long tube that connects the mouth with the anus.

  ileum

  The third part of the small intestine. A few remaining nutrients are absorbed in the ileum, as are salts that form from liver bile.

  inflammatory bowel disease

  Inflammation of the large intestine and, in some cases, other parts of the GI tract; includes Crohn’s disease and ulcerative colitis.

  irritable bowel syndrome (IBS)

  A disorder in which the large intestine is easily irritated.

  jejunum

  The second part of the small intestine; where most nutrients are absorbed into the blood.

  large intestine

  A relatively wide tube that connects the small intestine with the anus; consists of three parts: the cecum, colon, and rectum.

  liver

  A large organ next to the stomach; produces digestive substances that are carried by ducts, or tubes, to the small intestine and gall bladder.

  lower GI tract

  Segment of the GI tract that includes the small and large intestines.

/>   mechanical digestion

  The physical breakdown of chunks of food into smaller pieces; takes place mainly in the mouth and stomach.

  mucous membranes

  Moist tissues that can secrete and absorb substances.

  pancreas

  A gland below the stomach; produces digestive substances that are carried by a duct to the small intestine.

  peristalsis

  A rapid, involuntary, wave-like contraction of muscles; pushes food through the GI tract.

  pharynx

  Connects the mouth to the rest of the digestive tract; also connects the mouth and nose to the rest of the respiratory system.

  rectum

  The third part of the large intestine; where feces accumulates. As the rectum fills, the feces become compacted. The feces are stored in the rectum until they are eliminated from the body.

  small intestine

  A narrow tube leading away from the stomach; made up of three parts: the duodenum, jejunum, and ileum; the site of most chemical digestion and virtually all absorption.

  stomach

  A saclike organ located between the end of the esophagus and the beginning of the small intestine. In the stomach, food is further digested both mechanically and chemically.

  stomach ulcer

  A sore in the lining of the stomach.

  upper GI tract

  Segment of the GI tract that includes the mouth, esophagus, and stomach.

  Points to Consider

  The large intestine eliminates the waste that remains after food is digested. More waste is produced when cells break down nutrients for energy and building materials. How is this waste removed from the body? Is it eliminated by the large intestine? Is it removed in some other way?

  Lesson 23.3: Excretory System

  Lesson Objectives

  Define homeostasis and excretion, and explain why they are necessary for life.

  Describe the urinary system, kidneys, and nephrons; summarize the processes involved in excretion.

  Identify roles of the kidneys in homeostasis.

  Name diseases of the urinary system, and explain how dialysis helps treat kidney failure.

  Introduction

  If you exercise on a hot day, you are likely to lose a lot of water in sweat. Then, for the next several hours, you may notice that you do not pass urine as often as normal and that your urine is darker than usual. Do you know why this happens? Your body is low on water and trying to reduce the amount of water lost in urine. How does the body know when it is low on water? How does it control the amount of water lost in urine? The answers to both questions are the kidneys and the glands that control them.

  Homeostasis and Excretion

  The kidneys are the body’s main organs of homeostasis and excretion. Homeostasis is the body’s attempt to maintain a constant internal environment. One of the major ways the body achieves homeostasis is through excretion. Excretion is the process of removing wastes and excess water from the body.

  Homeostasis

  Homeostasis is a fundamental characteristic of all living things. Internal body conditions must be kept within certain limits for the normal functioning of cells. Homeostasis involves keeping many internal factors at more or less constant levels. The factors include body temperature and properties of the blood. For example, the blood must have certain levels of acidity, salts, and nutrients in order for cells to function normally.

  A variety of homeostatic mechanisms help maintain stability of the internal environment. Each mechanism involves the interaction of at last three components: a receptor, a control center, and an effector.

  The receptor senses changes in the internal environment and sends the information to the control center.

  The control center processes the information, determines the appropriate action, and sends a command to the effector.

  The effector responds to the command and changes conditions in the internal environment.

  An example of a homeostatic mechanism in humans is the regulation of body temperature. This is represented by the diagram in Figure below. Temperature receptors in the skin send information about skin temperature to the brain. The brain is the control center. It determines whether the temperature is too high or too low and sends appropriate commands to effectors that control body temperature. Effectors include blood vessels near the surface of the body. If the temperature is too high, the brain commands the blood vessels to dilate, which helps the body lose heat. If the temperature is too low, the brain commands the blood vessels to constrict, which helps the body retain heat. These actions help return body temperature to normal.

  Negative Feedback and Body Temperature

  Figure 23.9

  Regulation of body temperature is an example of negative feedback. When body temperature deviates from normal, this information feeds back to the brain and sets in motion changes that return body temperature to normal.

  The regulation of body temperature is an example of negative feedback. Negative feedback is a type of homeostatic mechanism in which change in one direction results in a counteractive change in the opposite direction. Negative feedback reverses the direction of change to bring conditions back to normal. Most of the mechanisms that control homeostasis in the human body involve negative feedback.

  Positive feedback mechanisms also exist, but they are not common in the human body. Positive feedback accelerates or amplifies a change and pushes levels farther away from normal. One example of a positive feedback mechanism in the body is blood clotting, which is described in the chapter titled Circulatory and Respiratory Systems.

  If homeostasis is disturbed, a homeostatic imbalance results. This may result in cells getting too much or not enough of certain substances. Many diseases are caused by homeostatic imbalances. For example, diabetes mellitus is a disease in which the blood contains too much glucose. This can have serious consequences for cells throughout the body. It may lead to damaged blood vessels, heart disease, blindness, and kidney failure.

  Excretion

  Excretion is an essential process in all forms of life. When cells metabolize—or break down—nutrients, waste products are produced. For example, when cells metabolize proteins and nucleic acids, nitrogen wastes such as ammonia, urea and uric acid are produced. Ammonia is a toxic substance and must be removed from the blood and excreted from the body. Urea is removed through urine, which is produced in the kidney. Excretion is also necessary to remove excess water, salts, and many other substances from the body.

  Although the kidneys are the main organs of excretion of wastes from the blood, several other organs are also involved in excretion, including the large intestine, liver, skin, and lungs.

  The large intestine eliminates solid wastes that remain after the digestion of food in the gastrointestinal tract (as discussed in Lesson 23.2: Digestive System).

  The liver breaks down excess amino acids in the blood to form ammonia, and then converts the ammonia to urea, a less toxic substance. The liver also breaks down other toxic substances in the blood, including alcohol and drugs.

  The skin eliminates water and salts in sweat.

  The lungs exhale water vapor and carbon dioxide.

  Kidneys and Excretion

  The kidneys are part of the urinary system. The kidneys work together with other urinary system organs in the function of excretion. The urinary system is shown in Figure below.

  Figure 23.10

  The urinary system.

  Urinary System

  In addition to the kidneys, the urinary system includes the ureters, bladder, and urethra. The main function of the urinary system is to filter waste products and excess water from the blood and remove them from the body. The two kidneys, which are described in detail below, filter the blood and form urine. Urine is the liquid waste product of the body that is excreted by the urinary system.

  From the kidneys, urine enters the ureters, which carry it to the bladder. Each ureter is a muscular tube about 25 centimeters long.
Peristaltic movements of the muscles of the ureter send urine to the bladder in small spurts.

  The bladder is a hollow organ that stores urine. It can stretch to hold up to 500 milliliters. When the bladder is about half full, the stretching of the bladder sends a nerve impulse to the sphincter that controls the opening to the urethra. In response to the impulse, the sphincter relaxes and lets urine flow into the urethra.

  The urethra is a muscular tube that carries urine out of the body. Urine leaves the body through another sphincter in the process of urination. This sphincter and the process of urination are normally under conscious control.

  Kidneys

  The kidneys participate in whole-body homeostasis. As mentioned above, one of the promary roles of the kidney is to remove nitrogenous wastes. The kidneys are a pair of bean-shaped, reddish brown organs about the size of a fist. They are located just above the waist at the back of the abdominal cavity, on either side of the spine. As shown in Figure below, the kidneys are protected by the ribcage. They are also protected by a covering of tough connective tissues and two layers of fat, which help cushion them.