Why circulation?
Cells need a steady supply of nutrients and oxygen and must get rid of wastes. Simple organisms such as sponges use water from their surroundings. Larger animals use special fluids, blood and lymph, moved around the body by a circulatory system.
Blood
Blood is a special connective tissue consisting of a fluid plasma and formed elements.
Plasma (about 55% of blood) is a straw-coloured fluid, about 90–92% water and 6–8% proteins:
fibrinogen is needed for clotting,
globulins are involved in defence,
albumins help maintain osmotic balance.
Plasma also carries glucose, amino acids, lipids, minerals and hormones. Plasma without clotting factors is serum.
Formed elements (about 45%):
Cell | Number (per mm³) | Features | Function |
|---|---|---|---|
Red blood cells (erythrocytes) | 5–5.5 million | biconcave, no nucleus in most mammals; made in red bone marrow; live about 120 days; destroyed in the spleen ("graveyard of RBCs") | carry O₂ using haemoglobin (12–16 g per 100 mL blood) |
White blood cells (leucocytes) | 6000–8000 | nucleated, colourless, short-lived | defence |
Platelets (thrombocytes) | 1.5–3.5 lakh | cell fragments from megakaryocytes | clotting |
Types of WBCs:
Type | Share of WBCs | Role |
|---|---|---|
Neutrophils | 60–65% (most abundant) | phagocytes that destroy foreign organisms |
Lymphocytes | 20–25% | B and T cells, responsible for immune responses |
Monocytes | 6–8% | phagocytes |
Eosinophils | 2–3% | resist infections; involved in allergic reactions |
Basophils | 0.5–1% (least abundant) | release histamine, serotonin and heparin; involved in inflammation |
Blood groups
ABO grouping depends on the antigens (A and B) on red blood cells and the natural antibodies in plasma.
Blood group | Antigen on RBCs | Antibodies in plasma | Can receive from |
|---|---|---|---|
A | A | anti-B | A, O |
B | B | anti-A | B, O |
AB | A and B | none | A, B, AB, O (universal recipient) |
O | none | anti-A and anti-B | O only (universal donor) |
Rh grouping: about 80% of people have the Rh antigen (Rh⁺). An Rh⁻ person exposed to Rh⁺ blood makes antibodies against it.
A special case is erythroblastosis fetalis. If an Rh⁻ mother carries an Rh⁺ fetus, a small amount of fetal blood can enter her circulation at the first delivery, and she makes anti-Rh antibodies. In a later pregnancy with an Rh⁺ fetus, these antibodies can cross the placenta and destroy the fetus's red blood cells, causing severe anaemia and jaundice. It is prevented by giving the mother anti-Rh antibodies immediately after the first delivery.
Coagulation
When a blood vessel is injured, a clot forms to stop bleeding.
Damaged tissue and platelets release factors that activate the enzyme thrombokinase.
Thrombokinase converts inactive prothrombin into thrombin.
Thrombin converts soluble fibrinogen into insoluble fibrin threads.
The fibrin network traps blood cells, forming a clot.
Calcium ions play a very important role in clotting.
Lymph
As blood passes through capillaries, some water and small solutes leak out into the spaces between cells. This tissue fluid (interstitial fluid) exchanges nutrients and gases with the cells. It is collected by lymphatic vessels and drained into major veins; the fluid in this system is lymph.
Lymph contains specialised lymphocytes for immunity. Lymph vessels in the intestinal villi, called lacteals, absorb fats.
Circulatory pathways
Open circulation (arthropods and molluscs): blood is pumped into open spaces (sinuses).
Closed circulation (annelids and chordates): blood always flows in vessels, allowing precise control.
The heart has evolved along with vertebrates:
Animal | Heart chambers |
|---|---|
Fish | 2 (single circulation) |
Amphibians and most reptiles | 3 (two atria, one ventricle; incomplete double circulation) |
Crocodiles, birds, mammals | 4 (complete double circulation) |
The human heart
The heart is a muscular organ about the size of a clenched fist, lying between the lungs, slightly tilted to the left. It is protected by a double-walled membrane, the pericardium.
It has four chambers: two upper atria and two lower, thicker-walled ventricles.
The tricuspid valve guards the opening between the right atrium and right ventricle.
The bicuspid (mitral) valve guards the opening between the left atrium and left ventricle.
Semilunar valves guard the openings from the ventricles to the pulmonary artery and the aorta.
The valves let blood flow only one way.
Nodal tissue: the sinoatrial (SA) node in the right atrium wall generates the electrical impulses that start each heartbeat. It is the pacemaker. The impulse spreads to the atrioventricular (AV) node, then through the bundle of His and Purkinje fibres into the ventricles. The normal rate is 70–75 beats per minute.
The cardiac cycle
One cardiac cycle (one heartbeat) takes about 0.8 seconds.
Joint diastole: all four chambers are relaxed. Blood flows from the veins into the atria and on into the ventricles through the open tricuspid and bicuspid valves.
Atrial systole: the SA node fires; the atria contract and push about 30% more blood into the ventricles.
Ventricular systole: the ventricles contract. The tricuspid and bicuspid valves close, then the semilunar valves open and blood is pumped into the pulmonary artery and aorta.
Ventricular diastole: the ventricles relax, the semilunar valves close, and the cycle starts again.
Each ventricle pumps about 70 mL per beat (the stroke volume). Cardiac output = stroke volume × heart rate ≈ 70 mL × 72 ≈ 5 litres per minute.
Heart sounds: the first sound, "lub", comes from the closure of the tricuspid and bicuspid valves. The second, "dub", comes from the closure of the semilunar valves.
The ECG
An electrocardiograph records the electrical activity of the heart as an electrocardiogram (ECG).
P wave: depolarisation of the atria, leading to atrial contraction.
QRS complex: depolarisation of the ventricles, starting ventricular contraction.
T wave: the return of the ventricles to normal (repolarisation); the end of the T wave marks the end of systole.
Counting QRS complexes over a period gives the heart rate. Deviations from the normal shape point to heart problems.
Double circulation
Pulmonary circulation: right ventricle → pulmonary artery → lungs (oxygenated) → pulmonary veins → left atrium.
Systemic circulation: left ventricle → aorta → body tissues → veins → venae cavae → right atrium.
Blood passes through the heart twice in one complete circuit. A special hepatic portal system carries blood from the intestine to the liver before it returns to the heart.
Regulation of the heart
The heart is myogenic: its beat starts in its own muscle (the SA node). The rate can be adjusted by a centre in the medulla oblongata: sympathetic nerves and the hormone adrenaline speed it up, while parasympathetic nerves slow it down.
Disorders
Hypertension: blood pressure persistently above 140/90 mm Hg (normal is 120/80). It can damage the heart, brain and kidneys.
Coronary artery disease (atherosclerosis): deposits of calcium, fat and cholesterol narrow the arteries supplying the heart muscle.
Angina: chest pain when not enough oxygen reaches the heart muscle.
Heart failure: the heart can't pump enough blood to meet the body's needs. It is different from cardiac arrest (the heart stops) and heart attack (heart muscle is damaged by a sudden lack of blood supply).
Summary
Blood is plasma plus RBCs, WBCs and platelets; neutrophils are the most common WBCs.
ABO groups depend on antigens and antibodies; O is the universal donor, AB the universal recipient.
Clotting: thrombokinase → thrombin → fibrin, with calcium ions.
The SA node is the pacemaker; a cardiac cycle lasts about 0.8 s; cardiac output is about 5 L/min.
ECG: P wave (atria), QRS (ventricles), T wave (recovery).