Term
| What is the normal physiological osmolality of body fluids and how is it maintained? |
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Definition
285 mosmoles/kg water
**Preferred to osmolarity as a clinical measure because it is independent of temperature and the volueme occupied by dissolved solutes**
To maintain osmolality, water intake (thirst) must match water excretion (regulated by the kidney) |
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Term
| Why is water weight 50% in women and 60% in men? |
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Definition
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Term
| Explain the distribution of body fluids between intra/extracellular compartments |
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Definition
2/3 intracellular and 1/3 extracellular
- 3/4 of intracellular (1/2 of total) is interstitial
- 1/4 of intracellular is intravascular. |
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Term
| How does the concept of "tonicity" differ from "osmolality"? |
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Definition
Tonicity is similar to osmolality, but is "effective measure" of particles ability to exert osmotic force.
Tonicity is calculable, but not measurable.
For example, Urea, ethanol and methanol will increase osmolality, but NOT tonicity, because they diffuse freely and rapidly across membranes ("ineffective osmoles") |
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Term
| What are the principle anions/cations of the ECF and ICF? |
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Definition
1) ECF
- Sodium (140 mM) is cation
- Chloride and Bicarbonate are anions
2) ICF
- Potassium (160mM) and Magnesium (40mM) are cations
- Proteins and phosphate in muscles are anions |
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Term
| How can osmolarity regulate cell volume? |
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Definition
1) Hypoosmolar ECF will draw water into the ICF, causing cellular swelling.
- To compensate, cell will lose KCl and other osmolytes from ICF
2) Hyperosmolar ECF will draw water out of the ICF, causing cell shrinkage.
- To compensate, cell will gain NaC and other osmolytes |
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Term
| What are the 3 basic requirements for renal H20 reabsorption and excretion? |
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Definition
1) Excretion
- Fluid must be filtered at glomerulus
- Salt must be removed from ultrafiltrate (TAL)
- "Free" water must transverse through tubules that are H20 tight.
2) Absorption
- Osmotic gradient must be generated to allow H20 to be reabsorbed
- Gradient established by generating and maintaining hypertonic medullary interstitium
- Vasopressin must be present to allow H20 re-absoprtion in collecting duct by increasing H20 permeability in the apical membrane. |
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Term
| How does ADH act on the collecting duct? |
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Definition
Increases apical membrane permeability to water by cytoplasmic aquaporin insertion into the apical membrane, leading to H20 reabsorption from tubular fluid.
It also acts on the late distal convoluted tubule, which is normally impermeable to water |
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Term
| How is water handled at each part of the nephron? |
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Definition
1) Glomerulus- water is freely filtered (150L daily)
2) Proximal tubule- Solutes and H20 reabsorbed isotonically (60-70% normally, or up to 90% in depletion)
3) Loop of Henle (15-20%)
- Thin descending water is reabsorbed
- Thin (passive NaCl diffusion) and thick ascending (active NaCl transport via Na/K/2Cl) is impermeable
4) Distal Nephron
- Distal convoluted tubule is H20 impermeable, unless vasopressin acts on late distal tubule.
- Collecting duct also depends on ADH via aquaporin insertion |
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Term
| How does EC blood volume influence proximal fluid reabsorption? |
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Definition
At low ECBV, renal capcity for excretion will decrease and up to 90% of H20 and salt will be reabsorbed in the proximal tubules
**In extremem cases, GFR can go down** |
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Term
How does urea transport influence medullary osmolarity?
What does it have to do with vasopressin? |
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Definition
1) Diffuses out of medullary collecting duct (especially important in states of active water reabsoprtion to maintain osmolarity).
2) Vasopressin activates urea transporters in medullary collecting duct, and in the absence of vasopressin, we are unable to concentrate urine. |
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Term
| Explain the concept of "Countercurrent Multiplier" |
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Definition
Basically, water goes out of the descending limb of henle and NaCl goes out in the TAL.
1) Active transport of NaCl in TAL by Na-K-2Cl in apical membrane and Na-K-ATPase in basolateral membrane leads to the removal of salt from the lumen (making tubular fluid hypotonic)
2) Water may also be reabsorbed in cortical and medullary collecting duct via ADH if the kidney is trying to conserve water.
3) Relative impermeability of cortical and medullary collecting duct to urea allows urea to build up in tubules until it reaches the inner medulla, where ADH-regulated urea permeability allows it out (generates hypertonic environment)
4) Osmotic water loss throughout descending limb of LOH allows luminal NaCl concentration to get high by the hairpin turn
5) As NaCl reaches ascending limb, it can passively diffuse out at first, and then via Na,K,2Cl co-transporter in TAL
The end goal is to concentrate the medullary interstitium |
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Term
| Why/How is H20 removed from the medullary interstitium interstitium? |
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Definition
Remember, water may leave the collecting tubule via ADH-mediated aquaporin insertion during hypovolemic states.
It needs to get out of there though, so as not to dilute interstitium
Its removed by the VASA RECTA (COUNTERCURRENT EXCHANGER), which flows slowly toward the deep medulla, allowing solute trapping to maintain hypertonic medullary interstitium |
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Term
| How does the kidney maximally dilute vs. concentrate urine? |
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Definition
ADH paves the way!
1) Dilute-
- LOH generates hypertonic interstitium while delivering dilute urine to distal nephron. Without ADH, water will not be reabsorbed in the collecting duct.
2) Concentrate
- Vasopressin (ADH) release will allow water reabsorption in the collecting duct |
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Term
| What is the major regulator of body tonicity? How is it produced/regulated? |
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Definition
1) Vasopressin (ADH) synthesized in supraoptic nucleus of hypothalamus and secreted by posterior pituitary.
2) Secretion from pituitary is regulated by plasma osmolality of ECF (small increases above 280 mOsm/kgH20 will cause secretion)
3) ADH can also be release in response to
- Volume depletion
- Drugs (barbiturates, narcotics, nicotine)
- Stress
- CNS disease
4) Suppressed by hypotonicity, alpha adrenergic stimulation, ethanol and volume expansion. |
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Term
| How does vasopressin secretion alter water reabsorption? |
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Definition
1) Released from posterior pituitary and binds V1 and V2 receptors in the basolateral membrane of collecting tubular cells, resulting in increased cAMP
2) cAMP causes Aquaporin 2 to move to apical membrane, causing increased apical permeability. |
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Term
| How does plasma osmolarity relate to thirst? |
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Definition
1) If osmolality increases above 285 mOsm, thirst and ADH release are stimulated
** Also caused by decrease in ECF or "effective arterial volume"** |
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Term
| What is Free H2O clearance and how is it used clinically? |
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Definition
Clearance of osmotically active particles= (Uosm X V)/Posm
Of the final urine, urine can either = Posm, or it can be "free H20," which = V-Cosm
During water diuresis, CH20 has positive value
During antidiureses, CH20 is negative |
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Term
| What is the difference between Dehydration and Volume Depletion? |
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Definition
IMPORTANT- because treatments are different
**Correct volume deficits first, then correct water imbalances**
1) Dehydration= free water deficit= hypernatremia
- Treat with free water replacement (slow)
2) Volume depletion= ECF or effective circulating blood volume depletion
- Treat with isotonic fluid |
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Term
| How does Osmoregulation differ from Volume regulation in terms of effectors and affected processes? |
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Definition
1) Osmoregulation
- ADH regulates water excretion
- Thirst regulates water intake
2) Volume regulation
- SNS, RAAS, ANP and ADH regulation sodium excretion |
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