Renal system: L37 - Function of the renal tubule Flashcards
What are examples of solutes that are only reabsorbed (and not secreted) in the renal tubule?
Glucose is 98% reabsorbed in PT and 2% in LoH.
Water, Na+, Cl-, PO4-, Ca2+ are reabsorbed at varying rates. 2/3 of water and sodium are absorbed in PCT.
What are examples of solutes that are only secreted (not reabsorbed)?
Organic cations e.g monoamines (dopamine,histidine) and drugs like morphine.
Organic anions e.g endogenous compounds (bile salt) and drugs like penicillin.
What are examples of solutes that are secreted AND reabsorbed in renal tubules?
K+, NH3, H+, HCO3-: secretion and reabsorption is regulated according to homeostatic requirements.
Urea (has N) is unregulated.
Both reabsorption and secretion must occur the tubular epithelium. Name some features of this epithelium.
The epithelium has a variety of types across the renal tubules depending on function. The cells have microvilli in the Proximal tubule which increases surface area. Cells are held together by tight junctions.
Where are the transcellular and paracellular pathways of water and solute reabsorption mainly occurring in the renal tubules?
Transcellular route is more selective and may be under hormonal control. Reabsorption in this manner tends to occur in the more distal parts of the tubule (fine-tuning areas).
Paracellular route is useful for reabsorbing large amounts of water and solutes so it happens a lot in the proximal tubule.
What does the proximal convoluted tubule reabsorb?
Handles most of the reabsorption of the nephron. It reabsorbs all the glucose and most of the amino acids. Although at very high levels of plasma glucose (e.g diabetes) glucose will appear in the urine. The proximal tubule reabsorbs about 65% of water, sodium and chloride. Most K+, PO4 and Ca2+ are reabsorbed in the proximal tubule, as is half of urea.
How is reabsorption in the proximal tubule driven by sodium?
Sodium concentration in the cells is kept low by Na+/K+ ATPase pumps, so sodium moves down concentration gradient. Like in the GI system, it can take with it solutes like glucose and amino acids in secondary active transport.
Is filtered bicarbonate absorbed to be replaced?
Filtered bicarbonate is not actually reabsorbed, a new molecule of bicarbonate is synthesised by the cells of the PT and is released into the blood. This helps keep pH balance.
What does the proximal tubule secrete?
H+, drugs and organic acids.
What is the function of the distal tubule?
Fine-tuning of the electrolytes and balance pH by secreting K+ and H+. Reabsorption of sodium in the distal tube depends on aldosterone.
What is the function of the collecting duct?
Collecting ducts fine-tune water reabsorption, along with distal tubule, they reabsorb most of the remaining sodium and chloride (10%). Reabsorption of sodium in the collecting duct depends on aldosterone.The collecting duct reabsorbs most of the remaining water (15%). Reabsorption of water in the collecting duct depends on antidiuretic hormone (ADH). We reabsorb water by inserting aquaporins into the collecting duct. These work by osmosis: the filtrate is sucked towards the hyperosmotic medulla made by the countercurrent from the Loop of Henle.
What are the effects of ADH, the loop of Henle and HOMG?
ADH causes the insertion of aquaporins to the apical surface of collecting ducts. However, water will not flow through the aquaporins without a osmolarity gradient. The gradient is achieved by the loop of Henle, which accumulates salt and urea in the medulla of the kidney, increasing osmolarity up to 1200 mosmol/L. Ultimately water is reabsorbed from the collecting duct down this gradient depending on the permeability of the lumen, which as we have seen determined by the ADH.
Explain the countercurrent multiplication and the resulting hyperosmotic medullary gradient in the loop of Henle.
Water in the descending limb of the loop reabsorbs and moves from the filtrate to the hyperosmotic medulla. The ascending limb of the loop ‘actively’ transports Na+ from the filtrate to the medulla interstitium, but is relatively impermeable to water. Thus a process of countercurrent multiplication, the interstitial fluid in the medulla is made hyperosmotic as water flows from descending limb to medulla interstitium resulting in the hyperosmotic medullary gradient (HOMG).
There is a special arrangement of the blood vessels in the medulla (vasa recta) with their own countercurrent exchange mechanism. This prevents salt in the medulla from being removed in the blood, ensuring HOMG is maintained. Due to many ions being removed from the filtrate in the loop to generate the gradient, the filtrate that leaves the loop is very dilute. This allows water reabsorption in the collecting duct as water can move down the osmotic gradient.
