Anatomy and Physiology: Cardiovascular Dynamics

1. To define the following: blood flow; viscosity; peripheral resistance; systole; diastole; end diastolic volume; end systolic volume; stroke volume; cardiac output.
2. To explore cardiovascular dynamics using an experimental setup to simulate a human body function.
3. To understand that heart and blood vessel functions are highly coordinated. 4. To comprehend that pressure differences provide the driving force that moves blood through the blood vessels.
5. To recognize that body tissues may differ in their blood demands at a given time.
6. To identify the most important factors in control of blood flow. 7. To comprehend that changing blood vessel diameter can alter the pumping ability of the heart.
8. To examine the effect of stroke volume on blood flow.

The physiology of human blood circulation can be divided into two distinct but remarkably harmonized processes: (1) the pumping of blood by the heart, and (2) the transport of blood to all body tissues via the vasculature, or blood vessels. Blood supplies all body tissues with the substances needed for survival, so it is vital that blood delivery is ample for tissue demands.

The Mechanics of Circulation
To understand how blood is transported throughout the body, let’s examine three important factors influencing how blood circulates through the cardiovascular system: blood flow, blood pressure, and peripheral resistance. Blood flow is the amount of blood moving through a body area or the entire cardiovascular system in a given amount of time. While total blood flow i s determined by cardiac output (the amount of blood the heart is able to pump per minute), blood flow to specific body areas can vary dramatically in a given time period. Organs differ in their requirements from moment to moment, and blood vessels constrict or dilate to regulate local blood flow to various areas in response to the tissue’s immediate needs. Consequently, blood flow can increase to some regions and decrease to other areas at the same time. Blood pressure is the force blood exerts against the wall of a blood vessel. Owing to cardiac activity, pressure is highest at the heart end of any artery. Because of the effect of peripheral resistance, which will be discussed shortly, pressure within the arteries (or any blood vessel) drops as the distance (vessel length) from the heart increases. This pressure gradient causes blood to move from and then back to the heart, always moving from high- to low-pressure areas.

Peripheral resistance is the opposition to blood flow resulting from the friction developed as blood streams through blood vessels. Three factors affect vessel resistance: blood viscosity, vessel radius, and vessel length. Blood viscosity is a measure of the “thickness” of the blood, and is caused by the presence of proteins and formed elements in the plasma (the fluid part of the blood). As the viscosity of a fluid increases, its flow rate through a tube decreases. Blood viscosity in healthy persons normally does not change, but certain conditions such as too many or too few blood cells may modify it. Controlling blood vessel radius (one-half of the diameter) is the principal method of blood flow control. This is accomplished by contracting or relaxing the smooth muscle within the blood vessel walls. To see why radius has such a pronounced effect on blood flow, we need to explore the physical relationship between blood and the vessel wall. Blood in direct contact with the vessel wall flows relatively slowly because of the friction, or drag, between the blood and the lining of the vessel. In contrast, fluid in the center of the vessel flows more freely because it is not “rubbing” against the vessel wall. When we contrast large- and small-radius vessels, we see that proportionately more blood is in contact with the wall of small vessels, hence blood flow is notably impeded in small-radius vessels.

Although vessel length does not ordinarily change in a healthy person, any increase in vessel length causes a corresponding flow decrease. This effect i s principally caused by friction between blood and the vessel wall. Consequently,

given two blood vessels of the same diameter, the longer vessel will have more resistance, and thus a reduced blood flow.

The Effect of Blood Presure and Vessel Resistance on Blood Flow Poiseuille’s equation describes the relationship between pressure, vessel radius, viscosity, and vessel length on blood flow:
Blood Flow = DP p r 4 / 8 h l
In the equation, DP is the pressure difference between the two ends of the vessel and represents the driving force behind blood flow. Viscosity (h) and blood vessel length (l) are not commonly altered in a healthy adult. We can also see from the equation that blood flow is directly proportional to the fourth power 4

of vessel radius (r ), which means that small variations in vessel radius

translate into large changes in blood flow. In the human body, changing blood vessel radius provides an extremely effective and sensitive method of blood flow control. Peripheral resistance is the most important factor in blood flow control, because circulation to individual organs can be independently regulated even though systemic pressure may be changing.

Vessel Resistance
Imagine for a moment that you are one of the first cardiovascular researchers interested in the physics of blood flow. Your first task as the principal investigator for this project is to plan an effective experimental design simulating a simple fluid pumping system that can be related to the mechanics of the cardiovascular system. The initial phenomenon you study is how fluids, including blood, flow through tubes or blood vessels. Questions you might ask include:

1. What role does pressure play in the flow of fluid?
2. How does peripheral resistance affect fluid flow?

The equipment required to solve these and other questions has already been designed for you in the form of a computerized simulation, which frees you to focus on the logic of the experiment. The first part of the computer simulation indirectly investigates the effects of pressure, vessel radius, viscosity, and vessel length on fluid flow. The second part of the experiment will explore the effects of several variables on the output of a single-chamber pump. Follow the specific guidelines in the exercise for collecting data. As you do so, also try to imagine alternate methods of achieving the same experimental goal.

The opening screen you will be working with looks like this:

The primary features on the screen when the program starts are a pair of glass beakers perched atop a simulated electronic device called the equipment control unit, which is used to set experiment parameters and to operate the equipment. When the Start button (beneath the left beaker) i s clicked, the simulated blood flows from the left beaker (source) to the right beaker (destination) through the connecting tube. Clicking the Refill button refills the source beaker after an experimental trial. Experimental parameters can be adjusted by clicking the plus (1) or minus (2) buttons to the right of each display window.

The equipment in the lower part of the screen is called the data collection unit. This equipment records and displays data you accumulate during the experiments. The data set for the first experiment (Radius) is highlighted in the Data Sets window. You can add or delete a data set by clicking the appropriate button to the right of the Data Sets window. The Record Data button at the lower right part of the screen activates automatically after an experimental trial. Clicking the Delete Line or Clear Data Set buttons erases any data you want to delete.

You will record the data you accumulate in the experimental values grid in the lower middle part of the screen.

Activity: Studying the Effect of Flow Tube Radius on Fluid Flow Our initial study will examine the effect of flow tube radius on fluid flow. 1. Conduct the initial equipment setup. The Radius line in the data collection unit should be highlighted in bright blue. If it is not, choose it by clicking the Radius line. The data collection unit will now record flow variations due to changing flow tube radius. If the data grid is not empty, click Clear Data Set to discard all previous values.