Transport
🎯What you need to be able to do
- Explain how capillaries are adapted for exchange, and distinguish arteries from veins in micrographs.
- Explain how arteries withstand and maintain high pressure, and how veins return blood to the heart.
- Measure pulse rate, and evaluate epidemiological data on coronary heart disease, including correlation.
- Explain how transpiration pulls water up through xylem, and how xylem vessels are adapted for this.
- Draw plan diagrams of transverse sections of a dicot stem and root.
- AHL Explain the formation, exchange and drainage of tissue fluid, and compare single and double circulation.
- AHL Explain how the heart is adapted to pump blood, and interpret the cardiac cycle and blood pressure data.
- AHL Explain root pressure and how phloem sieve tubes and companion cells translocate sap.
📚The biology
Capillaries: adapted for exchange
Capillaries are where exchange happens between blood and the tissues (or, in the lungs and gut, the external environment). Their adaptations:
They branch repeatedly into huge networks, and their narrow diameter (about the width of one red blood cell) gives a large surface relative to the volume of blood. No cell is far from a capillary.
The wall is a single layer of flattened endothelial cells, giving a very short diffusion distance.
In some capillaries — in the kidney glomerulus, the intestine and endocrine glands — the wall has small pores that allow particularly rapid exchange of fluid and solutes.
Narrow capillaries also slow blood flow, giving more time for exchange.
Arteries and veins
Both have walls with three layers: an inner endothelium, a middle layer of smooth muscle and elastic fibres, and an outer layer of tough collagen. They differ in proportions, and you should be able to tell them apart in a micrograph:
Thick wall relative to the lumen; narrow, round lumen; thick layer of muscle and elastic tissue; the endothelium often appears folded.
Thin wall relative to the lumen; wide lumen, often flattened or irregular in section; little muscle and elastic tissue; valves may be visible.
Arteries carry blood away from the heart
Blood leaves the heart in pulses at high pressure. Arteries are adapted to withstand and even out that pressure:
- Elastic fibres stretch as each surge of blood arrives, so the wall is not damaged, then recoil between beats, pushing the blood on. This keeps pressure high between heartbeats and smooths the flow.
- Smooth muscle makes the wall thick and strong, and can contract to narrow the lumen, maintaining pressure and controlling where blood goes.
- Collagen in the outer layer resists overstretching.
- A narrow lumen helps maintain high pressure.
Measuring pulse rate
Each heartbeat sends a pressure wave along the arteries that can be felt as a pulse where an artery lies near the surface over a bone or firm tissue. Place two fingertips (not the thumb, which has its own pulse) on the radial artery at the wrist or the carotid artery in the neck, count beats for 15 or 30 seconds, and multiply to get beats per minute. Traditional counting can be compared with digital methods such as pulse oximeters and smartwatches, which use light to detect changes in blood volume in the skin.
Veins return blood to the heart
By the time blood reaches the veins, its pressure is low. Veins are adapted to return it anyway:
- Valves prevent backflow, so blood can only move towards the heart.
- The flexible, thin wall can be compressed by surrounding skeletal muscles when they contract. The squeezed blood can only go one way through the valves — towards the heart.
- A wide lumen offers little resistance to flow.
Coronary heart disease
The heart muscle is supplied with blood by the coronary arteries. In coronary heart disease, these arteries become narrowed or blocked (occluded).
Causes: fatty deposits (atheroma, containing cholesterol) build up under the endothelium, forming plaques that narrow the lumen and harden the artery. If a plaque ruptures, a blood clot can form and block the artery completely. Risk factors include high blood cholesterol, a diet high in saturated fat, smoking, high blood pressure, obesity, lack of exercise, diabetes, age and genetics.
Consequences: the heart muscle beyond the blockage receives less oxygen. Partial blockage causes chest pain during exertion (angina); complete blockage causes the cells to die — a heart attack (myocardial infarction).
Much of the evidence is epidemiological: comparing the incidence of disease across populations with different lifestyles. Evaluating such data needs care. A correlation coefficient quantifies how strongly two variables are related. A low coefficient, or no correlation, is evidence against a hypothesis. But even a strong correlation — such as that between saturated fat intake and coronary heart disease — does not prove causation: another factor might cause both, or the relationship might run the other way. Causation needs further evidence, such as a plausible mechanism and controlled trials.
Water transport in xylem
Plants have no pump for water. It is pulled up from the roots by transpiration:
- Water evaporates from the moist walls of mesophyll cells into the air spaces of the leaf and diffuses out through stomata (B3.1).
- This loss is replaced by water drawn through the cell walls by capillary action, because water adheres to the cellulose (A1.1).
- This draws water out of the nearest xylem vessels, creating tension (a negative pressure potential) in the xylem.
- The tension pulls water up the xylem. Because water molecules are held together by cohesion, the column stays continuous all the way from the roots to the leaves.
This is the cohesion–tension mechanism. Energy for it comes from the Sun, which evaporates the water; the plant uses no ATP to move water up the stem.
Adaptations of xylem vessels
Xylem vessels are dead when mature: no cytoplasm, nucleus or membranes, so water flows unimpeded.
Cells are joined end to end with their end walls broken down, forming long continuous tubes.
Walls thickened with lignin, often in rings or spirals, are strong enough to withstand tension without collapsing inwards.
Gaps in the lignin where only the thin primary cell wall remains allow water to enter and exit the vessels sideways.
Tissues in a dicot stem
In a plan diagram of a transverse section of a dicotyledonous stem, from the outside in:
- Epidermis — outer protective layer, with a waxy cuticle to reduce water loss.
- Cortex — a wide region of packing cells beneath the epidermis, providing support and sometimes storing food.
- Vascular bundles arranged in a ring near the edge of the stem, each with phloem on the outside (transports sugars) and xylem on the inside (transports water and mineral ions, and gives support).
- Pith — the central region of packing cells.
The ring arrangement of vascular bundles near the outside helps the stem resist bending.
Tissues in a dicot root
- Epidermis — with root hairs that increase surface area for water and mineral uptake.
- Cortex — a wide region through which water passes to the centre.
- Vascular tissue in the centre of the root: xylem often in a star or cross shape, with phloem between the arms of the star.
A central core of vascular tissue suits a root, which must resist being pulled rather than bent.
Tissue fluid: formation and reuptake AHL
Cells are bathed in tissue fluid, through which substances pass between blood and cells. Tissue fluid forms by pressure filtration:
- At the arteriole end of a capillary bed, blood arriving from the arterioles is at relatively high pressure. This forces plasma out through the capillary walls into the spaces between cells.
- At the venule end, blood pressure is lower. Tissue fluid drains back into the capillaries.
Plasma and tissue fluid AHL
Tissue fluid is essentially plasma without the large proteins. Water, glucose, amino acids, fatty acids, ions, oxygen and hormones pass through the capillary wall, but plasma proteins are too large, and red blood cells and platelets remain in the blood.
Exchange between tissue fluid and cells happens across the plasma membrane by the usual mechanisms — diffusion, facilitated diffusion, active transport — so that oxygen and nutrients move into cells, while carbon dioxide, urea and other wastes move out into the tissue fluid, and from there back into the blood.
Lymph AHL
Not all tissue fluid returns to the capillaries. The excess drains into lymph ducts, where it is called lymph. Lymph ducts have:
- thin walls with gaps between the cells, so tissue fluid can enter easily;
- valves that prevent backflow, so lymph moves in one direction, squeezed along by surrounding muscles.
Lymph ducts join into larger vessels, which return lymph to the blood in veins near the heart. Without this drainage, fluid would accumulate in the tissues.
Single and double circulation AHL
heart → gills → body organs → heart
Blood passes through the heart once per circuit. Pressure drops greatly in the gill capillaries, so blood flows slowly to the body.
heart → lungs → heart → body organs → heart
Blood passes through the heart twice per circuit: a pulmonary circulation to the lungs and a systemic circulation to the body. After the lungs, the heart re-pressurizes the blood, so it reaches the body at high pressure, supplying the high metabolic demands of an endotherm.
The mammalian heart AHL
You should be able to identify these features on a diagram of the heart in the frontal plane (as if looking at a person facing you, so the left side of the heart is on the right of the diagram), and state how each is adapted:
Contracts without fatigue throughout life; branched cells spread the signal so the chambers contract as a unit.
A group of cells in the wall of the right atrium that initiates each heartbeat.
Thin-walled chambers that receive blood from veins and pump it a short distance into the ventricles.
Thick muscular walls to pump blood out at high pressure. The left ventricle wall is much thicker than the right, as it pumps blood round the whole body; the right pumps only to the nearby lungs.
Between atria and ventricles. Close when ventricles contract, preventing backflow into the atria.
At the bases of the aorta and pulmonary artery. Close when ventricles relax, preventing backflow into the ventricles.
Wall dividing left and right sides, keeping oxygenated and deoxygenated blood separate.
Supply the heart muscle itself with oxygen and nutrients.
Tracing the flow: vena cava → right atrium → atrioventricular valve → right ventricle → semilunar valve → pulmonary artery → lungs → pulmonary veins → left atrium → atrioventricular valve → left ventricle → semilunar valve → aorta → body.
The cardiac cycle AHL
Each heartbeat is initiated by the sinoatrial node, which sends an electrical signal through the walls of the atria. On the left side of the heart:
- Atrial systole. The atria contract, raising their pressure slightly and pushing the remaining blood through the open atrioventricular valve into the ventricle.
- Ventricular systole. After a short delay (while the signal passes to the ventricles), the ventricle contracts. Its pressure rises above that in the atrium, so the atrioventricular valve closes. When ventricular pressure rises above the pressure in the aorta, the semilunar valve opens and blood is ejected into the aorta.
- Diastole. The ventricle relaxes and its pressure falls. When it drops below aortic pressure, the semilunar valve closes. When it drops below atrial pressure, the atrioventricular valve opens, and blood flows from the atrium (which has been filling from the pulmonary veins) into the ventricle.
The key principle: valves open and close because of pressure differences on either side of them. Blood always flows from higher to lower pressure, and valves stop it flowing backwards.
Blood pressure is recorded as two numbers, such as 120/80 mmHg. The higher, systolic pressure is the peak in the arteries during ventricular systole; the lower, diastolic pressure is the minimum during diastole, maintained by the elastic recoil of the arteries.
Root pressure AHL
Sometimes transpiration is too slow to move water — when high humidity prevents it, or in spring before the leaves of deciduous trees have opened. Plants can then generate root pressure, a positive pressure potential in the xylem.
Cells in the root actively transport mineral ions into the xylem, using ATP. This lowers the water potential of the xylem sap, so water follows by osmosis from surrounding cells. The build-up of water pushes sap up the xylem. Root pressure is why sap oozes from a cut stem, and why droplets can appear at leaf margins in the early morning.
Phloem and translocation AHL
Translocation is the transport of sap — mainly sucrose and amino acids dissolved in water — in phloem, from sources (where carbon compounds are made or released, such as leaves, or storage organs being used up) to sinks (where they are used or stored, such as roots, fruits, growing tips).
Sieve plates: end walls with large pores, letting sap flow between cells.
Reduced cytoplasm and organelles, and no nucleus, leaving an open channel for flow.
Many mitochondria, supplying ATP for active loading of sucrose.
A nucleus and full set of organelles, keeping the sieve tube element alive.
Plasmodesmata: cytoplasmic connections to the sieve tube element, through which substances pass.
At a source, companion cells use ATP to actively load sucrose into sieve tubes. This lowers the water potential of the sap, so water enters by osmosis from nearby xylem, raising the pressure. At a sink, sucrose is unloaded and used, water leaves, and pressure falls. Sap flows along the pressure gradient from source to sink. These adaptations ease the flow of sap and enhance the loading and unloading of carbon compounds.
✏️Worked example
(a) Calculate her heart rate in beats per minute, and the time taken for one cardiac cycle.
(b) Her stroke volume (volume pumped by the left ventricle per beat) is 70 cm3. Calculate her cardiac output in dm3 per minute.
(c) Explain why she should count for at least 15 seconds rather than 5.
(d) AHL During the cardiac cycle, the pressure in the left ventricle rises from 10 to 120 mmHg while the pressure in the aorta is 80 mmHg. State which valves are open and which closed when ventricular pressure is (i) 50 mmHg, rising, and (ii) 110 mmHg, rising.
(a) 15 s is a quarter of a minute, so heart rate = 18 × 4 = 72 beats per minute. Time for one cycle = 60 ÷ 72 = 0.83 s.
(b)
(c) Counting for only 5 seconds gives very few beats, so missing or adding a single beat, or starting and stopping the count slightly early or late, causes a large percentage error, which is then multiplied by 12. A longer count reduces the effect of these errors and gives a more accurate rate.
(d) (i) At 50 mmHg the ventricle is contracting and its pressure is above the atrium’s, so the atrioventricular valve is closed; it is still below aortic pressure (80 mmHg), so the semilunar valve is closed. Both valves are closed. (ii) At 110 mmHg ventricular pressure exceeds aortic pressure, so the semilunar valve is open and blood is being ejected; the atrioventricular valve is closed.
📝Practise
Work through these on paper, then reveal the answer. Questions 5 and 6 are AHL.
1. Describe how you could distinguish an artery from a vein in a light micrograph of a transverse section.
2. Explain how the structure of arteries enables them to maintain high blood pressure.
3. A study finds a strong positive correlation between saturated fat intake and coronary heart disease across twenty countries. Explain why this does not prove that saturated fat causes the disease.
4. Explain how water moves from the roots to the leaves of a tree.
5. AHL Explain how tissue fluid is formed and returned to the blood.
6. AHL Explain how the structure of phloem sieve tubes and companion cells is adapted for translocation.
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- British Heart Foundation — clear diagrams and animations of the heart, the cardiac cycle and coronary heart disease.
- Histology Guide (University of Leeds) — micrographs of arteries, veins and capillaries, and of dicot stem and root sections.
- Science & Plants for Schools (SAPS) — practical guides to transpiration and phloem transport.