51

key concepts

51.1

Excretory Systems Regulate Osmotic and Ionic Concentrations

51.2

Animals Excrete Nitrogen as Ammonia, Urea, or Uric Acid

51.3

Invertebrate Excretory Systems Use Filtration, Secretion, and Reabsorption

51.4

The Nephron Is the Basic Functional Unit of Vertebrate Excretory Systems

51.5

The Mammalian Kidney Can Produce Concentrated Urine

51.6

Kidney Function Is Regulated

How Vampire Bats Use Blood as Fast Food

Blood, sweat, and tears have the composition of extracellular fluid and taste salty. The composition and volume of the extracellular fluid are regulated by the excretory system. The challenges to an animal’s excretory system depend on its environment and lifestyle. Some desert animals rarely encounter free water, so they must conserve water by excreting their excess ions and nitrogenous wastes with as little water loss as possible. Animals living in fresh water have the opposite challenge: water continuously enters their bodies by osmosis and with their food, so they must excrete water and conserve ions. Vertebrate animals living in the ocean face a challenge similar to that of the desert dwellers; they lose body water by osmosis and therefore need to conserve water and excrete ions.

The physiological mechanisms animals have to maintain salt and water balance are similar, but they are used in different ways to solve the problems unique to each species. Consider vampire bats, which feed on the blood of animals such as goats and cattle. The bats use their sharp incisor teeth to make a small incision (usually on the ankle or leg of a sleeping victim) and then lap up the blood. A vampire bat feeds only once a day, and has to do so quickly, before the victim awakens and shakes it off.

Blood contains nutritious protein but consists mostly of water. To meet its daily energy needs, a vampire bat must take in a huge volume of blood relative to its body size, and it has to be able to fly back to its roost. To maximize protein intake and still be able to fly, the bat gets rid of water fast. The warm fluid flowing down the victim’s leg is not blood!

Back in the roost, the bat’s challenge is reversed. To digest a high-protein meal, the bat must excrete a lot of nitrogenous waste with minimal loss of water, as it may not get more water until its next meal. So within a few hours, the bat’s excretory system has to switch from rapidly excreting water to conserving water, in spite of the need to excrete nitrogenous wastes in the urine. The vampire bat’s excretory system rapidly switches from producing lots of dilute urine to producing small amounts of very concentrated urine.

How does the excretory system of vampire bats process their high-protein liquid diet?