Comparative Circulation and Gas Exchange in Animals

Core Idea

Diffusion is fast only over short distances, so any animal more than a few cell layers thick needs a way to bring oxygen close to every cell. Very small or thin animals rely on diffusion alone, while larger ones use a circulatory system, either open (fluid bathes the organs directly) or closed (blood stays inside vessels). Gas exchange surfaces include skin, gills, tracheal tubes and lungs, each matched to life in water or air. Among vertebrates, hearts range from two chambers with a single circuit in fish to four chambers with two fully separate circuits in birds and mammals.

How It's Best Learned

Line up a flatworm, an insect, a fish, a frog and a mammal and trace the path of one oxygen molecule from the outside world to a muscle cell in each. Draw the vertebrate hearts side by side with arrows for oxygen-rich and oxygen-poor blood. A model of countercurrent flow, with two streams passing in opposite directions, shows why gills extract oxygen so efficiently.

Common Misconceptions

• Insect blood carries oxygen to the cells, as ours does. (Insects deliver air directly to their tissues through branching tracheal tubes; their hemolymph mainly carries nutrients and wastes.)
• An open circulatory system is a primitive failure. (Open systems need less pressure and less energy and serve arthropods and most molluscs well.)
• Amphibians and reptiles have "imperfect" hearts that mix blood by mistake. (Their three-chambered hearts allow blood to bypass the lungs when useful, such as during diving, and fit their lower metabolic rates.)

Explainer

Oxygen reaches cells by diffusion, and diffusion is quick over a fraction of a millimeter but very slow over centimeters. The rate also rises with surface area and with the difference in oxygen concentration, and it falls as the barrier gets thicker. Every gas exchange and circulatory system can be read as a way to increase area and gradient and to shrink distance.

Small, thin animals need no special system. A flatworm is so flat that most cells are close to the surface. Cnidarians such as hydras have a gastrovascular cavity that brings water close to inner cells.

Larger animals pump fluid through the body. In an open circulatory system, found in arthropods and most molluscs, the heart pumps a fluid called hemolymph into spaces around the organs, where it bathes the cells directly before returning. In a closed system, found in annelids, octopuses and squids, and vertebrates, blood stays inside vessels and exchanges materials across capillary walls. Closed systems can keep higher pressures and send blood quickly to active tissues.

Gas exchange surfaces must be moist, thin and large. Earthworms and many amphibians exchange gases through their skin. Fish use gills, thin folds bathed by water. In gills, water and blood flow in opposite directions. This countercurrent exchange keeps the water next to the blood richer in oxygen along the whole capillary, so a fish can extract a large fraction of the dissolved oxygen. Insects use a tracheal system of air-filled tubes that open at spiracles and branch to the cells. Lungs are internal pockets. Mammals ventilate them by negative pressure, drawing air in and out of the same path. Birds have air sacs that drive air in one direction through rigid lung tubes, which gives highly efficient exchange.

Vertebrate hearts show a clear series. Fish have a two-chambered heart (one atrium, one ventricle) and a single circuit: heart, then gills, then body. Pressure drops in the gill capillaries, so blood reaches the body slowly. Amphibians have three chambers (two atria, one ventricle) and a double circulation, with one circuit to the lungs and skin and one to the body, though some mixing occurs in the ventricle. Most reptiles also have three chambers with a partly divided ventricle; crocodilians have four. Birds and mammals have four chambers and two fully separate circuits. Oxygen-rich blood is pumped at high pressure to the body, and the lungs receive blood at low pressure. This supports the high metabolic rates of endotherms. Birds and mammals evolved their four-chambered hearts independently.