Swamps of Capillaries

Capillaries are the exchange vessels and have the smallest inside diameters of all the vessels. However, there are so many capillaries that their combined inside diameters are greater than the inside diameters of the arteries, so all arterial blood can pass through capillaries without pooling. Arterioles reduce the perfusion pressure of arterial blood to the lower blood pressures of capillaries. Blood flow in capillaries is very slow and is not uniform. In one capillary, flow may slow down,...

Countercurrent Heat Exchanger

Honeybees fly at ambient temperatures up to forty-five degrees Celsius when other 'bee-sized' insects cannot. Bees employ an efficient counter-current heat exchanger that apposes two oppositely directed flows of hemolymph in close proximity to control heat loss and heat gain to maintain heat balance. If one flow is of a higher temperature, heat passively flows downhill from higher to lower temperature across the wall separating the flows. At warm-up, hemolymph flowing aft conveying heat from...

Feedback and Bee Stings

Negative feedback opposes change and restores stability. Positive feedback amplifies a disturbance's effects. Both types of feedback control a disturbance and may occur within the same system. A predator receives a bee-sting. If he moves away from the hive, it is negative feedback. But if the disturbance continues, angry bees may entice others to sting, until the entire colony emerges stinging. If the predator and the angry bees all die, together they have displayed positive feedback.