What you'll learn
Gas exchange and transport explains how organisms obtain oxygen and remove carbon dioxide, and how those gases are carried in the blood. The organising principle is that diffusion alone is adequate only for small organisms, and that as size increases the surface area to volume ratio falls until specialised exchange surfaces and a transport system become necessary. At CAPE level the detail extends to the interpretation of oxygen dissociation curves, the Bohr effect and the chloride shift. By the end of this topic you should be able to explain the significance of surface area to volume ratio, describe the features of an efficient exchange surface, describe gas exchange in mammals, fish, insects and plants, interpret oxygen dissociation curves including shifts, and explain the three ways carbon dioxide is transported.
Key terms and definitions
Diffusion — net movement of particles from a higher to a lower concentration
Surface area to volume ratio — the relationship determining whether diffusion alone is adequate
Fick's law — the rate of diffusion is proportional to surface area multiplied by concentration difference, divided by diffusion distance
Alveolus — a microscopic air sac in the lung, the site of gas exchange
Ventilation — the movement of air or water over a respiratory surface, maintaining the concentration gradient
Counter-current flow — flow of blood and water in opposite directions, maintaining a gradient along the whole length
Tracheal system — the network of air-filled tubes supplying gases directly to insect tissues
Stoma — a pore in a leaf surface, bounded by two guard cells
Haemoglobin — the pigment in red blood cells that binds oxygen
Oxyhaemoglobin — haemoglobin with oxygen bound
Partial pressure — the pressure contributed by one gas in a mixture, a measure of its concentration
Cooperative binding — the effect whereby binding of the first oxygen molecule makes subsequent binding easier
Bohr effect — the reduced affinity of haemoglobin for oxygen at higher carbon dioxide concentration
Chloride shift — the movement of chloride ions into red blood cells to maintain electrical neutrality
Core concepts
Surface area to volume ratio
As an organism increases in size, its volume increases faster than its surface area, so the surface area to volume ratio falls.
A small organism has a large ratio and a short diffusion distance, so diffusion across the body surface supplies its needs adequately.
A large organism has a small ratio, a long diffusion distance to internal tissues, and a higher total metabolic demand. Diffusion alone becomes far too slow, so a specialised exchange surface and a transport system are required.
Fick's law formalises the requirements: the rate of diffusion is proportional to the surface area and the concentration difference, and inversely proportional to the diffusion distance. Every efficient exchange surface therefore shows the same three adaptations — a large surface area, a maintained concentration gradient, and a short diffusion distance — and identifying these three in any example is what examination questions reward.
Gas exchange in mammals
Air passes through the trachea, which divides into two bronchi, then into progressively smaller bronchioles, ending in clusters of alveoli.
The alveoli provide the exchange surface, and their adaptations map exactly onto Fick's law.
There are several hundred million alveoli, giving a total surface area of around seventy square metres, which satisfies the surface area requirement.
The alveolar wall is a single layer of flattened epithelial cells, and the capillary wall is likewise one cell thick, so the diffusion distance is only about two cell membranes.
A dense capillary network carries oxygenated blood away and brings deoxygenated blood, and ventilation continually replaces the air, so steep concentration gradients are maintained for both gases.
The alveolar surface is covered by a thin film of moisture in which gases dissolve before diffusing, and surfactant reduces surface tension and prevents the alveoli collapsing on exhalation.
Ventilation itself depends on pressure changes. During inspiration the external intercostal muscles contract, raising the ribs upwards and outwards, and the diaphragm contracts and flattens. The volume of the thorax increases, so the pressure within it falls below atmospheric pressure, and air moves in down the pressure gradient. During normal expiration these muscles relax, the elastic tissue of the lungs recoils, thoracic volume decreases and pressure rises, so air moves out. Forced expiration additionally involves contraction of the internal intercostal muscles and abdominal muscles.
Gas exchange in fish
Water contains far less dissolved oxygen than air, and is denser and more viscous, so fish require a highly efficient system.
Water enters the mouth, passes over the gills and out through the operculum. Each gill consists of many gill filaments, and each filament carries numerous lamellae arranged at right angles to it, giving a very large total surface area. The lamellae are richly supplied with capillaries and have very thin walls.
The key adaptation is counter-current flow. Blood in the capillaries of a lamella flows in the opposite direction to the water passing over it.
The consequence is that along the whole length of the lamella, the water always has a higher oxygen concentration than the adjacent blood, so a concentration gradient exists throughout and diffusion occurs along the entire surface. Around eighty per cent of the dissolved oxygen can be extracted.
If the flow were parallel, the two would reach equilibrium partway along, and no further diffusion could occur beyond that point, so only about fifty per cent of the oxygen could be extracted. Being able to explain this difference is the standard question on fish gas exchange.
Gas exchange in insects
Insects have an open circulatory system that does not transport oxygen, so gases are delivered directly to the tissues by a tracheal system.
Air enters through pores called spiracles along the sides of the body. These lead into tracheae, tubes strengthened with rings of chitin to prevent collapse, which branch into finer tracheoles.
The tracheoles are extremely narrow, have very thin walls and end directly among the cells, so oxygen diffuses the last short distance straight into the tissue without any intermediate transport.
Gases move along the tracheal system mainly by diffusion, aided in larger or more active insects by rhythmic abdominal contractions that ventilate the system. During activity, fluid at the ends of the tracheoles is drawn into the tissues by the accumulation of solutes from anaerobic respiration, exposing more surface for gas exchange.
The spiracles can be closed by valves, which reduces water loss, and this is an important compromise: insects must balance the need for gas exchange against the risk of desiccation, which is why the spiracles are closed when the insect is inactive.
Gas exchange in plants
Leaves are adapted for gas exchange as well as for photosynthesis.
Gases enter and leave through stomata, pores in the lower epidermis, each bounded by two guard cells that control its aperture.
Inside, the spongy mesophyll contains large air spaces that provide a very large internal surface area and allow gases to diffuse readily to and from all the photosynthesising cells.
The leaf is thin, so the diffusion distance is short, and the moist cell surfaces allow gases to dissolve.
Guard cells open the stoma when they take up water by osmosis and become turgid. Because their inner walls are thicker and less elastic than their outer walls, and because the cellulose microfibrils are arranged so that the cell cannot readily expand in width, the cell curves outwards as it swells, opening the pore. Loss of water makes them flaccid and the pore closes.
Stomata therefore mediate a compromise similar to that of the insect spiracle: they must open for carbon dioxide uptake but water is lost through them by transpiration, so they close in conditions of water stress even at the cost of reduced photosynthesis.
Haemoglobin and the dissociation curve
Haemoglobin has four polypeptide chains, each with a haem group containing iron, so each molecule can bind four oxygen molecules.
The oxygen dissociation curve plots percentage saturation of haemoglobin against the partial pressure of oxygen. It is S-shaped, and the shape is itself explained by cooperative binding.
The curve is shallow at low partial pressures because the first oxygen molecule binds with difficulty: the haemoglobin's shape makes the haem groups relatively inaccessible.
Binding of the first oxygen changes the tertiary and quaternary structure of the molecule, making the remaining haem groups more accessible, so the second and third bind far more readily. This produces the steep middle section, and it is why a small fall in partial pressure in respiring tissues causes a large release of oxygen.
At high partial pressures the curve levels off as the haemoglobin approaches full saturation and few binding sites remain.
In the lungs, where the partial pressure of oxygen is high, haemoglobin is almost fully saturated and loads oxygen. In respiring tissues, where the partial pressure is low, it unloads oxygen.
Shifts in the curve
A curve lying to the left indicates a higher affinity for oxygen: the pigment saturates at lower partial pressures and releases oxygen less readily. Foetal haemoglobin lies to the left of adult haemoglobin, which is essential, since it must take up oxygen from the mother's blood at the placenta where the partial pressure is already reduced. Organisms living at high altitude or in oxygen-poor environments similarly have left-shifted curves.
A curve lying to the right indicates a lower affinity: the pigment releases oxygen more readily. Active organisms with high metabolic rates have right-shifted curves, since ready unloading matters more to them than maximal loading.
The Bohr effect is a right shift caused by carbon dioxide. Actively respiring tissue produces carbon dioxide, which lowers the pH of the blood, and the resulting change alters the tertiary structure of haemoglobin and reduces its affinity for oxygen. More oxygen is therefore released precisely where respiration is most active, which is an elegant self-regulating mechanism and a favourite examination topic.
Carbon dioxide transport
Carbon dioxide is carried in the blood in three ways, and the proportions are examinable.
About five per cent dissolves directly in the plasma.
About ten to twenty per cent binds to haemoglobin, forming carbaminohaemoglobin. Note that it binds to the polypeptide chains, not to the haem groups, so it does not compete directly with oxygen for the same site.
The majority, about eighty-five per cent, is transported as hydrogencarbonate ions. Carbon dioxide diffuses into the red blood cell and reacts with water, catalysed by the enzyme carbonic anhydrase, to form carbonic acid, which dissociates into hydrogen ions and hydrogencarbonate ions.
The hydrogencarbonate ions diffuse out of the red blood cell into the plasma. To maintain electrical neutrality, chloride ions move from the plasma into the red blood cell, and this is the chloride shift.
The hydrogen ions remaining in the red blood cell are taken up by haemoglobin, which acts as a buffer, preventing a large fall in pH. It is this combination with hydrogen ions that causes the reduction in oxygen affinity underlying the Bohr effect.
In the lungs the whole sequence reverses: the low carbon dioxide concentration causes hydrogencarbonate to recombine with hydrogen ions to form carbon dioxide, which diffuses into the alveoli and is exhaled.
Worked examples
Example 1: Explaining counter-current flow (5 marks)
Explain why counter-current flow allows a fish to extract more oxygen from water than parallel flow would.
In counter-current flow, water passes over the lamella in the opposite direction to the flow of blood in the capillaries beneath.
As a result, blood that is already partly oxygenated meets water that has lost little of its oxygen, and blood that has just arrived and contains very little oxygen meets water that has already given up much of its oxygen. At every point along the lamella, therefore, the oxygen concentration in the water is higher than that in the adjacent blood.
A concentration gradient exists along the entire length of the exchange surface, so oxygen diffuses from water to blood throughout, and around eighty per cent of the dissolved oxygen can be absorbed.
With parallel flow, the two would equalise partway along the lamella. Beyond that point no gradient would exist and no further diffusion could occur, so only about fifty per cent of the oxygen could be extracted.
Example 2: Interpreting a dissociation curve shift (5 marks)
Explain the position of the foetal haemoglobin dissociation curve relative to that of adult haemoglobin, and why this is necessary.
The foetal haemoglobin curve lies to the left of the adult curve. This means that at any given partial pressure of oxygen, foetal haemoglobin has a higher percentage saturation, and therefore a higher affinity for oxygen.
This is necessary because the foetus obtains oxygen from the mother's blood at the placenta, where the partial pressure of oxygen is already reduced, since the mother's haemoglobin has released some of its oxygen.
If foetal haemoglobin had the same affinity as adult haemoglobin, it could not take up oxygen efficiently at that partial pressure. Because its affinity is higher, it binds oxygen from the maternal blood, so oxygen is transferred from mother to foetus across the placenta.
Example 3: Explaining the Bohr effect (5 marks)
Explain how the Bohr effect increases oxygen delivery to an actively respiring muscle.
An actively respiring muscle produces a large quantity of carbon dioxide. The carbon dioxide diffuses into the red blood cells and reacts with water, catalysed by carbonic anhydrase, forming carbonic acid, which dissociates to give hydrogen ions and hydrogencarbonate ions.
The hydrogen ions combine with haemoglobin. This alters the tertiary and quaternary structure of the haemoglobin molecule and reduces its affinity for oxygen.
The dissociation curve is therefore shifted to the right, meaning that at the same partial pressure of oxygen the haemoglobin has a lower percentage saturation and releases more of its oxygen.
More oxygen is consequently unloaded exactly where carbon dioxide concentration is highest, which is where respiration is most active and demand is greatest. The uptake of hydrogen ions by haemoglobin also buffers the blood, preventing a damaging fall in pH.
Common mistakes and how to avoid them
The most frequent error is describing the exchange surface without linking its features to Fick's law. Large surface area, short diffusion distance and maintained concentration gradient should each be named and explained.
Students often say that air moves into the lungs because the diaphragm pulls it in. Air moves down a pressure gradient created by the increase in thoracic volume.
Another common slip is stating that a left-shifted curve releases oxygen more readily. A left shift means higher affinity and therefore less ready release.
Many candidates state that carbon dioxide binds to the haem group. It binds to the polypeptide chains, forming carbaminohaemoglobin.
In chloride shift questions, answers frequently omit the reason. Chloride ions enter to maintain electrical neutrality after hydrogencarbonate ions leave.
Finally, candidates often describe counter-current flow without explaining what parallel flow would achieve. The comparison is what demonstrates understanding.
Exam technique for "Gas exchange and transport"
For any exchange surface, structure the answer around the three requirements of Fick's law and give a specific structural feature for each.
When interpreting dissociation curves, always quote partial pressure values from the graph and state the corresponding saturations rather than describing the curves qualitatively.
Use the terms loading and unloading, and state where each occurs. Loading in the lungs at high partial pressure, unloading in respiring tissue at low partial pressure.
For the Bohr effect, take the explanation through the full chain: carbon dioxide, carbonic anhydrase, hydrogen ions, change in haemoglobin structure, reduced affinity, right shift, more oxygen released.
In comparison questions between organisms, use the same headings for each — surface, area, distance, gradient maintenance — so the comparison is genuinely parallel.
Quick revision summary
As organisms grow, surface area to volume ratio falls, so specialised exchange surfaces are needed, all showing large surface area, short diffusion distance and maintained concentration gradient in accordance with Fick's law. Alveoli provide about seventy square metres of surface, walls one cell thick, a dense capillary network and ventilation, with surfactant preventing collapse; ventilation results from pressure changes as the intercostal muscles and diaphragm alter thoracic volume. Fish gills use counter-current flow so a gradient exists along the whole lamella, extracting about eighty per cent of dissolved oxygen against about fifty per cent for parallel flow. Insects deliver gases directly through spiracles, chitin-ringed tracheae and fine tracheoles, closing spiracles to limit water loss. Leaves exchange gases through stomata controlled by turgid guard cells, with spongy mesophyll air spaces giving a large internal surface. Haemoglobin binds four oxygen molecules, and cooperative binding produces the S-shaped dissociation curve; left shifts indicate higher affinity as in foetal haemoglobin, and right shifts lower affinity as in active organisms. The Bohr effect shifts the curve right as carbon dioxide lowers pH, releasing more oxygen where respiration is greatest. Carbon dioxide travels about five per cent dissolved, ten to twenty per cent as carbaminohaemoglobin and eighty-five per cent as hydrogencarbonate, with the chloride shift maintaining electrical neutrality.