What you'll learn
This topic examines how organisms exchange materials with their environment and transport these substances around the body. You'll study surface area to volume ratios, gas exchange systems, the circulatory system, and blood components—all essential for maintaining life in animals.
Key terms and definitions
Diffusion — the net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient, as a result of their random movement
Surface area to volume ratio — the relationship between the outer surface area of an organism and its internal volume; smaller organisms have larger SA:V ratios than larger organisms
Alveoli — tiny air sacs in the lungs where gas exchange occurs between air and blood
Haemoglobin — the red protein in red blood cells that binds reversibly with oxygen to transport it around the body
Double circulatory system — a circulation system where blood passes through the heart twice for each complete circuit of the body (pulmonary and systemic circuits)
Plasma — the liquid component of blood that transports dissolved substances including glucose, amino acids, carbon dioxide, urea, hormones and heat energy
Capillaries — the smallest blood vessels with walls one cell thick, allowing efficient exchange of materials between blood and tissues
Ventilation — the process of moving air in and out of the lungs through breathing movements
Core concepts
Surface area to volume ratio and exchange surfaces
As organisms increase in size, their volume increases faster than their surface area. This creates a problem: small organisms can rely on diffusion alone across their body surface, but larger organisms require specialized exchange surfaces and transport systems.
Calculating SA:V ratio:
- For a cube with side length 2 cm: SA = 6 × 2² = 24 cm²; V = 2³ = 8 cm³
- SA:V ratio = 24:8 = 3:1
Adaptations of effective exchange surfaces:
- Large surface area (e.g., folded membranes, root hair cells, alveoli, villi)
- Thin walls/short diffusion distance (often one cell thick)
- Efficient transport system to maintain concentration gradients (e.g., good blood supply, ventilation)
- In animals, moist surfaces for gases to dissolve
Single-celled organisms like amoeba have a large SA:V ratio, so diffusion across their cell membrane is sufficient for gas exchange and waste removal. Larger organisms like mammals have a small SA:V ratio and need specialized systems.
Gas exchange in humans
The respiratory system enables gas exchange between air and blood. Air enters through the nose/mouth, passes through the trachea, bronchi, bronchioles, and reaches the alveoli.
Structure of the alveoli:
- Clustered in grape-like arrangements to maximize surface area (approximately 300 million alveoli in human lungs)
- Walls one cell thick for short diffusion distance
- Surrounded by dense capillary networks for efficient gas transport
- Moist lining to dissolve gases
- Good ventilation maintains steep concentration gradients
Gas exchange mechanism:
- Oxygen diffuses from alveolar air (high O₂ concentration) into blood capillaries (low O₂ concentration)
- Carbon dioxide diffuses from blood (high CO₂ concentration) into alveolar air (low CO₂ concentration)
- Both gases move down their concentration gradients by diffusion
Ventilation mechanism:
Inhalation (breathing in):
- External intercostal muscles contract, internal intercostal muscles relax
- Ribs move up and out
- Diaphragm contracts and flattens
- Volume of thorax increases
- Pressure inside thorax decreases below atmospheric pressure
- Air moves into lungs
Exhalation (breathing out):
- External intercostal muscles relax, internal intercostal muscles contract
- Ribs move down and in
- Diaphragm relaxes and moves up (becomes dome-shaped)
- Volume of thorax decreases
- Pressure inside thorax increases above atmospheric pressure
- Air moves out of lungs
The circulatory system
Humans have a double circulatory system consisting of two circuits:
- Pulmonary circulation: right ventricle → lungs → left atrium (carries deoxygenated blood to lungs, returns oxygenated blood)
- Systemic circulation: left ventricle → body → right atrium (delivers oxygenated blood to tissues, returns deoxygenated blood)
This system is more efficient than single circulation as blood can be pumped at higher pressure to the body after being oxygenated.
Heart structure and function:
The heart is a muscular organ with four chambers:
- Two atria (upper chambers) receive blood from veins
- Two ventricles (lower chambers) pump blood into arteries
- Left ventricle has thicker muscular wall than right ventricle (pumps blood at higher pressure around entire body)
- Valves prevent backflow of blood (atrioventricular valves between atria and ventricles; semilunar valves in arteries)
Blood flow through the heart:
- Deoxygenated blood enters right atrium via vena cava
- Blood flows to right ventricle
- Right ventricle contracts, pumping blood to lungs via pulmonary artery
- Oxygenated blood returns to left atrium via pulmonary vein
- Blood flows to left ventricle
- Left ventricle contracts, pumping blood to body via aorta
The heart's natural pacemaker (located in the right atrium) controls heart rate by sending electrical impulses that cause coordinated contractions.
Coronary arteries supply the heart muscle with oxygenated blood. Blockage of coronary arteries leads to heart attacks as heart muscle is deprived of oxygen.
Blood vessels
Three types of blood vessels form the circulatory system, each adapted to its function:
Arteries:
- Carry blood away from the heart
- Thick muscular walls to withstand high pressure
- Elastic tissue allows stretching and recoil to maintain blood pressure
- Small lumen relative to wall thickness
- Carry oxygenated blood (except pulmonary artery)
- No valves needed (except where they leave the heart)
Veins:
- Carry blood toward the heart
- Thinner walls (blood under lower pressure)
- Larger lumen to ease blood flow
- Contain valves to prevent backflow
- Carry deoxygenated blood (except pulmonary vein)
- Blood moved by skeletal muscle contraction squeezing veins
Capillaries:
- Connect arteries to veins
- Walls one cell thick (endothelium only) for short diffusion distance
- Very narrow lumen (often only one red blood cell wide)
- Permeable walls allow exchange of materials (oxygen, glucose, carbon dioxide, urea)
- Form extensive networks to increase surface area for exchange
- Low pressure allows time for exchange
Blood composition and function
Blood is a tissue consisting of plasma with suspended cells.
Plasma (55% of blood volume):
- Straw-colored liquid
- Transports dissolved substances: glucose, amino acids, mineral ions, hormones, urea, carbon dioxide, heat energy, proteins (including antibodies and fibrinogen)
Red blood cells (erythrocytes):
- Transport oxygen from lungs to respiring tissues
- Biconcave disc shape increases surface area for oxygen diffusion
- No nucleus, providing more space for haemoglobin
- Contain haemoglobin, which binds reversibly with oxygen to form oxyhaemoglobin
- In lungs: haemoglobin + oxygen ⇌ oxyhaemoglobin
- In tissues: oxyhaemoglobin ⇌ haemoglobin + oxygen
White blood cells (leukocytes):
- Part of immune system
- Contain nucleus
- Two main types:
- Phagocytes: engulf and digest pathogens (phagocytosis)
- Lymphocytes: produce antibodies specific to antigens on pathogens
Platelets:
- Small cell fragments without nuclei
- Essential for blood clotting
- At wound sites, platelets and damaged tissue release clotting factors
- These trigger conversion of fibrinogen (soluble plasma protein) to fibrin (insoluble threads)
- Fibrin forms mesh that traps red blood cells, forming a clot
- Prevents blood loss and pathogen entry
Transport of materials
Oxygen transport:
- Oxygen diffuses into red blood cells in lung capillaries
- Combines with haemoglobin forming oxyhaemoglobin
- In tissue capillaries, oxyhaemoglobin releases oxygen
- Oxygen diffuses into respiring cells
Carbon dioxide transport:
- Produced by respiring cells
- Diffuses into blood plasma
- Most transported as hydrogen carbonate ions in plasma
- Some carried by haemoglobin
- Diffuses from blood into alveoli to be exhaled
Glucose and amino acids:
- Absorbed from small intestine into blood plasma
- Transported dissolved in plasma to all body cells
- Glucose used for respiration; amino acids for protein synthesis
Urea:
- Produced in liver from breakdown of excess amino acids
- Transported in plasma to kidneys for excretion
Worked examples
Example 1: Calculate the surface area to volume ratio for a cube with side length 4 cm. Explain how this ratio changes as organisms increase in size. [4 marks]
Answer:
- Surface area = 6 × 4² = 96 cm² ✓
- Volume = 4³ = 64 cm³ ✓
- SA:V ratio = 96:64 = 1.5:1 or 3:2 ✓
- As organisms increase in size, their volume increases faster than surface area, so SA:V ratio decreases ✓
Example 2: Describe how the structure of alveoli is adapted for efficient gas exchange. [6 marks]
Answer:
- Large surface area / many alveoli present, so more diffusion can occur ✓
- Walls one cell thick / thin walls, providing short diffusion distance ✓
- Moist lining, allowing gases to dissolve ✓
- Good blood supply / surrounded by capillaries, maintains concentration gradient / removes oxygen / brings carbon dioxide ✓
- Good ventilation, maintains concentration gradient / brings oxygen / removes carbon dioxide ✓
- Permeable walls, allow gases to pass through ✓
Example 3: Explain why the left ventricle has a thicker muscular wall than the right ventricle. [3 marks]
Answer:
- Left ventricle pumps blood to the (whole) body / around the body ✓
- Right ventricle pumps blood (only) to the lungs ✓
- Left ventricle needs to generate higher pressure / pump blood further / pump blood to more places ✓
Common mistakes and how to avoid them
Confusing arteries and veins with oxygenated/deoxygenated blood: Remember that arteries carry blood away from the heart (not always oxygenated). The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.
Saying "oxygen is picked up by red blood cells": Be specific—oxygen combines with haemoglobin inside red blood cells to form oxyhaemoglobin. This is a reversible reaction.
Describing diffusion as "balancing out concentrations": Diffusion is the net movement of particles down a concentration gradient. Particles continue to move randomly even at equilibrium, but there is no net movement.
Writing that the heart "oxygenates" blood: The heart pumps blood; gas exchange occurs in the lungs at the alveoli. Keep the functions of organs separate.
Confusing ventilation with gas exchange: Ventilation is the mechanical process of moving air in and out of lungs. Gas exchange is diffusion of oxygen and carbon dioxide between alveoli and blood.
Using vague language about surface area: Don't just write "large surface area"—explain why this adaptation is beneficial (e.g., "allows more diffusion to occur" or "more particles can diffuse at the same time").
Exam technique for "Exchange and Transport in Animals"
"Explain" questions require reasons: When asked to explain an adaptation, always link structure to function (e.g., "thin walls provide short diffusion distance, so diffusion is faster"). This comparative or causal language secures higher marks.
Use precise biological terms: Write "concentration gradient" not "difference in concentration," "haemoglobin" not "red pigment," "ventilation" not "breathing." Examiners reward specific terminology.
In calculation questions, show your working: For SA:V ratios, write out the formula and each step. You can gain method marks even if your final answer is incorrect.
For 6-mark questions on gas exchange or circulation, cover multiple points: These questions assess your ability to provide detailed, coherent answers. Aim for 6-7 distinct points, ensuring you mention structure, function, and mechanism where appropriate.
Quick revision summary
Larger organisms have smaller surface area to volume ratios and require specialized exchange surfaces and transport systems. Gas exchange in humans occurs at alveoli, adapted with large surface area, thin walls, and good blood supply. The double circulatory system pumps blood through the heart twice per circuit. Blood vessels (arteries, veins, capillaries) have structures matched to their functions. Blood contains plasma, red blood cells with haemoglobin for oxygen transport, white blood cells for immunity, and platelets for clotting.