Large multicellular organisms cannot rely on diffusion across their body surface for gas exchange, for three main reasons:
• They have a low surface area to volume ratio, so their outer surface is too small to supply all their cells.
• Many cells are far from the surface. Diffusion is only fast over very short distances, so it is far too slow across the long distances inside a large body.
• Many large animals, especially mammals and birds, are active and keep a constant body temperature. They have a high metabolic rate, so they need a lot of oxygen per unit mass.Large organisms solve this in two ways.1. A specialised gas exchange surface. This is folded or branched to give a very large surface area in a small space, and is thin to keep the diffusion distance short. It is often inside the body to reduce water loss. Examples:
• lungs with alveoli in mammals;
• gills with filaments and lamellae in fish;
• tracheae in insects, a system of air-filled tubes that deliver air directly to the tissues;
• the spongy mesophyll and stomata of plant leaves.2. A mass transport system. In most large animals, a circulatory system pumps blood between the gas exchange surface and the tissues. This is a mass transport system: the whole fluid moves, carrying gases much faster than diffusion could. Haemoglobin increases how much oxygen the blood can carry.Ventilation movements, such as breathing in mammals or pumping water over gills in fish, keep fresh air or water at the surface. Together with blood flow, this keeps the concentration gradients steep.
• They have a low surface area to volume ratio, so their outer surface is too small to supply all their cells.
• Many cells are far from the surface. Diffusion is only fast over very short distances, so it is far too slow across the long distances inside a large body.
• Many large animals, especially mammals and birds, are active and keep a constant body temperature. They have a high metabolic rate, so they need a lot of oxygen per unit mass.Large organisms solve this in two ways.1. A specialised gas exchange surface. This is folded or branched to give a very large surface area in a small space, and is thin to keep the diffusion distance short. It is often inside the body to reduce water loss. Examples:
• lungs with alveoli in mammals;
• gills with filaments and lamellae in fish;
• tracheae in insects, a system of air-filled tubes that deliver air directly to the tissues;
• the spongy mesophyll and stomata of plant leaves.2. A mass transport system. In most large animals, a circulatory system pumps blood between the gas exchange surface and the tissues. This is a mass transport system: the whole fluid moves, carrying gases much faster than diffusion could. Haemoglobin increases how much oxygen the blood can carry.Ventilation movements, such as breathing in mammals or pumping water over gills in fish, keep fresh air or water at the surface. Together with blood flow, this keeps the concentration gradients steep.