Nephron Physiology Flashcards
How does the kidney regulate systemic RAS (renin-angiotensin system) activity?
The RAS system activity maintains effective arterial blood volume, adequate tissue perfusion and blood pressure. The RAS system activity is regulated based on the stretching of the afferent arteriole. If there is inadequate stretching of the afferent arteriole then renin is released. Inadequate stretching may be caused by decreased NaCl, decreased arterial pressure, decreased ECF (extracellular fluid), increased stress/trauma. Renin release allows for angiotensin to increase aldosterone, cause vasoconstriction and increase sodium reabsorption.
How does the kidney eliminate endogenous and exogenous waste?
Elimination is determined by selective convection as a result of a hydrostatic pressure gradient (+/- reabsorption/metabolism) in the glomerulus.
There is a high hydrostatic pressure in the capillaries relative to Bowmans capsule resulting in the movement of waste from capillaries to capsule (this also includes water and electrolytes that will be reabsorbed later). The convection is selective as RBCs and proteins should not be filtered out (in healthy kidneys).
How do we quantify convection in the kidneys?
Glomerular Filtration Rate (GFR) quantifies convection. A normal GFR = 90-120 ml/min/1.73m2 BSA.
Clearance of X = (Urine [X]*Urine volume)/Plasma [X]
- If X is freely filtered, not reabsorbed or secreted then clearance = GFR
- Exogenous candidates for X = inulin, I*131 iothalamate, DTPA
- Endogenous marker for X = creatinine
What determines GFR?
GFR Determinants:
- Renal plasma flow (RPF)
- Filtration surface area
- Net ultrafiltration pressure
- Starling forces
What is the role of the proximal convoluted tubule?
The PCT is the principal site of reabsorption and secretion via the Na+/K+ ATPase pump. The pump moves Na+ out of the cell to create a concentration gradient that moves sodium, water, glucose etc. via sodium coupled transport. This co-transport system works for positive and negative charge molecules.
How is the kidney involved in hormone production and regulation?
The kidneys produce EPO and are involved in vitamin D and insulin metabolism.
The interstitium of the kidney exists in a hypoxic state relative to other tissues and is therefore highly sensitive when a person becomes hypoxic. The kidney adjusts to a hypoxic state by producing EPO.
The insulin requirement of patients with T1 and T2 diabetes was found to be reduced as creatinine clearance rate declined.
What is the function of the Loop of Henle (LoH)?
LoH is directly and indirectly involved in:
- Na+ reabsorption
- H20 reabsorption
- Urine dilution
- Urine concentration
How is the hypertonic interstitial created and maintained?
Countercurrent multiplier
- Uses energy
- Builds and maintains gradient
Countercurrent exchanger (vasa recta equilibration and maintenance + oxygen delivery)
- Uses no energy
- Uses physical principal of diffusion to maintain gradient
How does the LoH control Na loss?
The LoH influences GFR via tubuloglomerular feedback.
The macula densa (located near the top of the ascending LoH as well as the blood supply) senses the level of sodium and based off of this information will regulate the blood flow to that glomeruli.
A high Na+ will result in increased Cl- and adenosine on the blood side of the channels. This results (via paracrine signalling) in constriction of the afferent arteriole and reduce the amount of blood going into that glomerulus. If a nephron is not appropriately absorbing sodium less blood will be directed there (similar to V/Q matching in the lungs).
What is the function of the distal convoluted tubule (DCT) and connecting segment?
The DCT and connecting segment regulate solute reabsorption (fine tuning).
What is the main function of the collecting duct?
The CD is the principal site of fine tuning in the nephron.
CD is where the final decision is made on how much sodium will be reabsorbed and potassium will be excreted. Hormones that regulate sodium and potassium will act here, most importantly aldosterone and ANP.
The intercalated cell determines pH with H+ secretion.
ADH is the hormone that is dedicated to water handling. ADH stimulates aquaporin to insert water channels.
