Remember, oxygen is the final electron acceptor in cellular respiration (O 2 combines with H + & electrons to form H 2 O).
Diffusion can effectively distribute substances over about 0.5 millimeters. Earthworms & amphibians have circulatory systems that transport gases between cells & the body surface.
Gills are not suited to a terrestrial environment because they tend to collapse in air. Problem with external gills is that they are easily damaged. Sea stars & sea urchins have dermal gills that project from body wall. Gills are ventilated by ciliated epidermal cells. Parapodia of polychaetes function as gills & in locomotion.
Countercurrent flow maximizes amount of oxygen extracted from the water.
A First lungs appeared in lobe-finned fishes (smooth-walled sacs associated with capillary beds). Lungs of lobe-finned fishes & lungfishes (bichir) supplemented gills in breathing air. B & C Amphibian lungs have few subdivisions. Gills &/or skin supplement lungs. Lungs of reptiles (D), birds (E) & mammals (F) are highly subdivided & are used exclusively for respiration. Turtles are an exception - rigid shell restricts ventilation, so they supplement lung breathing with gas exchange across moist surfaces in mouth & cloaca. Note: bird lungs are unusual in that they contain air sacs that enable them to move air through the lungs in a one-way direction.
Air leaving pharynx enters larynx. Food leaving pharynx enters esophagus. Larynx is composed of 9 cartilages, one of which is the epiglottis. When you swallow, the larynx rises up as the epiglottis curves downward. These actions combine to close the opening to the larynx (glottis), causing food to enter the esophagus. Males begin producing relatively large amounts of testosterone during puberty. Testosterone causes the vocal cords to lengthen & thicken - results in deepening of voice.
Note dome shape of relaxed diaphragm. When diaphragm contracts it flattens. Contraction of external intercostal muscles causes rib cage to expand. The action of both these muscles increases the volume of the thoracic cavity. When volume increases, pressure decreases. Air rushes inward because the air pressure inside the thoracic cavity is less than that of the atmosphere. Air will stop rushing in when the 2 air pressures equalize.
Normal exhalation is a passive process because it occurs without contracting any muscles. Muscles relax & lungs recoil, returning to their original shape. Air is pushed out because air pressure inside the thoracic cavity is greater than that of the atmosphere. Air will stop rushing outward when the 2 air pressures equalize. Once you have taken your first breath, you can never force all of the air out of your lungs. The air remaining after a maximal exhalation is called residual air.
At rest, most adults breath an average of 12 times/ minute. Thus, the medulla sends out 12 impulses/minute.
To a lesser degree, the increase in arterial CO 2 is detected by chemoreceptors in the carotid arteries & aorta. [Chemoreceptors in the carotid arteries & aorta mediate 30% of the response]
Countercurrent Mechanism for Gas Exchange Draw a similar picture showing blood flow and water flow in the same direction. How would oxygen diffusion change? Nearly 80% of the O 2 is removed from the water as it passes over the gills. Is this more or less efficient than gas exchange at the lungs? Why ? p. 984