What Are Blood Vessels?

Blood vessels are intricate networks of hollow tubes that transport blood throughout the entire body. This is an essential function as blood delivers valuable nutrients to and removes wastes from our cells. Blood vessels are constructed of layers of connective tissue and muscle. The inner blood vessel layer is formed of endothelium. In capillaries and sinusoids, endothelium comprises the majority of the vessel.

Blood vessel endothelium is continuous with the inner tissue lining of organs such as the brain, lungs, skin, and heart. In the heart, this inner layer is called the endocardium.

Types of Blood Vessels

• Arteries
  Arteries are elastic vessels that transport blood away from the heart. Pulmonary arteries carry blood from the heart to the lungs where oxygen is picked up by red blood cells. Systemic arteries deliver blood to the rest of the body.
   
• Veins
  Veins are elastic vessels that transport blood to the heart. Veins can be categorized into four main types: pulmonary, systemic, superficial, and deep veins.
   
• Capillaries
  Capillaries are extremely small vessels located within the tissues of the body that transport blood from the arteries to the veins. Fluid exchange between capillaries and body tissues takes place at capillary beds.
   
• Sinusoids
  Sinusoids are extremely small vessels located within the liver, spleen, and bone marrow.

Circulation

Blood is circulated through the body via the cardiovascular system.

This system is comprised of the heart and the circulatory system. Blood vessels carry blood from the heart to all areas of the body. The blood travels from the heart via arteries to smaller arterioles, then to capillaries or sinusoids, then to venules, to veins, and back to the heart.

Microcirculation deals with the flow of blood from arterioles to capillaries or sinusoids to venules. As blood moves through the capillaries, substances such as oxygen, carbon dioxide, nutrients, and wastes are exchanged between the blood and the fluid that surrounds cells.