[2] Some tissues such as cartilage, epithelium, and the lens and cornea of the eye are not supplied with blood vessels and are termed avascular.

When blood vessels connect to form a region of diffuse vascular supply, it is called an anastomosis.

Veins can have valves that prevent the backflow of the blood that was being pumped against gravity by the surrounding muscles.

[9] Early estimates by Danish physiologist August Krogh suggested that the total length of capillaries in human muscles could reach approximately 100,000 kilometres (62,000 mi) (assuming a high muscle mass human body, like that of a bodybuilder).

[10] However, later studies suggest a more conservative figure of 9,000–19,000 kilometres (5,600–11,800 mi) taking into account updated capillary density and average muscle mass in adults.

These can include paracrine factors (e.g., prostaglandins), a number of hormones (e.g., vasopressin and angiotensin[26]) and neurotransmitters (e.g., epinephrine) from the nervous system.

The most prominent vasodilator is nitric oxide (termed endothelium-derived relaxing factor for this reason).

This is a result of the left and right sides of the heart working together to allow blood to flow continuously to the lungs and other parts of the body.

Oxygen-rich blood from the lungs enters through the pulmonary veins on the left side of the heart into the aorta and then reaches the rest of the body.

The capillaries are responsible for allowing the blood to receive oxygen through tiny air sacs in the lungs.

The greater amount of contact with the wall will increase the total resistance against the blood flow.

[31] Blood vessels play a huge role in virtually every medical condition.

Cancer, for example, cannot progress unless the tumor causes angiogenesis (formation of new blood vessels) to supply the malignant cells' metabolic demand.

[32] Atherosclerosis represents around 85% of all deaths from cardiovascular diseases due to the buildup of plaque.

[33] Coronary artery disease that often follows after atherosclerosis can cause heart attacks or cardiac arrest, resulting in 370,000 worldwide deaths in 2022.

These eddies create abnormal fluid velocity gradients which push blood elements, such as cholesterol or chylomicron bodies, to the endothelium.