Understanding Sweat, Urine, and Osmoregulation

Sweat

Sweat glands are located in the skin, opening to the epidermis through pores. These glands are mainly concentrated in areas like the forehead, armpits, palms, and soles of the feet. Each gland consists of a thin tube called a glomerulus, which extracts fluid from blood capillaries. This fluid contains compounds like urea and water, similar to urine but more dilute. From the glomerulus, sweat travels through channels to the pores and is released as droplets.

Regulating Urine Concentration

Urine concentration depends on the permeability of the collecting tubules, regulated by the antidiuretic hormone vasopressin. Increased osmotic pressure stimulates vasopressin secretion, increasing the permeability of the collecting tubules. This leads to greater water reabsorption and more concentrated urine. Conversely, lower vasopressin secretion decreases tubule permeability, resulting in less water reabsorption and more dilute urine. Increased blood osmotic pressure also triggers thirst, prompting the body to drink. The thirst center, located in the hypothalamus, is also stimulated by moisture receptors in the mouth and pharynx.

Osmoregulation

• Freshwater Fish

  Living in a hypotonic environment, freshwater fish constantly absorb water through osmosis and their gills. To remove excess water, their kidneys reabsorb salts but very little water, resulting in plentiful, dilute urine.

• Saltwater Fish

  Inhabiting a hypertonic environment, saltwater fish experience constant water loss through osmosis. Bony fish reabsorb almost all water in their kidneys, producing small amounts of concentrated urine. Cartilaginous fish maintain an internal environment nearly isotonic (slightly hypertonic) to seawater by accumulating urea. They excrete nearly isotonic urine, removing excess salt through a gland in the posterior intestine.

• Terrestrial Vertebrates

  These animals must conserve water and eliminate nitrogenous waste. Reptiles and birds excrete uric acid; through intense tubular reabsorption, they minimize water loss. Mammals produce hypertonic urine thanks to the metanephros, which has long, complex tubules that facilitate reabsorption.