Compliance is the ability of a hollow organ (vessel) to distend and increase volume with increasing transmural pressure or the tendency of a hollow organ to resist recoil toward its original dimensions on application of a distending or compressing force. The reciprocal of compliance iselastance, a measure of the tendency of a hollow organ to recoil toward its original dimensions upon removal of a distending or compressing force.

The terms elastance and compliance are of particular significance incardiovascular physiology andrespiratory physiology. In compliance, an increase in volume occurs in a vessel when the pressure in that vessel is increased. The tendency of thearteries andveins to stretch in response to pressure has a large effect on perfusion and blood pressure. This physically means that blood vessels with a higher compliance deform easier than lower compliance blood vessels under the same pressure and volume conditions.^[1] Venous compliance is approximately 30 times larger than arterial compliance.^[2] Compliance is calculated using the following equation, where${\displaystyle \mathrm{\Delta }V}$ is the change in volume (mL), and${\displaystyle \mathrm{\Delta }P}$ is the change in pressure (mmHg):^[3]

${\displaystyle C=\frac{\mathrm{\Delta }V}{\mathrm{\Delta }P}}$

Physiologic compliance is generally in agreement with the above and adds$\frac{dP}{dt}$ as a common academic physiologic measurement of both pulmonary and cardiac tissues. Adaptation of equations initially applied torubber andlatex allow modeling of the dynamics of pulmonary and cardiac tissue compliance.

Veins have a much higher compliance than arteries (largely due to their thinner walls.) Veins which are abnormally compliant can be associated withedema.Pressure stockings are sometimes used to externally reduce compliance, and thus keep blood from pooling in the legs.

Vasodilation andvasoconstriction are complex phenomena; they are functions not merely of thefluid mechanics of pressure and tissueelasticity but also of activehomeostatic regulation withhormones andcell signaling, in which the body producesendogenous vasodilators and vasoconstrictors to modify its vessels' compliance. For example, themuscle tone of thesmooth muscle tissue of thetunica media can be adjusted by therenin–angiotensin system. In patients whose endogenous homeostatic regulation is not working well, dozens ofpharmaceutical drugs that are alsovasoactive can be added. The response of vessels to such vasoactive substances is called vasoactivity (or sometimes vasoreactivity). Vasoactivity can vary between persons because ofgenetic andepigenetic differences, and it can be impaired by pathosis and by age. This makes the topic ofhaemodynamic response (including vascular compliance andvascular resistance) a matter of medical andpharmacologic complexity beyond merehydraulic considerations (which are complex enough by themselves).

The relationship between vascular compliance, pressure, and flow rate is:$${\displaystyle Q=C\frac{\mathrm{d}P}{\mathrm{d}t}=(\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e})(\mathrm{c}{\mathrm{m}}^3/\mathrm{s}\mathrm{e}\mathrm{c})}$$

The classic definition by MP Spencer and AB Denison of compliance (${\displaystyle C}$) is the change in arterial blood volume (${\displaystyle \mathrm{\Delta }V}$) due to a given change inarterial blood pressure (${\displaystyle \mathrm{\Delta }P}$). They wrote this in the "Handbook of Physiology" in 1963 in work entitled "Pulsatile Flow in the Vascular System". So,${\displaystyle C=\frac{\mathrm{\Delta }V}{\mathrm{\Delta }P}}$.^[4]

Arterial compliance is an index of theelasticity of large arteries such as thethoracic aorta. Arterial compliance is an important cardiovascular risk factor. Compliance diminishes with age and menopause. Arterial compliance is measured byultrasound as a pressure (carotid artery) and volume (outflow intoaorta) relationship.^[5]

Compliance, in simple terms, is the degree to which a container experiences pressure or force without disruption. It is used as an indication ofarterial stiffness. An increase in the age and also in the systolicblood pressure (SBP) is accompanied with decrease on arterial compliance.^[6]

Endothelial dysfunction results in reduced compliance (increased arterial stiffness), especially in the smaller arteries. This is characteristic of patients withhypertension. However, it may be seen in normotensive patients (with normal blood pressure) before the appearance of clinical hypertension. Reduced arterial compliance is also seen in patients withdiabetes and also in smokers. It is actually a part of a vicious cycle that further elevates blood pressure, aggravatesatherosclerosis (hardening of the arteries), and leads to increased cardiovascular risk. Arterial compliance can be measured by several techniques. Most of them are invasive and are not clinically appropriate.Pulse contour analysis is a non-invasive method that allows easy measurement of arterial elasticity to identify patients at risk for cardiovascular events.^[7]

• Cardiovascular System Dynamics Society
• Windkessel effect

• Compliance at the U.S. National Library of MedicineMedical Subject Headings (MeSH)

• Respiratory physiology
• Cardiovascular physiology

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