Renal 4 Flashcards
What is the definition of osmolarity, and how can it be used to understand water concentration?
Total solute concentration of a solution
A greater osmolarity means a lower water concentration
Water is _____ filtered and is ______ reabsorbed.
freely
mostly
The majority (how much?) of water is reabsorbed in the ___________.
about 2/3 in the proximal tubule
Major ________ control of water reabsorption occurs in the _______. This is (the same/ different) from sodium.
Hormonal
Collecting ducts
Different than sodium
In the proximal tubule, what is water reabsorption dependent on?
Depends on Na reabsorption, follows the set gradient.
Explain the steps of water reabsorption in the PROXIMAL TUBULE
- Na/K pump creates a gradient for Na to move from the tubule lumen to the capillary
- The osmolarity of the lumen decreases, and the osmolarity in the interstitial fluid increases
- Difference in osmolarity causes net diffusion of water from the tubule to the capillary via 2 pathways
- bulk flow carries everything from the interstitial fluid to the capillary
What are the 2 pathways water can take to reach the interstitial fluid in the PROXIMAL TUBULE?
Transcellular (tubular cell plasma membrane)
Paracellular (tight junction)
What happens in the medullary collecting duct regarding water?
Maintenance of water balance with hormone regulation
What allows the control of urine concentration?
Countercurrent multiplier system
How much can the kidneys concentrate urine?
1400 mOsm/L
How to do Henle’s loop and the MCD work together in water reabsorption?
Henle’s loop builds the gradient
MCD concentrates the urine under ADH control
Urinary concentration depends on…
the hyperosmolarity (low water) of the interstitial fluid.
What is the permeability of the descending limb of Henle’s loop?
Permeable to water, no NaCl
What is the permeability of the ascending limb of Henle’s loop?
Impermeable to water, actively reabsorbs NaCl
Explain how the osmotic gradient is created in the medulla
Water is reabsorbed into the interstitial fluid in the descending limb, making the tubular fluid hyperosmotic.
In the ascending, solutes are ACTIVELY reabsorbed, more than water was, making the tubular fluid hyposmotic.
When the fluid reaches the medullary collecting duct, this gradient will allow water to want to flow in (to the hyperosmotic medullary interstitial fluid) if there is vasopressin
What characteristics can change the osmolarity of the tubular fluid?
the flow, speed and length of Henle’s loop
What are the vasa recta?
the blood vessels following Henle’s loop form to reabsorbed things from the interstitial fluid
In the wholes tubular system, what is water reabsorption dependent on?
Depends on water permiability
T or F
Water permeability is different based on the segment of the tubule
True, the number of aquaporins will differ
How is water permeability changed throughout the tubule? (PT, Loop, CD)
the proximal tubule is always permeable to water, the amount will depend on solute reabsorption
The loop has a fixed permiability
Collecting ducts permeability depends on the presence of vasopressin
What’s another name for vasopressin?
Anti-diuretic hormone
Where is vasopressin released from?
The posterior lobe of the pituitary
What is the function of vasopressin?
stimulates the insertion of aquaporins on the luminal membrane of the collecting ducts to increase the water permeability
What happens if the vasopressin system does not work?
It can cause diabetes insipidus
What happens in the absence of vasopressin?
The collecting ducts will not be permeable to water. It’s going to stay in the collecting duct and make a very hypoosmotic tubular fluid
What are the two mechanism that can regulate vasopressin so it can itself regulate water reabsorption?
Osmoreceptor control (most important) and Baroreceptor control
How does the osmoreceptor control function?
Excess H2O
Body fluids are hypoosmotic
Osmoreceptors decrease firing
Posterior pituitary decreases vasopressin secretion
Less tubular permeability to H2O
More H2O excretion
Why is the osmoreceptor control more precise?
Because the osmoreceptors are more sensible to osmolarity than the baroreceptors
How does baroreceptor control function?
Plasma volume decreases
Pressure decreases
Posterior pituitary increases vasopressin secretion
More tubular permeability to H2O
Less H2O excretion
4 things allow us to feel thirst. What are they and how do they work?
Decreased plasma volume activates baroreceptors to increase angiotensin 2 secretion and cause thirst
Increased plasma osmolarity activates the osmoreceptors and causes thirst
Dry mouth and throat directly activate thirst
sensing of water intake by GIT acc inhibits thirst
Why do older adults not feel thirst as much (to a dangerous level)?
Because the osmoreceptors get more and more insensitive, decreasing feelings of thirst
