O B J E C T I V E S 1. To understand the relationships among blood flow, pressure gradient, and resistance 2. To define resistance and describe the main factors affecting resistance 3. To describe Poiseuille’s equation and how it relates to cardiovascular dynamics 4. To define diastole, systole, end systolic volume, end diastolic volume, stroke volume, isovolumetric contraction, and ventricular ejection 5. To describe Starling’s Law and its application to cardiovascular dynamics 6. To design your own experiments using the lab simulation for pump mechanics 7. To understand what is meant by the term compensation
The cardiovascular system is composed of a pump—the heart—and blood vessels that distribute blood containing oxygen and nutrients to every cell of the body. The principles governing blood flow are the same physical laws that apply to the flow of liquid through a system of pipes. For example, one very elementary law in fluid mechanics is that the flow rate of a liquid through a pipe is directly proportional to the difference between the pressures at the two ends of the pipe (the pressure gradient) and inversely proportional to the pipe’s resistance (a measure of the degree to which the pipe hinders or resists the flow of the liquid): Flow pressure gradient/resistance P/R
This basic law applies to blood flow as well. The “liquid” is blood, and the “pipes” are blood vessels. The pressure gradient is the difference between the pressure in arteries and that in veins that results when blood is pumped into arteries. Blood flow rate is directly proportional to the pressure gradient, and inversely proportional to resistance. Recall that resistance is a measure of the degree to which the blood vessel hinders or resists the flow of blood. The main factors governing resistance are 1) blood vessel radius, 2) blood vessel length, and 3) blood viscosity.
Radius. The smaller the blood vessel radius, the greater the resistance, due to frictional drag between the blood and the vessel walls. Contraction, or vasoconstriction, of the blood vessel results in a decrease in the blood vessel radius. Lipid deposits can cause the radius of an artery to decrease, preventing blood from reaching the coronary tissue and result in a heart attack. Alternately, relaxation, or vasodilation, of the blood vessel causes an increase in the blood vessel radius. As we will see, blood vessel radius is the single most important factor in determining blood flow resistance.
Length. The longer the vessel length, the greater the resistance—again, due to friction between the blood and vessel walls. The length of a person’s blood vessels change only as a person grows; otherwise, length generally remains constant. Viscosity. Viscosity is blood “thickness,” determined primarily by hematocrit— the fractional contribution of red blood cells to total blood volume. The higher the 63
hematocrit, the greater the viscosity. Under most physiological conditions, hematocrit does not vary by much, and blood viscosity remains more or less constant. A fourth factor in resistance is the manner of blood flow. In laminar flow, blood flows calmly and smoothly along the length of the vessel. In turbulent flow, blood flows quickly and roughly. Most blood flow in the body is laminar, and the experiments we will conduct in this lab focus on laminar flow. Poiseuille’s equation expresses the relationships among blood pressure, vessel radius, vessel length, and blood viscosity on laminar blood flow: Blood flow ( Q) or Blood flow ( Q) where P r l Pr4 8 l Pr4/8 l
You may also adjust the pressure by clicking the ( ) and ( ) buttons for pressure on top of the left beaker. Clicking Refill will empty the right beaker and refill the left beaker. At the bottom of the screen is a data recording box. Clicking Record Data after an experimental run will record that run’s data in the box. A C T I V I T Y 1
The Effect of...