Practice Problems # 4

Fick's law of diffusion applies to all systems of gas exchange. A. According to Fick's law, which factors would yield a high rate of diffusion and maximize gas exchange? B. Describe two examples.

2A. -increase surface area (A) -increase partial pressure difference (P1 - P2) -decrease path length (L) B. Answers will vary and include: -small path length for gas exchange through the cell membrane of amoeba or lamella of fish -high surface area of gills and lungs -countercurrent flow of water (oxygen source) and blood in fish

Carbon dioxide produced by tissues as a by-product (waste) from cellular respiration is transported in the blood to the lungs as bicarbonate ion. A. Describe chemical reaction of CO2 as it is transported from tissue to blood to lung. B. When oxygen is transported through the blood, it is not changed. Why is it necessary for CO2 to convert to carbonic acid as it is transported through the blood?

5A. First, between body tissue and blood capillary: As CO2 moves to lower PCO2 in blood plasma, enzyme carbonic anhydrase catalyzes formation of carbonic acid which dissociates into H+ and bicarbonate ion. Then, between blood capillary and lung: The reaction is reversed to produce CO2. B. Since the PCO2 between the external and internal environments has a much smaller difference than that of PO2, a conversion to carbonic acid is necessary. By converting CO2 to carbonic acid with the help of carbonic anhydrase, PCO2 in the blood is reduced, thereby maintaining diffusion of CO2 from tissues. Otherwise, a buildup of CO2 in the blood may decrease the diffusion of CO2 from body tissue.

Hemoglobin transports respiratory gases through the blood and contains four heme groups. Each heme binds or releases oxygen based on the blood PO2. A. Below which PO2 of blood does hemoglobin release a lot of oxygen? B. How does positive cooperativity and probability phenomenon influence oxygen loading?

6A. 40 mmHg B. When one molecule of oxygen is bound to hemoglobin, the affinity for additional oxygen molecules increases due to conformational changes of hemoglobin. This is known as positive cooperativity. Once three of the four heme subunits have oxygen bound, it becomes more difficult for the forth oxygen to find and bind the last available heme subunit. This is known as probability phenomenon. C. Positive cooperativity: 20-40 mmHg Probability phenomenon: 40-100 mmHg

South American camelids—llamas, alpacas, guanacos, and vicuñas—are native to the Andes Mountains. In the natural habitat of these mammals, more than 5,000 m above sea level, the PO2 is below 85 mm Hg, and the PO2 in the camelids' lungs is about 50 mm Hg. The hemoglobins of these camelids are different from that of humans; the figure shows the O2-binding/dissociation curves for human and llama hemoglobins. A. Compared with human hemoglobin, does llama hemoglobin have a higher or a lower affinity for O2? B. Why would llama hemoglobin be advantageous at high altitudes?

7A. The llama hemoglobin has a higher affinity for O2. B. Llama hemoglobin would be advantageous at high altitudes because it can become 100 percent saturated at the low PO2 of the high-altitude environment. Therefore the hemoglobin can carry a full load of O2 to the tissues

Diffusion is the mechanism for gas exchange between air and gas exchange surfaces. Air contains 21% oxygen and 0.04% carbon dioxide among other gases. The partial pressure of air at sea level is 760 mmHg. A. What is the partial pressure of oxygen (PO2) and carbon dioxide (PCO2)? B. Because of the relatively high altitude of Antonito, Colorado, the town has a normal barometric pressure of about 600 mmHg. What is the partial pressure of O2 in Antonito's air? C. Explain why an increase in altitude reduces the oxygen supply for air breathers. D. Explain why carbon dioxide has no problem diffusing out of human lungs.

A. PO2 =(0.21) (760 mmHg) = 159 mmHg PCO2 = (0.04) (760mmHg) = 30 mmHg B. 126 mmHg C. The total atmospheric pressure at higher altitude decreases relative to sea level (760mmHg). As a result, the partial pressure of oxygen decreases at higher altitude. This decreases the partial pressure differences between air breathing organisms, which limits oxygen intake at higher altitudes relative to sea level. Higher altitude does changes the percent of oxygen in air, which is 21%. D. Due to the large partial pressure difference outside of human lungs relative to inside, there is always a gradient for diffusion to occur.

At what point in the breathing cycle would the pleural cavity pressure be going down while the alveolar pressure is going up?

Close to the end of inhalation when the thoracic cavity is fully expanded, the pleural cavity pressure is reaching its maximum negative value, which means the pressure is going down. At the same time, the alveolar pressure is rising back up to being the same as the atmospheric pressure.

Countercurrent flow in fish maximizes gas exchange with the surrounding environment. Describe the mechanism of countercurrent flow and explain how it is more effective than concurrent flow. You may include an illustration to describe the mechanism of countercurrent flow.

Countercurrent flow in fish is the movement of water and blood in opposite directions. As a result, blood flowing with either low or high oxygen saturation is located in areas of the gill lamella that are in close proximity to a gas exchange surface that has higher oxygen saturation in the water flowing. Countercurrent flow is more effective at gas exchange because the gradient established by maximal partial pressure difference covers the entire area of gill lamella. As a result, gas exchange will occur throughout all gas exchange surfaces of gill lamella.

The autonomic nervous system regulates unconscious actions. Describe when the autonomic nervous system neurotransmitters: norepinephrine and acetylcholine are released and explain how they modulate rhythmic activity of the heart?

Sympathetic nerves release norepinephrine, which stimulates "fight or flight". This decreases interval between action potentials causing the heart to beat faster and prepares the body for action. Parasympathetic nerves release acetylcholine, which stimulates "rest and digest" or "feed and breed". This increases the interval between action potentials causing the heart to beat slower and calms the body.

The "lub-dub" sounds of the cardiac cycle correspond to the closing of heart valves. The rhythmic contraction of ventricles is called the cardiac cycle, which consists of two phases: systole and diastole. Describe each phase of the cardiac cycle. In your answer, describe the activity of heart valves during the cardiac cycle.

Systole and diastole are the two phases of cardiac cycle. The ventricles are relaxed during diastole blood is filling the ventricles from the atria. During systole, ventricles contract and blood is pumped to the body from the heart. At the start of systole, there is greater pressure in the ventricles than atria. As a result, both atrioventricular valves slam shut ("lub") and pressure in ventricles builds up which causes aortic and pulmonary valves to open. At the end of systole, there is greater pressure in the aorta and pulmonary artery. As a result, aortic and pulmonary valves slam shut ("dub") and pressure in atria builds up which causes AV valves to open.

The hearts of most mammals stop beating when their temperature falls more than a few degrees below 20C, but the hearts of hibernating mammals can beat at 0C. What adaptations of cardiac muscle might explain this capacity of the hearts of hibernators?

The cardiac muscle must be capable of generating and conducting action potentials. This may involve different variants of the ion channels involved in action potential generation and conduction. The cardiac muscle must be able to convert the action potential into the opening of Ca2+ channels in the sarcoplasmic reticulum, so there could be adaptive changes in the DHP and ryanodine receptors. Once Ca2+ is released into the sarcoplasm, it has to be taken back up into the sarcoplasmic reticulum, and that Ca2+ pump is likely to be adapted to operate at lower temperatures in the hibernator

Is there a time in the mammalian cardiac cycle when all four heart valves are open? Explain.

There is no time when all four heart valves are open at the same time; if there were, the heart could not pump efficiently. Throughout diastole, the aortic and pulmonary valves are closed and the atrioventricular valves are open. At the beginning of systole, the atrioventricular valves close. There is a brief moment when all four valves are closed, until the aortic and pulmonary valves open, and stay open until the end of systole.

How is action potential initiated in the heart?

Without help from the nervous system, spontaneous depolarization of the plasma membrane of modified cardiac muscle cells called pacemaker cells initiates action potential.