B3.2 Transport

Blood flows from the heart through arteries, which branch into narrow arterioles, then through capillaries, and back to the heart through venules and veins. Each type of vessel has a wall suited to its job.

Capillaries: adapted for exchange

Capillaries are where materials are exchanged between the blood and the tissues (or, in the lungs and gut, with the outside world). They have three key adaptations:

• Large surface area. Capillaries branch and rejoin repeatedly to form dense capillary networks, so their total surface area is enormous and every active cell is close to one. Their narrow lumen (about 5–10 µm) means red blood cells pass in single file, close to the wall.
• Thin, permeable wall. The wall is a single layer of very thin endothelial cells on a basement membrane, so the diffusion distance is tiny. Small gaps (pores) between the cells let water and small solutes leak out to form tissue fluid, while blood cells and most plasma proteins stay in.
• Fenestrations. Fenestrated capillaries also have larger pores through the endothelial cells themselves. They are found where exchange must be especially rapid, such as the glomerulus of the kidney (filtration of blood), the villi of the small intestine (absorption) and endocrine glands (release of hormones).

Telling arteries from veins in micrographs

Each time the ventricles contract (systole), a surge of blood at high pressure enters the arteries; between beats (diastole) the heart pumps nothing into them. Artery walls are built to cope with this.

• Collagen fibres in the tunica externa (and media) are very strong in tension, so the artery does not bulge outwards (an aneurysm) or burst under the highest pressures.
• Elastic fibres (elastin) in the tunica media are stretched by each surge, storing potential energy. When the ventricles relax, the fibres recoil, squeezing the blood onwards. This keeps arterial pressure from falling to zero between beats (it stays around 80 mmHg in large arteries) and makes the flow more continuous.
• Smooth muscle in the tunica media is arranged in circles. Its contraction narrows the lumen (vasoconstriction); relaxation widens it (vasodilation). Arterioles have the most muscle relative to their size, so they control how much blood each tissue receives.
• A thick wall and a relatively narrow lumen help to maintain high pressure; a smooth endothelium in the tunica intima reduces friction.

The stretch and recoil of the artery wall with every heartbeat is what you feel as the pulse. By the time blood reaches the capillaries, its pressure has fallen and the pulse has smoothed out.

There is one pulse for every beat of the heart, so the pulse rate equals the heart rate. You can feel it wherever an artery runs close to the skin over something firm:

• the radial pulse, on the palm side of the wrist below the thumb;
• the carotid pulse, in the neck in the soft groove beside the windpipe (press gently and on one side only).

Method

1. Rest quietly for a few minutes first if you want a resting rate.
2. Place the tips of your index and middle fingers on the artery. Do not use your thumb, which has a pulse of its own.
3. Count the beats for a fixed time and convert to beats per minute (bpm): ×4 for 15 s, ×2 for 30 s, ×1 for 60 s.
4. Repeat and calculate a mean.

A resting adult heart rate is usually about 60–100 bpm.

Traditional versus digital methods

Digital methods include pulse oximeters, which clip on a fingertip, shine light through it and detect the change in light absorption as each pulse of blood arrives; fitness watches, which use a similar optical sensor; and chest-strap monitors, which detect the heart’s electrical activity. They are quick, give continuous readings and avoid counting errors, but they can be thrown off by movement, cold fingers or a loose fit. Counting by hand needs no equipment but depends on the counter. A short count is quicker, but each missed or extra beat changes a 15 s result by 4 bpm.

By the time blood has passed through capillaries, its pressure is low and steady: there is no pulse in veins. Veins are adapted to get this slow-moving blood back to the heart.

• Valves. Many veins, especially in the limbs, contain pocket valves: flaps of tissue attached to the wall like pockets. Blood flowing towards the heart pushes the flaps against the wall, so the valve is open. If blood starts to flow backwards, it fills the pockets, which meet in the middle and close the vein. Blood can therefore only move one way, towards the heart.
• Thin, flexible walls. Veins have a thin tunica media with little muscle or elastic tissue. When nearby skeletal muscles contract, they bulge and press on the veins, squeezing them flatter and pushing blood along; the valves make sure it goes towards the heart. This is sometimes called the skeletal muscle pump, and it is one reason why walking helps prevent blood pooling in the legs.
• Wide lumen. A large lumen gives little resistance to flow, so the low pressure is enough to move a large volume of blood. At any one time most of the blood in the body is in the veins.

Cardiac muscle works without stopping, so it needs a constant supply of oxygen and glucose for aerobic respiration. This comes not from the blood inside the heart’s chambers but from the coronary arteries, the first branches of the aorta, which run over the surface of the heart.

Causes

In atherosclerosis, fatty deposits called plaques (atheroma), made of lipids including cholesterol, fibrous tissue and white blood cells, build up in the walls of arteries just beneath the endothelium.

1. The plaque narrows the lumen and makes the artery wall less elastic, so blood flow is reduced.
2. If the surface of a plaque ruptures, a blood clot (thrombus) forms there. The clot can block the artery completely: occlusion.

Consequences

• A narrowed coronary artery may supply enough blood at rest but not during exercise, when cardiac muscle needs more oxygen. The result is chest pain called angina and shortness of breath on exertion.
• If a coronary artery is occluded, the cardiac muscle it supplies receives no oxygen. It cannot respire aerobically, stops contracting and begins to die: a myocardial infarction (heart attack). The heart may beat irregularly or stop.

Together these conditions are called coronary heart disease (CHD).

Evaluating epidemiological data

Epidemiology studies the patterns and causes of disease in populations. It has identified risk factors for CHD, including high blood pressure, smoking, high blood cholesterol (especially LDL), obesity, lack of exercise, diabetes, older age, being male and a family history of CHD.

A correlation coefficient (r) quantifies how strongly two variables are linearly related. It runs from −1 (perfect negative correlation) through 0 (no linear correlation) to +1 (perfect positive correlation).

• A low or zero correlation coefficient is evidence against a hypothesis that one variable affects the other.
• A high correlation coefficient supports a hypothesis but does not prove causation. For example, countries where people eat more saturated fat tend to have higher CHD rates, but those countries may also differ in smoking, exercise, wealth or other foods. These confounding variables could explain the pattern.

Plants move large volumes of water from the roots to the leaves in xylem vessels, with dissolved mineral ions. Most of this water is lost from the leaves by transpiration. Remarkably, the movement is entirely passive.

1. Evaporation. In the leaf, water evaporates from the moist cell walls of the spongy mesophyll cells into the air spaces. The water vapour then diffuses out through the stomata into the drier air outside (see B3.1 Gas exchange).
2. Capillary action in cell walls. Cellulose in cell walls is hydrophilic. The water lost is replaced by water drawn through the narrow spaces between the cellulose fibres, by adhesion of water to cellulose and cohesion between water molecules. This water is pulled from the nearest xylem vessels in the leaf veins.
3. Tension in the xylem. Pulling water out of the xylem puts the water inside under tension (a negative pressure). Because of cohesion (hydrogen bonding between water molecules, see A1.1 Water), the whole column of water in the xylem is pulled upwards as a continuous thread, from the roots to the leaves. Adhesion to the vessel walls also helps.
4. Water enters the roots. As water is pulled up, it is replaced by water moving across the root from the soil.

This is the transpiration pull, or cohesion–tension mechanism. The energy comes from the heat that causes evaporation, ultimately from the Sun; no ATP is used to move water, and the vessels themselves are dead.

Xylem vessels develop from columns of cells arranged end to end. As they mature, the cells die and their structure is modified:

• No cell contents. The plasma membranes, cytoplasm and nuclei break down, leaving an empty tube. Nothing obstructs the flow, and the movement of water must be passive.
• No end walls (or incomplete ones). The end walls between the cells are removed, so each vessel is a long continuous tube in which flow of xylem sap is unimpeded.
• Lignified walls. The side walls are thickened and impregnated with lignin, a strong, rigid polymer. Under tension, water pulls inwards on the walls; lignin stops the vessels collapsing. In young xylem the thickening is laid down as rings or a spiral; in older xylem it covers most of the wall.
• Pits. Gaps in the lignified thickening, where only a thin layer of cellulose wall remains, let water move sideways into and out of the vessel, for example into neighbouring vessels or out to the cells of a leaf.

Lignified xylem also gives the plant mechanical support: wood is mostly xylem.

A plan diagram shows the outlines of tissues in an organ, without drawing individual cells. You need to draw plan diagrams of transverse sections (TS) of the stem and root of a dicotyledonous plant from micrographs, and annotate them with the functions of the tissues.

Stem

In a young dicot stem, the vascular bundles are arranged in a ring near the outside. In each bundle, phloem is on the outer side and xylem on the inner side, separated by a thin layer of cambium. Outside the ring is the cortex, covered by the epidermis; inside the ring is the pith.

Root

In a dicot root the vascular tissue is in the centre. The xylem forms a star or cross shape, with groups of phloem between its arms. The vascular tissue is surrounded by a ring of endodermis, then a wide cortex and an outer epidermis, many of whose cells grow root hairs.