Urine Production and Micturition Flashcards
How is regulation of plasma osmolarity achieved?
by varying the amount of water excreted relative to the amount of solute excreted (i.e. by varying urine osmolarity).
What is the corticopapillary osmotic gradient?
the gradient of osmolarity from the cortex (300 mOsm/L) to the papilla (1200 mOsm/L), and is composed primarily of NaCl and urea. It is established by countercurrent multiplication and urea recycling and maintained by countercurrent exchange in the vasa recta.
Why is the high tubular osmotic concentration at the bottom of the loop of henle so important?
bc the pumps can only pump against a difference of 200 mOsm/L, and thus as we ascend there will be a 200 mOsm/L difference at every level along this limb.
Will the osmolality at the top of the ascending limb be greater than, equal to, or less than the osmolality of the blood?
LESS THAN ALWAYS (100 mOsm/L; HYPOOSMOLAR).
*Remember the only purpose of the ascending limb is to concentrate the INTERSTITIUM of the MEDULLA. This does not change if you are dehydrated or overhydrated.
When the filtrate leaves the PCT, is it hypo, hyper, or isoosmolar?
ISOOSMOLAR
What happens when we are dehydrated?
osmoreceptors in the posterior pituitary pick this up immediately and thus cause secretion of ADH into the blood stream. ADH acts on receptors in the DCT and collecting duct basolateral membranes. ADH makes these impermeable areas on the luminal membrane permeable to water by opening aquaporin molecules. Because of the hypertonic interstitium, water will easily flow out of the tubules and into this area and hence the blood. Water is conserved and the urine is made more concentrated.
How does the urea cycle act as a second mechanism to assist in the concentration of urine?
- 50% of filtered UREA (produced by the liver as a byproduct of amino acid catabolism; measured as blood urea nitrogen (BUN)) is reabsorbed passively in the PCT.
- The other portions of the nephron are impermeable to urea (except for the ascending limb). However, ADH increases the urea permeability of the late collecting ducts (inner medullary collecting ducts) allowing it to enter the interstitium, and then freely diffuse back into the filtrate of the ASCENDING LIMB of the loop of henle, and equilibrate between the interstitium and the filtrate. This helps to pull water into the interstitium when you are dehydrated :) Aka: the urea is being cycle around between the ascending limb and the late part of the collecting tubule (remember only with ADH).
How do the vasa recta function?
Not like the glomerular capillary beds, which rely on hydrostatic pressure, these act more like traditional capillary beds where the absorptive forces predominate over the filtering forces (but they still exchange nutrients and waste products), which is good. So they are well suited for an exchange of water and solute at each level in the medullary interstitium.
***The secrete to the vasa recta not destroying the medullary interstitium gradient is the countercurrent arrangement of the vasa recta. Aka any solute that the vasa recta pick up on the descending limb, they will lose on the ascending limb (and the opposite goes for water movement). So there is a nice balance.
** When do you see the peak osmolality of the filtrate and under what conditions?
(USMLE QUESTION)
at the tip of the loop of henle in the presence of ADH
** What causes the large INCREASE in INTERSTITIAL osmolality when under the influence of ADH?
(USMLE QUESTION)
UREA (you would intuitively think the interstitial osmolality would decrease, but instead it increases due to the movement of urea also with the water)!
** Do medullary or cortical nephrons absorb more water?
USMLE QUESTION
CORTICAL because there are so many more (remember ISOosmotic here)!
The job of the juxtamedullary nephrons is only to concentrate the urine).
What is micturition?
the process of expelling the final tubular fluid (urine) from the body. Ureters move urine via smooth muscle contraction to the bladder, which is under the control of parasympathetics.
What makes the wall of the bladder waterproof?
cerebrosides coating the walls.
What are the 2 main afferent pathways of the bladder?
- afferent neurons carry information about wall distention.
- afferents originating in the urethra carry information to the spinal cord via the pudendal nerves.
* Together they signal pressure and volume changes in the urethra and elicit a local spinal reflex to initiate voiding (normally suppressed by information reaching the sacral spinal cord from higher brain centers).
What are the 2 efferent pathways?
- parasympathetic innervation= via pelvic nerves, which innervate the detrusor and the internal sphincter. Stimulation elicits constriction of the detrusor muscle and relaxation of the internal sphincter.
- somatic motor control of the system is tonic via the pudendal nerves. The tonic activity of these nerves produces a tonic constriction of the external sphincter.
