Glossary: Transport in Humans and Animals

This chapter's terms in context

Chapter 10 organises five terms around how blood is moved through, and exchanges substances with, the human body: artery, vein, capillary, haemoglobin and double circulation. An artery is a blood vessel that carries blood away from the heart, and because it must withstand the high pressure generated directly by each heartbeat, its wall is thick, muscular and elastic, with a narrow lumen; the elastic recoil of an artery wall between heartbeats is exactly why a pulse can be felt. A vein carries blood back towards the heart at much lower pressure, so its wall is thinner and less muscular, its lumen is wider, and, critically, it contains valves at intervals along its length to prevent the backflow of blood, a structural feature arteries do not need because blood inside them is always moving forward under pressure from the heart.

A capillary is the smallest type of blood vessel, connecting the smallest arteries (arterioles) to the smallest veins (venules), and its wall is only one cell thick, allowing the direct diffusion of oxygen, nutrients, carbon dioxide and other waste products between the blood and the surrounding body tissues, capillaries are therefore the only blood vessels across which exchange actually happens, while arteries and veins exist purely to transport blood to and from them. A common exam error is describing exchange as occurring in arteries or veins, when structurally their thick walls make this impossible; only the capillary's single-cell-thick wall permits it.

Haemoglobin is the iron-containing pigment inside red blood cells that gives blood its red colour and carries oxygen from the lungs to respiring tissues throughout the body, binding to oxygen where oxygen concentration is high (in the lungs, forming oxyhaemoglobin) and releasing it where oxygen concentration is low (in respiring tissues). A red blood cell is biconcave in shape and lacks a nucleus specifically to maximise the space available for haemoglobin and increase the surface area for gas exchange, a structure-function link examiners frequently ask candidates to explain in full rather than simply state.

Double circulation describes the specific arrangement of the human circulatory system in which blood passes through the heart twice for each complete circuit of the body: once through the pulmonary circulation, where deoxygenated blood is pumped from the heart to the lungs to be oxygenated and returns to the heart, and once through the systemic circulation, where oxygenated blood is pumped from the heart to the rest of the body and returns deoxygenated. This double-pass arrangement keeps oxygenated and deoxygenated blood almost entirely separate and allows blood to be re-pressurised by the heart between the lungs and the rest of the body, delivering oxygen more efficiently than a single circulation would, a comparative point frequently tested against the single circulatory system of a fish.

Together, these five terms explain not only what each vessel and molecule does individually, but why the human circulatory system is built as a closed, double, pressurised system rather than a simpler open one, a design question examiners return to across several chapters, including immunity and the lymphatic system, where the same vessels reappear in a different functional context.

Paper 2 questions on this chapter frequently ask a candidate to explain why a particular blood vessel is suited to its function, supplying an unfamiliar cross-section diagram rather than a memorised one, and the strongest answers describe wall thickness, lumen size and the presence of valves together rather than naming only one feature. A recurring scoring gap appears when candidates correctly identify a vessel as an artery or vein from its structure but fail to explain why that specific feature (a thick muscular wall, for instance) suits the pressure the vessel experiences, treating structure and function as separate facts rather than a connected explanation. The double circulation diagram is also frequently tested as a labelling task on an unfamiliar heart diagram, so practising the four chambers and major vessels in both directions of blood flow closes a common source of lost marks.