# Vascular functional anatomy

The vascular system has two functions: distribution and exchange.

• Distribution: the transport of blood to and away from organs.
• Exchange: the movement of fluid, nutrients and gases between blood and tissues.

## The vascular network and vessel layers

The heart's left ventricle ejects blood into the aorta, which distributes blood flow throughout the body using a network of branching arterial vessels, the arterial tree. Arteries branch into successively smaller vessels, until they become the smallest vascular units, capillaries, which are found within tissues and organs. Capillaries then converge into successively larger vessels, venules and veins, until blood returns to the heart.
Vessels can be modeled as having three layers, or "tunicas" (the Latin tunica referred to a "tunic", an undergarment worn by both sexes, and used in biology in the figurative sense of "a membrane"):

• The vessels' innermost tunica intima is made up of the endothelium, which lines the cardiovascular system and is in contact with bloood, and the underlying supporting internal elastic lamina. Regarding the etymology of endothelium, it comes from "epithelium". The word epithelium was coined by Frederik Ruysch, who dissected the lip of a cadaver, and used the Greek words epi- ("on top of") and -thele ("nipple"). The "-thele" part came from the fact that microscopic visualization of the lip tissue showed papillary (nipple-like) projections (i.e. the papillary dermis), and in analogy with "epithelium" the "-thele" was retained for the layer of cells within blood- and lymph vessels and the heart, the Greek endon- meaning "within", thus forming the word endothelium.
• The middle layer is the smooth muscle layer, the tunica media.
• The vessels' outer tunica externa, or tunica adventitia (the Latin "adventicius" means "coming from outside") is an anchoring layer supported by a fibroelastic external elastic lamina.

## Vessel types

Vessels can crudely be divided into three types: arteries, capillaries, and veins. In general, arteries are those vessels that lead away from the heart to capillary beds (the common misconception that all arteries carry oxygenated blood and veins deoxygenated blood is not true, as exemplified by the pulmonary and umbilical arteries, which carry deoxygenated blood), but there are exceptions to the heart-to-capillary-rule (e.g. the renal circulation is a sort of "portal system" that has an artery (afferent arteriole) followed by a capillary bed (glomerulus) that leads to an arterial system (efferent arterioles) that in turn supply downstream capillary beds (the peritubular capillaries and vasa recta)). Veins generally lead to the heart. The calibers (circular section diameter) of blood vessels determine the proportion of smooth muscle, elastin, elastic fibers and collagen fibers present in the vessel wall.
Vessels can be grouped into four groups based on their primary function: distribution, resistance, exchange and capacitance.

### Distribution vessels

The main distributing vessel is the body's largest artery, the aorta. Because of the aorta's compliance properties, it also has a function in dampening the pulsatile pressure that results from intermittent cardiac blood ejection. From the aorta, large arteries branch off to distribute blood to specific organs or regions (i.e. a. carotis communis, aa. mesentericae, aa. renales). In normal physiology, the large arteries serve no function in regulating blood pressure or blood flow, despite having the capability to constrict and dilate. Arterial walls are generally thicker than walls in veins, since arteries need to withstand (1) higher pressures and (2) a pulsatile flow. As arteries become smaller, so wall thickness decreases. Large elastic arteries (e.g. the aorta, a. carotis, a. iliaca communis, aa. pulmonales) have a tunica media with abundant elastic fibers, enabling elastic expansion in systole and recoil during diastole.

### Resistance vessels

When distributing arteries reach the organs to which they supply blood, they are called small arteries. Continuous branching eventually gives rise to the smallest arteries, called arterioles. Arterioles have only a few smooth muscle layers. Small arteries and arterioles are the body's primary resistance vessels. Such vessels regulate vessel diameter (i.e. resistance), and thus indirectly control arterial blood pressure and blood flow. Resistance vessels have abundant autonomic innervation, especially sympathetic adrenergic innervation, and they constrict and dilate in response to changes in nerve activity. The wealth of receptors also includes angiotensin II ("angiotensinergic") receptors. The abundance of receptors means that circulating hormones also influence vessel diameter. A variety of substances produced locally (in endothelium or vessel-surrounding tissues) also influence resistance vessels' diameter.

### Exchange vessels

Arterioles successively become smaller in diameter. At a diameter of <10 µm the smooth muscle layer is lost. When vessels have no smooth muscle, and only a single layer of endothelial cells on a basement membrane (i.e. only a "tunica intima" with no other layers), they are called capillaries. Inside organs and tissues, large networks of capillaries form capillary beds. The arteriole-capillary transition is made up of a special arteriole called a metarteriole (Greek meta means "in the middle"), which has a precapillary sphincter through which the blood flow can be reduced or shut down.
Capillaries are the circulation's smallest vessels, but also its' most numerous, which means that the cross-sectional area of capillaries is the greatest of all vessel types. The volumetric flow rate equation states that flow ($F$) is the product of mean velocity ($v$) and cross-sectional area ($A$), so that $F = vA$. In normal physiology, the total blood flow in all capillaries in the body combined is the same as the flow in the aorta as it leaves the heart. Since the cross-sectional area of the capillaries is circa 1000 times that of the aorta's cross-section, this means that flow velocity of the capillaries is about 1000 times less than that of the aorta.
Capillaries have the greatest surface area for exchange in the body. Capillary endothelium facilitates the exchange of O2, CO2, H2O, electrolytes, proteins, metabolic substances, and hormones between blood plasma and tissue interstitial fluid. As capillaries converge, they form postcapillary venules, which have a high permeability, and function as exchange vessels for fluids and macromolecules.

### Capacitance vessels

Small postcapillary venules converge and unite to form larger venules. These venules have smooth muscle, and are thus capable of regulating their diameter. Changes in venular diameter regulates capillary pressure and venous blood volume. Large venules converge to form larger veins. Venules and veins have a large capacitance (defined as $V/P$, and different from compliance, which is $\Delta V/\Delta P$), which can be defined as the total quantity of blood that can be stored in the circulation at each pressure. Venous capacitance is much higher than arterial capacitance, and veins thus serve as the blood's primary capacitance vessels, and contain the majority of the total blood volume as a mobilizable "reserve pool" of blood. Constriction of veins (venoconstriction) decreases venous blood volume and increases venous blood pressure, which can affect right atrial pressure and ventricular preload, and thus alter cardiac output.
Compared to arteries, veins have thinner walls and larger diameters, meaning that the venous lumen is larger. This contributes to veins' capacitance. The venous tunica intima and externa have less smooth muscle and elastic tissue, meaning that they have less elastic recoil and constriction capacity than arteries.
The forces that propel blood through veins are (1) contractions of surrounding muscles, (2) pressure gradients associated with respiration. Reverse venous flow during periods of no pressure difference is prevented by venous valves. The composition of the venous walls means that veins are succeptible to collapse and compression (i.e., veins are more "flaccid" than arteries).

## Vascular anatomical motifs

Certain anatomical patterns are found in the healthy circulatory system, such as portal systems, anastomoses and accompanying veins, and also as parts of disease (e.g. shunts) and man-made manipulation (e.g. surgical anastomoses).