3) Basically in diastole the pressure is such that the venous return to the heart will allow blood to fill the ventricles and the atria. This happens because the pressure in the veins is high compared to the pressure in the heart. When the atria contract additional blood is ejected from them into the ventricles (pressure in the ventricle is increasing at this point). Ventricular contraction is next and at the start is isometric in nature, causing the pressure to soar. This contraction will cause enough pressure to both close the AV valves (at the start of the contraction) and eject the blood into the aorta, once the aortic valve opens.

4) Stroke volume of the heart is influenced by a number of factors. Contractility of the heart is one. This basically just means that the heart can contract harder under certain conditions and will eject more blood. Secondly, the pressure in the systemic or pulmonary circulation will also be a factor. If the pressure is high in these arteries then they will resist further ejection of blood. There are many more factors since the definition of stroke volume is the amount of blood ejected from the heart. These include venous filling pressure, atria contractility, and ventricular distensibility.

5) Nervous innervation to the heart is designed to modulate the heart rate, the speed of the conduction wave, and the contractility of the muscle. Nervous input does not create the excitation that gives rise to action potentials.

6) Compliant pericardiums do not cause significant pressure changes inside the pericardial cavity. Their main purpose is to protect the heart, and in some cases facilitate movement of the heart as a result of lubricants between the layers. In animals with non-compliant pericardiums, the heartbeat causes a pressure change inside the rigid structure which can facilitate the flow of blood.

7) A partially divided ventricle helps to separate blood flow from the systemic and pulmonary systems, while also allowing blood to be shunted from one side to the other if necessary. For more information see pages 480 and 484.

8) There are blood shunts in the unborn mammalian fetus. The ductus arteriosus lets blood leaving in the pulmonary circulation enter into the systemic circulation. The foramen ovale allows the blood from the right atrium enter into the left atrium. Remember that the right atrium serves the pulmonary circulation. Prior to birth the function of pulmonary circulation is mostly unnecessary. At birth the body shuts down both of these shunts, allowing blood to flow through the pulmonary circulation.

9) Poiseuille's Law is functional when predicting the flow of a fluid through a rigid pipe. As the circulatory system is not made of rigid pipes, this equation is applicable to circulation only at certain times. Typically, it is an accurate estimate of the relationship between flow and pressure in the small terminal arteries and veins. A fudge factor/second equation is used to determine when this law can be applied. This is discussed further on page 490.

10) The arterial system has many functions. In a general sense the arterial system is there to deliver the blood to the body (nutrients, oxygen, and hormone delivery). The second purpose of the arteries is to serve as a pressure reserve and to dampen the pressure oscillations resulting from each heartbeat. The arteries also contain receptors which help monitor such things as pH and oxygen levels. Heat conservation is also served by the arterial system. The list goes on and on.

11) The blood in an arterial is exposed to two forces. The pressure inside the arteriole is the force that drives the blood to leave the arteriole. The second force lies in the colloidal osmotic pressure set up by large proteins that can not leave the arterioles. In general the pressure inside the arterial is larger than the colloidal pressure and this will lead to an excess of fluid leaving the capillary. Thank goodness we have lymph vessels. I'm wondering if they were also hinting at wanting to know about pre-capillary sphincters and the small diameter which causes larger cells (white blood cells) to get stuck. Both of these factors would reduce blood flow through the capillary bed. The former can be controlled through nervous input.

12) These receptors are found in the heart, arteries and veins. The medullary cardiovascular center is responsible for integrating the input from these receptors. Their role is to regulate the blood at certain blood pressures. Mechanoreceptors are concerned with other variables in blood flow such as interruptions in blood flow.

13) Diving animals are obviously unable to breathe. Their heart rate and cardiac output are reduced during the dive. Diving animals must then rely on oxygen stores in the body and will, of course, create an oxygen debt. A person breathing less oxygen will increase their ventilation for more oxygen. We discussed the body's response to decreased oxygen intake in recitation covering ch13.

14) During exercise many changes can be seen. Increased cardiac output, increased venous return, increased and decreased blood flow to different areas of the body, and of course an increase in ventilation are all changes which occur. Also seen is an increase in both heart rate and contractility. Pg 512.

15) Increasing the arterial blood pressure will increase the amount of fluid lost by the capillaries. A decrease in blood pressure will decrease the amount of fluid exiting the capillaries. As for cardiac function, an increase in arterial pressure will resist the cardiac output of the heart. This would probably lead to some type of compensation such as increased contractility or a decrease in the tonicity of the arteries. A decrease in arterial pressure will increase the cardiac output of the heart.

16) There are 3 different kinds of capillaries (see question 17). Sinusoidal capillaries are found in areas that require blood cells and large proteins to enter the circulation. Fenestrated capillaries are found in areas that are concerned with increased exchange or filtration, such as the glomerulus in the kidney. Lastly there are continuous capillaries that are very selective as to what passes. These can be found in muscles, nerves, and many other areas as well.

17) Substances may cross capillaries through sinuses, pores, fenestrations, or by transcytosis. Pores are small holes that only allow very small molecules to pass through. Fenestrations are large cracks between epithelial cells that allow substances as big as proteins to pass. Sinuses are just what the name implies. They allow anything to pass, including cells. Transcytosis is a process by which cells can endocytose a substance on one side and ship it in a vesicle across the cell for exocytosis. This process is independent of lyposomal interactions.

18) Blood return. Blood reservoir.

19) Gravity will cause the blood to pool in the lower extremities, decreasing the return of blood to the heart. Ultimately changes in the vasculature (constriction of vessels closer to the ground, dilation of those farther away) and increased contractility of the heart are necessary to maintain circulation. This will cause the heart to do more work. In water this is not an issue because to density of the two is so similar. Therefore the hydrostatic pressure can counteract the effect of gravity.

20) Laplace's law states that the transmural pressure is equal to twice the wall tension divided by the radius (p479). Although the vascular system is not made of very defined tubes and spheres this law can still hold true on a basic level. For example, a heart twice the size of another must have twice the tension in the walls to develop the same pressure.

21) The lymphatic system functions to remove fluid lost from circulation back to the heart. Different parts of the body will then have different demands due to a difference in the amount of lost fluid. For example, the legs have an increase in fluid loss from the circulation due to gravity's effect on the transmural pressure in capillaries. The lymphatic system has other functions that we should explore at another time.