A mass transport system moves substances in bulk from one part of an organism to another by mass flow: a whole body of fluid moves, carrying dissolved and suspended substances with it. This is much faster than diffusion over long distances.Mass transport systems in animals share several features:• A transport medium: a fluid that carries substances, such as blood. It is mainly water, which is a good solvent and flows easily.
• Vessels: tubes that carry the medium and direct it to where it is needed, such as arteries, capillaries and veins.
• A pump: a way of creating a pressure difference so the medium flows, such as the heart. In plants, differences in water potential and pressure play this role instead.
• Valves: structures that keep the flow moving in one direction, such as the valves in the heart and veins.
• Exchange surfaces: places where substances enter and leave the transport system, such as the alveoli, the villi of the small intestine and the capillaries in every tissue.Some systems also carry respiratory pigments, such as haemoglobin, which greatly increase the amount of oxygen the medium can carry.Mass flow and diffusion work together. Mass flow carries substances over long distances, keeping concentration gradients steep at each end. Diffusion then moves substances across the short final distances, from the capillaries into cells or from the alveoli into the blood.Heat is also distributed around the body by the transport medium, helping to keep the temperature of all tissues similar.
• Vessels: tubes that carry the medium and direct it to where it is needed, such as arteries, capillaries and veins.
• A pump: a way of creating a pressure difference so the medium flows, such as the heart. In plants, differences in water potential and pressure play this role instead.
• Valves: structures that keep the flow moving in one direction, such as the valves in the heart and veins.
• Exchange surfaces: places where substances enter and leave the transport system, such as the alveoli, the villi of the small intestine and the capillaries in every tissue.Some systems also carry respiratory pigments, such as haemoglobin, which greatly increase the amount of oxygen the medium can carry.Mass flow and diffusion work together. Mass flow carries substances over long distances, keeping concentration gradients steep at each end. Diffusion then moves substances across the short final distances, from the capillaries into cells or from the alveoli into the blood.Heat is also distributed around the body by the transport medium, helping to keep the temperature of all tissues similar.