A transport network
Cells throughout the body need a steady supply of oxygen and nutrients and a way to dispose of waste such as carbon dioxide. Diffusion alone works only over very short distances, so larger animals use a circulatory system: a pump (the heart), a network of tubes (blood vessels) and a fluid (blood). The cardiovascular system also helps distribute hormones and immune cells and moves heat around the body.
Adults have roughly five liters of blood, although the amount depends on body size. Blood is mostly a watery fluid called plasma, in which red cells, white cells and platelets are suspended. Red cells carry oxygen bound to a protein called hemoglobin.
Two connected loops
Circulation is often described as a figure eight. In the pulmonary circuit, the right side of the heart sends blood to the lungs to pick up oxygen and release carbon dioxide. In the systemic circuit, the left side sends oxygen-rich blood out to the rest of the body. Blood returns to the heart and the loop begins again. The heart’s design is covered in its own article; here the focus is on the vessels.
Arteries: carrying blood away
Arteries carry blood away from the heart, usually under relatively high pressure. Because the heart pumps in pulses, the large arteries must withstand a surge with each beat. Their walls are thick and contain elastic tissue that stretches as blood is pushed in, then recoils, which helps smooth out the pulses into a steadier flow downstream.
Arteries branch into smaller ones and then into arterioles, narrow vessels whose walls contain smooth muscle. This muscle can contract or relax, changing the vessel’s diameter. Since resistance to flow rises steeply as a tube becomes narrower, arterioles are the main control point for how much blood reaches each region and for the overall resistance the heart pumps against. That resistance is a major part of blood pressure.
Capillaries: where exchange happens
Arterioles lead into capillaries, the smallest vessels, with walls generally one cell thick and widths barely enough for red cells to pass in single file. They are extremely numerous, forming dense networks through almost all tissues, and their combined surface area is vast.
Capillaries are where the circulation does its real work. Oxygen and nutrients pass out of the blood, down their concentration differences, into the surrounding tissue, while carbon dioxide and wastes pass in. Water and small dissolved substances also move across the walls, driven by a balance between the pressure pushing fluid out and the pull of proteins in the blood that retain it. Some fluid stays in the tissue, and is returned by the lymphatic system, a network of vessels that collects it and eventually empties it back into the blood.
Different tissues have different capillary types. Some have tight walls to limit what crosses, as in the brain, while others have pores to allow rapid exchange, as in the kidneys.
Veins: bringing blood back
From the capillaries, blood flows into small venules and then larger veins that lead back toward the heart. Pressure is much lower here, so veins have thinner walls and larger interiors, and at any moment they hold a large portion of the body’s blood.
Returning blood to the heart from the legs requires working against gravity when a person stands. Two mechanisms help. Many veins contain one-way valves that stop blood sliding backward. And as skeletal muscles contract during walking, they squeeze nearby veins, pushing blood forward in a “muscle pump.” Breathing movements also assist. Prolonged standing or sitting may let blood pool in the legs, one reason that regular movement is widely encouraged.
Controlling where blood goes
The body does not deliver equal blood to every organ all the time. Nerves and local chemical signals adjust vessel width to match demand. During exercise, vessels in working muscle widen and flow there increases. After a meal, flow to the digestive tract rises. When the body is hot, skin vessels widen to release heat. Hormones and the nervous system also fine-tune overall blood pressure.
Vessels can also adapt over longer periods, growing new branches in response to sustained demand, a process researchers study in wound healing and exercise training.
When vessels are affected
Blood vessels can change with age and with disease. In atherosclerosis, fatty deposits build up in artery walls, narrowing the passage and sometimes triggering clots; factors such as cholesterol levels are widely studied in that process. Persistently high pressure may damage vessel walls over time. In diabetes, small vessels are often affected. Weakened vein valves can contribute to varicose veins. The risk factors and their relative importance differ across individuals, and research continues. Staying physically active is generally associated with healthier vessels, and regular exercise is a standard recommendation, but individual advice should come from a clinician.
This article is a general science overview and not medical advice.