44.1. How Animals Exchange Gases (p. 790)
A. Respiration
1. Breathing includes inspiration (bringing air in) and expiration (moving air out).
2. External respiration involves gas exchange with the external environment at the respiratory surface.
3. Internal respiration found in more complex animals involves gas exchange between blood and tissue fluid.
4. Aerobic respiration involves production of ATP in cells.
B. Diffusion Accompanies Gas Exchange
1. To be effective, the gas exchange region must be moist, thin, and large in relation to the size of the body.
2. Some animals are small and shaped to allow the surface of the animal to be the gas-exchange surface.
3. Other animals are complex and have a specialized gas-exchange surface.
4. Effectiveness of diffusion improves with vascularization; delivery is promoted if blood contains hemoglobin.
C. In the Water
1. It is more difficult for animals to obtain O2 from water than from air.
a. Water fully saturated with air contains only a fraction of the amount of O2 as air.
b. Water is more dense than air; aquatic animals expend more energy to breathe than do terrestrial animals; fish use up to 25% of energy to breathe, but land mammals use only 1-2% of their energy output to breathe.
2. Hydras and planaria have a large surface area in comparison to their size. (Fig. 44.1) [transp. 238]
a. Gas exchange occurs directly across their whole body surface.
b. Hydra's outer layer of cells contacts environment; inner layer exchanges gases with gastrovascular cavity.
c. In planaria, a flattened body permits ready exchange of gases with the external environment.
3. A tubular shape and parapodia extensions (polychaete worms) provide surface areas for diffusion.
4. Most large aquatic animals pass water over gills.
a. Gills are finely divided and vascularized outgrowths of either the outer or the inner body surface.
b. Among clams and some snails, water is drawn into the mantle cavity to pass over the gills.
c. Decapod gills are located in brachial chambers under exoskeleton; water circulates by special mouthparts.
d. Gills of fishes are outward extensions of the pharynx organized into arches. (Fig. 44.2)
[transp. 239]
e. Ventilation in fish is result of combined action of mouth and gill covers. (Fig. 44.2a)
[transp. 239]
f. When the mouth is open, the opercula are closed and water is drawn in; the mouth then closes and the opercula open, drawing water from the pharynx through the gill slits located between the gill arches.
g. On the outside of gill arches are gill filaments folded into platelike lamellae, each of which contains capillaries; the result is a tremendous surface area for gas exchange. (Fig. 44.2b, c) [transp. 239]
1) Blood in the capillaries of gill lamellae flows in the direction opposite to that of water.
2) This countercurrent flow of water and blood increases the amount of O2 and CO2 exchanged.
3) The countercurrent mechanism allows about 80-90% of initial dissolved O2 in water to be extracted.
D. On the Land
1. Although air is a more concentrated source of O2 than water, it tends to have a drying effect on respiratory surfaces; humans lose about 350 ml of water per day when the air has 50% relative humidity.
2. The earthworm is an invertebrate that uses its body surface for respiration.
a. Earthworms expend much energy to secrete mucus and release fluids from excretory pores to stay moist.
b. The earthworm is behaviorally adapted to stay in moist soil during the day when air is driest.
3. Terrestrial insects utilize tracheal systems. (Fig. 44.3) [transp. 240]
a. Oxygen enters the tracheal system at spiracles, which are valvelike openings on each side of the body.
b. The tracheae branch and rebranch, ending in tiny tracheoles that are in direct contact with body cells.
c. Larger insects have air sacs located near major muscles to keep air moving in and out of the trachea.
d. Tracheae are effective enough in delivering oxygen to cells; circulatory system has no role in gas transport.
4. Terrestrial vertebrates use lungs for gas exchange.
a. Lungs are vascularized outgrowths of the lower pharyngeal region.
b. Lungs of amphibians are simple, saclike structures, that connect to the external environment by way of two bronchi, which connect to a short trachea. (Fig. 44.4a)
1) Amphibians' gas exchange occurs through skin kept moist by mucus produced by numerous glands.
2) During winter in temperate climates, amphibians burrow in mud; all gas exchange occurs across skin.
3) Frogs use positive pressure to force air inside; nostrils shut and floor of mouth forces air into the lungs.
c. Reptiles, birds, and mammals use negative pressure to move air into the lungs.
1) They have jointed ribs that can be raised and a muscular diaphragm that is flattened to expand lungs.
2) As the thoracic cavity expands, lung volume increases; air flows in due to difference in air pressure.
3) By lowering the ribs, pressure is exerted on the lungs, which forces air out.
d. Lungs of reptiles, amphibians and mammals are not completely emptied each breathing cycle.
1) With incomplete ventilation, entering air mixes with used air in the lungs.
2) This conserves moisture but decreases gas-exchange efficiency.
e. The high oxygen requirements of flying birds requires a complete ventilation system. (Fig. 44.5)
1) Incoming air is carried past the lungs by a bronchus that takes it to set of posterior air sacs.
2) Air then passes forward through lungs into a set of anterior air sacs and is finally expelled.
3) This one-way flow of oxygen-rich air does not mix with used air and maximizes gas exchange.
44.2. How Humans Exchange Gases (p. 794)
A. Human Respiratory System
1. The human respiratory system includes all structures that conduct air to and from lungs; lungs lie deep within the thoracic cavity, where they are protected from drying out. (Fig. 44.6) [transp. 240] (Table 44.1)
2. Air moves into the nose and then flows past the pharynx to the trachea, bronchi and to the lungs.
a. This process filters debris, warms the air and adds moisture.
b. When air reaches the lungs, it is at body temperature and saturated with water.
c. The trachea and bronchi are lined with cilia that beat upward, carrying mucus, dust and particles.
d. The hard and soft palates separate the nasal cavities from the mouth.
e. Air and food passages cross in the pharynx; the danger of choking is offset by providing an alternative path for breathing during congestion, and increasing air intake during exercise.
f. Air flows past the pharynx through the glottis into the larynx, which is protected by the epiglottis.
g. At edges of glottis are vocal cords; when air is passed across them, tissues vibrate creating vocal sounds.
h. From the larynx, air flows down the trachea to the bronchi.
1) The larynx is held open by cartilage that forms the Adam's apple.
2) The trachea walls are reinforced with C-shaped rings of cartilage.
3) When food is swallowed, the larynx rises and glottis is closed by a flap of tissue called the epiglottis.
4) Backward movement of the soft palate covers the entrance to the nasal passages; food is directed down.
i. The trachea divides into two bronchi; the C-shaped rings of cartilage diminish as the bronchi branch.
j. Within the lungs, each bronchus branches into numerous bronchioles that conduct air to alveoli.
k. The alveoli are microscopic air sacs.
B. Breathing In and Out
1. Humans breathe using negative pressure just as all other mammals. (Fig. 44.7) [transp. 242]
2. During inhalation, lowering the diaphragm and raising the ribs form negative pressure by increasing the volume of the thoracic cavity, which allows air under greater outside pressure to flow into the lung.
3. Increases in CO2 and H+ concentrations in the blood are the primary stimuli that increase breathing rate.
a. Chemical content of the blood is monitored by chemoreceptors that are very sensitive to increases in CO2 and H+ concentrations of the blood, but minimally sensitive to decreases in O2 concentration.
1) Aortic bodies are chemoreceptors located in the wall of the aortic arch.
2) Carotid bodies are chemoreceptors located in the wall of the carotid arteries.
4. Information from these goes to the respiratory center in the medulla oblongata that increases breathing rate when CO2 or H+ concentration increases; this respiratory center is also sensitive to blood reaching the brain.
C. Exchanging and Transporting Gases (Fig. 44.8) [transp. 243]
1. Gas exchange in the lungs and the tissues is brought about primarily by diffusion.
2. Atmospheric air contains little CO2, but blood flowing in pulmonary capillaries is almost saturated with CO2; therefore, CO2 diffuses from a region of higher concentration in air in the blood, across the epithelial walls of the alveoli and capillaries, to a region of lower concentration in the air in the alveoli.
3. Blood coming into pulmonary capillaries is oxygen poor, and alveolar air is oxygen rich; oxygen diffuses from higher concentration in alveoli, across walls of alveoli and capillaries, to lower concentration in the blood.
D. Transporting O2 and CO2 (Fig. 44.8) [transp. 243]
1. Most O2 entering the blood combines with hemoglobin (Hb) to form oxyhemoglobin (HbO2).
Hb + O2
HbO2
Hb +
oxyhemoglobin
2. A hemoglobin molecule has four polypeptide chains; each chain is folded over an iron-containing heme.
a. The iron atom of a heme group loosely binds with an O2 molecule.
b. Each RBC has 250 million hemoglobin molecules; over a billion molecules of O2 can be carried per RBC.
3. Oxygen-binding ability of hemoglobin is studied using oxyhemoglobin dissociation curves. (Fig. 44.9)
a. Percentage of oxygen-binding sites of hemoglobin carrying O2 varies directly with partial pressure of O2 (PO2) in the immediate environment.
b. The partial pressure is the amount of pressure exerted by a particular gas among all the gases present.
c. At normal partial pressures of O2 in lungs, hemoglobin becomes practically saturated with O2; but at the O2 partial pressures in the tissues, oxyhemoglobin quickly unloads much of its O2.
(Fig. 44.8)
HbO2
Hb + O2
d. Acid pH and warmer temperature of tissues promote this dissociation.
4. In tissues, some hemoglobin combines with CO2 to form carbaminohemoglobin. (Fig. 44.8)
Hb + CO2
HbCO2
Hb +
carbaminohemoglobin
5. However, most of the CO2 is transported in the form of the bicarbonate ion (HCO3-).
a. First CO2 combines with water, forming carbonic acid (H2CO3).
b. Then this dissociates to a H+ and a HCO3-:
CO2 + H2O
H2CO3
H+ + HCO3 B
CO2 + H
carbonic
H2
bicarbonate
CO2 + H
acid
H2COB
ion
c. Carbonic anhydrase, an enzyme present in red blood cells, catalyzes this reaction.
d. Released H+ ions could drastically lower the pH of the blood; however, hydrogen ions are absorbed by globin portions of the hemoglobin, and HCO3 B diffuses out of the RBCs into the plasma.
e. Hemoglobin combines with a H+ ions as reduced hemoglobin (HHb); HHb plays a vital role in blood pH.
f. As blood enters pulmonary capillaries, most CO2 is in plasma as HCO3. (Fig. 44.8) [transp. 242]
g. The little free CO2 remaining diffuses out of blood across walls of pulmonary capillaries and into alveoli.
h. Decrease in plasma CO2 concentration causes the following reaction also catalyzed by carbonic anhydrase:
H+ + HCO3-
H2CO3
CO2 + H2O
i. At the same time, hemoglobin unloads H+ and HHb becomes Hb.
44.3. Keeping the Respiratory Tract Healthy (p. 798)
A. The Respiratory Tract
1. The entire respiratory tract has a warm, wet, mucous membrane lining exposed to environmental air.
B. The Tract Gets Infected
1. Droplets from single sneeze carry billions of bacteria or viruses.
2. The mucous membranes of the respiratory tract are protected by mucus and the constant beating of cilia.
3. If the number of infectious agents is large and/or resistance is inadequate, respiratory infections can result.
4. Viral infections spread to sinuses, to middle ears (otitis media), to larynx (laryngitis), and to bronchi.
5. Acute bronchitis caused by a secondary bacterial infection involves much coughing; responds to antibiotics.
6. Chronic bronchitis can be caused by constant irritation of the lining of bronchi; it is often seen in smokers.
7. Strep throat is a severe infection caused by Streptococcus pyogenes; as a bacterial infection causing high fever and difficult swallowing, it can be treated with antibiotics to prevent rheumatic fever and damage to heart valves.
C. The Lungs Have Disorders
1. Pneumonia and tuberculosis are infections controlled by antibiotics; emphysema and lung cancer are not.
2. Most forms of pneumonia are caused by a bacterium or a virus that has infected the lungs.
3. AIDS patients are subject to a rare form of pneumonia caused by the protozoan Pneumocystis carinii.
4. Pneumonia can be localized in specific lobules that become nonfunctional as they fill with mucus and pus.
D. Tuberculosis Is Back
1. Pulmonary tuberculosis is caused by the tubercle bacillus, a type of bacterium.
2. In a person with tuberculosis, the alveoli burst and are replaced by inelastic connective tissue.
3. A TB skin test is a highly diluted extract of the bacilli injected into the patient's skin; if a person has been exposed, the immune response causes an area of inflammation.
4. Bacilli that invade lung tissue are isolated by lung tissue in tiny capsules called tubercles.
5. If the person is resistant, the imprisoned bacteria die.
6. If the person is not resistant, the bacteria can eventually be liberated.
7. A chest X ray detects active tubercles.
8. Appropriate drug therapy can ensure localization and eventual destruction of live bacterial organisms.
9. Tuberculosis killed about 100,000 people in the U.S. annually before the middle of the twentieth century.
10. Resurgence has occurred due to increases in AIDS, homeless, and poor; new strains are often resistant.
E. Emphysema Reduces Capacity (Fig. 44.10)
1. Emphysema involves destruction of lung tissue, with accompanying ballooning of lungs due to trapped air.
2. This stems from the damage to and collapse of bronchioles, which traps air in alveoli.
3. Trapped air frequently causes the alveolar walls to rupture, and a loss of elasticity makes breathing difficult.
4. Since the surface area for gas exchange is reduced, insufficient O2 reaches the heart and the brain.
5. This triggers the heart to work furiously to force more blood through lungs; this can lead to a heart condition.
6. Lack of oxygen to the brain makes the patient feel depressed, sluggish, and irritable.
F. Pulmonary Fibrosis is Dangerous
1. Inhaling particles such as silica, coal dust, and asbestos can lead to pulmonary fibrosis.
2. These agents cause irritation to lung tissue and result in build up of fibrous connective tissue in the lungs.
3. Asbestos was use widely for fireproofing and widespread exposure has occurred; a possible 2 million deaths could be caused by asbestos between 1990 and 2020. (Fig. 44.11)
G. Lung Cancer Due to Smoking
1. Lung cancer was more common in men; now it has surpassed breast cancer as a cause of death in women.
2. Lung cancer develops in lung tissue and involves the following steps.
a. First, a thickening and callusing of the cells lining the bronchi appears.
b. There is a loss of cilia so that it is impossible to prevent dust and dirt from settling in the lungs.
c. Next, cells with atypical nuclei appear in the callused lining.
d. A tumor, consisting of disordered cells with atypical nuclei develops as cancer in situ.
e. When some of the tumor cells break free and penetrate other tissue (metastasis), the cancer spreads.
f. The tumor may grow until the bronchus is blocked, cutting off air supply to the lung.
g. Entire lung collapses, secretions trapped in lung become infected, and pneumonia or a lung abscess results.
3. The only treatment is surgery (pneumonectomy) where a lobe or whole lung is removed.
4. Current research indicates passive smoking also causes lung cancer and other illnesses.
5. Stopping smoking can result in a return to healthy tissues.