Your Body’s Highway System
The vascular system consists of blood vessels that permeate the human body and convey all essential materials, both cellular and molecular. Some vessels are subjected to high blood pressure (arteries), which their structure must resist, while others (veins) must assist the extremely low-pressure blood in returning to the heart. Other vessels are only one red blood cell in diameter, which allows the direct exchange of materials between the blood and the body’s tissues.
How extensive is the vascular system in the human body?
Connected end to end, all of the blood vessels in the body would extend approximately 60,000 miles. For reference, the circumference of the earth at the equator is just under 25,000 miles.
Arteries
The vessels that transport blood away from the heart are called arteries. Not all of the blood carried in arteries is oxygen-rich; the pulmonary arteries transport oxygen-depleted blood from the right ventricle to the lungs. Arteries, like all types of vessels, have a lining layer of epithelium called the endothelial layer. These cells, because of their overall net negative surface charge and molecular composition, provide a low-friction surface for blood cells and platelets to flow across.
The three layers of an artery include:
· The endothelium (the name of the epithelial lining, or endothelial layer, of vessels) and the underlying connective tissue is grouped together into a layer termed the tunica intima. This is the innermost of three layers present in most arteries and veins.
· The middle layer, which is most prominent in arteries, is the tunica media and consists of varying numbers of smooth muscle cells and sheets of elastic fibers (elastic laminae). Together the elastic component (which rebounds following high pressure) and the smooth muscle cells (which control the narrowing or expansion of the vessel diameter) assist in maintaining both blood pressure and the flow of blood through different vessels.
· The outermost layer of the vessels is the tunica adventitia and is composed of the connective tissue layer surrounding the vessel.
The largest-diameter arteries, such as the aorta, are defined as elastic arteries because of the large number of elastic layers present in the tunica media. Pressure is highest when the blood immediately leaves the heart, and these vessels require the elastic fibers to resist and rebound to the high-pressure stretching. Most of the intermediate-sized vessels have layers of smooth muscle in the tunica media that may reach up to 40–50 layers thick.
As blood flows farther away from the heart, the layers of the arterial wall become fewer and the diameter of the vessel decreases until all that remains of the tunica media is one or two smooth muscle cells along with the tunica media. These are called arterioles (small branches of an artery) and immediately precede capillaries, tiny vessels that make up the body’s microcirculation. Often the smooth muscle of these vessels acts as a pressure regulator to prevent the blood pressure from being too high before entering the capillary network.
Capillaries
The capillaries are the small vessels that allow the direct exchange of materials between vessel and tissue. They are composed of a single endothelial layer with little, if any, connective tissue.
Most of the capillaries in the body are categorized as continuous capillaries. The endothelial cells of these capillaries form tight complexes between cells and only material passed through the cell is transported. There is no place in the body where this is more evident than in the brain, where the continuous capillaries are a part of the blood-brain barrier. In other areas, material needs to be transported more rapidly and the specificity (e.g., type of molecule being transported) isn’t critical. The endothelial cells in areas such as in the kidney have small pores in the endothelial cells that allow larger material to be transported more quickly. These openings, called fenestrae, may be open or covered with a thin membrane to further specify (that is, define the type of material) what can pass. In either case, these capillaries are called fenestrated capillaries. The final type of capillary is somewhat like Swiss cheese in that there are almost more holes than cells in these vessels. In organs such as the liver, in which cells are in contact with much of the plasma component of the blood, only the cells are restricted from access to open areas called sinuses or sinusoids. All other material can pass through these large holes. Thus, these are termed sinusoidal capillaries.
Veins
By the time the blood passes through the capillaries, the blood pressure has been drastically decreased. In the aorta, the blood pressure was approximately 100 mmHg. When the blood finally returns to the heart, the pressure falls to almost 0 mmHg. The exchange of materials in the capillaries and their small diameter allow this low-pressure blood to enter and return back to the heart.
What does mmHg mean?
In medicine, mmHg is used as a measurement of force per unit. It is based on an old method of comparing the amount of pressure that mercury (a heavy metal) exerts to other types of pressure (such as blood pressure): 1 mmHg is the amount of pressure generated by a column of mercury 1 millimeter high. It is the equivalent of 1/760 atmosphere, an atmosphere being a unit of measure equaling about 14.7 pounds per square inch.
Vessels called venules are primarily a single endothelial layer. However, their diameter is larger than that of capillaries. They usually are found adjacent to an arteriole. From the venules, the blood moves into increasingly larger-diameter veins. This increase in diameter causes the blood to pool and also decreases its pressure.
Unlike arteries, the thickest layer of veins is the adventitia, or the outer connective tissue layer. Little smooth muscle is present in the tunica media of veins. Another identifying characteristic of veins is their diameter, which is much greater than the thickness of the vessel wall (the opposite is true for arteries). Since the blood flow in the veins lacks sufficient pressure to return blood to the heart, veins possess unidirectional valves. With each contraction of the heart, blood is moved upward through a set of valves, which then close as the heart relaxes, keeping the blood from falling backward due to gravity.