Structure of Blood Vessels

ArteriesCapillariesVeins

Muscular Walls

ArteriesThese vessels receive blood at high pressure from

the heartThick Muscular walls help them to withstand that

pressureThe 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).They maintain the blood pressure to all parts of the

body

ArteriolesArterioles are tiny branches of arteries that

lead to capillaries. These are also under the control of the sympathetic nervous system, and constrict and dilate, to regulate blood flow.

Transport blood from arteries to capillaries;Arterioles are the main regulators of blood

flow and pressure.

Capillaries Walls are only one cell thick which permits exchanges of material

between the contents of the capillary and the surrounding tissue. Permeable to small particles eg. Molecules of gas, nutrients and

waste. Capillaries branch repeatedly to form a network within tissues. 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.

Function is to supply tissues with substances in the blood & substances 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.

VeinsReturn blood at low pressure to the heart from the tissuesTheir wide lumens helps to reduce the resistance of blood

flowReduced pressure means that they have only thin wallsThe walls (outer structure) of veins consist of three layers

of tissues that are thinner and less elastic than the corresponding layers of arteries.

Veins contain valves that aid the return of blood to the heart by preventing blood from flowing in the reverse direction. One way flow is maintained.

Transport blood towards the heart;Transport deoxygenated blood only (except in the case of

the pulmonary vein).

VenulesVenules 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

Comparison between Arteries and VeinsArteries 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).

Veins 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.

Major Organs and Associated Blood Vessels

Formation of Tissue fluid

Tissue Fluid

Formation of Tissue Fluid cont...

Tissue Fluid (Interstitial Fluid)

Tissue FluidBlood flow slows down in these vesselsHigh Blood Pressure at the arterial end forces

the plasma out through the capillary walls and the dissolved substances in it

The plasma that baths the cells is know as tissue fluid

No cell is more than 100 micrometers from a capillary

Low Blood Pressure at the venous end allows some of the tissue fluid to re-enter the veins

Tissue Fluid cont...The blood is more concentrated at the venous

endThe rest of the tissue fluid enters the

lymphatic capillaries which make up part of the lymphatic system

The lymphatic system eventually rejoins the circulatory system and its contents are returned

Table summarizing blood VesselsArteries Capillaries Veins

Blood carried

Pressure

Blood Flow

Gases

Valves

Position of Vessels