Arteries, Capillaries & Veins

Explain means cause and effect, and the dot point asks you to work through a model. A model earns its place only if you also say what it cannot show, so the limitation is part of the answer rather than an afterthought.

6 marks. One structure-function link for each vessel, a model used to demonstrate them, and one honest limitation.

A simple physical model uses three everyday materials: a thick, springy rubber tube for an artery, a bundle of very fine porous tubing for a capillary network, and a soft floppy tube containing a paper flap for a vein. Laid side by side, the three make the differences in wall thickness, lumen width and internal structure immediately visible.

An artery has a thick, muscular and elastic wall and a narrow lumen, and it carries blood away from the heart. The thick elastic wall lets it withstand the high pressure of blood pumped from the heart without bursting, and smooths out the surges between beats: the wall stretches as blood is forced in and recoils afterwards, keeping blood moving between contractions. The narrow lumen helps maintain that pressure. Arteries have no valves, because the pressure itself keeps blood moving in one direction. In the model, the stiff springy tube demonstrates this directly — squeeze it and it resists, then springs back.

A capillary has a wall only one cell thick and a very small lumen, and its function is exchange with the surrounding tissue. The single-cell wall gives an extremely short diffusion distance, so oxygen, glucose and wastes cross quickly, and the vast number of capillaries provides an enormous total surface area for that exchange. The lumen is so narrow that red blood cells pass almost single file, which keeps every cell close to the wall. In the model, fine porous tubing shows the narrow bore and permeability.

A vein has a thinner, less muscular wall, a wide lumen and valves along its length, and it returns blood to the heart. By the time blood reaches the veins the pressure is low, so a thick wall is unnecessary, and the wide lumen offers little resistance to that low-pressure flow. Because pressure alone is insufficient to prevent blood falling back, the one-way valves close if blood begins to flow backward, so that blood squeezed along by the surrounding skeletal muscles can only travel toward the heart. In the model, the paper flap inside the floppy tube demonstrates exactly this, allowing flow one way and blocking it the other.

The model demonstrates the relationships clearly but simplifies in ways worth naming. No everyday material is anywhere near one cell thick, so the model cannot represent the capillary wall to scale; the rubber tube is not living elastic tissue and a paper flap is far stiffer than a real venous valve; and the model is static, so it cannot show exchange actually occurring or elastic recoil driving flow. A model of this kind demonstrates structure-function relationships, not the processes themselves.

Analyse asks you to identify the components of a situation and the relationships between them. The apparent paradox here is the point of the question: intuition says a narrow tube speeds flow up, and intuition is looking at the wrong quantity.

5 marks. Name why the intuition fails, identify the quantity that actually governs flow speed, apply it, then link the result to the capillary's function.

The intuition comes from a single tube: narrowing one pipe while the same volume flows through does make the fluid move faster. But the capillaries are not one narrow pipe in place of the artery. A single artery branches into arterioles and then into an immense number of capillaries that carry the blood in parallel.

Flow speed depends on the total cross-sectional area of all the vessels at that level, not on the width of any one of them. Although each capillary is extremely narrow, there are so many billions of them that their combined cross-sectional area is far greater than that of the artery supplying them — by a very large factor.

The same volume of blood per second must pass every level of the circulation. Spread across a far larger total cross-sectional area, that volume moves much more slowly through the capillary bed than through the artery. Blood then speeds up again as the capillaries converge into venules and veins and the total cross-sectional area falls.

The slow flow is not an inefficiency but an adaptation. The capillary is the only site where exchange with tissue cells occurs, and diffusion takes time, so blood lingering in the capillary bed maximises the opportunity for oxygen and glucose to leave and for carbon dioxide and wastes to enter. Slow flow therefore works with the capillary's other features — a wall one cell thick for a short diffusion distance, and a huge total surface area — so that all three combine to make exchange as complete as possible. By contrast the artery, whose job is rapid delivery rather than exchange, has a narrow lumen and fast flow, and the vein, whose job is low-resistance return, has a wide lumen.