Mass Transport: The Mammalian Circulatory System
The Need for a Circulatory System
Single-celled organisms exchange substances directly with their environment by diffusion across the cell surface membrane. This is sufficient because they have a large surface-area-to-volume ratio and short diffusion distances.
Larger multicellular organisms have a small surface-area-to-volume ratio relative to their metabolic demands. Diffusion alone would be too slow to supply oxygen and nutrients to deep tissues or remove waste products. They therefore require a mass transport system — the circulatory system — to move substances rapidly over long distances by bulk flow.
Structure of the Mammalian Circulatory System
Mammals have a closed, double circulatory system:
- Closed — blood is contained within vessels at all times (unlike the open system of insects)
- Double — blood passes through the heart twice in each complete circuit
The Two Circuits
| Circuit | Route | Function |
|---|---|---|
| Pulmonary | Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium | Deoxygenated blood is oxygenated; CO₂ is removed |
| Systemic | Left ventricle → aorta → body tissues → vena cava → right atrium | Oxygenated blood delivers O₂ and nutrients to tissues; deoxygenated blood returns |
The advantage of a double circulation is that blood pressure is boosted by passing through the heart a second time before reaching the body tissues. The pulmonary circuit operates at lower pressure (to protect the delicate gas exchange surfaces in the lungs), while the systemic circuit operates at higher pressure to drive blood through the extensive network of capillaries in the body.
The Heart
The heart is a muscular pump made largely of cardiac muscle (myocardium). It has four chambers: two thin-walled atria (receiving chambers) and two thick-walled ventricles (pumping chambers).
Key Structural Features
- The left ventricle has a much thicker wall than the right ventricle because it must generate higher pressure to pump blood around the entire systemic circulation
- The septum separates the left and right sides, preventing mixing of oxygenated and deoxygenated blood
- Atrioventricular (AV) valves (tricuspid on the right, bicuspid/mitral on the left) prevent backflow from ventricles to atria
- Semilunar valves in the aorta and pulmonary artery prevent backflow into the ventricles
- Coronary arteries branch from the aorta and supply the heart muscle itself with oxygenated blood
The Cardiac Cycle
The cardiac cycle is the sequence of events in one complete heartbeat, lasting approximately 0.8 seconds at rest (75 bpm).
Atrial systole (0.1 s):
- Atria contract, pushing blood through the open AV valves into the ventricles
- Atrial pressure exceeds ventricular pressure
Ventricular systole (0.3 s):
- Ventricles contract from the apex upward
- Ventricular pressure rises above atrial pressure → AV valves close (producing the first heart sound, "lub")
- Ventricular pressure exceeds arterial pressure → semilunar valves open → blood is ejected into the aorta and pulmonary artery
Diastole (0.4 s):
- Ventricles relax; ventricular pressure drops below arterial pressure → semilunar valves close (producing the second heart sound, "dub")
- Atria fill with blood returning via veins
- When atrial pressure exceeds ventricular pressure, AV valves open and blood flows passively into ventricles (accounting for ~70% of ventricular filling)
Cardiac Output
Cardiac output = stroke volume × heart rate
- Stroke volume — volume of blood ejected per beat (~70 mL at rest)
- Heart rate — beats per minute (~75 bpm at rest)
- Typical resting cardiac output: 70 × 75 = 5,250 mL/min (~5.25 L/min)
Control of Heart Rate
The heart is myogenic — cardiac muscle contracts rhythmically without nervous stimulation. The intrinsic rhythm is set by the sinoatrial node (SAN), the heart's natural pacemaker.
Conduction pathway:
1. SAN (in the wall of the right atrium) generates electrical impulses
2. Impulse spreads across both atria, causing atrial systole
3. Impulse reaches the atrioventricular node (AVN), which delays it by ~0.1 s — ensuring atria empty before ventricles contract
4. Impulse travels down the bundle of His in the septum
5. The bundle branches into Purkyne fibres that spread through the ventricular walls from the apex
6. Ventricles contract from the apex upward, efficiently ejecting blood into the arteries
Nervous regulation via the medulla oblongata adjusts heart rate:
- Sympathetic nerve releases noradrenaline → increases heart rate
- Vagus nerve (parasympathetic) releases acetylcholine → decreases heart rate
- Baroreceptors in the aortic arch and carotid sinus detect blood pressure changes
- Chemoreceptors in the aortic and carotid bodies detect changes in blood CO₂, pH, and O₂
Blood Vessels
Structure Related to Function
| Vessel | Wall | Lumen | Blood pressure | Speed of flow |
|---|---|---|---|---|
| Arteries | Thick; elastic fibres + smooth muscle; tough outer collagen layer | Narrow (relative to wall thickness) | High; pulsatile | Fast |
| Arterioles | Thinner; more smooth muscle relative to size | Very narrow; can constrict/dilate (vasoconstriction/vasodilation) | Moderate; controlled | Moderate |
| Capillaries | One cell thick (endothelium only); no muscle or elastic tissue | Very narrow (~8 μm); red blood cells squeeze through single file | Low | Very slow (maximises exchange time) |
| Venules | Thin walls; little muscle | Wider | Low | Slow |
| Veins | Thin walls; less elastic tissue and smooth muscle than arteries; valves present | Wide lumen | Very low; non-pulsatile | Moderate |
Capillary Exchange
Capillaries are the site of substance exchange between blood and tissues. Their thin walls (one endothelial cell thick) and narrow lumen ensure a short diffusion distance. The slow blood flow allows maximum time for exchange. Capillary networks form dense capillary beds to provide a large surface area.
Tissue fluid is formed at the arterial end of capillary beds by ultrafiltration: high hydrostatic pressure forces water and small solutes out through gaps between endothelial cells. Plasma proteins are too large to leave, so blood oncotic (osmotic) pressure due to proteins draws water back in at the venous end, where hydrostatic pressure is lower. Excess tissue fluid drains into lymph capillaries.
Exam Tips
- When explaining why a double circulation is advantageous, emphasise the pressure boost — blood loses pressure in the capillary beds of the lungs, and the left side of the heart restores high pressure for the systemic circuit
- AQA pressure-change graph questions require you to read the graph and explain valve opening/closing in terms of pressure differences between chambers
- Remember the conduction pathway sequence: SAN → atrial muscle → AVN → bundle of His → Purkyne fibres → ventricular muscle from apex upward
- Always link structure to function when describing vessels — e.g. thick elastic walls of arteries stretch and recoil to smooth pulsatile blood flow