40.1 The Nature of Respiration
A. The Basis of Gas Exchange
1. Respiratory systems rely on the diffusion of gases down pressure gradients.
a. Partial pressures for each gas in the atmosphere can be calculated; for example, oxygen's is 160 mm Hg.
b. Gases will diffuse down a pressure gradient across a respiratory surface if it is permeable and moist.
2. According to Fick's Law, the amount of diffusion depends on the surface area of the membrane and the differences in partial pressure.
B. Which Factors Influence Gas Exchange?
1. Surface-to-Volume Ratio
a. As an animal grows, its surface area increases at a lesser rate than its volume, making diffusion of gases into the interior a problem.
b. Therefore, animals either must have a body design that keeps internal cells close to the surface (flatworms) or must have a system to move the gases inward.
a. Animals have adaptations to move the air, or water, over the respiratory surfaces.
b. Bony fish move the covers over the gills; sponges move the flagella on their collar cells; humans move the muscles of the thorax to expand and contract the chest cavity and move air in and out of the lungs.
3. Transport Pigments
a. Hemoglobin is the main transport pigment.
b. It binds four molecules of oxygen in the lungs (high concentration) and releases them in the tissues where oxygen is low.
40.2 Invertebrate Respiration
A. Integumentary exchange is used by small invertebrates.
1. In animals such as flatworms which have a low metabolic rate, the epidermis at the body surface is used for integumentary exchange.
2. For terrestrial animals, like the earthworm, mucus helps keep the surface moist to allow the oxygen to diffuse inward through the thin epidermis.
B. Gills are used by invertebrates that live in aquatic habitats.
1. A gill has a moist, thin, vascularized epidermis.
2. The highly folded gill walls greatly increase the respiratory surface.
C. Tracheal respiration is used by arthropods in terrestrial settings.
1. Tracheal respiration in insects and spiders, utilizes fine air-conducting tubules to provide gaseous exchange at the cellular level.
2. In most cases no participation by the circulatory system is needed, neither are any respiratory pigments needed.
40.3 Vertebrate Respiration
A. Gills of Fishes and Amphibians
1. 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.
2. Water flows over the gills and blood circulates through them in OPPOSITE DIRECTIONS.
3. This mechanism, called countercurrent flow, is highly efficient in extracting oxygen from water, whose oxygen content is lower than air.
B. Evolution of Paired Lungs
1. Lungs contain internal respiratory surfaces shaped as a cavity or sac.
2. Simple lungs evolved about 450 million years ago to assist respiration in oxygen-poor habitats; some evolved into swim bladders, others into complex respiratory organs.
3. Lungs provide a membrane for gaseous exchange with blood.
a. Air moves by bulk flow into and out of the lungs.
b. Gases diffuse across the inner respiratory surfaces of the lungs.
c. Pulmonary circulation enhances the diffusion of dissolved gases into and out of lung capillaries.
d. In body tissues, oxygen diffuses from blood --> interstitial fluid --> cells; carbon dioxide travels the route in reverse.
4. Lungs also participate in sound production by forcing air to pass through the glottis opening causing the vocal cords on either side to vibrate.
40.4 Human Respiratory System
A. What Are the System's Functions?
1. Ventilation alternately moves air into and out of a pair of lungs inside of which are the tiny air sacs called alveoli where gas exchange takes place.
2. Breathing is necessary for speech.
3. Limited amounts of excess heat and water are eliminated here.
4. It also adjusts the body's acid-base balance.
5. The respiratory system has mechanisms to deal with airborne foreign matter than enters the system.
B. From Airways Into the Lungs
1. Air enters or leaves the respiratory system through nasal cavities where hair and cilia filter out dust and particles; blood vessels warm; and mucus moistens the air.
2. Air moves via this route: pharynx --> larynx (route blocked by epiglottis during swallowing) --> vocal cords (space between is glottis) --> trachea --> bronchi --> bronchioles --> alveoli.
3. When air is exhaled through the glottis, the folds of the cords vibrate to produce sounds which are under regulation by nerve commands to the elastic ligaments that regulate the glottal opening.
4. Human lungs are a pair of organs in the rib cage above the diaphragm.
a. Each lung lies in a thin-walled pleural sac, which leaves a very thin intrapleural space between the membranes.
b. Inside the lungs, respiratory bronchioles bear outpouchings of their walls called alveoli, which are usually clustered as alveolar sacs.
c. Alveoli provide a tremendous surface area for gaseous exchange with the blood located in the dense capillary network surrounding each alveolar sac.
40.5 Breathing--Cyclic Reversals in Air Pressure Gradients
A. The Respiratory Cycle
1. 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.
2. In exhalation, the actions listed above are reversed; the elastic lung tissue recoils passively.
B. Lung Volumes
1. The maximum volume that can be moved in or out is called the vital capacity, but the lungs cannot be completely emptied.
2. About 500 ml of air enters and leaves with each breath (tidal volume).
40.6 Gas Exchange and Transport
A. Exchanges at the Respiratory Membrane
1. Each alveolus consists of a single layer of epithelial cells through which gases can readily diffuse to and from interstitial fluid and blood capillaries.
2. The partial pressure gradients are sufficient to move oxygen in and carbon dioxide out of the blood, passively.
B. Oxygen Transport
1. Blood cannot carry sufficient oxygen and carbon dioxide in dissolved form to satisfy the body's demands.
2. Hemoglobin is a protein with four heme groups that bind oxygen.
a. Oxygen diffuses down a pressure gradient into the blood plasma --> red blood cells --> binds to hemoglobin (4 molecules of oxygen/hemoglobin to form oxyhemoglobin).
b. Hemoglobin gives up its oxygen in tissues where partial pressure of oxygen is low, blood is warmer, partial pressure of carbon dioxide is higher, and pH is lower; all four conditions occur in tissues with high metabolism.
C. Carbon Dioxide Transport
1. Because the concentration of carbon dioxide is higher in the body tissues, it diffuses into the blood.
2. Ten percent is dissolved in plasma, 30 percent binds with hemoglobin to form carbaminohemoglobin, and 60 percent is in bicarbonate form.
3. Bicarbonate and carbonic acid formation is enhanced by the enzyme carbonic anhydrase, which is located in the red blood cells.
D. How to Match Air Flow With Blood Flow?
1. Gas exchange in the alveoli is most efficient when air flow equals the rate of blood flow.
2. The nervous system controls oxygen and carbon dioxide levels for the entire body by adjusting contraction rates of the diaphragm and chest wall muscles.
3. The brain monitors input from carbon dioxide sensors in the bloodstream and from receptors sensitive to decreases in oxygen partial pressure (carotid bodies and aortic bodies).
40.7 Focus on Health: When the Lungs Break Down
40.8 High Climbers and Deep Divers
A. Respiration at High Altitudes
1. At high altitudes, the partial pressure of oxygen is lower than at sea level.
2. The lungs of permanent residents of high mountains have more alveoli and blood vessels plus larger ventricles in the heart and more mitochondria in muscle tissue.
3. Acclimatization allows newcomers to high altitudes to adjust.
a. The kidney cells release erythropoietin which induces production of more red blood cells.
b. Increased blood cell numbers creates more resistance to flow, which in turn makes the heart work harder to pump the blood.
B. Carbon Monoxide Poisoning
1. Carbon monoxide is a colorless, odorless gas produced as a byproduct of combustion.
2. It combines with hemoglobin 200 times greater than does oxygen.
C. Respiration in Deep Water
1. The respiratory system of the whale is modified to collect and store oxygen by several mechanisms including the use of myoglobin, an oxygen-binding pigment in muscle.
2. If a diver ascends too rapidly, the change in pressures will force the nitrogen to leave the tissues of the body and pass into the blood, often as bubbles, causing pain in the joints known as "the bends" or decompression sickness.