Cardiovascular physiology: Blood pressures & volumes
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Blood pressures
Blood pressure is highest during systole as it leaves the left ventricle and enters the aorta. The rise in blood pressure during systole stretches the vessel walls and drives blood flow. During diastole elastic recoil of the vessel wall helps maintain blood flow. Blood then travels through the arterial system moving through progressively smaller diameter vessels until it reaches the capillary bed. The capillaries then join venules which drain into larger diameter veins. Pressure progressively drops as blood moves through the vascular system as shown in the graph below: | |
| Site | Typical blood pressure (mmHg) |
|---|---|
| Left ventricle | 120 |
| Start of arterioles | 65 |
| Arterial side (start) of capillaries | 25 |
| Venous side (end) of capillaries | 15 |
| Vena cava at level of heart | close to zero (2-8) |
Mean arterial pressure
Pressure in the arteries fluctuates between the minimum diastolic pressure when the ventricles are filling and a maximum systolic pressure when the ventricles are contracting and forcing blood into the aorta and systemic circulation.
For approximately 60% of the cardiac cycle the heart is in diastole. The mean arterial pressure (MAP) is a reflection of the average arterial pressure during the cardiac cycle.
It is calculated by the equation below:
MAP = Diastolic pressure + 1/3 pulse pressure
Note: Pulse pressure = systolic - diastolic pressure
Note: Pulse pressure = systolic - diastolic pressure
MAP is determined by the total peripheral resistance (TPR) and the cardiac output (CO).
MAP = CO x TPR
Volumes
- Total blood volume is approximaetly 5 litres in adults
- Total blood volume is 70 to 75 mL per kg bodyweight in humans
- Cardiac output in an average male at rest is approx ~ 5 litres/min
- Stroke volume is approx 70 mls in an average male
Note cardiac output (CO) is the amount of blood the heart pumps through the circulatory system in a minute. Stroke volume (SV) is the volume of blood pumped out of the heart with each heart beat. Therfore:
CO = SV x HR