Blood Vessels

Structure

Functions

Arteries

The walls (outer structure) of arteries contain smooth muscle fibre that contract and relax under the instructions of the sympathetic nervous system.

• Transport blood away from the heart
• Transport oxygenated blood only (except in the case of the pulmonary artery).

Arterioles

Arterioles are tiny branches of arteries that lead to capillaries. These are also under the control of the sympathetic nervous system, and constrict and dialate, to regulate blood flow.

• Transport blood from arteries to capillaries
• Arterioles are the main regulators of blood flow and pressure.

Capillaries

Capillaries are tiny (extremely narrow) blood vessels, of approximately 5-20 micro-metres
(one micro-metre = 0.000001metre) diameter.
There are networks of capillaries in most of the organs and tissues of the body. These capillaries are supplied with blood by arterioles and drained by venules. Capillary walls are only one cell thick (see diagram), which permits exchanges of material between the contents of the capillary and the surrounding tissue.

• Function is to supply the tissues of the body with the components of blood, and (carried by the blood), and also to remove waste from the surrounding cells ... as opposed to simply moving the blood around the body (in the case of other blood vessels)
• Exchange of oxygen, carbon dioxide, water, salts, etc., between the blood and the surrounding body tissues.

Venules

Venules are minute vessels that drain blood from capillaries and into veins. Many venules unite to form a vein.

• Drains blood from capillaries into veins, for return to the heart.

Veins

The walls (outer structure) of veins consist of three layers of tissues that are thinner and less elastic than the corresponding layers of arteries.
Veins include valves that aid the return of blood to the heart by preventing blood from flowing in the reverse direction.

• Transport blood towards the heart.
• Transport deoxygenated blood only (except in the case of the pulmonary vein).

Arteries

Veins

• Transport blood away from the heart
• Carry Oxygenated Blood
  (except in the case of the Pulmonary Artery)
• Have relatively narrow lumens (see diagram above)
• Have relatively more muscle / elastic tissue
• Transports blood under higher pressure (than veins)
• Do not have valves (except for the semi-lunar valves of the pulmonary artery and the aorta).

• Transport blood towards the heart
• Carry De-oxygenated Blood
  (except in the case of the pulmonary vein)
• Have relatively wide lumens (see diagram above)
• Have relatively less muscle / elastic tissue
• Transports blood under lower pressure (than arteries)
• Have valves throughout the main veins of the body. These are to prevent blood flowing in the wrong direction, as this could (in theory) return waste materials to the tissues.