Blood Vessels
Types of Blood Vessels
There are three main types of vessels that circulate blood :
- Veins;
- Arteries;
- Capillaries.
Arterial blood, with oxygen and nutrients, is carried from the heart to the body’s tissues, and venous blood, with carbon dioxide and other waste products, goes from the body to the heart.
Arteries have more elastic walls than veins. This helps control blood pressure.
Veins, in turn, have valves to prevent blood from flowing back. Capillaries are very thin vessels that only have the innermost layer of endothelial cells.
Arteries
Arteries form a network of branching vessels that carry arterial blood from the heart to the body. Blood is pumped from the left ventricle and distributed through the body’s main artery: the aorta . Arterial branches branch out from it, which branch out further and further to irrigate all tissues.
The pulmonary artery is the only artery that carries blood rich in carbon dioxide. The blood leaves the heart directly to the lungs to be oxygenated, in a process known as hematosis.
Large-caliber arteries are called elastic, medium-caliber arteries are muscular and the thinnest arteries are arterioles.
Veins
Veins are vessels that carry blood rich in carbon dioxide (CO2 ) from the body to the heart, via the right atrium. Like the pulmonary artery, the pulmonary veins are the only ones that carry oxygen-rich blood from the lungs to the heart.
There are deep and superficial veins, as the name suggests, the former are found in deeper regions; while the latter are on the surface of the skin, being easily visible.
The thinnest veins are called venules and provide communication between vessels.
Capillaries
Capillaries are vessels with a very small diameter that branch out to form a wide network of tubules. They provide communication with other vessels and are also responsible for gas exchange.
Read also:
- Blood pressure
- Circulatory System
- Cardiovascular System
Features
They have a similar structure from a certain diameter (caliber). However, in the same vessel these characteristics vary along the way, and it is not always easy to distinguish one from the other. They branch out and become thinner, which happens gradually.
The vessels are formed by three layers, also called tunics. They are:
- The inner layer or tunica intima: consists of a layer of endothelial cells and a layer of loose connective tissue. It is separated from the next layer (tunica media) by an internal elastic lamina , which has small holes through which nutrients pass to the cells of the deeper layers;
- The intermediate layer or tunica media: formed by smooth muscle cells, between which there are collagen fibers. There is an external elastic lamina that separates this layer from the next.
- The outermost layer or tunica adventitia: composed basically of collagen and elastic fibers.
Larger blood vessels (arteries and veins) have more cell layers and thicker walls, while smaller vessels are very thin (capillaries), usually with only one layer.
Large veins have valves that prevent blood backflow. They help the blood to follow its path, which is very important in distant regions such as the legs. If the vein valves do not work properly, this leads to venous insufficiency, which causes varicose veins.
Very fine vessels form a well-branched network of tubes, called vasa vasorum, which helps to nourish the adventitial layer of larger vessels.
