You were the top student in your physiology class last semester. For this reason, your professor has asked that you prepare some lectures for her physiology class this semester. Specifically, she would like you to discuss the similarities and differences between the cardiovascular and respiratory systems, in terms of pressure gradients and fluid flow.

Question

A. Explain how you will describe the role of the following structures: the pump and the tubes.
B. Explain how you will describe the importance of an open vs. closed system and the relevant differences between liquids and gases.
C. What is fundamentally different about the respiratory pump compared to the cardiac pump, and why does this difference exist?

Answer 1

A. In both systems, fluids flow down a pressure gradient, within different-sized tubes of variable resistance; the
pressure gradient is generated by the action of a muscular pump. In the cardiovascular system, the tubes vary from
elastic arteries that minimize pressure extremes and fluctuations, muscular arteries and arterioles whose diameter
is changeable to regulate flow and that account for most of the resistance, finally to the large number of tiny
capillaries in parallel, with veins to conduct the blood back to the pump. In the respiratory system, the larger tubes
are rigid and account for most of the resistance, and lead to a large number of tiny tubes in parallel whose diameter
is adjusted to regulate flow.
B. In the closed cardiovascular system, the highest pressure is generated within the left ventricle, and an
incompressible liquid flows in a continuous fashion. High pressure is created when the ventricle contracts,
squeezing the blood it contains; low pressure results when the ventricle relaxes. In the open respiratory system, the
highest pressure during normal breathing is generated when the respiratory muscles relax, decreasing the volume
of the alveoli as air flows out; low pressure results when the muscles contract. Air is compressible, thus Boyle's law
of inverse relationship between volume and pressure is important. The flow of air is intermittent, flowing into the
lungs then stopping, and flowing out of the lungs then stopping again.
C. The driving pressure for fluid flow in the cardiovascular system is created directly by contraction of the heart
chamber, and depends upon the total peripheral resistance. The driving pressure for fluid flow in the respiratory
system is created indirectly by contraction of respiratory muscles, which produces a volume increase that creates a
pressure decrease relative to atmospheric pressure. The compressibility of air and the incompressibility of blood, as
well as the fact that the respiratory system is open and thus dependent on atmospheric pressure, while the
cardiovascular system is closed and thus wholly responsible for pressure, explains these differences.