Renal Lecture 4 Flashcards
Topic 4: Renal System
If one says that the renal clearance of a substance is 140 (assume normal GFR), what does this mean?
a. 140 mg of the substance is filtered out of the blood every minute.
b. 140 mg of the substance is reabsorbed by the kidney tubules every minute.
c. The substance is freely filtered and some is reabsorbed.
d. The substance is freely filtered and some is also secreted.
e. The normal plasma concentration of the substance is 140 mg/ml.
d. The substance is freely filtered and some is also secreted.
Alcohol acts as a diuretic because it:
a. is not reabsorbed by the tubule cells
b. increases the rate of glomerularfiltration
c. promotes excess excretion of solute, causing water to follow
d. inhibits the release of ADH
e. damages water channels
inhibits the release of ADH
Regulation of micturition requires 3 things:
To pee (micturition):
Bladder muscle (detrusor) must contract.
Both sphincters (internal & external) must relax/open.
At rest:
Detrusor is relaxed, sphincters are closed.
When peeing:
Detrusor contracts, sphincters open.
200 mL of urine stretches the bladder.
Nerves send signals to the spine and brain.
Parasympathetic nerves send signals back →
Bladder, detrusor, contracts, internal sphincter opens.
Brain feels the urge to pee.
You voluntarily relax the external sphincter to urinate.
If you don’t pee, the reflex fades and comes back later.
See diagram
Incontinence: Loss of voluntary control, due to weak pelvic muscles, pregnancy, or nerve issues.
Stress incontinence: Urine leakage from pressure (e.g., coughing).
Urinary retention: Inability to expel urine, caused by anesthesia or prostate hypertrophy.
Renal failure: Reduced or stopped filtrate formation due to damaged nephrons.
Causes: Kidney infections, injuries, muscle pressure, poor blood delivery.
Symptoms: Nitrogen buildup, acidity, diarrhea, vomiting, edema, breathing issues, anemia.
Treatment: Dialysis needed below 25% kidney function. Hemodialysis: 3-5 hrs/session, several times a week.
Peritoneal Dialysis: Uses peritoneal membrane for filtration.
Done at home/work, no hospital visits.
Dialysate (salts & sugar) infused, wastes removed from blood.
Diagram
CAPD (Continuous Ambulatory Peritoneal Dialysis):
4-5 times/day.
Dialysate (2L) infused, stays for 4-5 hrs, then drained and replaced.
CCPD (Continuous Cycling Peritoneal Dialysis):
Done at home with a cycler machine.
Multiple exchanges (1-1.5 hrs each), typically during sleep.
see diagram
CAPD: Happens during the day, with 4-5 hour exchanges done manually several times a day.
CCPD: Happens at night, with the machine doing multiple exchanges while the patient sleeps.
The 4-5 hours for CAPD is the optimal time for the dialysate to stay in the abdomen to allow for proper filtration. During this period:
Waste products and excess fluid move from the blood into the dialysate through the peritoneal membrane.
After 4-5 hours, the effectiveness of filtration starts to decrease, so the dialysate is drained and replaced with fresh fluid.
Here’s how it works:
The machine does an exchange (fills the abdomen with dialysate, lets it sit for a while, then drains it).
Each exchange takes about 1-1.5 hours.
Multiple exchanges are done during the night while you sleep, typically over an 8-10 hour period.
Fluid Compartments:
Intracellular Fluid (ICF): 60%/40% of total body fluid, inside cells.
Extracellular Fluid (ECF): 40%/20% of body fluid, divided into:
Plasma: ~20% of ECF
Interstitial Fluid (IF): ~80% of ECF (includes lymph, CSF, eye fluids, synovial fluid, GI secretions).
role of the kidneys in fluid & electrolyte balance
The intracellular fluid (ICF) compartment holds about 62.5% of the body’s water, or 40% of the body’s total weight, making it the largest of the three compartments.
Electrolytes vs Non-electrolytes:
Electrolytes have greater osmotic power because they dissociate into ions, creating more particles in solution.
Key Electrolytes:
ECF:
Chief cation: Na+
Chief anion: Cl-
ICF:
Chief cation: K+
Chief anion: HPO42⁻ (phosphate)
Na+/K+ Pumps: Maintain low [Na+] and high [K+] inside cells using ATP. Changes in plasma solute concentrations affect intracellular fluid volume.
See diagram
See diagram
Na+ is the most abundant cation in the interstitial fluid.
Ca2+ is the least abundant ion when summing all three compartments. It is particularly scarce in the intracellular compartment, although some cells, such as muscle, do sequester large amounts.
Water Balance: Water intake must equal water output.
Intake:
Liquids, foods (30%), cellular metabolism (10%), beverages (60%)
Output:
~60% via kidneys; also lungs (28%), skin (28%), sweat (8%), feces (4%).
Response to Increased Plasma Osmolality (280-300 mOsm):
Thirst increases, leading to higher water intake.
ADH stimulates renal water reabsorption.
Response to Decreased Plasma Osmolality:
Thirst not stimulated.
ADH secretion is inhibited.
ADH prompts the kidney to reabsorb and thus conserve water, reducing urine output. A rise in plasma osmolality causes ADH release, but ADH is not the cause of what drives us to seek greater water intake.
ADH increases water reabsorption at the collecting ducts in the kidneys.
Thirst Mechanism:
Increase in Plasma Osmolality (1-2%): Dry mouth triggers osmoreceptors in the hypothalamus, causing thirst.
Decrease in Plasma Volume (5-10%): Baroreceptors detect low volume, triggering thirst.
Dampening Thirst: Once the mouth/throat is moistened, thirst decreases to prevent overdrinking.
See diagram
An increase in blood pressure would reduce the hormones and responses that normally trigger the thirst mechanism.
Centers for detecting osmolarity changes and other signs of water balance are in the hypothalamus.
Obligatory water losses include:
Insensible loss through the lungs and skin.
Water loss through feces.
Minimal sensible urinary loss of about 500 ml/day.
Thirst Mechanism:
Increase in ECF Osmolality:
Osmoreceptors trigger thirst → drink → water absorbed → lowers ECF osmolality & increases plasma volume.
Decrease in Plasma Volume:
Low blood pressure activates renin-angiotensin → thirst → drink → water absorbed → lowers ECF osmolality & increases plasma volume.
Renin release from the kidney and subsequent activation of angiotensin is the most important trigger for aldosterone release.
When the ECF becomes hypertonic (more concentrated), water moves from the intracellular fluid (ICF) inside the osmoreceptor cells to the extracellular fluid (ECF), causing the cells to shrink and triggering the thirst response.
Water Balance Disorders:
Dehydration: Loss of fluid or fluid + salts.
Hypotonic Hydration: Dilutes sodium in ECF, water moves into cells.
Edema: Fluid (and salts) accumulate in interstitial fluid.
Dehydration Consequences:
Water loss → ECF osmotic pressure rises → cells shrink.
Hypotonic Hydration Consequences:
Water gain → ECF osmotic pressure falls → cells swell.
Water loss leads to an increase in ECF osmotic pressure, causing water to be drawn out of the cells, leading to cell shrinkage.
Hypotonic hydration (water gain) lowers the ECF osmotic pressure, causing water to move into the cells, resulting in cell swelling.
The higher osmotic pressure in the blood would draw water from the interstitial fluid, resulting in a higher blood volume and increased blood hydrostatic pressure.
Sodium & Balance Influences:
Sodium:
90-95% of ECF solutes, affecting plasma osmolality and blood volume.
ECF sodium concentration remains stable via water adjustments (e.g., salty meal raises blood pressure).
Aldosterone:
Regulates sodium in kidneys, promoting reabsorption in DCT & collecting ducts.
Essential for sodium balance, reabsorbs almost all remaining Na+.
Osmolarity: Concentration of solute particles in a solution.
Tonicity: Ability of a solution to affect cell volume by osmosis.
Isotonic: Solution with equal solute concentration to the cell, causing no net movement of water.
Hydrostatic pressure: Pressure exerted by a fluid against the walls of its container (e.g., blood vessel).
Other Influences:
Baroreceptors: Detect blood pressure changes, adjust sodium/water retention.
ADH: Controls water reabsorption, affecting sodium concentration.
ANF: Reduces sodium reabsorption, increasing sodium excretion.
Both ANF and ANP (Atrial Natriuretic Peptide) help lower blood pressure through these mechanisms:
Promoting Sodium Excretion: They inhibit sodium reabsorption in the kidneys, leading to increased sodium excretion in urine.
Vasodilation: They relax blood vessel walls, which lowers vascular resistance and decreases blood pressure.
Inhibiting Renin-Angiotensin-Aldosterone System (RAAS): They reduce renin release, which lowers aldosterone and decreases sodium and water retention.
Aldosterone Secretion Pathways:
Renin-Angiotensin System
High K+ or low Na+
Renin Secretion Triggers:
Renin is released when:
Sympathetic Nervous System activates.
Filtrate osmolarity is low (low solute concentration).
Afferent arteriole is stretched less (low blood pressure).
Addison’s Disease:
Low aldosterone → Na+ and water loss, high K+.
Renin-Angiotensin: Stimulates aldosterone → increases Na+ reabsorption, K+ secretion.
ADH (Water Regulation):
Function: Regulates water reabsorption, influencing plasma sodium.
Osmoreceptor Response:
Low [Na+]: Reduced ADH → dilute urine.
High [Na+]: Increased ADH → conserve water.
Effectiveness: ADH needs adequate body water to work; low water requires intake for effectiveness.
See diagram
Atrial Natriuretic Factor (ANF):
Released from heart’s atria when blood pressure is elevated.
Effects:
Inhibits Na+ reabsorption in DCT & collecting duct.
Reduces ADH, renin, and aldosterone release.
Induces vasodilation.
Overall Effect: Lowers blood pressure.
Other Hormones:
Estradiol: Aldosterone-like effect.
Progesterone: Mild diuretic effect.
Cortisol: Aldosterone-like effect.
Cardiovascular Baroreceptors:
High BP: Reduced sympathetic output → increased GFR → more urine.
Low BP: Opposite effect, less urine.
Function: Regulates sodium and water balance based on blood pressure.
See graph
When sympathetic activity decreases due to high blood pressure, the afferent arteriole dilates, increasing blood flow to the glomerulus. This leads to a higher glomerular filtration rate (GFR), allowing more fluid to be filtered into the renal tubules. As a result, more urine is produced to help lower blood pressure by excreting excess fluid.
Which of the following statements about fluid movement is NOT correct?
- Exchange between the plasma and the intracellular fluid occurs across the cell membrane.
Under normal circumstances, lymph vessels help maintain fluid balance, especially between the plasma and the interstitial fluid.
Exchange between interstitial fluid and intracellular fluid occurs across the plasma membrane.
Exchange between plasma and interstitial fluid happens between capillary walls.
Fantastic!
More information
Exchange between the plasma and the intracellular fluid would have to first involve exchange through the interstitial fluid.
Increasing osmolality of ECF stimulates the hypothalamic thirst center by both increasing dry mouth and directly activating osmoreceptors in the hypothalamus.
What type of intravenous infusion would you give to a runner who has collapsed after drinking too much water during the course of her marathon and why?
A hypertonic saline solution to pull water out of her cells
Low Blood Pressure:
ADH, Aldosterone, Renin increase.
ADH: Retains water to increase blood volume.
Aldosterone: Retains sodium and water.
Renin: Activates RAAS, raises blood pressure.
Hyperkalemia (High K+):
Damaged kidneys: Cannot excrete potassium.
Spleen loss: Does not affect potassium.
Increased ADH: No significant effect on potassium.
Increased aldosterone: Lowers potassium (increases excretion).
