Capacitance is defined as the distensibilty of a blood vessel. In other words, capacitance is the ability of an object to get stretched. In contrast, elasticity is defined as the object’s ability to recoil or return to its previous shape after being stretched. Elastance is produced by elastin fibers present in the vascular wall. Take for example the aorta; it has the most layers (around 50 layers) of elastin fibers in its tunica media which makes it the most elastic blood vessel in the body. If the aorta is compressed or stretched, it will recoil back to its normal shape.
Compliance of a vessel is the opposite of its elastance. The veins are said to be compliant because if you keep increasing the volume of blood in the veins, their walls will distend allowing for more blood to be accommodated.
Arteries and veins possess relative percentage of both, elasticity and compliance. However, arteries are said to be relatively more elastic and less compliant owing to the fact that they have more elastic fibers. This explains why the aorta resists distension when blood at high pressure is pumped into it by the heart. The increase in pressure causes a small stretch in the wall (due to compliance) of aorta which is immediately diffused as the aorta recoils back (due to elasticity) to its original shape. Greater the amount of elastic tissue in a vessel wall, the higher is the elastance, and the lower is the compliance of the blood vessel. As the person’s age increases, the arteries become stiffer and further lose their compliance. Pathological conditions such as atherosclerosis also further decreases the arterial compliance.
The opposite is true for the veins. The walls of the veins, being very compliant, will keep on stretching as more blood is added to the veins. There will be a small degree of recoil as well but the resultant force is not as strong as that in the arteries.
- Compliance = Volume (ml)/ Pressure (mm Hg)
As seen in the formula, compliance can be written as a ratio of volume and pressure. This shows that more compliant vessels will allow more volume of blood to be added without causing any changes in pressure. On the other hand, the arteries try to resist changes in volume by generating more pressure as they are less compliant. This is the reason why the volume of blood in arteries is called stressed volume whereas the volume in veins is called unstressed volume. . Systemic veins are 20 times more compliant relative to systemic arteries. Due to this, the veins easily distend in response to high volumes of blood. Because of their ability to capacitate, the veins accommodate roughly 70% of the systemic blood volume. For this reason the systemic veins are a major blood reservoir of the body.
The table below contrasts the different properties of arteries and veins and the effect they have on hemodynamics. The result are further explained below:
Elastance (Recoil ability)
- Compliance: Arteries are less compliant and veins are more compliant.
- Elastance: The arteries are more elastic. This property makes up the basis for “windcastle effect” in the aorta. The aorta distends in response to the high pressure blood pumped by the heart and recoils back thereby maintaining the pressure between 120/80 mm Hg. The elastance of veins is low.
- Stress: The arteries face stress as the blood running in them is at high pressure. The stress on the walls of the veins is negligible.
- Volume: The arteries are not capable of housing large volumes of blood. The veins have more capacity as they are more distensible and compliant in the face of volume changes.
- Pressure: The pressure in the arteries is greater than that in the veins. Arterial wall recoils due to presence of elastic fibers, and this recoil is responsible for the arterial pressure. Veins on the other hand don’t have that much of an elastic component and therefore lack recoil ability and consequently the blood pressure is relatively low in the veins.
- Flow: The flow is equal in both, the arteries and the veins, as the blood is flowing in a closed circuit. As the blood moves in a closed circuit, the flow is equal at all points.
COMPLIANCE GRAPHS FOR VOLUME VS PRESSURE
X-axis = Volume (mL)
Y-axis = Pressure (mm Hg)
The graphs in the lecture compare the compliance of aorta with that of systemic veins. For understanding purpose, compliance can be referred to as the pressure change in the blood vessel when unit volume of blood is added to the vessel (aorta or the veins). The compliance of each vessel can be calculated as the gradient of the graph at any point.
Compliance = ΔV/ΔP
For Aorta, when unit volume of blood is added to it, there’s a significant rise in its pressure. The gradient of the volume vs. pressure curve is relatively less steep. This suggests that the aorta has a low compliance. Increase in pressure can also be explained by the greater recoil (elastance) of the aorta.
For veins, when blood is added to them, there’s no significant change in pressure. The gradient of the volume vs. pressure is very steep, which suggests that their compliance is very high. The veins can hold greater volume of blood without any increase in their pressure because they are very distensible. However, as we continue to add blood, there comes a point when the curve starts to flatten out and the vein cannot accommodate any more blood. The compliance at this point is zero.