Basic principles of blood vessels

The circulatory system consists of the heart and blood vessels. Blood vessels transport blood around the body and help maintain blood pressure. The main types of vessels are as follows:

1) Arteries and arterioles

2) Veins and venules

3) Capillaries

Structure of blood vessels

Vessels can have 3 distinct layers: tunica intima, tunica media, tunica adventitia (tunica externa).

All vessels, except capillaries, are made up of these 3 layers, but differ in the size and components of these.

Tunica intima: Innermost layer and consists of a layer of endothelial cells which are in direct contact with the blood. Endothelial cells secrete many substances which affect the vascular permeability and vasoconstriction.

Tunica media: The middle layer consists of smooth muscle cells with an abundance of elastin and collagen. Elastin enables the blood vessel to stretch and recoil while the smooth muscles, innervated by the sympathetic nerves, control the diameter of vessels by contracting and relaxing.

Tunica externa (or adventitia): Composed of connective tissues (mainly collagen) that serve to protect the vessel. The outer wall is supplied by a network of small blood vessels called the vasa vasorum. This small vessel can be found in large arteries and veins.


Blood flow

Blood flow

There are two main types of flow which occur in tubes.

i) Laminar flow

ii) Turbulent flow

This process of haemodynamic flow ensures the transportation of oxygen, metabolic waste, nutrients, hormones, drugs etc. Although there are many theoretical laws that regulate the flow of fluid, it is important to remember that they do not strictly apply to blood flow as:

1) Blood is a non-Newtonian fluid, i.e. the viscosity of this suspension changes when the gradient in flow speed changes. It is independent of sheer stress and sheer rate.

2) The flow of blood is pulsatile and non-continuous due to pumping of the heart

Nonetheless, it is useful to draw some conclusions from these laws that govern the fluid mechanics. In large vessels it behaves like a Newtonian fluid, which means the viscosity is constant and it assumes the Hagen-Pouiseille equation of laminar flow.



As Q=(change in pressure/resistance), it can be seen that R (resitance of flow) is proportional to viscosity and length of cylinder, and inversely to the radius based on the above relationship.

In other words, to control the flow of blood, changing the radius is a key mechanism (doubling the radius reduces the resistance of flow by 16 fold!) Of note, these equations only applied to laminar flow, which is predominantly the mechanism of flow in the blood vessel.

The flow to tissues is usually supplied by a network of vessels as opposed to a single tube.

Heart> Arteries > Arterioles> Capilaries (where the vessel forms a parallel branch) > Venule> Vein> Heart .

The total resistance across the parallel tube is as shown above.

So, as the total number of vessels increases, resistance decreases. Another major implication of these relationships is that the total resistance is not affected by small changes of resistance in a vessel. This allows maintenance of blood pressure whilst controlling blood flow to certain areas of the body (as in haemorrhage/shock where blood is redistributed).





i) Act as the conduits for the distribution of blood at high pressure from the heart to tissues.

ii) Have thick elastic layer to allow stretching. Blood pressure is maintained by elastic recoil.

iii) Classified as elastic artery in large vessel (10-20mm e.g. aorta) or muscular artery in medium/small sized vessels ( brachial, popliteal artery).

iv) The diameter and thus the blood flow are controlled by smooth muscles in tunica media (vascular myocyte).

v) The strength of vessel is provided by collagen connective tissues in the outer layer. (also contained vaso vasorum in the outer layer of large vessel which supply the vessel).



i) Drain blood back to the heart from the capillary bed.

ii) Larger lumen compared to arteries, act as a reservoir of blood (70%)

iii) Low pressure blood, venous return is assisted by skeletal muscle contraction.

iv) Valves prevent backflow of blood. Weakened valves causes varicose veins.

v) The vena cava returns the blood to the heart. At the right atrium, the blood pressure is negligible (almost zero).

vi) Venous thrombi form in veins as a result of Virchow's triad: Blood stasis, hypercoagulability, and endothelial cell injury.



Most tissues receive nutrients from the exchange vessels which comprise of the arterioles, capillaries and post-capillary venules. The vast majority of meterial transport takes place across the capillary.

Arteriole: Control the distribution of blood to tissues by vasoconstriction and act as a pressure reservoir (maintenance of pressure).

Venule: return blood from capillary beds to vein.



There are three main types: Continuous, fenestrated, and discontinuous/ sinusoidal.


Continuous: Most common. These have tight junctions and only allow small molecules such as ions and water to diffuse across the membrane. Found in skin, muscles, fat.

Fenestrated: Contain pores and have greater permeability. Found in sites of substantial filtration/absorption e.g. intestinal musoca and kidney tubules.

Discontinuous: Filled with gaps between the endothelial cells. Primarily located at the liver, and haematopoietic organs such as spleen, bone marrow. Also found in lymph nodes and the adrenal glands. It enables the passage of white and red blood cells, serum proteins e.g. albumin.


Properties of capillaries:

i) The pre-capillary sphincter controls the blood flow to each capillary. Originates from the metarteriole (vessel that link arteriole and post-capillary vein). The sphincter also regulates the blood pressure at the capillary and damage to it can result in high capillary pressure and the formation of oedema in the interstitial space.

ii) Arteriovenous anastomoses (AV shunt) allows the bypass of blood through the capillary beds allowing collateral circulation of blood.


Control of blood pressure

Short term: Baroreceptors located at the carotid sinus and arch of aorta. They respond to the stretch of wall due to increased pressure. Parasympathetic system stimulation is increased and the sympathetic respond is decreased, resulting in vasodilation and a drop in blood pressure.

Long term: Kidney via renin-angiotensin-aldosterone pathway. Overall, sodium and water reabsorption is increased in respond to low blood pressure, increasing the circulating volume in the body.

Central chemoreceptors responds to high level of blood carbon dioxide while peripheral chemoreceptors respond to low level of oxygen.



i) Maintain vascular tone, permeability, inflammatory responses.

ii) The endothelium regulates hormonal control.

Relaxing factors produced by endothelium include prostacyclin and nitrous oxide.

The release of releasing factor is triggered by Bradykinin, acetylcholine, adrenalin ,and prostaglandin.

Contracting factors produced by the endothelium include prostaglandin H2, thromboxane A2, and endothelin-1.

Triggered by angiotensin 2, thrombin, serotonin and noradrenaline in the blood.



Fluid Volume

Fluid Volume

60% body weight is made up of water.

Intracellular volume: 2/3 of body water

Extracellular volume: 1/3 of body water


Extracellular volume is sub-divided into: plasma, interstitial fluid, and transcellular fluid (CSF,peritoneal and synovial fluid)


 In a 70kg man, there is approximately 5L of blood in which 60% is made up of plasma and 40% as cells, also known as packed cell volume (PCV). 


Fluid distribution

Fluid Volume










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