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Everything you need to know about your blood.
Introduction
The average adults has about 6 litres of blood, which is roughly 7 to 8 percent of total body weight. It contains about 55% plasma and 45% solids.
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| Blood after settling |
Blood has many function the main ones are listed below:
- carries oxygen from the lungs to all the other tissues in the body
- carries waste products, predominantly carbon dioxide, back to the lungs where they are released into the air. Also carries urea and uric acid to the kidneys
- Transportation of food such as proteins, fats, and carbohydrates from the digestive system to the tissues
- Transportation of hormones, that regulate certain body functions.
- Assists with control of infection
- Blood is vital in maintaining a stable body temperature by transferring heat externally by dilating blood vessels in the skin or restricting blood flow to extremities thus maintaining the core temperature.
Blood also carries the means for stopping itself from leaking out of the body after an injury. The blood does this by carrying special cells (platelets) and proteins, known as the coagulation system, that start to form clots within a matter of seconds after injury.
Blood Components
It is composed of the following components:
- Red cells (erythrocytes) are packed with Haemoglobin (a protein) blue in colour but combines well with oxygen to form Oxyhaemoglobin which gives blood its red colour. The membrane, or outer layer, of the red
 blood cell is flexible, like a soap bubble and is able to bend in many directions without breaking. This is important because the red blood cells must be able to squeeze through the tiniest blood vessels, the capillaries, to deliver oxygen wherever it is needed. The capillaries are so narrow that the red blood cells, normally shaped like a disk with a concave top and bottom (see micrograph), must bend and twist to manoeuvres single file through them.
- White Cells (leukocytes), which fight infection and allergies.
- Platelets (thrombocytes), which form clots to prevent bleeding and are the smallest of the blood cells.
- Plasma - consists of water, salts, sugars and special proteins. The kidneys carefully maintain the salt concentration in plasma because small changes in its concentration will cause cells in the body to function improperly. The pH of plasma, the common measurement of the plasma's acidity, is critical and carefully controlled by the kidneys. It lies between within the neutral range of 7.35 to 7.45. A PH outside of this range will cause death within a few hours.
Erythrocytes release O2 due to lower partial pressure at tissues, and pick up CO2 for same reason. An enzyme in the red blood cell combines with CO2 to form bicarbonate and this accounts for about 75% of the CO2 carried in the blood. A further 5% is carried in the plasma and the remaining 20% in haemoglobin.
Most of the O2 is carried in the blood by the red blood cells (erythrocytes) however a little O2 is carried in blood physically dissolved in solution (only 0.3ml for every 100ml litre blood). The O2 binds with haemoglobin in the red cells to form Oxyhaemoglobin.
A reduction in oxygenation blood is called Hypoxia. An increase in CO2 is called Hypercapnia and a reduction in CO2 is called Hypocapnia.
Blood Circulation
The circulatory system consists of the heart and blood vessels. Together, these provide a continuous flow of blood to your body, supplying the tissues with oxygen and nutrients. Arteries carry blood away from the heart; veins return blood to the heart.
Blood pressure
Blood pressure needs to remain constant to avoid rupture and to maintain a good supply to tissues. It relies on rate and force of contractions of heart and restrictions of arteries to control blood flow. These are under the nervous system control which is under pressure sensitive organs (called pressure receptors or baroreceptors).
Blood pressure is measured by two readings of pressure: Systolic and Diastolic. Systolic is the pressure of the blood as the heart contracts and Diastolic is the measurement when the heart is not contracting i.e., between beats. Normal resting blood pressure 120-140mm Hg (Systolic) and 70-80mm Hg (Diastolic), usually written as 120/70.
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