Kidney

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Blood enters the system via the afferent arteriole

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Blood enters the glomerulus, which is a bundle of thin blood vessels found inside the Bowman's capsule

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Due to a difference in diameter, there is a high hydrostatic pressure

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This hydrostatic pressure forces out water, urea, sodium ions, chloride ions and glucose out into the Bowman's capsule

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These substances first pass through pores in the capillary endothelium, then through the basement membrane and finally through slits in the Bowman's capsule epithelium

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The remaining blood, including red blood cells, white blood cells and plasma proteins, carry on in the efferent arteriole

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The process of these substances passing into Bowman's capsule is known as ultrafiltration and the fluid formed is called the golmerular filtrate/tubular fluid

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The golmerular filtrate enters the proximal convoluted tubule for selective re-absorption

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Here, all glucose (unless the person has diabetes or other medical issues) is absorbed back into the body via co-transport. It uses the sodium ions which are part of the tubular fluid

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(Facilitated diffusion can also take place here if fructose is present)

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Glucose and other sugars are absorbed back into the blood since there is a large capillary bed over the nephron

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The remaining solution now enters the loop of Henle

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At the top of the ascending limb, sodium ions are actively transported out into the interstitial space and then transported into the top of the descending limb via facilitated diffusion

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Since the descending limb is impermeable to sodium, the solution keeps going until it reaches the bottom of the loop of Henle

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Here, it is extremely permeable to sodium ions, so most of them diffuse out into the interstitial space down their concentration gradient

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A concentration gradient of sodium has now been created, from the top to the bottom

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This concentration gradient allows water to pass out of the descending limb throughout the whole of its length due to a difference in water potential via osmosis

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This stage, along with the next one, is known as osmoregulation

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The remaining solution then enters the distal convoluted tubule and the collecting duct. Because the nephron is so tightly packed, water can still leave the DCT and collecting duct via osmosis due to the difference in water potential

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If osmoreceptors in the hypothalamus detect there isn't enough water (they themselves get flaccid and send chemical messages to the hypothalamus, which then sends chemical messages to the posterior pituitary gland), then antidiuretic hormone is released

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ADH causes aquaporins in the DCT and collecting duct to fuse with the membrane. These are special channel proteins to allow for more water to leave by osmosis