Animal Transport — A-Level Biology Revision Notes
Comprehensive revision notes covering circulatory systems, heart structure and the cardiac cycle, ECG interpretation, blood vessel structure and function, tissue fluid formation, oxygen dissociation curves, the Bohr shift, and carbon dioxide transport. Written by Tyrone — Chartered Biologist and former WJEC/Eduqas & Edexcel examiner with 25+ years of teaching experience.
Last updated: February 2026
Types of Circulatory System
All animals with a circulatory system share three basic components: a pump (the heart), a transport fluid (the blood), and a network of tubes (the blood vessels). However, the complexity and arrangement of these components varies greatly between organisms, and understanding these differences is essential for A-Level Biology.
Open vs Closed Circulatory Systems
In an open circulatory system, the blood vessels do not form a continuous circuit. Instead, they have open ends that empty blood directly into a body cavity called the haemocoel. Blood bathes the organs directly before returning to the heart through valve-controlled openings called ostia. Insects are the key example — their blood is called haemolymph and does not contain haemoglobin because it does not transport oxygen (gas exchange is handled by the tracheal system instead). Open circulatory systems generate low hydrostatic pressure, so blood flows slowly — this makes them inefficient for oxygen transport but energetically cheap to maintain.
In a closed circulatory system, blood remains within blood vessels at all times, forming a continuous circuit. Blood vessels are connected via capillaries in the body organs. Because the blood is contained, it can be maintained at a much higher hydrostatic pressure, allowing rapid transport of oxygen and nutrients to the tissues.
Single vs Double Circulatory Systems
In a single circulatory system (as found in fish), blood passes through the heart once per complete circuit. Deoxygenated blood is pumped from the ventricle to the gills, where it becomes oxygenated. It then flows directly to the body organs before returning to the atrium. The problem is that blood pressure drops significantly as it passes through the gills, so blood reaches the body organs at low pressure and low velocity. Furthermore, the organs are arranged in series — each successive organ receives blood with less oxygen and at lower pressure than the one before it.
In a double circulatory system (as found in mammals), blood passes through the heart twice per complete circuit. The pulmonary circulation carries deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side. The systemic circulation then pumps this oxygenated blood at high pressure to the body organs, which are arranged in parallel — so each organ receives blood at the same high pressure and oxygen concentration.
Comparing Circulatory Systems Across Organisms
| Organism | Type | Heart Chambers | Key Features |
|---|---|---|---|
| Insect | Open | Tubular hearts along dorsal vessel | Haemolymph, no haemoglobin, haemocoel, ostia, low pressure |
| Earthworm | Closed, single | 5 pseudo-hearts | Dorsal to ventral blood flow, relatively high pressure, capillaries present |
| Fish | Closed, single | 2 (1 atrium, 1 ventricle) | Organs in series, pressure drop at gills, cold-blooded |
| Mammal | Closed, double | 4 (2 atria, 2 ventricles) | Organs in parallel, high systemic pressure, warm-blooded |
Structure of the Mammalian Heart
The mammalian heart is a muscular organ divided into left and right sides by a wall of muscle called the septum. Each side has an upper chamber (the atrium) and a lower chamber (the ventricle). The bottom of the heart is called the apex.
The Four Chambers and Their Connections
The right atrium receives deoxygenated blood from the body via the vena cava (superior and inferior). Blood passes through the tricuspid valve (right atrioventricular valve) into the right ventricle, which pumps it to the lungs through the pulmonary semi-lunar valve into the pulmonary artery.
Oxygenated blood returns from the lungs via the pulmonary vein into the left atrium. Blood passes through the bicuspid valve (mitral valve / left atrioventricular valve) into the left ventricle, which pumps it out through the aortic semi-lunar valve into the aorta for distribution around the body.
The Heart’s Own Blood Supply
The heart muscle itself requires a constant supply of oxygen and nutrients. This is provided by the coronary blood vessels. The coronary arteries branch from the aorta and supply oxygenated blood to the heart muscle. The coronary veins remove deoxygenated blood. Blockage of a coronary artery (by atherosclerosis or a blood clot) deprives the heart muscle of oxygen, leading to a myocardial infarction (heart attack).
The Cardiac Cycle
Cardiac Cycle
The cardiac cycle is the sequence of events that occurs in the heart during one complete heartbeat. It consists of alternating periods of contraction (systole) and relaxation (diastole) of the atria and ventricles.
Key Principles
Before working through the cardiac cycle, you need to understand the relationship between contraction, pressure and volume. When a chamber contracts (systole), its volume decreases and pressure increases. When a chamber relaxes (diastole), its volume increases and pressure decreases. Valves open and close in response to pressure differences — they open when pressure behind them exceeds pressure in front, and close when pressure in front exceeds pressure behind.
Atrial Systole / Ventricular Diastole
Both atria contract simultaneously, increasing pressure in the atria above that in the ventricles. The tricuspid and bicuspid valves open, and blood is forced into the ventricles. At the same time, the ventricles are relaxing — their volume is increasing and pressure decreasing. The pressure in the ventricles remains lower than that in the aorta and pulmonary artery, so the semi-lunar valves remain closed.
Ventricular Systole / Atrial Diastole
Both ventricles contract, rapidly increasing pressure. The pressure in the ventricles quickly exceeds the pressure in the aorta and pulmonary artery, so the aortic and pulmonary semi-lunar valves open and blood is forced out. At the same time, the atria are relaxing — the right atrium fills with deoxygenated blood from the vena cava, and the left atrium fills with oxygenated blood from the pulmonary vein.
Cardiac Output
Cardiac Output (CO)
CO = Stroke Volume (SV) × Heart Rate (HR). Typical values: SV = 70 ml/beat, HR = 75 beats/min, giving CO = 5,250 ml/min. Stroke volume is the volume of blood pumped from the left ventricle per heartbeat.
Electrical Activity of the Heart and ECG Interpretation
The Cardiac Conduction System
The heart is myogenic — it can initiate its own contraction without external nervous stimulation. The heartbeat is coordinated by a specialised conduction system consisting of four components:
Reading an ECG Trace
An electrocardiogram (ECG) records the electrical activity of the heart. A normal ECG trace has three main components:
The P wave represents atrial depolarisation — the wave of electrical activity leaving the SAN and spreading across both atria, causing atrial systole.
The QRS complex represents ventricular depolarisation — the electrical activity passing from the AVN, down the bundle of His, and up through the Purkinje fibres, causing ventricular systole. The QRS complex is larger than the P wave because the ventricular muscle mass is much greater.
The T wave represents ventricular repolarisation — the ventricles recovering from contraction and entering diastole.
Abnormal ECG Traces
| Condition | ECG Appearance | What It Indicates |
|---|---|---|
| Tachycardia | Fewer squares between QRS complexes (~2) | Heart rate above 75 bpm; heartbeats occurring more rapidly |
| Bradycardia | More squares between QRS complexes (~6) | Heart rate below 60 bpm; heartbeats occurring more slowly |
| First degree heart block | Prolonged PR segment | Delayed conduction at the AVN |
| Second degree heart block | Multiple P waves before each QRS complex | Problem with bundle of His or Purkinje fibres; not every atrial contraction leads to ventricular contraction |
| Atrial fibrillation | No clear P waves; irregular peaks between QRS complexes | Uncoordinated electrical activity in atria; atria do not contract properly |
| Ventricular fibrillation | No P waves, no QRS complexes, no T waves; chaotic trace | Uncoordinated ventricular activity; no cardiac output; medical emergency |
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Book a Free ConsultationBlood Vessel Structure and Function
Five types of blood vessel form the circulatory system: arteries, arterioles, capillaries, venules, and veins. Each has a structure precisely adapted to its function.
The Three Layers
Arteries and veins share three structural layers: the tunica externa (adventitia) — the outermost layer of connective tissue; the tunica media — the middle layer containing varying proportions of elastic fibres, smooth muscle and collagen; and the tunica intima — the innermost layer including a smooth endothelium lining the lumen.
Comparing Blood Vessels
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Lumen | Narrow, circular | Wide, irregular shape | Very narrow (~7 µm) |
| Tunica media | Thick — abundant elastic fibres & smooth muscle | Thin — less elastic fibre & muscle | Absent |
| Valves | None (high pressure prevents backflow) | Present (prevent backflow at low pressure) | None |
| Wall thickness | Thick | Thinner | One cell thick (endothelium only) |
| Fenestrations | No | No | Yes — gaps between endothelial cells |
Functional Adaptations
Arteries close to the heart have a high proportion of elastic tissue. During ventricular systole, the artery wall expands to accommodate the stroke volume. During diastole, the elastic tissue recoils, converting the intermittent ejection of blood from the ventricles into a continuous, smooth flow. Collagen prevents overexpansion. Smooth muscle in arteries further from the heart can vasoconstrict to maintain blood pressure and regulate flow.
Arterioles have a high proportion of smooth muscle in the tunica media, enabling vasodilation (widening) and vasoconstriction (narrowing). This regulates blood flow to individual organs — for example, during exercise, arterioles supplying skeletal muscles dilate while those supplying the gut constrict.
Capillaries consist of a single layer of endothelium with fenestrations (gaps) between cells. This structure allows the formation of tissue fluid and the exchange of nutrients, gases, and waste products between blood and tissues. The very narrow lumen brings red blood cells close to the capillary wall, minimising diffusion distance.
Veins have valves to prevent backflow and a wide lumen that reduces friction, offering low resistance to blood flow. The thin, flexible walls allow veins to change shape — during exercise, surrounding muscles compress the veins, forcing blood back towards the heart more rapidly.
Blood Pressure, Velocity and Cross-Sectional Area
Blood pressure, velocity, and the total cross-sectional area of blood vessels are interrelated and change systematically as blood travels through the circulatory system.
Pressure Changes
Pressure is highest in the aorta (typically ~120 mmHg during ventricular systole, dropping to ~80 mmHg during diastole). It decreases progressively through the arteries, arterioles, and capillaries. In the arteries and arterioles, pressure fluctuates with each heartbeat — this can be felt as a pulse. The fluctuations diminish with distance from the heart. By the time blood reaches the veins, pressure is very low and there is no pulse.
Cross-Sectional Area and Velocity
The total cross-sectional area of the blood vessels increases dramatically from the aorta to the capillaries because of branching. Although each individual capillary has a tiny lumen, the combined cross-sectional area of all capillaries is far greater than that of the aorta. This increase in cross-sectional area causes a drop in blood pressure and a significant reduction in blood velocity. The slow velocity at the capillaries is functionally important — it provides time for the exchange of nutrients, gases, and waste products, and for the formation of tissue fluid.
Total cross-sectional area is calculated as A = πr² multiplied by the number of vessels. For example, the aorta (radius ~12.5 mm) has a cross-sectional area of approximately 491 mm². A single arteriole (radius ~50 µm) has a tiny cross-sectional area, but with approximately 120 arterioles per organ, the total cross-sectional area of the arterioles is roughly twice that of the aorta.
Tissue Fluid Formation and Reabsorption
Tissue Fluid
Tissue fluid is a liquid that leaves the capillaries and bathes the cells of a tissue. It delivers nutrients (glucose, amino acids, oxygen) to the cells and removes their waste products (carbon dioxide, urea). Tissue fluid is formed from blood plasma but does not contain red blood cells or large plasma proteins.
Formation at the Arteriole End
At the arteriole end of the capillary, the hydrostatic pressure (blood pressure) is high — higher than the opposing osmotic pressure and tissue fluid pressure. This imbalance forces fluid out of the capillary through the fenestrations between endothelial cells. The fluid that leaves is plasma minus the large proteins (which are too big to pass through the gaps). This newly formed tissue fluid bathes the surrounding cells, and exchange of nutrients and waste products occurs by diffusion.
Reabsorption at the Venule End
As blood flows through the capillary, hydrostatic pressure drops (due to friction and the loss of fluid). At the venule end, the hydrostatic pressure has fallen below the osmotic pressure. The high concentration of plasma proteins remaining in the blood creates an osmotic pressure gradient (water potential gradient), drawing tissue fluid back into the capillary by osmosis.
Any excess tissue fluid that is not reabsorbed is drained by the lymphatic system and eventually returned to the blood.
Clinical Relevance — Oedema
Kwashiorkor is a protein-deficiency condition where the concentration of plasma proteins is too low to generate sufficient osmotic pressure at the venule end. Tissue fluid accumulates in the tissues, causing visible swelling (oedema), particularly in the abdomen. Elephantiasis occurs when parasitic worms block the lymphatic vessels, preventing excess tissue fluid from draining away, causing severe swelling in the limbs.
Venous Return — Getting Blood Back to the Heart
Blood returns to the right atrium via the vena cava under very low pressure. Three mechanisms assist venous return:
Valves in the veins prevent backflow. They are one-way, opening only towards the heart. Without valves, blood in the legs would pool under gravity.
The musculoskeletal pump operates when skeletal muscles in the legs contract during movement. The muscles press against the veins, squashing them and creating a localised pressure increase that pushes blood upward through the open valve above. When the muscles relax, the vein springs back to shape and the valve below closes, preventing backflow.
Negative intrathoracic pressure during inspiration (breathing in) aids venous return. As the chest volume increases during inspiration, the pressure in the thoracic cavity drops below atmospheric pressure. This allows the right atrium to expand more than usual, effectively drawing blood up the vena cava.
Oxygen Transport and Dissociation Curves
Haemoglobin and Oxygen Binding
Oxygen is transported in the blood bound to the haem group (containing Fe²⁺) of haemoglobin inside red blood cells. Each haemoglobin molecule can carry a maximum of four oxygen molecules. When fully loaded, it is called oxyhaemoglobin — Hb(O₂)₄.
Oxygen binding is reversible — oxygen associates (binds) in the lungs where partial pressure of oxygen is high, and dissociates (releases) in the respiring tissues where partial pressure of oxygen is low. This reversibility is what makes haemoglobin effective as a transport molecule.
Cooperative Binding
The binding of the first oxygen molecule to haemoglobin occurs slowly. However, this first oxygen changes the shape of the haemoglobin protein, making it easier for the second, third and fourth oxygen molecules to bind. This phenomenon is called cooperative binding — each oxygen molecule that binds increases haemoglobin’s affinity (readiness to bind) for the next. The reverse is also true: when oxygen begins to dissociate, the remaining oxygen molecules are released more readily.
The Oxygen Dissociation Curve
When percentage saturation of haemoglobin with oxygen is plotted against partial pressure of oxygen (in kPa), the result is a characteristic S-shaped (sigmoidal) curve. The S-shape is a direct consequence of cooperative binding.
Region 1 (low pO₂, 0–3 kPa): The curve is almost flat. Haemoglobin has a very low affinity for oxygen. Only the first oxygen molecule is binding (slowly), and percentage saturation is very low (~1–2%).
Region 2 (mid pO₂, 3–15 kPa): The curve is steep. Cooperative binding means the second, third and fourth oxygen molecules bind rapidly. This region represents the respiring tissues — small changes in partial pressure cause large changes in oxygen release. At 15 kPa, haemoglobin approaches full saturation (~98–100%).
Region 3 (high pO₂, above 15 kPa): The curve plateaus. Haemoglobin is fully saturated as oxyhaemoglobin. Further increases in pO₂ have no effect on saturation.
The Bohr Shift
Bohr Shift
The Bohr shift is the shift of the oxygen dissociation curve to the right, caused by an increase in the partial pressure of carbon dioxide (and a corresponding decrease in pH). It indicates a lowering of haemoglobin’s affinity for oxygen, resulting in more oxygen being released to the respiring tissues.
During physical activity, the rate of aerobic respiration in the muscles increases. This produces more carbon dioxide, which is an acidic gas — when it dissolves in water inside the red blood cell, it forms carbonic acid, lowering the pH. The lower pH causes a conformational change in haemoglobin that reduces its affinity for oxygen. Oxygen dissociates more readily, and the dissociation curve shifts to the right.
The greater the level of physical activity, the further the curve shifts to the right:
| Activity Level | pCO₂ | pH | Affinity | % Saturation at Unloading Tension |
|---|---|---|---|---|
| Rest | Low | ~7.9 | High | ~50% |
| Moderate activity | Medium | ~7.2 | Medium | ~27% |
| High activity | High | ~6.5 | Low | ~13% |
The biological significance is clear: when tissues need more oxygen (during exercise), the Bohr shift ensures that haemoglobin releases more of its oxygen. At rest, when the percentage saturation is 50%, haemoglobin retains half its oxygen. During intense exercise, the percentage saturation drops to just 13% — meaning 87% of the oxygen is released to the muscles. This is an elegant self-regulating mechanism.
Carbon Dioxide Transport and the Chloride Shift
Carbon dioxide produced by aerobic respiration is transported back to the lungs by three methods: 85% as sodium hydrogencarbonate in the plasma, 10% bound to haemoglobin as carbaminohaemoglobin, and 5% dissolved directly in the plasma.
The Chemistry Step by Step
Additionally, some CO₂ binds directly to amino acid groups on haemoglobin to form carbaminohaemoglobin (10%), and a small amount simply dissolves in the plasma water (5%).
Adaptations to Low Oxygen Environments and Myoglobin
High-Affinity Haemoglobin
Organisms that live in environments with low partial pressures of oxygen have evolved haemoglobin with a higher affinity for oxygen than adult human haemoglobin. Their dissociation curves are shifted to the left, meaning their haemoglobin becomes fully saturated at lower partial pressures.
| Organism | Environment | Why High Affinity? |
|---|---|---|
| Llama | High altitude (Andes, up to 7,000 m) | Atmospheric pO₂ decreases with altitude; must load oxygen efficiently in thin air |
| Lugworm | Stagnant water in sand burrows at low tide | Water in burrow becomes oxygen-depleted when tide is out; must extract O₂ from low-pO₂ water |
| Foetus | The uterus (oxygen supplied via placenta from mother’s blood) | Must obtain oxygen from mother’s oxyhaemoglobin; foetal haemoglobin (with γ chains instead of β chains) has higher affinity than adult haemoglobin |
Myoglobin
Myoglobin is a protein found in mammalian muscle tissue with a very high affinity for oxygen — its dissociation curve lies far to the left of adult haemoglobin. Myoglobin binds oxygen so tightly that it functions as an oxygen store in the muscles. During intense physical activity, when the circulatory system cannot deliver oxygen fast enough, the increased CO₂ and lactic acid production lowers myoglobin’s affinity, causing it to release its stored oxygen to the muscle cells.
Diving mammals such as the Weddell seal have exceptionally high myoglobin concentrations in their muscles — approximately twice the amount per kilogram of body mass compared to humans. This stored oxygen sustains aerobic respiration in their organs during extended dives.
Surface Area to Volume Ratio
Small mammals like mice have a large surface area to volume ratio, meaning they lose body heat rapidly. To compensate, they must maintain a high metabolic rate (high rate of aerobic respiration), requiring a high oxygen demand. Their haemoglobin has a lower affinity for oxygen compared to larger mammals like elephants — the dissociation curve is shifted to the right, ensuring oxygen is released more readily to the tissues.
Where Animal Transport Appears on Your Specification
| Exam Board | Unit / Module | Topic Area |
|---|---|---|
| AQA | Paper 1 (Year 1) | Topic 3: Organisms exchange substances with their environment — Mass transport |
| Edexcel A (Salters Nuffield) | Paper 1 (Year 1) | Topic 1: Lifestyle, Health and Risk — Cardiovascular system |
| OCR A | Paper 1 (Year 1) | Module 3: Exchange and Transport — Transport in animals |
| OCR B (Advancing Biology) | Paper 1 (Year 1) | Module 3: Transport and Gas Exchange |
| WJEC | Unit 2 | Adaptations for Transport |
| Eduqas | Component 3 (A-Level only) | Adaptations for Transport |
| IB Biology | Topic 6 (SL & HL) | Human Physiology — The transport system |
| CIE 9700 | Paper 2 (AS) | Transport in mammals |
Related Resources on This Site
Frequently Asked Questions
In an open circulatory system (e.g. insects), blood vessels are open-ended and blood empties into a body cavity called the haemocoel, bathing the organs directly. Blood returns to the heart through valve-controlled openings called ostia. This generates low pressure and slow flow. In a closed circulatory system (e.g. fish, mammals), blood remains within blood vessels at all times, connected via capillaries. This allows higher pressure and faster, more efficient delivery of oxygen and nutrients.
In a single circulatory system (e.g. fish), blood pressure drops as it passes through the gills, so organs receive blood at low pressure. In a double circulatory system (e.g. mammals), blood returns to the heart after the lungs to have its pressure boosted before being sent to the body. This means organs receive oxygenated blood at high pressure. Additionally, organs are arranged in parallel rather than in series, so each receives blood with the same oxygen concentration. This is essential for warm-blooded animals with high metabolic rates.
The curve plots percentage saturation of haemoglobin with oxygen (y-axis) against partial pressure of oxygen in kPa (x-axis). The S-shape reflects cooperative binding. At low pO₂ (like in respiring tissues), haemoglobin has low affinity and releases oxygen. At high pO₂ (like in the lungs), haemoglobin has high affinity and binds oxygen. The loading tension (~16 kPa) is where haemoglobin becomes fully saturated. The unloading tension (~3–5 kPa) is where haemoglobin is 50% saturated. A curve shifted to the right indicates lower affinity (more oxygen released); a curve shifted to the left indicates higher affinity.
The Bohr shift is the rightward shift of the oxygen dissociation curve caused by increased carbon dioxide concentration (and decreased pH). During exercise, muscles produce more CO₂, which lowers blood pH. This reduces haemoglobin’s affinity for oxygen, so more oxygen is released to the tissues that need it most. It’s a self-regulating mechanism — the harder a tissue works, the more CO₂ it produces, and the more oxygen it receives. At rest, haemoglobin may retain 50% of its oxygen. During intense exercise, this drops to about 13%, meaning 87% is delivered to the muscles.
Tissue fluid forms at the arteriole end of capillaries where hydrostatic pressure (blood pressure) exceeds the osmotic pressure of the blood. This forces plasma (minus large proteins and blood cells) out through fenestrations in the capillary wall. The fluid bathes surrounding cells, exchanging nutrients and waste by diffusion. At the venule end, hydrostatic pressure has dropped below osmotic pressure (due to the high concentration of remaining plasma proteins), so tissue fluid is drawn back into the capillary by osmosis. Excess fluid is drained by the lymphatic system.
A normal ECG has three main features: the P wave (atrial depolarisation/contraction), the QRS complex (ventricular depolarisation/contraction — larger because the ventricles have more muscle), and the T wave (ventricular repolarisation/relaxation). The interval between P and QRS represents the AVN delay. You can calculate heart rate by counting the squares between two QRS complexes. Abnormalities include: tachycardia (fast rate, fewer squares between QRS), bradycardia (slow rate, more squares), heart block (multiple P waves per QRS), and fibrillation (chaotic, irregular traces).
Foetal haemoglobin has gamma (γ) chains instead of the beta (β) chains found in adult haemoglobin. This structural difference gives it a higher affinity for oxygen — its dissociation curve is shifted to the left of adult haemoglobin. The higher affinity is essential because the foetus must obtain oxygen from the mother’s oxyhaemoglobin across the placenta. If foetal haemoglobin had the same affinity as adult haemoglobin, it could not effectively compete for oxygen. Shortly after birth, foetal haemoglobin is replaced by adult haemoglobin.
Yes — animal transport is a core topic on every A-Level Biology specification. These notes cover the content shared by AQA, Edexcel A and B, OCR A and B, WJEC, Eduqas, IB Biology, and Cambridge International (CIE 9700). The biological principles — circulatory systems, heart structure, cardiac cycle, blood vessels, oxygen dissociation curves, the Bohr shift, and gas transport — are the same across all boards. The notes are written by a former WJEC/Eduqas and Edexcel examiner, but the science and exam technique applies universally.
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