The kidneys are remarkable organs responsible for filtering blood and creating urine. To learn more about how they do this, check out this YouTube video:

The evolution of glomerular filtration is what allows animals to regulate the disposal of ions such as calcium, magnesium, sodium, potassium, and sulfate. Some solutes are reabsorbed from the urine and returned to the body with the help of sodium ions, whereas other solutes are added to the urine and thus removed from the body.

In a recent paper published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, Dr. Roger Evans from Monash University in Melbourne, Australia examined various hypotheses that have been put forth through the years to explain how the complex system of urine formation in mammals evolved. Dr. Homer Smith (1895-1962) was a prominent renal physiologist who predicted that the glomerulus first evolved in a freshwater environment and allowed the fish to excrete dilute urine. The problem with this hypothesis is that more recent findings suggest that the first vertebrates evolved from a common marine ancestor sometime during the Cambrian explosion which took place between 541-530 million years ago. Seawater has high levels of chloride, sodium, magnesium, sulfate, calcium, as well as potassium. The first prototype of the nephron, referred to as a ‘first little kidney’ or protonephridia, appeared to have evolved in invertebrates. Examples are found in platyhelminths, nemerteans as well as annelid and mollusk larva whereas the glomerular kidney is thought to have evolved in ancestors of jawless fish such as marine hagfish as well as anadromous lampreys who spend time in freshwater as larva and adults whereas juveniles live in seawater.

Human kidneys are incredibly inefficient, requiring about 10% of the body’s energy. Most of that energy is used to reabsorb sodium from the urine. With each minute that passes, the kidneys of a human filter 125 mL of plasma and then reabsorb 99% of the sodium that was filtered in the glomerulus. Water passively follows salts in the body because of osmosis. So, the movement of sodium back into the body helps prevent water loss and dehydration. In fact, in just one day, the body reabsorbs 1.6 kg of sodium chloride and 180 liters of water! In his paper, Dr. Evans states, “No engineer would design a system like this, akin to a house cleaner first throwing all the contents of the house (including the dirt) onto the street and then painstakingly bringing everything but the dirt back inside.”

Dr. Evans suggested that the glomerulus may have actually first appeared in organisms living in saltwater environments as a way to get rid of salts from the body. It may also be what allowed organisms to eventually transition to life in freshwater environments and dry land by controlling electrolyte and water balance in the body. In fact, the kidneys of terrestrial organisms spend most of their energy on reabsorbing sodium (and water) back into the body.

Knowing that the human kidney requires so much energy to function, helps to explain why it is easily damaged by hypoxia and is prone to both acute and chronic diseases.

Source

RG Evans. Evolution of the glomerulus in a marine environment and its implications for renal function in terrestrial vertebrates. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 324(2): R143-R151, 2023.

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Tags: American Journal of Physiology, American Physiological Society, dehydration, kidney