Overview of the CV System

For information purposes only. Exercise at your own risk

The cardiovascular system serves a number of important functions in the body that generally fall into the categories of Delivery, Removal, transport, Maintenance and Prevention

The CV system consists of three major components – a fluid medium – the blood, a system of channels – the blood vessels, and a pump – the heart.

Blood typically consists of 55% plasma and 45% formed elements.

This plasma is 90% water, 7% plasma proteins and 3% other materials such as nutrients, enzymes, waste products and hormones. The plasma is a straw colour, due to bilirubin; a substance that accumulates due to the breakdown of haemoglobin. The plasma proteins that make up the 7%, include Albumin (a carrier protein that binds and transports nutrients, enzymes and hormones. It also includes another group of substances called ‘globulines’, which include antibodies; and clotting proteins such as fibrinogen.

Formed elements
More than 99% of the formed elements are red blood cells; the rest is white blood cells and platelets.

Less than 1% is white blood cells; the cells of the immune system, which are produced in thebone marrow. These can be separated into two categories – neutrophils (60% of all white blood cells). These engulf and destroy foreign proteins. Lymphocytes make up 20 – 30% of all white blood cells. These protect the body by producing antibodies.

Platelets are cell fragments that bind to collagen and form a ‘platelet plug’, which is the start of a blood clot. This then interacts with fibrinogen to form a mesh, that eventually turns into a scab.

Red blood cells have a 120 day life in circulation they are bi-concave disks that are produced in the bone marrow. Their production is controlled by erythropoietin (EPO), and is stimulated by an decrease in oxygen tension of the blood. RBC’s contain haemoglobin (Hb) that binds to oxygen and carbon dioxide. Per 100ml of blood, men typically have 16g of Hb, whereas women have 14g.

Haematocrit refers to:
‘the percentage of total blood volume consisting of red blood cells, found by centrifuging the whole blood and measuring the volume of red cells in a given volume of blood. Decreased haematocrit levels are associated with anaemia, hyperthyroidism, cirrhosis, bone marrow failure and numerous other pathogenic conditions’
www.haemochromatosis-ir.com/glossary2.html

males typically have a Haematocrit value of 40-50%, and females have 35-45% blood doping increases the haematocrit but can result in increased blood viscosity, leading to increased resistance to blood flow, and therefore decreased blood flow. This results in reduced performance and possible death.

http://dpi.radiology.uiowa.edu/nlm/app/pbf/movies.html The vascular system consists of the pulmonary (lungs) and systemic systems. Arteries are large, muscular and elastic, and carry blood away from the heart. Veins carry blood to the heart. Veins contain 60% of total blood volume at rest, the majority of which is in the legs.

Arterioles determine blood flow to individual organs. They are autoregulated to a certain extent, in that they are under intrinsic control. Theses blood vessels contain smooth muscle which dilates (vasodilation) in response to local stimuli, such as decreased oxygen levels, increased carbon dioxide levels, or increased potassium levels. Extrinsic control in the sympathetic (spelling) nervous system supplies ‘vasomotor tone. This along with noradrenalin release causes vasoconstriction, whereas a decrease in vasomotor tone can lead to vasodilation.

Capillaries are the smallest blood vessels. Their walls are one cell thick, which allows for the interchange of gases and nutrients. Most cells of the body are less than 0.1mm away from a capillary, which again allows interchange of gases and other substances, due to the limited diffusion distance.

Blood pressure
Blood pressure is greatest in the arteries, then arterioles, then capillaries, then venules then veins. This determines the direction of blood flow. To know the rate of flow, we also need to know the resistance. This is the measure of friction that impedes flow, and it is determined by fluid viscosity, vessel length and vessel internal radius. The resistance is proportional to the radius to the power of 4. so if the vessel radius was to half, the resistance would be 16 times greater.

Mean arterial blood pressure is the diving pressure for the rest of circulation, and is the pressure exerted upon the vessel walls. Systolic pressure (the top bit of a blood pressure reading) is the pressure during ventricular systole (contraction); diastolic pressure is the pressure during ventricular diastole (relaxation).

Blood returning to the heart
This is called venous return, and is 50% water. In order to overcome ‘hydrostatic pressure’, and prevent blood from pooling in the legs, venous valves prevent backflow of blood, whilst breathing changes pressure in the thoracic and abdominal cavities that aids return. In addition contracted muscles, squeeze blood upwards towards the heart (the blood cant go back down due to the venous valves, and so has to go up when the contracted muscles exert pressure on the blood).

In summary – the cardiovascular system, transports gases, nutrients, wastes, hormones and enzymes. It is also involved in controlling body temperature, pH and the prevention of infection. It has 3 major systems, the blood, blood vessels and the heart. Blood pressure = flow x resistance