Unit 3 Circulatory system in animals
Key unit competence: Relate the structures of the circulatory and lymphatic systems to their functions.
Key ideas
- Large, active animals need a transport system because of their small surface area to volume ratio, long diffusion distances and high metabolic rate. Open circulation (insects, molluscs): blood (haemolymph) is pumped into a body cavity (haemocoel) and bathes organs directly at low pressure. Closed circulation (annelids, vertebrates): blood stays in vessels, at higher pressure, giving faster, directed flow.
- Single circulation (fish): heart → gills → body → heart; blood passes through the heart once per circuit; pressure falls after the gills. Double circulation (mammals, birds): pulmonary circuit (heart → lungs → heart) and systemic circuit (heart → body → heart); blood passes the heart twice, so it can be re-pressurised for the body while keeping low pressure in the lungs. Amphibians have a three-chambered heart with some mixing.
- The human heart has four chambers: right and left atria (thin walls, receive blood) and ventricles (thick walls, pump blood). The left ventricle has the thickest wall because it pumps blood around the whole body. The septum separates oxygenated and deoxygenated blood. Valves: tricuspid (right) and bicuspid/mitral (left) atrioventricular valves, held by tendinous cords; semilunar valves at the bases of the aorta and pulmonary artery. The heart muscle is supplied by coronary arteries.
- Cardiac cycle (about 0.8 s at 75 beats min⁻¹): atrial systole (atria contract, blood into ventricles), ventricular systole (ventricles contract, AV valves close — "lub", semilunar valves open, blood into arteries), diastole (heart relaxes, semilunar valves close — "dub", atria fill). Valves open and close due to pressure differences.
- The heart is myogenic: the sinoatrial node (SAN), the pacemaker in the right atrium, sends waves of excitation across the atria; the atrioventricular node (AVN) delays the impulse (about 0.1 s) so atria empty first; the impulse passes down the bundle of His and Purkyne fibres to the ventricle apex, so ventricles contract from the bottom up. An ECG records these electrical changes (P wave, QRS complex, T wave).
- Control of heart rate: the cardiovascular centre in the medulla receives information from baroreceptors (pressure, in carotid sinus and aortic arch) and chemoreceptors (CO₂/pH). Sympathetic nerves (accelerator) increase rate; the vagus (parasympathetic) nerve decreases it. Adrenaline increases rate and force. Other factors: exercise, temperature, age, fitness, emotions, drugs (caffeine and nicotine increase rate; alcohol and beta-blockers decrease it). Cardiac output = stroke volume × heart rate.
- Blood vessels: arteries carry blood away from the heart at high pressure: thick walls with elastic tissue (stretch and recoil, smoothing flow) and smooth muscle, narrow lumen. Veins return blood at low pressure: thin walls, wide lumen, valves prevent backflow; skeletal muscles squeeze them. Capillaries: walls one cell thick (endothelium), narrow (red cells pass in single file), large total surface area for exchange.
- Body fluids: blood = plasma (water, glucose, amino acids, salts, hormones, urea, CO₂, plasma proteins) + red blood cells (biconcave, no nucleus, haemoglobin), white blood cells (phagocytes, lymphocytes) and platelets (clotting). Tissue fluid forms at the arterial end of capillaries where hydrostatic pressure exceeds the oncotic (osmotic) pressure of plasma proteins; it bathes cells and most returns at the venous end. Excess drains into lymph capillaries to form lymph, carried in lymph vessels with valves through lymph nodes (lymphocytes filter pathogens) and returned to the blood via the thoracic duct into the subclavian vein. Lacteals absorb fats. Blockage causes oedema.
- Transport of gases: O₂ binds haemoglobin (four haem groups, each binds one O₂) forming oxyhaemoglobin. The oxygen dissociation curve is S-shaped (sigmoid) due to cooperative binding. Bohr effect: high CO₂ shifts the curve to the right, so Hb releases more O₂ in active tissues. Fetal haemoglobin and myoglobin have a higher affinity (curve to the left); animals at high altitude or in low-O₂ habitats (llama, lugworm) also have left-shifted Hb. CO₂ is carried as hydrogencarbonate ions (~85%; carbonic anhydrase in red cells, chloride shift), as carbaminohaemoglobin (~10%) and dissolved in plasma (~5%).
- Blood clotting: damaged tissue and platelets release thromboplastin; with Ca²⁺ and vitamin K, prothrombin → thrombin; thrombin converts soluble fibrinogen into insoluble fibrin threads that trap cells to form a clot. Haemophilia: a clotting factor is missing. Cardiovascular diseases: atherosclerosis (fatty plaques narrow arteries), coronary heart disease (angina, myocardial infarction), hypertension, stroke, thrombosis. Risk factors: smoking, high-fat diet, obesity, inactivity, stress, diabetes, high salt, genetics.
Common misconceptions
- "Arteries always carry oxygenated blood." No: the pulmonary artery carries deoxygenated blood to the lungs; arteries carry blood away from the heart.
- "The right ventricle has the thickest wall." No: the left ventricle is thickest because it pumps to the whole body.
- "The heart beats because the brain tells it to." No: heart muscle is myogenic; nerves only change the rate.
- "Lymph is the same as blood plasma." No: lymph has fewer proteins and no red cells.
Worked examples
Example 1. A heart beats 72 times per minute with a stroke volume of 70 cm³. Calculate the cardiac output in dm³ per minute.
Solution
Cardiac output = stroke volume × heart rate = 70 × 72 = 5 040 cm³ min⁻¹ = 5.04 dm³ min⁻¹.
Example 2. Explain why the oxygen dissociation curve shifts to the right during exercise.
Solution
Active muscles produce more CO₂, which forms carbonic acid, lowering pH. H⁺ ions reduce haemoglobin's affinity for O₂ (Bohr effect), so at the same partial pressure Hb releases more oxygen to the respiring muscle cells.
Practice questions
Distinguish open and closed circulatory systems.
Show answer
Open: blood pumped into body cavity, bathes organs, low pressure, slow flow (insects). Closed: blood in vessels, high pressure, fast directed flow, exchange through capillaries (vertebrates, annelids).
Common mistake: Saying insects have capillaries.
Describe the cardiac cycle.
Show answer
Atrial systole: atria contract, blood to ventricles. Ventricular systole: AV valves close, semilunar valves open, blood to aorta and pulmonary artery. Diastole: heart relaxes, semilunar valves close, atria and ventricles fill.
Common mistake: Saying valves open by muscle action.
Explain how the heartbeat is initiated and coordinated.
Show answer
SAN generates impulse; wave of excitation across atria causing atrial systole; AVN delays; bundle of His and Purkyne fibres carry impulse to apex; ventricles contract from apex upwards.
Common mistake: Saying the brain starts each beat.
Explain how heart rate is controlled.
Show answer
Medulla cardiovascular centre; inputs from baroreceptors and chemoreceptors; sympathetic nerve increases rate; vagus nerve decreases; adrenaline increases rate and force.
Common mistake: Saying the vagus speeds up the heart.
Compare arteries, veins and capillaries.
Show answer
Arteries: thick elastic muscular wall, narrow lumen, high pressure, no valves. Veins: thin wall, wide lumen, valves, low pressure. Capillaries: one cell thick, very narrow, exchange.
Common mistake: Saying capillaries have valves.
Explain how tissue fluid is formed and returned.
Show answer
At arterial end hydrostatic pressure exceeds oncotic pressure; fluid forced out; at venous end hydrostatic pressure lower, water returns by osmosis; excess drains into lymph.
Common mistake: Saying proteins leave capillaries.
Explain how CO₂ is carried in the blood.
Show answer
85% as hydrogencarbonate: CO₂ + H₂O → H₂CO₃ (carbonic anhydrase) → H⁺ + HCO₃⁻; HCO₃⁻ diffuses to plasma, Cl⁻ enters (chloride shift); H⁺ buffered by Hb; 10% carbaminohaemoglobin; 5% dissolved.
Common mistake: Confusing with carboxyhaemoglobin.
Describe blood clotting.
Show answer
Damaged tissue and platelets release thromboplastin; with Ca²⁺ and vitamin K, prothrombin → thrombin; thrombin converts fibrinogen → fibrin; fibrin mesh traps cells to form clot.
Common mistake: Saying fibrin is in blood all the time.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Most CO₂ is carried in blood as: a. hydrogencarbonate ions 2. High CO₂ moves the dissociation curve to the right. This is the: b. lymph capillaries 3. Arteries have much elastic tissue to: c. stretch and recoil with each beat 4. Excess tissue fluid drains into: d. Bohr effect Show answer
1 → a (hydrogencarbonate ions) · 2 → d (Bohr effect) · 3 → c (stretch and recoil with each beat) · 4 → b (lymph capillaries)
- About 85%.
- More O₂ released.
- It evens out flow.
- Becomes lymph.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
prevent backflow of blood · sinoatrial node · raise blood pressure · stroke volume × heart rate- Cardiac output equals .
- Veins have valves to .
- The pacemaker of the heart is the .
Show answer
1: stroke volume × heart rate · 2: prevent backflow of blood · 3: sinoatrial node
- Volume per minute.
- Pressure is low.
- In the right atrium.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The AVN delays the impulse so that: a. vagus nerve 2. The nerve that slows the heart is the: b. atria empty before ventricles contract 3. Insoluble threads in a blood clot are: c. baroreceptors 4. Pressure receptors in the aorta are: d. fibrin Show answer
1 → b (atria empty before ventricles contract) · 2 → a (vagus nerve) · 3 → d (fibrin) · 4 → c (baroreceptors)
- About 0.1 s.
- Parasympathetic.
- They trap blood cells.
- Detect blood pressure.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
tissue fluid cannot drain away · myogenic · thoracic duct · aorta- Lymph returns to the blood through the .
- A blocked lymph vessel in a leg causes swelling because .
- The heart can beat without nerves because it is .
Show answer
1: thoracic duct · 2: tissue fluid cannot drain away · 3: myogenic
- Into the subclavian vein.
- Oedema.
- SAN generates impulses.