Fluids in Physiology - Fluids - MCAT Physics and Math Review

MCAT Physics and Math Review

Chapter 4: Fluids

4.4 Fluids in Physiology

As a future student of medicine, you may feel that the abstract application of physics and math can often seem unimportant or tedious. However, these disciplines are exceptionally important in physiology. The movement of blood, lymph, and air throughout the body and lungs follow basic principles of fluid dynamics and pressure, with some minor alterations. We will focus primarily on the circulatory system, but also briefly discuss pressure and flow as they relate to gas exchange.


The circulatory system is a closed loop that has a nonconstant flow rate. This nonconstant flow is a result of valves, gravity, the physical properties of our vessels (elasticity, in particular), and the mechanics of the heart. In particular, the nonconstant flow can be felt and measured as a pulse. In addition to these features, there is a loss of volume from the circulation as a result of a difference between osmotic (oncotic) and hydrostatic pressures. This fluid is eventually returned to the circulation as a result of lymphatic flow, but it is problematic for applications of the continuity equation. An important point to note is that despite these differences, blood volume entering the heart is always equal to blood volume leaving the heart during a single cycle.

As blood flows away from the heart, each vessel has a progressively higher resistance; however, the total resistance of the system decreases because the increased number of vessels are in parallel with each other. Like parallel resistors in circuits, the equivalent resistance is therefore lower for the capillaries in parallel than in the aorta. Return flow to the heart is facilitated by mechanical squeezing of the skeletal muscles, which increases pressure in the limbs and pushes blood to the heart, and the expansion of the heart, which decreases pressure in the heart and pulls blood in. Finally, the pressure gradients created in the thorax by inhalation and exhalation also motivate blood flow. Venous circulation holds approximately three times as much blood as arterial circulation. Heart murmurs, which result from structural defects of the heart, are heard because of turbulent blood flow.


The respiratory system is also mediated by changes in pressure, and follows the same resistance relationship as the circulatory system. During inspiration, there is a negative pressure gradient that moves air into the lungs. During expiration, this gradient reverses. An additional point to note is that when air reaches the alveoli, it has essentially no speed.

MCAT Concept Check 4.4:

Before you move on, assess your understanding of the material with these questions.

1. Under what conditions could the continuity equation be applied to human circulation?

2. During exhalation, how does the total resistance of the encountered airways change as air leaves the alveoli to escape the nose and mouth?

3. How does flow in the venae cavae relate to flow in the main pulmonary artery?