Short Answer (5pt each) Answer 2/3 or all 3 for extra credit –

1. Relate the anatomical differences between arteries, capillaries and veins to their different functions. How does the nervous system control the flow of blood through the capillary network?

Arteries transport blood away from the heart, while veins transport blood back to the heart; capillaries are very thin blood vessels that actually transfer the oxygen and chemicals in the blood to body tissues, as well as cellular waste products and carbon dioxide out into the circulatory system. Both arteries and veins have a three layer structure (tunica externa made of LC tissue, tunica media made of smooth muscle and collagen DC tissue, and squamous epithelium tunica interna) along with a basement membrane, endothelium and in the large vessels a lumen and vasa vasorum; however, there are key differences in the proportions of these structures that reflect the different functions of arteries and veins. Arteries have much thicker tunica mediae since they require smooth muscle to constrict and dilate in order to maintain blood flow in consistent diasto-systolic pressures for expedient cellular nutrition and oxygenation. Since veins do not constrict and dilate as arteries do, they also do not have as much elastic CT as arteries do, and have a separate system to keep blood flowing via skeletal movement and one-way valves, which are outcroppings of only venous endothelia. Veins have thinner walls than arteries because they do not require as much strength, since venous blood pressure is much lower than arterial blood pressure due to the force generated by the heart dissipateing into the many capillaries. And, capillaries themselves are unlike arteries or veins by their comparatively simple structure of only a basement membrane surrounding an epithelium – necessary to produce substance exchange between tissues and the circulatory system.

Neurons innervate only large blood vessels (that is enough to control blood flow), and they work involuntarily by the vasomotor center in the medulla oblongata. There are two types of neurons working in blood vessels – baroreceptors (blood pressure sensors) or chemoreceptors (chemical sensors). Baroreceptors inhibit or excite the vasomotor center, in degrees proportional to amount of blood pressure: vasoconstriction/dilation occurs as a response to overly low/high blood pressure, respectively (control of blood pressure is important because too high blood pressure leads to edema as it diffuses out of the capillaries, and the opposite effect of not enough blood exchanging nutrients and gases with cells if blood pressure is too low). Chemoreceptors respond to serum pH – if too low, then that means too much carbon dioxide and a signal is sent to the brain to make the lungs work harder; the chemoreceptors also excite the vasomotor center to constrict the blood vessels to increase blood pressure and thus gas exchange in the lungs. The circulatory system also can be adjusted through direct stimulation of the brain – if it is not receiving enough blood and nutrients, the cardiac and vasomotor centers will cause an increase in blood pressure and heartbeat (this is called the medullary ischemic reflex). Even psychological states like anger, depression, arousal, stress, etc. can change blood pressure and heart beat.

2. Explain how normal blood pressure is maintained through the regulation of cardiac output, peripheral resistance and blood volume. How does blood viscosity affect blood pressure?

Cardiac output of blood is defined as heart stroke volume multiplied by beats per time unit, so the regulation of either stroke volume or heart rate will regulate cardiac output. High blood

pressure is often caused by high cardiac output, so regulation of cardiac output (through neural signals or hormones like adrenaline, ADH, and ANF) maintains normal blood pressure.

Peripheral resistance in blood vessels is defined as mean arterial pressure divided by cardiac output – that is, peripheral resistance is the amount of deceleration effected on blood flow by mostly arterioles (since they lie between arteries and capillaries, and thus at the boundary between constriction/dilation and lack of it, they determine ultimate speed of blood flow). High peripheral resistance will increase blood pressure as blood moves more slowly and jostle against the endothelia. Looking at the formula, peripheral resistance is directly correlated with MAP and inversely correlated with cardiac output, so control of cardiac output (by aforementioned means) or vasodilation/constriction will control BP.

Blood volume is associated with blood pressure because the circulatory system is obviously finite in area and high blood density comes with high blood pressure. High blood pressure associated with high blood volume can be alleviated with the creation of new blood vessels (angiogenesis) and vasodilation.

Blood viscosity affects blood pressure much in the same way as blood volume affects BP, and viscosity is also related to peripheral resistance as there is decelerative internal friction between the closely packed erythrocytes and/or serum albumin.

3. George is in congestive heart failure. Because of his condition, his ankles and feet appear to be swollen and he has trouble breathing. What is congestive heart failure and what are some of its causes? What is the relationship between congestive heart failure and the accumulation of fluid in the feet and ankles? Why does he have trouble breathing? How should he be treated?

Congestive heart failure is the failure of the heart to pump blood adequately as a result of some vascular condition such as narrowed coronary artery disease, disruptive scar tissue from past MIs, high blood pressure, some kind of heart valve disease, congenital heart defects, and endocarditis (valve infection) or myocarditis (myocardium infection). Because in congestive heart failure the heart cannot pump enough blood, edema occurs in the feet and ankles (or, rather, wherever gravity takes the blood). George probably has some fluid buildup in his lungs as well, as that is also a major capillary bed, and so he should have some trouble breathing. While recovering, George cannot exert himself too much because his heart will not be able to keep up, but he should not spend all his time lying down either because that puts him at a greater risk for pulmonary edema. He needs to give his body time to recover and needs to start maintaining a healthy diet if he had been doing so before his heart failure. He will take drugs like Angiotensin-Converting Enzyme inhibitors and vasodilators to lower blood pressure and decrease resistance; beta blockers improve left ventricle function (the pump driving blood to rest of body); something like digitalis will increase cardiac output. He should also take diuretics to help kidney function, which is hurt by the low blood flow caused by congestive heart failure.