Blood Vessels

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Blood Vessels

Introduction

There are three types of blood vessels: arteries, capillaries, and veins. These are the pipes that circulate blood throughout the body. The lumen is the channel within these vessels through which blood flows.


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Arteries

The arteries are the large, thick-walled vessels that carry the blood away from the heart. The walls of arteries contain a thick layer of smooth muscle that can contract or relax to change the size of the arterial lumen. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. The largest artery, the aorta, begins from the left ventricle of the heart and carries oxygenated blood to all the body systems. The coronary arteries then branch from the aorta and provide blood to the myocardium, as they travel through the body, the arteries branch into progressively smaller-sized arteries. The smallest of the arteries, called arterioles, deliver blood to the capillaries.


Capillaries

Capillaries are a network of tiny blood vessels referred to as a capillary bed. Arterial blood flows into a capillary bed, and venous blood flows back out. Capillaries are very thin walled, allowing for the diffusion of the oxygen and nutrients from the blood into the body tissues. Likewise, carbon dioxide and waste products are able to diffuse out of the body tissues and into the bloodstream to be carried away. Since the capillaries are so small in diameter, the blood will not flow as quickly through them as it does through the arteries and veins. This means that the blood has time for an exchange of nutrients, oxygen, and waste material to take place. As blood exits a capillary bed, it returns to the heart through a vein.


Veins

The veins carry blood back to the heart, Blood leaving capillaries first enters small venules, which then merge into larger veins. Veins have much thinner walls than arteries, causing them to collapse easily. The veins also have valves that allow the blood to move only toward the heart. These valves prevent blood from backflowing, ensuring that blood always flows toward the heart. The two large veins that enter the heart are the superior vena cava, which carries blood from the upper body, and the inferior vena cava, which carries blood from the lower body. Blood pressure in the veins is much lower than in the arteries. Muscular action against the veins and skeletal muscle contractions helps in the movement of blood.




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