1. In external respiration, animals exchange oxygen and carbon dioxide between respiratory surfaces and blood. In internal respiration, these gases are exchanged between blood and cells. Aerobic cellular respiration uses oxygen to store nutrient energy in ATP.
2. CO2 forms as a by-product of aerobic ATP production and must be eliminated from the body. Gas exchange brings oxygen to and removes CO2 from cells.
3. In external respiration, oxygen and CO2 are exchanged by diffusion across a moist membrane. Body size, metabolic requirements, and habitat have affected the evolution of respiratory systems.
4. Simple organisms exchange oxygen and CO2 directly across the body surface. Larger or more active animals have circulatory systems to transport gases between cells and the environment.
5. Terrestrial arthropods bring the environment into contact with almost every cell through a highly branched system of tracheae.
6. Complex aquatic animals exchange gases across gill membranes, body surface extensions. In bony fishes, water flows over the gills in the direction opposite blood flow.
7. Vertebrate lungs create an extensive, moist internal surface for two-way gas exchange.
36.2 Components of the Respiratory System in Humans
8. In humans, the nose purifies, warms, and moisturizes inhaled air. The air then flows through the pharynx, larynx, and trachea, which is held open by cartilage rings. The trachea divides to form bronchi that deliver air to the lungs. The bronchi branch extensively to form tinier air tubules, bronchioles, which end in clusters of thin-walled, saclike alveoli.
9. Many capillaries surround the alveoli. Oxygen diffuses into the blood from the alveolar air, while CO2 diffuses from the blood into the alveolar air. Human lung surfactant, a chemical mixture that reduces surface tension, helps keep alveoli open.
36.3 Functions of the Respiratory System in Humans
10. Breathing brings oxygen-rich air into the lungs and removes air high in CO2. When the diaphragm and rib cage muscles contract, the thoracic cavity expands. This reduces air pressure in the lungs, drawing air in. Expiration results when these muscles relax and the thoracic cavity shrinks, raising air pressure in the lungs and pushing out air.
11. The volume of air moved in and out of the lungs during a respiratory cycle is the tidal volume. The amount of air that can be exhaled after a maximal inspiration is the vital capacity. Residual air remains in the lungs after expiration.
12. Almost all oxygen transported to cells is bound to hemoglobin in red blood cells. Increase in blood acidity (usually due to a rise in CO2 level) or elevated temperature due to metabolism increases the amount of oxygen reaching cells.
13. Some CO2 in the blood is carried bound to hemoglobin or dissolved in plasma. Most CO2 is transported as bicarbonate ion, generated from carbonic acid that forms when CO2 reacts with water and ionizes. Carbonic anhydrase speeds this reaction. The process reverses in the lungs, releasing CO2 for expiration.
14. During quiet breathing, the inspiratory center within the medulla's rhythmicity center spontaneously generates impulses that trigger inhalation. During heavy breathing, the inspiratory center remains active and the expiratory center becomes active, causing active expiration. Other brain regions can alter the basic breathing pattern.
15. Breathing rate adjusts to the body's demands chemically. When CO2 levels rise, chemoreceptors in the medulla, aorta, and carotid arteries sense the rise in CO2 concentration and blood acidity and increase breathing rate. Oxygen levels do not change breathing rate unless levels are critically low.