Lecture 47 - Urinary System: Regulation of Urine and the Creation and Role of the Medullary Osmotic Gradient Flashcards
Describe the anatomy of the Loop of Henle and the collecting duct
Loop of Henle:
- Thin limb: squamous and tight epithelia (no paracellular transport)
- Thick limb: cuboidal and tight epithelia
Collecting duct:
- 2 types of cells: intercalated cells (microvilli) and principal cells (no microvilli)
- cuboidal and tight epithelia
Describe the location of the Loop of Henle. How does its location affect its osmolarity gradient?
It descends into the hyperosmotic renal medulla, then returns to the cortex via the ascending limb. The deeper in the medullar, the higher the osmolarity
Describe the tubular flow from the PCT and through the Loop of Henle
Tubular fluid flows from the PCT to the thin descending limb and on into the thick ascending limb
What is the counter-current multiplication of the Loop of Henle?
Counter-current - tubular flow in the limbs of the loop go in opposite directions
Multiplication - the 2 limbs have different permeabilities for water and solute movement, which enhances overall reabsorption
What are the properties of the thin descending limb?
It is permeable to water, but impermeable to solutes
Thin limbs express aquaporin-1 (water channel) on their membranes that allow for water transport. Because the osmolarity of the ISF increases as the limb goes deeper, the direction of tubular flow enhances water reabsorption
What are the properties of the thick ascending limb?
It can pump ions out of the tubular fluid, but it’s impermeable to water
NKCC transporters (Na+K+/2Cl-) are on the apical surface, allowing the thick limb to pump Na+ and Cl- out of the tubular lumen and into the ISF. Water cannot diffuse back into the lumen due to the impermeability despite the osmotic gradient
How is the counter-current exchange further enhanced?
By the exchange across the parallel loops of the vasa recta
Exchange between the limbs and the vasa recta leads to counter-current multiplication and net reabsorption of both water and solutes
What is the osmolarity gradient in the deep medulla partly maintained by?
Urea permeability in the papillary duct:
Urea becomes a larger component of tubular fluid as other solutes and water are removed. Urea transporters (and thus permeability) are only present in the papillary duct, which runs through the renal papilla. Therefore, urea diffusion into the papillary ISF maintains the high osmolarity in the deep medulla
What is the purpose of the DCT (distal convoluted tubule) and collecting duct sites?
They are sites for facultative water and solute reabsorption, which means permeability/transport (ie. reabsorption) of solutes will only occur in these segments by hormonal signals (recall ADH and aldosterone from L44)
How do the epithelia properties of the DCT and collecting duct affect solute and water transportation?
The epithelia in the DCT and collecting duct have little to no paracellular transport and are impermeable to water “at rest”. Without hormone signalling, water would be retained in the lumen of the DCT and CD even while passing through the high osmolarity in the deep medulla
This would lead to large volume of dilute urine
What is the function of ADH (vasopressin)?
It binds to DCT and CD cells to promote the addition of aquaporins to their apical membranes
Where is ADH secreted from and how does it lead to the addition of aquaporins?
Anti-diuretic hormone (ADH, also vasopressin) is secreted from the posterior pituitary. ADH acts through AVPR2 receptors (found on the basolateral membrane of DCT/CD cells). AVPR activation signals for insertion of aquaporin-2 (into the apical side of the DCT/CD cells)
What is the purpose of aquaporins?
They create transmembrane permeability to water, allowing for osmotic reabsorption and the concentration of urine
How does ADH affect urine volume, concentration, and osmolarity?
When ADH is present, water is reabsorbed by the DCT and CD cells, which eventually goes into the ISF and back to the bloodstream. This increased reabsorption decreases urine volume, increases concentration, thereby increasing osmolarity
Why are ADH levels important?
They are the body’s key effector for homeostatic regulation of body fluid osmolarity