International A Level Biology / Mammalian transport systems

Arteries, veins and capillaries

Edexcel International A Level Biology (YBI11), chapter 2: Mammalian transport systems, specification point 1.7. Free lesson.

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Arteries, veins and capillaries have structures suited to the pressure of blood and the jobs they do. Understanding these adaptations also explains how tissue fluid forms around cells and returns to the blood.

How each layer of an artery wall helps

An artery carries blood away from the heart at high pressure. Each part of its wall helps it cope with that pressure or keep blood flowing. In exam answers, link each feature to its reason: saying a wall is thick describes it, but explaining what the thickness does earns the explanation. Endothelium is a smooth lining of flat cells, so there is little friction as blood flows. Elastic fibres stretch when a surge of blood arrives, then spring back. Smooth muscle can contract to narrow the artery and control flow. Collagen fibres make the outer wall tough, so it does not…

How elastic recoil smooths blood flow

Blood enters an artery in surges as the ventricle contracts. The artery wall stretches to accommodate the surge. When the ventricle relaxes, the stretched wall recoils and pushes blood onwards, so pressure does not fall sharply between beats. Elastic recoil evens out the surges, making blood flow smoother. It keeps artery pressure from dropping close to zero between heartbeats.

How veins and capillaries are adapted

Blood pressure in veins is low, so their structure helps blood keep moving and prevents it flowing backwards. Capillaries are the exchange vessels. Their thin wall allows substances to move between blood and tissues over a short distance. A vein lumen is wide, which creates less resistance to blood flow. A vein wall is thin, with little muscle or elastic tissue, because it does not need to withstand high pressure. Valves stop blood flowing backwards. Surrounding muscles squeeze blood towards the heart. A capillary wall is one endothelial cell thick, giving a short diffusion…

How tissue fluid forms and returns

Tissue fluid is plasma, the liquid part of blood, that leaves capillaries and bathes the cells. At the arterial end, hydrostatic pressure from the heart's pumping is high. It forces water and dissolved substances through gaps in the capillary wall. Large plasma proteins stay in the blood because they are too big to leave. The fluid carries water, glucose, amino acids, ions and oxygen to the spaces around cells. Cells take what they need and pass carbon dioxide and other wastes back. Plasma proteins remain in the blood and make its water potential lower than that of tissue fluid. This…

Comparing artery and vein wall thickness

To compare wall thickness with lumen diameter, subtract the wall thickness on both sides of the vessel from its external diameter. Then write the wall-to-lumen ratio and simplify it so one value is 1.

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