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The kidney is the major excretory organ of vertebrates. Its functional unit is the nephron, which includes a glomerulus that filters blood and a renal tubule that secretes and reabsorbs solutes, modifying the filtrate to produce urine. As seen in freshwater fishes, the nephron evolved as a mechanism for excreting excess water while conserving valuable solutes. Vertebrates have various adaptations for conserving water. Marine bony fishes produce very little urine but lose nitrogenous waste as ammonia across their gills and also excrete salts across their gills. Marine cartilaginous fishes produce urea and retain it, so their extracellular fluids are hyperosmotic to the environment and therefore take up water by osmosis. Many terrestrial vertebrates conserve water by excreting nitrogenous waste as uric acid, by minimizing evaporative water loss, and by active secretion of salts. Mammals have the ability to conserve water by producing urine that is more concentrated than their extracellular fluid.
learning outcomes
You should be able to:
Relate structural and behavioral features of amphibians and reptiles to their abilities to minimize water loss.
Describe the organization of blood vessels in the nephron.
Explain the mechanism of filtration of blood plasma from the glomerular capillaries into Bowman’s capsule.
Explain why the renal filtrate changes in composition as it passes through the nephron.
What is the connection between the need to conserve water and the evolution of internal fertilization?
To move into more arid terrestrial habitats, vertebrates had to minimize their water loss and their need to have access to standing water to practice external fertilization. Whereas both amphibians and reptiles evolved various adaptations to decrease evaporative water loss from their skins, only reptiles gained independence from needing standing water to reproduce, by the evolution of the amniote egg. However, the amniote egg requires fertilization before it is encased in a protective shell, and thus the evolution of internal fertilization paralleled the evolution of the amniote egg.
What are the roles of the afferent and efferent arterioles in the nephron?
The afferent arterioles bring blood into the glomerulus at high pressure to support filtration. The efferent arterioles carry the remaining blood from the glomerulus to the capillaries serving the renal tubules, where secretion and reabsorption of solutes occur.
What are the anatomical components of the filtration mechanism in the glomerulus?
The anatomical components of the glomerular filtration mechanism are: fenestrations in the glomerular capillary walls, the basal lamina of the capillary endothelial cells, and the slits between the processes of the podocytes of Bowman’s capsule.
How does the composition of the urine become different from the composition of the blood?
The composition of the urine is made different from the composition of the blood by processes of selective active reabsorption and secretion by the renal tubules.
The adaptations that enable the mammalian kidney to produce urine more concentrated than extracellular fluids were important steps in vertebrate evolution, and they were largely achieved through changes in the structure and regional functions of the renal tubules. These changes converted a kidney that evolved to excrete water into an organ that conserves water.