• The loop of Henle create concentration gradient by actively pumping sodium and chloride ions out of
the filtrate into the tissue fluid in the ascending limb (2nd part of the loop)
• The walls of the upper parts of the ascending limb is impermeable to water; this it the part where
active pumping of salts happen
• This raises the concentration of sodium and chloride ions in the tissue fluid of medulla
• causes water to leave descending limb
• The first part of the loop (the descending limb) is permeable to ions and water, but water leaves by
osmosis. This makes the filtrate more concentrated as it descends.
• The longer the loop, the more concentrated towards the bottom of the loop
• as the concentrated fluid now turns to flow up the ascending limb, sodium and chloride ions leave
readily down concentration gradient
• As the fluid continues up the ascending limb, it becomes gradually less concentrated
• Having the two limbs running side by side, the fluid flowing down in one and up in one enables
the maximum concentration to be built up both inside and outside the tube at the bottom of the loop
• This mechanism is known as a counter-current multiplier
4. Distal Convoluted tubule – Homeostasis and Secretion
• It is relatively short and has a brush border (i.e. microvilli) with numerous membrane pumps for
active transport.
• In the distal convoluted tubule sodium ions are actively transported from the filtrate into tissue fluid
• Potassium ions are transported into the tubule
• Secretion of substances occurs here and it is regulated by hormones, so this is the homeostatic part of
the kidney.
• Substances secreted include H+ (for pH homeostasis), K+ (for salt homeostasis), ethanol, toxins,
drugs and other “foreign” substances.