4 - Renal Physiology II Flashcards
How much water is reabsorbed along the entire length of the nephron?
600-1200 mOsm/L
Where along the length of the nephron is water reabsorbed?
Everywhere EXCEPT the ascending loop of Henle
- Proximal convoluted tubule
- Descending tubule
- Distal convoluted tubule
- Collecting duct
Describe kidney conservation of water by excreting concentrated urine
The ability of the kidney to form urine that is more concentrated than the plasma is essential for survival of mammals that live on land
The basic requirements for forming a concentrated urine are:
- A high osmolarity of the renal medullary interstitial fluid
- A high level of antidiuretic hormone (ADH) also called vasopressin
Describe the counter multiplier system in the loop of Henle for producing a hyperosmotic renal medulla
Loop of Henle is responsible for counter current multiplication which is necessary for concentration or dilution of urine
- Co-transport of Na+ and Cl- and other ions out of the thick ascending limb of the loop of Henle into the medullary interstitium
- Active transport of ions from the collecting ducts into the medullary interstitium
- Facilitated diffusion of large amount of urea from the medullary collecting ducts into the medullary interstitium
- Diffusion of only small amount of water from the medullary tubules into the medullary interstitium, far less than the re-absorption of solutes into the medullary interstitium
IMPORTANT ***
Describe how the vasa recta can serve the purpose of counter current exchanges
- The vasa recta serve as counter current exchangers minimizing washout of solutes
from the interstitium - The U-shaped structure of the vessels minimizes loss of solute from the interstitium
Describe the function of antidiuretic hormone
- An increase in fluid osmolality, plasma sodium concentration caused osmoreceptor cells to shrink
- Shrinkage of the osmoreceptor cells signals the supraoptic nuclei that release ADH
- In the kidney, ADH controls the degree of dilution or concentration of the urine
Describe the ADH feedback mechanism
- Regulation of extracellular fluid osmolarity in response to a water deficit
- Water is conserved in the body while sodium and other solutes continue to be excreted in the urine
- Opposite sequence of events occurs when the extracellular fluid become too dilute (hypo-osmotic)
- With excess of water ingestion and decrease
in extracellular osmolarity, less ADH is formed, the renal tubules decrease their
permeability, less water is re-absorbed and a large volume of urine is formed
Good flow chart on slide 44
Describe the relative osmolality of the tubule fluid along the nephron
- Proximal = urine is same as plasma
- Loop = more concentrated than plasma as it loses a lot of water in the descending limb, then becomes less concentrated than plasma when it comes back down due to losing ions in the ascending limb
- Classic distal tubule = starts off less concentrated than plasma (due to all the ions reabsorbed in the ascending limb), but then the concentration depends on ADH (hydration level)
- This difference in concentration based on level of hydration continues through to the excretion of urine
- If the body is dehydrated, it will reabsorb more water and urine will be concentrated
- If the body is very hydrated, it will secrete more water, making the urine dilute
- This variance is seen in the cortical collecting duct, medullary collecting duct, and urine excretion
What is the role of thirst in controlling extracellular fluid (ECF) osmolarity?
- Fluid intake is regulated by thirst mechanism
- The hypothalamus contains thirst osmoreceptors that sense ECF osmolarity
- These cells stimulate other hypothalamic neurons causing thirst and increased water intake in response to hyperosmolality
Describe the stimuli for thirst
- Large decreases in circulating blood volume and/or blood pressure
- Hemorrhage
- Dryness of the mouth and throat
- Thirst mechanisms are temporarily suppressed after drinking water. If insufficient water was drunk the person becomes thirsty again until the ECF osmolarity is returned to normal
What are the mechanisms of regulation of reabsorption and secretion
- Local Control (renal auto-regulation)
- Hormonal Control
- Nervous Control
Describe renal autoregulation (local control)
The kidney regulates fluid and electrolytes (sodium chloride) homeostasis by modulating the rapport between glomeruli and tubules
- The tubules respond to glomeruli with the Glomerulotubular Balance (GTB)
- The glomeruli respond to tubules through the Tubuloglomerular Feedback (TGF)
What do the GTB and TGF feedback systems aim to stabilize?
- single nephron GFR
- distal fluid and electrolytes delivery
Describe the function of GTB
Glomerulotubular Balance (GTB) - GTB enables the function of a tubule to compensate automatically for the fluctuation in filtration rate in the glomerulus to which it is attached to
GTB is achieved by tubules via
- changes in hydrostatic and colloid osmotic forces
- luminal factors
Describe a calculation of GTB
GFR is about 180 L/day and tubular reabsorption is 178.5 L/day, leaving 1.5L/day of fluid to be excreted in the urine.
An increase in GFR to 225 L/day with no tubular reabsorption compensation would determine an urine flow of 46.5 L/day
This is why it is so important
