1 AP Biology March 2008 Respiration Chapter 42 Gas exchange occurs across specialized respiratory surfaces. 1) Gas exchange: the uptake of molecular oxygen (O2) from the environment and the discharge of carbon dioxide (CO2) to the environment. a) Relies on the diffusion of gases down pressure gradients. i) At sea level, atmosphere exerts a total pressure of 760 mm Hg. ii) This is the downward force equal to that exerted by a column of mercury 750 mm high. iii) Since the atmosphere is 21% O2 (by volume) Then the partial pressure of oxygen is 0.21 x 760 = 160 mm Hg b) Gases will diffuse down a pressure gradient across a respiratory surface if it is: i) permeable ii) moist 2) The amount of diffusion depends on the surface area of the membrane and the differences in partial pressure. Factors Influencing Gas Exchange 1) Surface-to-volume ratio: animals must have a surface area large enough to allow the exchange of gases in large enough quantities to sustain life. 2) Ventilation a) Many animals have developed adaptations to move air, or water, over respiratory surfaces. 3) Transportation pigments
2 a) Aids in the movement of gases throughout the body, increasing the quantity of molecules the blood can carry. b) Hemoglobin is the respiratory pigment found in almost all vertebrates. i) It consists of four subunits, each of which is a heme group with an embedded iron atom. ii) The iron atom binds O2, thus one hemoglobin can carry 4 oxygen molecules. c) Hemocyanin is a transport pigment found in many invertebrates. i) Contains copper, the oxygen-binding component. ii) Common in arthropods and many mollusks. iii) Dissolved in hemolymph rather than being confined to cells. Invertebrate Respiration 1) Respiratory medium: the source off the oxygen. a) Air for terrestrial animals b) Water for aquatic animals 2) Respiratory surface: the part of an animal where oxygen from the environment diffuses into living cells and carbon dioxide diffuses out. a) Always moist b) O2 and CO2 will first dissolve in the moist environment surrounding the cells and then diffuse through the membrane. 3) Integumentary Exchange a) Used by small invertebrates, such as flatworms and earthworms. b) Epidermis at the body surface is used for integumentary exchange. c) Earthworms have a mucus coating on their surface that helps keep the surface moist to allow the O2 to diffuse inward through the epidermis. 4) Gills a) Used by invertebrates in aquatic environments. b) A gill has a thin, moist, vascularized epidermis. c) Gill walls are highly folded to increase surface area.
3 5) Tracheal respiration a) Used by terrestrial arthropods. b) Consists of internal tubes that branch repeatedly and deliver air directly to body cells. c) Air enters the tracheae through openings called spiracles on the body surface and passes into smaller tubes called tracheoles. i) Tracheoles terminate on the plasma membranes of individual cells. d) In most cases no participation by the circulatory system is needed, neither is any respiratory pigments needed. Vertebrate Exchange 1) Gills of Fishes and Amphibians a) Gills: variously shaped outfoldings of the body surface specialized for gas exchange. i) The total surface are of the gills is often much greater than that of the rest of the body. b) The internal gills of adult fishes are positioned where water can enter the mouth and then flow over them as it exits just behind the head. c) Water flows over the gills and blood circulates through them in OPPOSITE DIRECTIONS. i) Countercurrent Exchange: the opposite flow of adjacent fluids that maximizes transfer rates. d) This mechanism is highly efficient in extracting O2 from water whose oxygen content is lower than air. 2) Lungs a) Lungs contain internal respiratory surfaces shaped as a cavity or sac. b) Lungs provide a membrane for gaseous exchange with blood. i) Air moves by bulk flow into and out of the lungs. ii) Gases diffuse across the inner respiratory surfaces of the lungs. iii) Pulmonary circulation enhances the diffusion of dissolved gases into and out to lung capillaries. iv) In body tissues, O2 diffuses from blood ------> interstitial fluid ------> cells
4 CO2 travels the route in reverse. c) Lungs also participate in sound production by forcing air to pass through the glottis opening causing the vocal cords on either side to vibrate. d) Amphibians have small lungs or no lungs, and they rely heavily on the diffusion of gases across other body surfaces. i) Frogs: the skin supplements gas exchange. e) Turtles are another vertebrate exception. i) Rigid shell restricts breathing movements. ii) Supplement with gas exchange across moist epithelial surfaces in their mouth and anus. Human Respiratory System 1) Anatomy of respiratory system a) Located in the thoracic (chest) cavity. b) Air enters through the nostrils. c) Then filtered by hairs, warmed, humidified, and sampled for odors as it flows through a maze of spaces in the nasal cavity. d) Nasal cavity leads to the pharynx. i) An intersection where the paths for air and food cross. ii) When food is swallowed, the larynx (the upper part of the respiratory tract) moves upward and tips the epiglottis over the glottis (the opening of the wind pipe). iii) This allows food to go down the esophagus to the stomach. iv) The rest of the time the glottis is open and we can breath. e) From the larynx, air passes into the trachea, or windpipe. f) The trachea forks into two bronchi. i) One leading to each lung. g) Within the lung, the bronchus branches repeatedly into finer and finer tubes called bronchioles. h) At their tips, the tiniest bronchioles dead-end as a cluster of air sacs called alveoli. i) The thin epithelium of the millions of alveoli in the lung serves as the respiratory surface.
5 i) O2 in the air conveyed to he alveoli dissolves in the moist film and diffuses across the epithelium and into a web of capillaries that surrounds the alveolus. ii) CO2 diffuses from the capillaries, across the epithelium of the alveolus, and into the air space. Breathing Cyclic Reversals in Air Pressure Gradients 1) The Respiratory Cycle a) In inhalation, the diaphragm contracts and flattens, muscles lift the rib cage upward and outward, the chest cavity volume increases, internal pressure decreases, air rushes in. i) Negative pressure breathing: works like a suction pump, pulling air, instead of pushing it, down into the lungs. ii) Results from changes in the volume of the lungs. iii) Diaphragm: a sheet of skeletal muscle that forms the bottom wall of the chest cavity. b) In exhalation, the actions listed above are reversed; the elastic lung tissue recoils passively. i) Rib muscles and diaphragm relax. ii) The lung volume is reduced. iii) The increase in air pressure within the alveoli forces air up the breathing tubes and out through the nostrils. 2) Lung Volumes a) Vital capacity: the maximum volume that can be moved in or out. b) Lungs cannot be completely emptied. c) Tidal volume: the amount of air that enters and leaves with each breath -- about 500 ml. Gas Exchange and Transport 1) Gas Exchange a) Each alveolus consists of a single layer of epithelial cells through which gases can readily diffuse to and from interstitial fluid and blood capillaries. b) The partial pressure gradients are sufficient to move O2 in and CO2 out of the blood passively. 2) Oxygen transport
6 a) Blood cannot carry sufficient O2 and CO2 in dissolved form to satisfy the body s demands. b) Hemoglobin is a protein with four heme groups that bind oxygen. c) O2 diffuses down a pressure gradient into the blood plasma, then into the red blood cells, where it binds reversibly to hemoglobin i) Each hemoglobin molecule can bind 4 O2 molecules. ii) Called oxyhemoglobin. iii) Consists of four subunits, each with a cofactor called a heme group that has an iron atom at its center. iv) Iron actually binds the oxygen. d) Hemoglobin gives up its O2 in tissues where: i) partial pressure of O2 is low. ii) blood is warmer iii) partial pressure of CO2 is higher iv) ph is lower 3) Carbon Dioxide Transport a) Because the concentration of CO2 is higher in body tissues, it diffuses into the blood. b) Ten percent is dissolved in plasma. c) 30 percent binds with hemoglobin to form carbaminohemoglobin. d) 60 percent is in bicarbonate form. 4) Matching Air Flow with Blood Flow a) Gas exchange in the alveoli is most efficient when airflow equals the rate of blood flow. b) The nervous system controls O2 and CO2 levels for the entire body by adjusting contraction rates of the diaphragm and chest wall muscles. c) The brain monitors input from CO2 sensors in the bloodstream and from receptors sensitive to decreases in O2 partial pressures (carotid bodies and aortic bodies) i) Breathing is under automatic control from the brain. ii) Breathing control center located in the medulla oblongata and the pons. iii) Medulla oblongata sets the basic rhythm iv) Pons moderates it.