Surface area : volume ratio

Substances move in and out of cells by diffusion, osmosis and active transport across the surface. As an organism gets bigger, its volume grows faster than its surface area, so its SA:V ratio falls.

Why it matters: single-celled organisms have a large SA:V and can rely on diffusion alone. Large animals cannot, so they need specialised exchange surfaces (lungs, gut villi) and a transport system (blood).

Your turn — SA:V ratio

Features of a good exchange surface

Efficient exchange surfaces all share the same adaptations, which make diffusion fast:

• Large surface area — more room for substances to cross.
• Thin walls (short diffusion distance) — often one cell thick.
• A good blood supply (in animals) — keeps a steep concentration gradient.
• Ventilation / moist surface — in the lungs, air is refreshed and the surface is moist so gases dissolve.

Key link: the rate of diffusion is higher when the surface area is bigger, the distance is shorter, and the concentration gradient is steeper. Every exchange-surface adaptation ties back to one of these.

Gas exchange in the alveoli

Gas exchange happens in the alveoli — millions of tiny air sacs in the lungs. Oxygen diffuses into the blood; carbon dioxide diffuses out.

Alveoli are excellent exchange surfaces because they have a huge total surface area, very thin (one-cell-thick) walls, a rich blood supply from surrounding capillaries, and a moist lining so gases dissolve.

Why are alveolar walls thin?

The heart & double circulation

The heart is a muscular pump with four chambers: two upper atria and two lower ventricles. Valves stop blood flowing backwards.

Humans have a double circulation: the right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the rest of the body. Blood passes through the heart twice for each full circuit.

Why is the left ventricle thicker? It must generate a higher pressure to push blood all around the body, whereas the right ventricle only pushes blood the short distance to the lungs.

The thickest wall

Arteries, veins & capillaries

Three types of vessel carry the blood, each with a structure suited to its job:

• Arteries — carry blood away from the heart at high pressure. Thick, muscular, elastic walls; a small lumen. (Usually oxygenated — except the pulmonary artery.)
• Veins — carry blood back to the heart at low pressure. Thinner walls, a large lumen, and valves to stop backflow.
• Capillaries — tiny vessels with one-cell-thick walls where substances are exchanged with the tissues.

Whose feature is it?

Tap a feature, then tap the vessel it belongs to.

Components of blood

Blood is a tissue made of four parts, each with a job:

• Red blood cells — carry oxygen. Packed with haemoglobin; a biconcave shape gives a big surface area; they have no nucleus, leaving more room for haemoglobin.
• White blood cells — part of the immune system; engulf pathogens (phagocytosis) and make antibodies. They keep their nucleus.
• Platelets — small cell fragments that help the blood clot at a wound.
• Plasma — the straw-coloured liquid that carries cells, CO₂, urea, glucose, hormones and heat around the body.

Nail this: red blood cells have no nucleus and no mitochondria, which is why they carry so much haemoglobin. White blood cells do keep their nucleus.

Carrying oxygen

Match each job to the part of blood

Tap a job on the left, then its matching component on the right.

Double circulation

Your turn — cardiac output

The big ideas to know

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