Body Fluids and Circulation

Cells deep inside the body cannot get food and oxygen directly from outside, so the body uses a fluid transport system. The main body fluid is blood, a special connective tissue made of a liquid part (plasma) and cells suspended in it.

• Plasma — the pale yellow liquid (about 55% of blood); it is mostly water and carries dissolved food, salts, hormones, wastes and proteins.
• Red blood cells (RBCs / erythrocytes) — contain haemoglobin and carry oxygen; they have no nucleus when mature and are made in the bone marrow.
• White blood cells (WBCs / leucocytes) — fight germs and defend the body (immunity).
• Platelets (thrombocytes) — help the blood to clot and stop bleeding from a wound.

Human blood is of four blood groups — A, B, AB and O — based on antigens on the RBCs. Group O is the "universal donor" and AB the "universal recipient." Another important grouping is the Rh factor: people are Rh-positive or Rh-negative. Matching blood groups is essential before a transfusion.

Lymph is another body fluid — a colourless fluid that leaks out of blood capillaries into the tissues, bathes the cells, and is collected by lymph vessels and returned to the blood. It carries nutrients and fights infection but has no RBCs.

The heart is a muscular organ, about the size of a fist, that pumps blood throughout the body. It lies in the chest between the two lungs and is made of a special muscle (cardiac muscle) that never tires. The human heart has four chambers: two upper atria (auricles) that receive blood, and two lower ventricles that pump blood out. The thick-walled left ventricle pumps blood to the whole body. Valves between the chambers and at the exits keep the blood flowing in one direction only and prevent back-flow.

Humans have a double circulation — blood passes through the heart twice in one complete round:

• Pulmonary circulation — the heart pumps de-oxygenated blood to the lungs, where it picks up oxygen, and returns it to the heart.
• Systemic circulation — the heart then pumps the oxygen-rich blood to the rest of the body, and the de-oxygenated blood returns to the heart.

This keeps oxygen-rich and oxygen-poor blood completely separate, which is very efficient. The blood vessels are: arteries (carry blood away from the heart, usually oxygenated, thick-walled), veins (carry blood back to the heart, have valves) and capillaries (tiny, thin-walled vessels where exchange of materials with cells happens).

One full sequence of the heart relaxing and contracting is the cardiac cycle; the contraction is called systole and the relaxation diastole. The familiar "lub-dub" heart sounds are made by the valves closing. A healthy adult heart beats about 70–72 times per minute.

The heart and blood vessels can develop problems, especially with age and an unhealthy lifestyle. Heart disease is one of the leading causes of death worldwide, so understanding and preventing it matters. Some important circulatory disorders:

• High blood pressure (hypertension) — the blood pushes too hard against the artery walls. It often has no symptoms but damages the heart, brain and kidneys over time. Normal blood pressure is about 120/80 mm Hg.
• Coronary artery disease (atherosclerosis) — the arteries supplying the heart muscle get narrowed by deposits of fat and cholesterol, reducing blood flow to the heart.
• Heart attack (myocardial infarction) — a part of the heart muscle is damaged because its blood supply is suddenly blocked.
• Angina — chest pain that occurs when the heart muscle does not get enough oxygen.

Common risk factors include smoking, a fatty diet, obesity, lack of exercise, stress, diabetes and a family history of heart disease. The good news is that much heart disease is preventable. To keep the heart healthy: eat a balanced diet low in saturated fat and salt, exercise regularly, maintain a healthy weight, avoid smoking and excess alcohol, manage stress, and get regular health check-ups (including blood pressure). These simple habits, started young, greatly lower the lifelong risk of heart disease.