When breathing is not possible, blood can be rerouted past the lungs. Thus, a loss in efficiency conveys an advantage in flexibility. This fundamental principle, in which efficiency and flexibility are traded against each other, is illustrated in many systems in living organisms. (23. ) The Human Cardiovascular System and Heart (23. 3-23. 6) 0 Students often expect that the blood flowing through the heart supplies the heart muscle. The need for coronary arteries and veins is not clear to them. (The thickness of the walls of the heart does not permit efficient diffusion, and furthermore, the oxygen content of the blood in the right atrium and ventricle is very low. ) (23. 4-23. 6) Students often develop an incorrect mental model of how atherosclerosis occurs. In a home, drainpipes grow narrower as materials accumulate on their inside surface.
However, in atherosclerosis, the blood vessels narrow through an accumulation of materials within the walls themselves. In the pipe analogy, atherosclerosis is a pipe with thicker walls, which shrink the size of the lumen. 23. 6) 0 When discussing the way blood flows through four-chambered hearts, it is helpful to remind students that the heart is essentially two pumps. The right side collects from the body and propels to the lungs; the left side propels from the lungs out to the body. Having them memorize this sequence as right-to-left helps students recall the correct trial and ventricular sequences. 23. 3) 0 Students often benefit from brief, concrete demonstrations of abstract ideas. When discussing the cardiac cycle, take the time to have students quickly take their own pulse as they are seated in lass to help them relate the lecture topic to their own anatomy. This very short activity will provide a small break in the lecture routine and refocus the attention of those students whose minds may have begun to wander. (23. 4) 0 Having students take their own pulses also provides an opportunity to stimulate further curiosity.
You may want to assign students to measure and record the variation in their pulse rates during the day’s different activities, perhaps a) upon arrival to a class and after 20 minutes sitting in the class, b) before and after drinking coffee, or c) prior to and urine exercise. (23. 4) 0 The specialized Junctions that promote signal conduction between cardiac cells are specifically identified in Figure 20. 6 in Chapter 20. (23. 5) 0 Before explaining the functions of the AS node, consider asking your students to explain why the atria contract before the ventricles contract.
Posing a question and asking for an explanation rather than simply lecturing students often generates a more active interest in the subject matter. (23. 5) 0 Strokes can result from the blockage to or rupture oat b vessel in the brain. Thus, clot-busting drugs may either help resolve a brain clot or lead to disastrous bleeding. (23. 6) 292 0 Cardiovascular disease affects more than the blood vessels of the heart and brain. Many of the same risk factors that promote cardiovascular disease are associated with poor circulation to a penis, potentially leading to erectile dysfunction. 23. 6) The Structure and Function of Blood Vessels (23. 7-23. 11) 0 Students may need to be reminded of the definitions of an artery and vein, especially when discussing blood flow to and from the heart. Although veins generally carry oxygenation blood, the pulmonary artery transports low-oxygen blood o the lungs. The main difference between arteries and veins is the direction of flow (away from or toward the heart). Due to their structure, arteries are better able to resist the higher pressures generated by ventricular contractions.
Veins generally experience lower pressure and are structurally less resistant. (23. 7) 0 Students often struggle to explain how blood is propelled up their legs to return to their hearts. Frequently, students will suggest that the heart itself must provide sufficient force to move blood completely around the body. However, such pressures would destroy elicited capillaries. Other student hypotheses might include attributing a negative, siphoning effect to the heart. (Although the heart can generate a small pull, it is not sufficient to return blood up their legs and trunk to the heart. Let them wonder long enough to stimulate critical thinking and motivate them to learn the answer. After explaining the role of skeletal muscles and one-way valves in veins, you might also note that it has been suggested that students will be more alert in class and even perform better on tests if they wiggle their legs. Challenge students to explain why his might work and why locking their knees when standing might have the opposite effect. (And enjoy watching some of your students deliberately wiggling their legs on the next exam! ) (23. 8) 0 The photo in Figure 23. A demonstrates the narrow width of capillaries. Notice that the diameter of the capillaries barely permits the passage of red blood cells. (Also note that Figure 23. B shows a capillary diameter much greater than in the photograph. ) Challenge your students to explain why such a small size is adaptive. (Answer: it increases the surface area of capillaries and places red blood cells adjacent to the capillary walls for efficient gas exchange. ) (23. 7) 0 One function of the circulatory system that is rarely discussed is the transport of heat.
Blood vessels near the surface of the body expand when the body is overheated, releasing some of this excess neat to the environment. Conversely, during periods to exposure to cold, blood is shunted away from the skin to conserve heat. (23. 7) 0 Veins on the back of our hands can reveal many of these same principles of venous blood flow. If students keep their hands down below their heart for several minutes, such as during note aging or typing, they might notice their veins starting to bulge. Students can watch the veins empty by simply lifting their hands up to eye level.
As we get older, such phenomena are even easier to see. Some instructors may be comfortable enough (and old enough! ) to demonstrate this effect to their students. (23. 8) 0 Contracting the hand into a fist helps propel blood back up the arms to the heart. Skin pulled tight on the back of the hand compresses veins against the underlying ligaments and 293 bones. With this example “in hand,” students may better understand the propulsive forces moving venous blood back to the heart. 23. 8) 0 Students may not relate the structure of the walls of arteries to blood pressure.
Consider noting the presence of smooth muscle in the walls of arteries (Figure 23. C). If these muscles contract, they narrow the arteries and increase pressure. (23. 7-23. 9) 0 Students might wonder why they are discouraged from swimming soon after eating a meal. Blood flow during exercise involves the diversion of blood away from the gut and to major muscle groups likely involved in swimming. This can lead to indigestion or muscle cramping. However, the greatest risk of swimming with a full stomach is more likely that even a mall amount of vomit could clog an air passageway. 23. 10) 0 Figure 23. BIB depicts the movements of fluid out of and back into capillaries because of changes in osmotic pressure. The text references Module 24. 3 for further discussion of the role of the lymphatic system in fluid removal. If you do not plan on addressing Chapter 24, consider including the role of lymphatic vessels in your discussion of Chapter 23. (23. 11) 0 Students who have little practice interpreting electron micrographs might benefit from a closer analysis of Figure 23. 1 IA, in which an electron micrograph is paired with explanatory fugue.
For example, simply recognizing nuclei in micrographs can be an important starting point in interpreting cellular details and gaining a sense of scale. (23. 11) Structure and Function of Blood (23. 12-23. 15) 0 Students with limited backgrounds in anatomy and physiology might not appreciate the diverse functions of plasma, instead thinking of blood as a transporter of oxygen and carbon dioxide. Figure 23. 12 lists the many functions performed by plasma. (23. 12) 0 Students might have heard about blood thinners, thinking that somehow these substances make blood more fluid (like watering down syrup).