What's the difference between capillaries, pulmonary artery and pulmonary vein?

The difference between capillaries, pulmonary artery, and pulmonary vein lies in their location, function, and structure within the circulatory system.

1. Capillaries: Capillaries are the smallest blood vessels in the body and serve as the connection between arteries and veins. They form an intricate network throughout tissues and organs. Capillaries are responsible for the exchange of oxygen, nutrients, and waste products between the blood and surrounding tissues. Their thin walls allow for efficient diffusion of substances. Capillaries are found throughout the body, except for a few areas such as cartilage and the epidermis (outermost layer of the skin).

2. Pulmonary Artery: The pulmonary artery is an important blood vessel that carries deoxygenated blood from the heart to the lungs. It is the only artery in the body that carries deoxygenated blood. After receiving blood from the right ventricle of the heart, the pulmonary artery branches into smaller vessels that ultimately reach the capillaries in the lungs. In the lungs, carbon dioxide is removed from the blood, and oxygen is replenished through the process of respiration. Once oxygenated, the blood returns to the heart through the pulmonary veins.

3. Pulmonary Vein: Unlike the pulmonary artery, the pulmonary veins carry oxygenated blood from the lungs to the heart. After the capillaries in the lungs have replenished the blood with oxygen, several pulmonary veins merge together to form four pulmonary veins (two from each lung). These veins transport the oxygenated blood back to the heart, specifically to the left atrium. From there, it is pumped to the rest of the body through the systemic circulation.

To recap:
- Capillaries facilitate the exchange of substances between blood and tissues.
- The pulmonary artery carries deoxygenated blood from the heart to the lungs.
- The pulmonary vein carries oxygenated blood from the lungs back to the heart.

Understanding the differences between these blood vessels helps us appreciate their essential roles in the circulation of oxygen and nutrients throughout the body.