Revision notes for Edexcel A A Level Biology Circulatory system and mass transport. Open the guide for explanations and worked examples. Written against the Edexcel A A Level Biology (9BN0) specification, so the content matches what's examinable rather than general Biology background.
Circulatory system and mass transport
What you'll learn
- Why large, active animals cannot rely on diffusion alone.
- How a mass transport system keeps cells supplied with oxygen and nutrients.
- How the human double circulatory system is organised.
- How blood vessels and cardiac output are adapted for efficient transport.
The basic problem: cells need exchange
Every living cell carries out metabolism, which means all the chemical reactions happening inside cells. These reactions need substances such as oxygen and glucose, and they produce wastes such as carbon dioxide.
Small organisms can often exchange substances directly with their surroundings by diffusion. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, due to random movement.
A concentration gradient is a difference in concentration across a distance. The steeper the gradient, the faster net diffusion tends to be.
Large animals have a problem: many of their cells are far away from the outside environment. Diffusion over long distances is too slow to meet the demands of active tissues such as muscle.
Surface area to volume ratio
As an organism gets larger, its volume increases faster than its surface area. Volume is linked to how many cells need supplying; surface area is linked to how much exchange can happen with the environment.
Definition
Surface area to volume ratio
Surface area to volume ratio, often written as SA:V, is the surface area of an organism or structure divided by its volume. A high SA:V means there is lots of exchange surface compared with the amount of living tissue.
At A level, you can explain the need for transport systems using the idea behind Fick’s law:
relative diffusion rate=surface area×concentration gradientdiffusion distance\text{relative diffusion rate} = \frac{\text{surface area} \times \text{concentration gradient}}{\text{diffusion distance}}relative diffusion rate=diffusion distancesurface area×concentration gradientSo diffusion is faster when there is a large surface area, a steep concentration gradient, and a short diffusion distance.
Key Idea
Why large animals need transport
Large, active animals have a low SA:V, long diffusion distances, and high metabolic demand. They need a mass transport system to move substances quickly and maintain steep concentration gradients at exchange surfaces.
Example
Comparing surface area to volume ratio
- For a cube with side length 1.0 mm, use A=6l2A=6l^2A=6l2 and V=l3V=l^3V=l3: A=6×(1.0 mm)2=6.0 mm2A=6 \times (1.0\ \text{mm})^2=6.0\ \text{mm}^2A=6×(1.0 mm)2=6.0 mm2 and V=(1.0 mm)3=1.0 mm3V=(1.0\ \text{mm})^3=1.0\ \text{mm}^3V=(1.0 mm)3=1.0 mm3, so SA:V is 6.0 mm−16.0\ \text{mm}^{-1}6.0 mm−1.
- For a cube with side length 10.0 mm: A=6×(10.0 mm)2=600 mm2A=6 \times (10.0\ \text{mm})^2=600\ \text{mm}^2A=6×(10.0 mm)2=600 mm2 and V=(10.0 mm)3=1000 mm3V=(10.0\ \text{mm})^3=1000\ \text{mm}^3V=(10.0 mm)3=1000 mm3, so SA:V is 0.600 mm−10.600\ \text{mm}^{-1}0.600 mm−1.
- The larger cube is 10 times longer, but its SA:V is 10 times smaller. This means each unit of volume has much less surface available for exchange.
Mass transport: moving substances in bulk
Definition
Mass transport
A mass transport system moves a transport medium, such as blood, in bulk from one part of an organism to another. In animals, this bulk movement is usually driven by pressure generated by a pump such as the heart.
Mass transport does not replace diffusion completely. Instead, it brings substances close to cells, so diffusion only has to happen across short distances at exchange surfaces.
In humans, the transport medium is blood. Blood carries:
- oxygen from the lungs to respiring tissues
- glucose, amino acids, fatty acids and glycerol from digestion to cells
- carbon dioxide from tissues to the lungs
- urea from the liver to the kidneys
- hormones from endocrine glands to target organs
- heat around the body
The human double circulatory system
The human circulatory system is made of the heart, blood, and blood vessels. It is a closed circulatory system, meaning blood stays inside vessels rather than directly bathing organs.
Humans have a double circulatory system. This means blood passes through the heart twice during one complete circuit of the body:
- the pulmonary circulation carries blood between the heart and lungs
- the systemic circulation carries blood between the heart and the rest of the body
The atrium is an upper heart chamber that receives blood. The ventricle is a lower chamber that pumps blood out of the heart. The septum is the wall separating the left and right sides, helping prevent oxygenated and deoxygenated blood from mixing.
Blood follows this route:
- Body tissues return deoxygenated blood to the right atrium through the vena cava.
- The right ventricle pumps blood to the lungs through the pulmonary artery.
- Blood becomes oxygenated in lung capillaries and returns through the pulmonary vein.
- The left atrium receives oxygenated blood.
- The left ventricle pumps blood to the body through the aorta.
The left ventricle has a thicker muscular wall than the right ventricle because it must generate enough pressure to pump blood around the whole body.
Common Mistake
Arteries are not always oxygenated
An artery carries blood away from the heart, and a vein carries blood towards the heart. The pulmonary artery carries deoxygenated blood, while the pulmonary vein carries oxygenated blood.
Blood vessels are adapted for their jobs
A lumen is the hollow space inside a blood vessel. The endothelium is the thin layer of cells lining the inside of blood vessels.
Arteries
Arteries carry blood away from the heart at high pressure. They have thick walls containing elastic tissue and smooth muscle. Elastic tissue allows the artery wall to stretch and recoil, helping maintain blood pressure between heartbeats.
Arterioles
Arterioles are small arteries that control blood flow into capillary networks. Their smooth muscle can contract or relax, changing the diameter of the lumen.
Capillaries
Capillaries are tiny vessels where exchange happens between blood and tissues. Their walls are one cell thick, giving a short diffusion distance. They form networks, giving a large surface area for exchange.
The fluid surrounding body cells is called tissue fluid. Oxygen and glucose diffuse from capillaries into tissue fluid, then into cells. Carbon dioxide diffuses in the opposite direction.
Veins
Veins carry blood back to the heart at low pressure. They have thinner walls, a larger lumen, and valves to prevent backflow. Movement of nearby skeletal muscles can help squeeze blood along veins.
Example
Identifying a blood vessel from its structure
A micrograph shows a vessel with a thick wall, a small lumen, and no valves.
- Compare the wall and lumen: a thick wall with a relatively small lumen suggests the vessel can withstand high pressure.
- Link the tissue structure to function: elastic and muscular tissue is typical of a vessel carrying blood away from the heart under pressure.
- Use the direction rule: vessels carrying blood away from the heart are arteries, so this vessel is most likely an artery.
Blood as the transport medium
Blood is a tissue made of cells suspended in plasma, the liquid part of blood. Plasma is mostly water and carries many dissolved substances.
Red blood cells, also called erythrocytes, contain haemoglobin, a protein that binds reversibly to oxygen. Their biconcave shape gives a large surface area for diffusion, and in mammals they have no nucleus, leaving more space for haemoglobin.
White blood cells are involved in immune defence, while platelets are cell fragments involved in clotting. These are important, but the main mass transport role belongs to plasma and red blood cells.
Cardiac output
Heart rate is the number of heart beats per unit time. Stroke volume is the volume of blood pumped out by one ventricle in one beat. Cardiac output is the volume of blood pumped out by one ventricle per unit time.
cardiac output=heart rate×stroke volume\text{cardiac output} = \text{heart rate} \times \text{stroke volume}cardiac output=heart rate×stroke volumeDuring exercise, cardiac output increases so that more oxygen and glucose reach respiring muscles, and carbon dioxide is removed faster.
Example
Calculating cardiac output during exercise
An athlete has a heart rate of 75 beats min⁻¹ and a stroke volume of 0.070 dm³ beat⁻¹ at rest. During exercise, these change to 150 beats min⁻¹ and 0.120 dm³ beat⁻¹.
- Calculate the resting cardiac output: COrest=75 beats min−1×0.070 dm3 beat−1=5.25 dm3 min−1\text{CO}_{\text{rest}}=75\ \text{beats min}^{-1}\times0.070\ \text{dm}^3\ \text{beat}^{-1}=5.25\ \text{dm}^3\ \text{min}^{-1}COrest=75 beats min−1×0.070 dm3 beat−1=5.25 dm3 min−1.
- Calculate the exercise cardiac output: COexercise=150 beats min−1×0.120 dm3 beat−1=18.0 dm3 min−1\text{CO}_{\text{exercise}}=150\ \text{beats min}^{-1}\times0.120\ \text{dm}^3\ \text{beat}^{-1}=18.0\ \text{dm}^3\ \text{min}^{-1}COexercise=150 beats min−1×0.120 dm3 beat−1=18.0 dm3 min−1.
- Compare the two values: 18.05.25=3.43\frac{18.0}{5.25}=3.435.2518.0=3.43, so cardiac output is about 3.4 times higher during exercise.
Tip
Unit check for cardiac output
Heart rate uses beats per unit time, and stroke volume uses volume per beat. When you multiply them, “beats” cancels, leaving volume per unit time.
Bringing it all together
The circulatory system solves the diffusion problem by moving blood rapidly around the body. The heart generates pressure, arteries distribute blood, capillaries allow exchange, and veins return blood to the heart.
The most important idea is that mass transport maintains steep concentration gradients. Blood arriving at tissues has high oxygen and glucose concentrations; blood leaving tissues has taken up carbon dioxide and other wastes. This keeps diffusion moving in the right direction.
Exam technique
In the exam
- For “why is a transport system needed?” mention low SA:V, long diffusion distances, high metabolic demand, and maintaining concentration gradients.
- Link vessel structure to function using “because”: thick elastic artery walls withstand high pressure; one-cell-thick capillary walls reduce diffusion distance.
- Define arteries and veins by direction of blood flow, not by oxygen content.
- In cardiac output calculations, write the formula, substitute values with units, and check that the final unit is volume per time.
Self review
Check yourself
- Why is diffusion alone too slow for a large active mammal?
- Trace the route of a red blood cell from the vena cava to the aorta.
- How would cardiac output change if heart rate increased but stroke volume stayed the same?
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