BIOL 4505 1st EditionExam # 4 Study Guide Lectures: 25 - 32Lecture 25 (October 29)There are three types of specialized breathing structures. Lungs are invaginated into the body and contain the environmental medium. External gills are evaginated from the body and project directly into the environmental medium. Internal gills are evaginated from the body and project into a superficial body cavity, through which the environment medium is pumped. Lungs are not the only breathing organs, some animals breathe with their skin, gills, tube feet (starfish). Brachial = gills, pulmonary = lungs. Insects and spiders have diffusion lungs, and dual breathers (bimodal breathers) can breathe in either air or water. Rate of O2 uptake (mL O2/minute) = Vmedium (CI – CE). Rainbow trout use 33%, yellowfin and skipjack tuna use 50-60%. The oxygen utilization coefficient during breathing is the percentage of the O2 in inhaled medium that an animal removes before exhaling the medium. The four major types of gas exchange that can occur during directional ventilation can be ranked in terms of their inherent ability to establish a high O2 partial pressure in blood exiting the breathing organ. Countercurrent gas exchange ranks highest. Cross-current gas exchange ranks second. Cocurrent and tidal gas exchange rank third. Because of the different capacitance coefficients of air and water, air breathers typically raise the CO2 partial pressure in air they breathe to roughly the same extent as they lower the O2 partial pressure. Water breathers, however, do not elevate the CO2 partial pressure in water they breathe to any great extent. Air breathers consequently tend to have much higher blood CO2 partial pressures than water breathers.The gill surface area of most fish of a given body size is similar to the lung surface area of amphibians and nonavian reptiles of the same size. Compared with the latter groups, mammals and birds have much more lung surface area—helping to meet their far higher needs for gas exchange. The barrier between the blood and the air or water in the breathing organs is notably thin in mammals and thinnest in birds. The skin can account for 25% or more of gas exchange in some fish, turtles, and other nonavian reptiles, and up to 100% in some amphibians. The skin is a minor contributor to gas exchange, however, in mammals and birds. The breathing muscles of vertebrates are skeletal muscles activated by motor nerve impulses. The breathing rhythm originates in a central pattern generator in the brainstem.Lecture 26 (October 31)The secondary lamellae are the principal sites of gas exchange in fish gills. Countercurrent gas exchange occurs in the lamellae. Water flow across the gills is essentially unidirectional. It is driven by a buccal pressure pump and an opercular suction pump that act in an integrated rhythm, so that the buccal pump drives water across the gills when the opercular pump is being emptied of water and the opercular pump sucks water across the gills while the buccal pump is being refilled with water. Some fish turn to ram ventilation when swimming fast enough. Others, such as tunas, are obligate ram ventilators and must swim all the time to avoid suffocation. A lowered O2 partial pressure in the blood is a more potent stimulus for increased ventilation in fish than an elevated CO2 partial pressure. Most of the 400 or so species of air-breathing fish have an air- breathing organ that is derived from the buccal cavity, opercular cavity, stomach, or intestines—or one that originates as an outpocketing of the foregut (e.g., swim bladder).Venous O2 concentration decreases as swimming speed increases. Arterial O2 concentration stays around the same. In tadpoles, gills and skin account for half the O2 exchange. In adult frogs, the lungs take up most O2 but the skin eliminates most CO2. There are three steps in breathing of a bullfrog. It fills its buccal cavity in preparation for inflation of its lungs before it empties its lungs; to do so, with itsglottis closed, it inhales air, which mostly comes to lie in a posterior depression of the buccal floor. Next, the glottis is opened, and pulmonary exhalant air passes in a coherent stream across the dorsal part of the buccopharyngeal cavity to exit through the nares. The fresh air in the depression of thebuccal floor is then driven into the lungs when the buccal floor is raised with the nares closed. An important effect of the buccopharyngeal pumping between breaths is that it washes residual pulmonary exhalant air out of the buccal cavity, so that when the next pulmonary ventilatory cycle begins, the buccal cavity is filled with a relatively fresh mixture. A bronchus allows air to flow to multiple chambers. The lungs of mammals consist of dendritically branching airways that end blindly in small, thin-walled, well-vascularized outpocketings, the alveoli. The airways exhibit 23 levels of branching in the human adult lung, giving rise to 300 million alveoli. The airways in a mammalian lung are categorized as conducting airways, where little gas exchange with the blood occurs, and respiratory airways, where most gas exchange with the blood takes place.  Because of the blind-ended structure of the mammalian lung, the gas in the alveoli always has a substantially lower O2 partial pressure and higherCO2 partial pressure than atmospheric air. Contraction of the diaphragm is a principal force for inhalation in mammals, especially large quadrupeds. External intercostal muscles may contribute to inhalation; internal intercostal muscles and abdominal muscles may contribute to exhalation. Inhalation occurs by suction as the lungs are expanded by contraction of inspiratory muscles. At rest, exhalation occurs passively by elastic rebound of the lungs to their relaxation volume when the inspiratory muscles relax. Lecture 27 (November 3)The breathing rhythm in mammals originates in a central pattern generator in the pre-Bötzinger complex in the medulla of the brainstem. The most potent chemosensory stimulus for increased ventilation in mammals is a rise in blood CO2 partial pressure and/or H+ concentration, sensed in the medulla. The blood O2 partial pressure, ordinarily a less influential factor in controlling ventilation, is sensed by the carotid bodies along the carotid arteries (humans) or by carotid and aortic bodies (certain other mammals). The control of ventilation during exercise involves stimuli generated in association with