1The idea in one paragraph

Diffusion is too slow to supply cells more than a fraction of a millimetre from a surface, so large organisms move fluids in bulk through pipes. Mammals pump blood from a heart through arteries, which are built to take high pressure, into capillaries, which are built for exchange, and back through veins, which are built to return blood at low pressure without letting it slip backwards. Plants have no pump. Water is pulled up xylem from root to leaf because it evaporates from the leaves, and the column holds together because water molecules cling to each other. At HL you add the fluid that leaks out of capillaries and back again, the heart that drives it all, and the phloem, where plants push sugar solution from where it is made to where it is used.

2Capillaries: built for exchange

Capillaries are the smallest blood vessels, and they are where the blood actually delivers and collects. Every other vessel only carries. Figure 1 shows a capillary bed and a capillary in section.

The guide names three adaptations.

• A large surface area, from branching and narrow diameters. An arteriole divides into a network of capillaries, each only about 5 to 10 µm across. Many narrow tubes have far more wall area than one wide one carrying the same blood, and they reach within a few cells of every cell in the body. Red blood cells pass in single file, pressed close to the wall, which also shortens the diffusion distance.
• Thin walls. The wall is a single layer of flattened endothelium cells. Oxygen, glucose and carbon dioxide diffuse through it quickly.
• Fenestrations in some capillaries. A fenestration is a pore through the endothelium. Where exchange must be particularly fast or involve larger volumes of fluid, the capillaries are fenestrated: in the kidney, where blood is filtered, and in the villi of the small intestine, where digested food is absorbed.

Capillaries exchange with the internal environment, the tissue fluid bathing the cells, and, in lungs, gut and kidney, with what is effectively the external environment.

3Arteries and veins

Arteries carry blood away from the heart; veins carry it back. Both have walls in three layers, and the difference between them is in the thickness of those layers. Figure 2 draws the two in section, with a capillary for comparison.

• Tunica intima: the inner lining of endothelium, smooth so that blood flows with little friction.
• Tunica media: the middle layer of smooth muscle and elastic fibres. Thick in arteries, thin in veins.
• Tunica externa: the outer layer, mainly tough collagen fibres that stop the vessel over-stretching.

Telling them apart in a micrograph. The guide asks you to use the thickness of the wall relative to the diameter of the lumen. An artery has a thick wall and a narrow, round lumen; its wall is often about as thick as the lumen is wide. A vein has a thin wall and a wide lumen, and because the wall has little to hold it open, the vein is often squashed into an irregular shape on a slide. If you are given measurements, calculate wall thickness ÷ lumen diameter for each vessel: the larger ratio is the artery.

How an artery copes with high pressure. Each contraction of the ventricles forces a surge of blood into the arteries at high pressure. Two tissues in the wall deal with it.

• Elastic fibres stretch as the surge arrives, storing energy, then recoil between beats. The recoil pushes the blood on and keeps the pressure up while the heart is refilling, so the flow becomes steadier as it travels away from the heart. Elastic tissue both withstands the peak and maintains the pressure after it.
• Smooth muscle contracts to narrow the lumen, which keeps the pressure up and controls how much blood reaches each organ. Together with the collagen, the thick muscle layer gives the wall the strength not to burst.

4Measuring the pulse

Each surge of blood stretches the artery wall, and you can feel that as a pulse wherever an artery runs close to the skin over something firm. The two places the guide names are the radial pulse, on the thumb side of the inside of the wrist, and the carotid pulse, at the side of the neck beside the windpipe.

Press gently with the first two fingertips, not the thumb, which has a pulse of its own. Count the beats for a fixed time and scale to a minute.

Counting for 15 s is quick but multiplies any counting error by four. Counting for 30 s or 60 s is more accurate, and repeating the count and taking a mean is more reliable still. Digital methods such as a pulse oximeter on a fingertip or the optical sensor in a smartwatch count continuously and remove human counting error, but they can be thrown by movement, a loose fit or cold fingers. A good comparison of the two methods takes both at the same time on the same person, several times.

5Veins: getting blood back at low pressure

By the time blood has passed through the capillaries its pressure is low, and much of it has to travel upwards against gravity. Veins have two adaptations for this, shown in Figure 3.

• Valves. Pocket-shaped flaps project into the lumen. When blood moves towards the heart it pushes the flaps flat against the wall and passes. If it starts to flow backwards, it fills the pockets, the flaps meet in the middle, and the vein is closed. Valves prevent backflow.
• A thin, flexible wall. Veins run between skeletal muscles. When those muscles contract, they bulge and squeeze the vein, and because the valves only let blood move one way, the squeeze pushes it towards the heart. A thin wall with little muscle is easy to compress; a thick artery wall would not be.

The wide lumen of a vein also matters: it offers little resistance, so blood flows even at low pressure.

6Coronary heart disease, and reading the evidence

Heart muscle works without stopping and needs a continuous supply of oxygen and glucose. It does not get them from the blood in its own chambers; it is supplied by the coronary arteries, which branch from the base of the aorta and run over the surface of the heart.

Occlusion means blockage. Figure 4 shows how it happens.

1. Atherosclerosis. Fatty material, including cholesterol, builds up under the endothelium of the artery wall, forming a plaque (atheroma). The wall thickens and the lumen narrows, so less blood gets through.
2. Thrombosis. If the plaque ruptures, the damaged lining triggers a blood clot. A clot in an already narrow coronary artery can block it completely.
3. Consequence. The part of the heart muscle beyond the blockage receives no oxygen, cannot respire aerobically and starts to die. This is a myocardial infarction, a heart attack. A partial blockage can cause chest pain on exertion, because the muscle's supply cannot keep up with its demand.

Risk factors that make atherosclerosis more likely include smoking, high blood pressure, a high blood concentration of LDL cholesterol, obesity, diabetes, lack of exercise, increasing age, being male, and a family history of heart disease.

Epidemiological data. Most of what we know about these risk factors comes from epidemiology, the study of disease in populations. A typical study plots a possible cause against the rate of disease, as in Figure 5.

The correlation coefficient, r, measures how closely the points follow a straight line: +1 is a perfect positive correlation, 0 is none, −1 a perfect negative one. In Figure 5, r = 0.89, a strong positive correlation: countries with more saturated fat in the diet tend to have more deaths from coronary heart disease.

Here is the nature-of-science point, and it earns marks every time. A correlation does not prove causation. Richer countries may eat more saturated fat and also smoke more, exercise less and have older populations, and any of those could be the real cause. That hidden variable is a confounding variable. What a correlation can do is test a hypothesis: if saturated fat caused heart disease, we would expect a correlation, and finding a weak one or none would be evidence against the hypothesis. Finding a strong one is consistent with it, but a causal link needs other evidence, such as a plausible mechanism and controlled trials.

When you evaluate epidemiological data in Paper 1B, check four things: the strength of the correlation (the value of r); the size and range of the sample; possible confounding variables; and whether the data describe populations or individuals.

7Water transport in plants: the transpiration pull

Water moves from the soil, through the root, up the stem in the xylem, and out of the leaves. No part of the plant pumps it. Figure 6 shows the sequence.

Follow it from the top, because that is where the force comes from.

1. Evaporation in the leaf. Water evaporates from the moist cell walls of the mesophyll cells into the air spaces, and diffuses out through the stomata. This is transpiration, from B3.1.
2. Capillary action in the cell walls. The cellulose walls hold water in their tiny spaces by adhesion. As water evaporates from the surface of a wall, more is drawn through the wall to replace it, and that water comes from the nearest xylem vessel.
3. Tension in the xylem. Pulling water out of the top of the xylem puts the water column under tension, a negative pressure. This tension draws water up the xylem from the roots. The guide is precise about this: it is the tension, generated by loss of water from the leaf cell walls, that draws water up.
4. Cohesion keeps the column continuous. Water molecules are held to each other by hydrogen bonds. This cohesion means that when the top of the column is pulled, the whole column moves up as one, rather than breaking. Adhesion to the xylem walls helps too.
5. Uptake in the root. Water enters root hair cells by osmosis and moves across the root to the xylem, replacing what has been pulled up.

This is called the cohesion–tension mechanism. The energy that drives it comes from the heat that evaporates water in the leaf, not from the plant's ATP. That is why transport of water in xylem is described as passive.

Xylem vessels are built to take tension. Figure 7 shows one.

• No cell contents. Mature xylem vessels are dead: no cytoplasm, no membranes, nothing in the way. Water flows unimpeded.
• Incomplete or absent end walls. Vessel elements are joined end to end and the walls between them have broken down, making a continuous tube many metres long.
• Lignified walls. The walls are thickened with lignin, in rings, spirals or as a solid layer. Because the water inside is under tension, the pull is inward, and without reinforcement the vessel would collapse. Lignin lets it withstand the tension. It also makes xylem a supporting tissue.
• Pits. Gaps in the lignin where only a thin cellulose wall remains. Water moves sideways through pits into and out of the vessel and between neighbouring vessels.

8Where the tissues are: stem and root

The guide asks for plan diagrams of a transverse section of a dicotyledonous stem and root, drawn from micrographs. Figure 8 shows both.

In the stem the vascular bundles sit in a ring near the outside. In each bundle the phloem is on the outer side and the xylem on the inner side. Outside the ring is the cortex, and outside that the epidermis; the centre is pith. Annotate each with its main function:

• epidermis: protects the stem and, with its cuticle, reduces water loss;
• cortex: fills and supports, and can store food;
• xylem: carries water and mineral ions up; the lignified vessels also support the stem;
• phloem: carries sugars and other organic compounds from sources to sinks.

Putting the bundles in a ring near the edge makes the stem stiff against bending, the same reason scaffolding poles are hollow tubes rather than solid rods.

In the root the vascular tissue forms a single central cylinder rather than separate bundles. The xylem forms a star or cross shape in the middle, with the phloem in the spaces between its arms. Around the cylinder is a wide cortex, and outside that the epidermis, whose cells carry root hairs that increase the surface for absorbing water and ions. Putting the vascular tissue in the centre suits a root, which is pulled rather than bent.