Question

Describe how 1) Mean Arterial Pressure, 2) Maximal Oxygen Consumption, and 3) Blood Flow are determined using its formula. In addition, you should explain what those factors are including cardiac output, stroke volume, heart rate, resistance, a-vO2 difference, etc.

Answer 1

1.

Mean arterial pressure can be determined from:^[3]

where:

• is cardiac output
• is systemic vascular resistance
• is central venous pressure and usually small enough to be neglected in this formula.

2.

VO₂ max is properly defined by the Fick equation:

, when these values are obtained during an exertion at a maximal effort.

where Q is the cardiac output of the heart, C_aO₂ is the arterial oxygen content, and C_vO₂ is the venous oxygen content.

(C_aO₂ – C_vO₂) is also known as the arteriovenous oxygen difference

3.

The relationship between pressure, flow, and resistance is expressed in the following equation:^[8]

Flow = Pressure/Resistance

When applied to the circulatory system, we get:

CO = (MAP – RAP)/TPR

where

• CO = cardiac output (in L/min)
• MAP = mean arterial pressure (in mmHg), the average pressure of blood as it leaves the heart
• RAP = right atrial pressure (in mmHg), the average pressure of blood as it returns to the heart
• TPR = total peripheral resistance (in mmHg * min/L)

Term definitions

• Cardiac output (CO), also known as heart output denoted by the symbols , or , is a term used in cardiac physiology that describes the volume of blood being pumped by the heart, by the left and right ventricle, per unit time. Cardiac output (CO) is the product of the heart rate (HR), i.e. the number of heartbeats per minute (bpm), and the stroke volume (SV), which is the volume of blood pumped from the ventricle per beat; thus, CO = HR × SV. Values for cardiac output are usually denoted as L/min. For a healthy person weighing 70 kg, the cardiac output at rest averages about 5 L/min; assuming a heart rate of 70 beats/min, the stroke volume would be approximately 70 ml.

• Stroke Volume (SV) is the volume of blood in millilitres ejected from the each ventricle due to the contraction of the heart muscle which compresses these ventricles.SV is the difference between end diastolic volume (EDV) and end systolic volume (ESV). Multiple factors can affect SV, eg. factors that change either EDV or ESV will change SV. The three primary factors that regulate SV are preload, afterload and contractility.

• Heart rate is the speed of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excrete carbon dioxide. It is usually equal or close to the pulse measured at any peripheral point. Activities that can provoke change include physical exercise, sleep, anxiety, stress, illness, and ingestion of drugs.
• Peripheral vascular resistance (systemic vascular resistance, SVR) is the resistance in the circulatory system that is used to create blood pressure, the flow of blood and is also a component of cardiac function. When blood vessels constrict (vasoconstriction) this leads to an increase in SVR.This is sometimes referred as total peripheral resistance (TPR).

• The arteriovenous oxygen difference, or a-vO₂ diff, is the difference in the oxygen content of the blood between the arterial blood and the venous blood. It is an indication of how much oxygen is removed from the blood in capillaries as the blood circulates in the body. The a-vO₂ diff and cardiac output are the main factors that allow variation in the body's total oxygen consumption, and are important in measuring VO₂. The a-vO₂ diff is usually measured in millilitres of oxygen per 100 millilitres of blood (mL/100 mL).

• Central venous pressure (CVP) is the blood pressure in the venae cavae, near the right atrium of the heart. CVP reflects the amount of blood returning to the heart and the ability of the heart to pump the blood back into the arterial system.

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