Transport Mechanisms
When renal cells transport solutes out of or into tubular fluid, they move specific substances in one direction only . Different types of transport proteins are present in the apical and basolateral membranes.
Recall …
Passive Diffusion requires no energy as substances move from an area of high concentration to an area of low concentration. A carrier may be needed to facilitate the diffusion. Primary Active Transport uses energy derived from the hydrolosis of ATP to “pump” a substance across a membrane. Example: Na+/K+ pump. Secondary Active Transport uses the energy stored in an ion’s electrochemical gradient to drive another substance across the membrane. Secondary active transport couples the movement of an ion down its electrochemical gradient to the “uphill” movement of a second substance against its electrochemical gradient. Symporters : membrane proteins that move 2 or more substances in the same direction across a membrane. Anitporter : membrane proteins that move 2 or more substances in opposite directions across a membrane. Cells lining the renal tubules have a low concentration of Na+ in their cytosol due to the activity of Na+/K+ pumps (like other cells throughout the body). These pumps are located in the basolateral membranes and eject Na+ from the renal tubule cells. The absence of these pumps in the apical layer ensures that reabsorption of Na+ is a one-way process. The amount of ATP used by the Na+/K+ pumps in the renal tubules is ~ 6% of the total ATP consumption of the body at rest. Solute reabsorption drives water reabsorption because all water reabsorption occurs via osmosis. ~ 90% of the reabsorption of water filtered by the kidneys occurs along with reabsorption of solutes such as Na+, Cl-, and glucose. Filtered fluid becomes Tubular Fluid once it enters the proximal convoluted tubules. The fluid that drains from the papillary ducts into the renal pelvis is Urine .
Osmolarity : is a measure of the total number of dissolved particles per litre of solution.
We will now follow the filtered fluid from the proximal convoluted tubule, into the loop of Henle, on to the distal convoluted tubule, and through the collecting ducts.
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