Chapter 17 - Respiration and Excretion

Section 1 - The Respiratory System

  The respiratory system moves oxygen from the outside environment into the body. It also removes carbon dioxide and water from the body.

  Respiration is the process in which oxygen and glucose undergo a complex series of chemical reactions inside cells. Respiration, which is also called cellular respiration, releases the energy that fuels growth and other cell processes. Respiration is different from breathing, which is the movement of air into and out of the lungs. Respiration also produces carbon dioxide and water, which are eliminated by the respiratory system.

  As air travels from the outside environment to the lungs, it passes through the following structures: nose, pharynx, trachea, and bronchi. Air enters the body through your nostrils and then moves into the nasal cavities. Some of the cells lining the nasal cavities produce mucus. Mucus cleans, warms, and moistens air you breathe. The inside of the nose is lined with cilia. Cilia (singular cilium) are tiny hairlike extensions that can move together. They sweep the mucus into the throat where it is swallowed.

  Air moves from the nose downward into the throat, or pharynx. The trachea, or windpipe, leads from the pharynx to the lungs. The walls of the trachea are made of rings of cartilage that keep it open. The trachea is lined with cilia and mucus. The cilia in the trachea move the mucus toward the pharynx, where it is swallowed.

  Air moves from the trachea into the bronchi (singular bronchus). The bronchi are passages that direct air into the lungs. The lungs are the main organs of the respiratory system. Inside the lungs, each bronchus divides into smaller and smaller tubes. At the end of the smallest tubes are bunches of alveoli (singular alveolus). Alveoli are tiny sacs of lung tissue specialized for the movement of gases between air and blood. Alveoli are surrounded by capillaries. After air enters an alveolus, oxygen passes through the wall of the alveolus and then through the capillary wall into the blood. Carbon dioxide and water pass from the blood into the alveoli. This process is known as gas exchange.

  The diaphragm is a large, dome-shaped muscle at the base of the lungs. When you breathe, the actions of your rib muscles expand or contract your chest, causing air to flow in or out.

  The larynx, or voice box, is located at the top of the trachea. Your vocal cords are two folds of connective tissue that stretch across the opening of the larynx. The vocal cords vibrate when air passes over them. This produces the sound of your voice.

Section 2 - Smoking and Your Health

  Some of the most deadly chemicals in tobacco smoke are tar, carbon monoxide, and nicotine.

  Tar is the dark, sticky substance produced when tobacco burns. When someone inhales tobacco smoke, tar settles on the cilia that line the respiratory organs. This makes the cilia clump together so they cannot prevent harmful materials from getting into the lungs. Tar also contains chemicals that have been shown to cause cancer.

  When tobacco is burned, a colorless, odorless gas called carbon monoxide is produced. Carbon monoxide is dangerous to inhale. Its molecules bind to hemoglobin in the red blood cells. This takes the place of some of the oxygen that the red blood cells normally carry. Smokers' blood may contain too little oxygen, causing a faster heartbeat and breathing rate.

  Nicotine speeds up the nervous system and heart. Nicotine produces an addiction, or physical dependence. Smokers crave a cigarette if they go without one.

  Some serious respiratory problems can result from long-term smoking. Over time, smokers can develop chronic bronchitis, emphysema, lung cancer, and atherosclerosis.

  Bronchitis is an irritation of the breathing passages. The small passages become narrower than normal and may be clogged with mucus. Long-term, or chronic, bronchitis can cause permanent damage to the breathing passages.

  The chemicals in tobacco smoke also damage lung tissue. Emphysema is a disease that destroys lung tissue and causes difficulty in breathing. People with emphysema do not get enough oxygen and cannot adequately eliminate carbon dioxide.

  Cigarette smoke contains over 50 chemicals that cause cancer. Tumors take away space in the lungs that are used for gas exchange. Lung cancer is difficult to detect early enough for effective treatment.

  Some of the chemicals in tobacco smoke get into the blood and cause problems in the circulatory system, including atherosclerosis. Compared to nonsmokers, smokers are more than twice as likely to have heart attacks.

  Smokers are not the only people to suffer from the effects of tobacco smoke. In passive smoking, people involuntarily inhale the smoke from other people's cigarettes, cigars, or pipes.

Section 3 - The Excretory System

  The excretory system is the system in the body that collects wastes produced by cells and removes the wastes from the body. The removal process is known as excretion.

  The structures of the excretory system that eliminate urea, water, and other wastes include the kidneys, ureters, urinary bladder, and urethra. The two kidneys are the major organs of the excretory system. The kidneys remove urea and other waste materials from your blood. The wastes are eliminated in urine, a watery fluid that contains the urea and other wastes. Urea is a chemical that comes from the breakdown of proteins.

  Urine flows from the kidneys through two narrow tubes called ureters. The ureters carry the urine to the urinary bladder, a sacklike muscular organ that stores urine. Urine flows from the body through a small tube called the urethra.

  Each kidney contains about a million nephrons. Nephrons are tiny structures that remove wastes from blood and produce urine. The nephrons filter wastes in stages. First, both wastes and needed materials, such as glucose, are filtered out of the blood. Then, much of the needed material is returned to the blood, and the wastes are eliminated from the body.

  Blood enters the kidneys and then flows through branching arteries into a cluster of capillaries in a nephron. The capillaries are surrounded by a hollow capsule connected to a tube. In the capillaries, urea, water, and glucose move from the blood into the capsule. From the capsule, the filtered substances pass into a long, twisting tube. As the filtered material moves through the tube, most of the water and glucose are reabsorbed into the blood. Most of the urea remains in the tube.

  Some medical problems can be detected by analyzing the chemicals in urine. Glucose in the urine may indicate that a person has diabetes. Protein in urine can be a sign that the kidneys are not functioning properly.

  Excretion maintains homeostasis by keeping the body's internal environment stable and free of harmful levels of chemicals. In addition to the kidneys, organs of excretion that maintain homeostasis include the lungs, skin, and liver. The kidneys help maintain homeostasis by regulating the amount of water in your body.

  When you exhale, carbon dioxide and some water are removed from the body by the lungs. Sweat glands in the skin excrete water and some chemical wastes in perspiration. The liver breaks down some wastes so they can be excreted.

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