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This MCAT post will cover a main component of the circulatory system: the blood vessels. Major features of the blood vessels include arteries, capillaries, and veins, as well as the presence of two blood vessel circuits: the pulmonary circulation and the systemic circulation. In this post, we'll discuss the difference between these components as well as how they function together to deliver oxygen and remove waste from tissues throughout the body,
The blood vessels of the cardiovascular system carry blood throughout the body. The vessels that carry blood away from the heart are called arteries, and vessels that carry blood to the heart are called veins.
Although it is common to think of arteries as carrying oxygenated blood and veins as carrying deoxygenated blood, this is not always the case. The body has two different systems of blood circulation – the systemic circulation and the pulmonary circulation – and the arteries and veins in either have different functions (Figure 1). The systemic circulation carries blood to the different tissues and organs in the body, and in this circulation, arteries carry oxygen-rich blood. In the pulmonary circulation, on the other hand, blood is carried to the lungs and then back to the heart. In this system, arteries carry deoxygenated blood to the lungs in order for the blood to get oxygenated. In this way, although arteries always carry blood away from the heart, they carry either oxygenated and deoxygenated blood, depending on the circulation in question.
Capillaries are the site of nutrient and waste exchange in the blood vessels. To facilitate this exchange, the capillary walls are ultra-thin so that nutrients and waste can pass through them. Figure 2 shows how capillaries work. As you can see, their function is different at their arterial ends and than at their venous ends.
Blood coming from the arterial circulation is at high pressure because blood pumped from the heart is at a higher pressure than blood going to the heart. Therefore, the arterial end of capillaries is also at high pressure. This high pressure then forces fluid out of the capillary walls. In this way, there is a net loss of fluid at the arterial ends of capillaries.
As blood travels through capillaries, not only is fluid lost but carbon dioxide and other nutrients, like glucose and oxygen, are being exchanged. However, anything too large to pass through the capillary walls, like proteins, are not being exchanged. In this way, as blood moves through the capillaries towards the venous end, the concentration of proteins increases, and the fluid volume decreases.
At some point, however, as blood moves towards the venous end of the capillaries, the osmotic pressure will become higher than the hydrostatic pressure, which will draw fluid back into the capillaries. Overall, however, there is a net loss of fluids, meaning more fluid is forced out of capillaries than drawn back in. The lymphatic system has an essential role in re-circulating the fluid that is lost from capillaries and will be discussed in greater detail elsewhere.
Veins are blood vessels that carry blood to the heart. Remember, they can either carry oxygenated or deoxygenated blood, depending on the particular circulation in the question. In the systemic circulation, veins carry deoxygenated blood back to the heart. In the pulmonary circulation, veins carry oxygenated blood back to the heart.
The blood in the veins is at much lower pressure than that in the arteries. This is because veins are further away from the pumping heart. For this reason, blood in the veins needs some extra assistance to return to the heart. Figure 3 illustrates the mechanism by which this occurs – something called the skeletal muscle pump. As shown in the figure, veins are situated between skeletal muscles, and when these muscles contract and relax, they squeeze the veins and mechanically push the blood through. Veins also have valves to prevent the backflow of blood, and these valves, in combination with skeletal muscle contractions, help bring blood back to the heart.
Varicose veins, a condition in which blood pools in the venous circulation, are the result of the partial failure of these blood-returning mechanisms. This pooling of blood in the veins can become a severe problem and is usually a result of another underlying medical condition such as pregnancy, obesity, or even just normal aging.