59 Vasculature Anatomy

Types of vasculature:

In any large building, there are hundreds of  pipes, each of varying thickness and diameter, specially designed to withstand different pressures. Similarly, cardiovascular system is equipped with three types of blood vessels: arteries, capillaries, and veins.

Figure 115 Various types of blood vessels ranging from arteries(left) to capillaries(middle) to veins(right)

Anatomy of arteries:

There are three types of arteries- elastic, muscular and the smallest of the arteries, the arteriole. Elastic arteries are built to withstand the highest pressures in the circulatory system and transport blood away from the heart. To accommodate the intense pressure propelled through the heart the largest arteries like the aorta and pulmonary artery have thick elastic walls. Smaller arteries have some elasticity but have muscular walls that can relax and constrict depending on changes in blood pressure.

Arterioles are the smallest  arteries and they protect delicate capillaries by regulating the blood pressure and flow experienced by the capillaries. Without arterioles, strong blood flow from arteries could be harmful to the much thinner and more fragile capillaries. They achieve this regulation through vasoconstriction (constriction or tightening of blood vessels) and vasodilation (dilation or expanding of blood vessels). Like muscular arteries, arterioles have muscular walls to regulate blood pressures but  the arteriole walls are relatively thin and are not very elastic.  The muscle in the walls regulate pressure and flowthrough vasoconstriction (constriction or tightening of blood vessels) and vasodilation (dilation or expanding of blood vessels).

Anatomy of capillaries:

While our building has massive water pipes of different types to carry large volumes of water to different areas, the smallest pipes are required to serve individual faucets. Similarly, as arteries branch off into smaller vessels, they eventually reach the smallest type of blood vessels, known as capillaries, whose walls may be just one cell wide and their lumen is only one RBC across. Their thin walls are paramount in allowing the exchange of nutrients, gases like oxygen as well as waste products like  carbon dioxide.

Anatomy of veins:

The clean water supplied all over our building has to return through wastewater pipes. In the body, blood from the capillaries, now relatively full of waste products and depleted in oxygen, must return back to the heart, which is facilitated by small veins called venules which feed into larger veins. These venules have considerably thinner walls and less muscle content than arterioles and arteries, making them far more distensible to accommodate changes in blood volumes. They can have thin walls because blood pressure has been dissipated in the transit across the capillaries. However, the blood still has to move back to the heart to complete the circuit and their not enough blood pressure to do it. To complete the circuit and return the  the blood to the heart, veins are equipped with one-way valves. These valves prevent the backflow of blood such that any pressure that does develop by, for example,  a muscle pushing on a vein can be exploited to propel the blood towards the heart, not towards the capillary beds.

Figure 116 Cross sections of arteries(top left), capillaries (bottom left) and veins (top right)