Blood: plasma and formed elements
Blood is a special connective tissue: a fluid matrix (plasma) with cells floating in it (formed elements).
Plasma (about 55%)
- A straw-coloured, sticky fluid; 90 to 92% water and 6 to 8% proteins.
- Fibrinogen (for clotting), globulins (for defence) and albumins (keep water balance) are the main proteins.
- It also carries glucose, amino acids, salts like Na+, Ca2+, Cl−, and hormones.
- Plasma without clotting factors is called serum.
Formed elements (about 45%)
- RBCs (erythrocytes): 5 to 5.5 million per mm3. Made in red bone marrow. Mammalian RBCs have no nucleus and are biconcave. They carry haemoglobin (12 to 16 g per 100 mL of blood). They live about 120 days and are broken down in the spleen, the 'graveyard of RBCs'.
- WBCs (leucocytes): 6000 to 8000 per mm3, have a nucleus, fight germs. Granulocytes: neutrophils (most, 60 to 65%, eat germs), eosinophils (2 to 3%, allergy), basophils (least, 0.5 to 1%, release histamine, serotonin, heparin). Agranulocytes: lymphocytes (20 to 25%, B and T cells, immunity) and monocytes (6 to 8%, eat germs).
- Platelets (thrombocytes): cell pieces from megakaryocytes, 1.5 to 3.5 lakh per mm3. They release clotting factors. Too few platelets cause heavy bleeding.
Blood groups: ABO and Rh
Red cells carry surface molecules called antigens. Plasma may carry antibodies that attack the antigens it does not have.
| Group | Antigen on RBC | Antibody 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 |
| O | none | anti-A and anti-B | O |
Group O is the universal donor; group AB is the universal recipient.
Rh group
About 80% of people carry the Rh antigen (Rh+). Rh− people make anti-Rh antibodies only after they are exposed to Rh+ blood. If an Rh− mother carries an Rh+ baby, her first pregnancy is usually safe, but she may make antibodies. In the next Rh+ pregnancy, these antibodies can cross the placenta and destroy the baby's RBCs: erythroblastosis foetalis. An anti-Rh injection given to the mother right after the first delivery prevents it.
Coagulation (clotting) of blood
- An injury makes platelets and damaged tissue release factors.
- These form the enzyme complex thrombokinase.
- Thrombokinase turns inactive prothrombin into active thrombin.
- Thrombin turns soluble fibrinogen into insoluble fibrin threads.
- Fibrin threads form a net that traps blood cells: this is the clot.
Calcium ions are needed at many steps. The dark reddish lump you see on a scraped knee is mostly this fibrin net with trapped cells.
Lymph (tissue fluid)
As blood moves through capillaries, some water and small molecules leak out into the spaces between cells. Large proteins and most cells stay in the blood. This leaked fluid is tissue fluid (interstitial fluid). It has the same salts as plasma.
Tissue fluid gives food and gases to cells and collects waste. It is then drained by thin lymphatic vessels, where it is called lymph. Lymph returns to the big veins.
- Lymph is colourless and has special lymphocytes for immunity.
- Lacteals in the intestinal villi absorb fats into the lymph.
- Lymph nodes filter germs.
Circulatory pathways and the human heart
Circulation can be open (blood leaves vessels and fills open spaces, as in arthropods and molluscs) or closed (blood always stays in vessels, as in annelids and chordates). Closed is more precise.
Fish have a 2-chambered heart, amphibians and most reptiles 3-chambered (crocodiles 4), birds and mammals 4-chambered.
The human heart
- Made of cardiac muscle, sits between the lungs, slightly to the left, the size of a closed fist. It lies in a double-walled sac, the pericardium, with fluid inside.
- Four chambers: two upper atria (receive blood) and two lower, thicker ventricles (pump blood). A wall called the septum separates left and right.
- Valves allow one-way flow: tricuspid (right atrium → right ventricle), bicuspid or mitral (left atrium → left ventricle), and semilunar valves at the openings of the pulmonary artery and aorta.
- Nodal tissue makes the heart auto-excitable: the SA node (sino-atrial, upper right atrium) is the pacemaker and fires 70 to 75 times a minute. The AV node passes the signal to the bundle of His and the Purkinje fibres in the ventricle walls.
Blood vessels
Arteries carry blood away from the heart (thick walls, high pressure). Veins bring blood back (thinner walls, valves). Capillaries are one cell thick and let exchange happen.
Cardiac cycle and cardiac output
One complete heartbeat, with contraction (systole) and relaxation (diastole) of atria and ventricles, is one cardiac cycle. At 72 beats a minute it lasts 60 ÷ 72 ≈ 0.8 s.
- Joint diastole: all four chambers relaxed. Tricuspid and bicuspid valves open; blood flows from veins through atria into ventricles. Semilunar valves are closed.
- Atrial systole: SA node fires; atria contract and push about 30% more blood into the ventricles.
- Ventricular systole: signal travels AV node → bundle of His → Purkinje fibres. Ventricles contract, AV valves shut (the first sound, lub), pressure rises, semilunar valves open and blood goes into the pulmonary artery and aorta.
- Ventricular diastole: ventricles relax, semilunar valves shut (the second sound, dub). AV valves open again and the cycle repeats.
Stroke volume: blood pumped out by each ventricle per beat, about 70 mL. Cardiac output = stroke volume × heart rate ≈ 70 × 72 ≈ 5 L per minute. Athletes can raise it a lot.
Electrocardiograph (ECG)
An ECG is a graph of the electrical activity of the heart during a cardiac cycle. Leads are placed on the wrists, ankle and chest.
- P wave: electrical excitation (depolarisation) of the atria → atrial contraction.
- QRS complex: depolarisation of the ventricles → start of ventricular contraction.
- T wave: return of the ventricles to rest (repolarisation) → end of systole.
Counting QRS complexes in a set time gives the heart rate. Any change in the shape or spacing of waves helps doctors find heart problems.
Double circulation
- Pulmonary circulation: right ventricle → pulmonary artery → lungs (blood gets O2) → pulmonary veins → left atrium.
- Systemic circulation: left ventricle → aorta → arteries → capillaries in tissues → veins → vena cavae → right atrium.
Blood goes through the heart twice in one full round, so it is called double circulation. Oxygen-rich and oxygen-poor blood never mix, so tissues get the most oxygen.
Hepatic portal system: the hepatic portal vein carries blood from the intestine to the liver before it goes to the body. Coronary circulation is a special set of vessels that feed the heart muscle itself.
Regulation of cardiac activity
- Normal beating is self-controlled by the nodal tissue, so the heart is called myogenic.
- A special centre in the medulla oblongata can adjust it through the autonomic nervous system.
- Sympathetic nerves raise heart rate, strength of beat and cardiac output.
- Parasympathetic nerves (vagus) lower heart rate, speed of conduction and cardiac output.
- Hormones from the adrenal medulla (adrenaline) also raise cardiac output.
Disorders of the circulatory system
- Hypertension (high blood pressure): normal BP is 120/80 mm Hg (systolic/diastolic). Readings of 140/90 or higher again and again mean hypertension. It damages the heart, brain and kidneys.
- Coronary artery disease (CAD) / atherosclerosis: fat, calcium, cholesterol and fibres build up in the vessels that feed the heart muscle, so the tube gets narrow.
- Angina (angina pectoris): sharp chest pain when the heart muscle does not get enough oxygen. It is more common in middle-aged and older people.
- Heart failure: the heart cannot pump enough blood for the body's needs. Lungs may fill with fluid (congestion). It is not the same as a cardiac arrest (heart stops beating) or a heart attack (heart muscle suddenly damaged by lack of blood).
Key formulas and definitions
- Cardiac output = stroke volume × heart rate
- Duration of cardiac cycle = 60 ÷ heart rate (s)
- Normal BP = 120/80 mm Hg (systolic/diastolic)
- Clotting: thrombokinase → prothrombin → thrombin → fibrinogen → fibrin
- Plasma = serum + clotting factors
Worked examples
1. Stroke volume = 70 mL and heart rate = 72 per minute. Find cardiac output.
CO = 70 × 72 = 5040 mL/min ≈ 5.04 L/min.
2. A heart beats 75 times a minute. How long is one cardiac cycle?
Time = 60 ÷ 75 = 0.8 s.
3. An athlete's cardiac output is 25 L/min at a heart rate of 180. Find stroke volume.
SV = CO ÷ HR = 25000 mL ÷ 180 ≈ 139 mL.
4. An ECG shows 18 QRS complexes in 15 seconds. Find heart rate.
Beats per minute = 18 × (60 ÷ 15) = 18 × 4 = 72 beats/min.
5. A person has 5 L of blood. With cardiac output 5 L/min, how long does all the blood take to pass through the left ventricle once?
Time = 5 L ÷ 5 L/min = 1 minute.
6. Heart rate rises from 70 to 120 while stroke volume rises from 70 to 100 mL. By how many times does cardiac output rise?
Before: 70 × 70 = 4900 mL/min. After: 120 × 100 = 12000 mL/min. Ratio = 12000 ÷ 4900 ≈ 2.4 times.
7. A person with group B blood needs blood. Which groups can they safely receive?
Group B has anti-A antibodies. So they can take B and O (O has no antigens). Not A or AB.
8. An Rh− mother is carrying her second Rh+ baby and did not get an anti-Rh injection after the first. What could happen and why?
Her body may have made anti-Rh antibodies after the first delivery. These can cross the placenta and destroy the baby's RBCs: erythroblastosis foetalis (severe anaemia and jaundice in the baby).
Common mistakes
- Saying all arteries carry oxygen-rich blood. The pulmonary artery carries oxygen-poor blood to the lungs.
- Mixing up serum and plasma. Serum is plasma without clotting factors.
- Calling AB the universal donor. O is the universal donor; AB is the universal recipient.
- Thinking the brain starts each heartbeat. The SA node in the heart does; nerves only speed it up or slow it down.