Urinary System: Electrolyte Regulation

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Questions and Answers

Which factor does NOT stimulate potassium secretion by principal cells?

Increased extracellular fluid potassium concentration

Decreased sodium intake (correct)

Increased aldosterone

Increased tubular flow rate

How does acute acidosis affect potassium secretion?

It stimulates sodium reabsorption

It has no effect on potassium secretion

It enhances potassium secretion

It inhibits potassium secretion (correct)

What is the primary site of calcium reabsorption in the kidneys?

Loop of Henle

Distal tubule

Proximal tubule (correct)

Collecting ducts

What role does parathyroid hormone (PTH) play in calcium regulation?

It increases intestinal calcium absorption through Vitamin D3 activation Signup and view all the answers

What is the typical percentage of filtered phosphate reabsorbed by the proximal tubule?

75 to 80 percent Signup and view all the answers

How does high sodium intake affect potassium excretion?

It reduces potassium excretion Signup and view all the answers

What is the primary storage form of calcium in the body?

Bone Signup and view all the answers

Which condition results in a decreased concentration of potassium in the extracellular fluid?

Hypokalemia Signup and view all the answers

What is the primary determinant of renal potassium excretion?

Rate of potassium filtration multiplied by plasma potassium concentration Signup and view all the answers

Which segment of the nephron is responsible for the majority of potassium reabsorption?

Proximal tubule Signup and view all the answers

What mechanism is used by intercalated cells for potassium reabsorption?

H+-K+ ATPase transport mechanism Signup and view all the answers

Which factor primarily influences potassium secretion by principal cells?

Activity of the Na+-K+ ATPase pump Signup and view all the answers

In which part of the nephron does active co-transport of potassium with sodium and chloride mainly occur?

Loop of Henle Signup and view all the answers

What role does the electrochemical gradient play in potassium secretion?

Facilitates passive flow from blood to tubular lumen Signup and view all the answers

What happens to potassium reabsorption during extracellular fluid potassium depletion?

It increases through active transport Signup and view all the answers

What is the primary role of potassium channels in the luminal membrane?

To specifically allow potassium ion permeability Signup and view all the answers

Study Notes

Urinary System: Regulation of Electrolytes

The urinary system regulates K+, Ca²⁺, PO₄³⁻, and Mg²⁺, crucial for maintaining homeostasis.
Potassium excretion is determined by filtration rate (GFR multiplied by plasma K+ concentration), reabsorption by tubules, and secretion by tubules.
- Approximately 65% of filtered potassium is reabsorbed in the proximal tubule, with an additional 25-30% being reabsorbed in the loop of Henle, particularly the thick ascending limb (active co-transport with Na+ and Cl-).
- Reabsorption in the late distal and collecting tubules involves H+-K+ ATPase, exchanging K+ for H+ ions, which then diffuse into the bloodstream.
- Under normal conditions, principal cells play a smaller role in potassium excretion and rely on the Na+-K+ ATPase pump and the electrochemical gradient.
- Factors affecting potassium secretion include extracellular fluid K+ concentration, aldosterone, and tubular flow rate. Increased extracellular fluid K+ and aldosterone increase potassium secretion; increased tubular flow rate offsets this.
- Acidosis inhibits potassium secretion; alkalosis reverses this effect.
Calcium excretion: About 99% of filtered calcium is reabsorbed by the tubules (65% in proximal, 25-30% in loop of Henle, 4-9% in distal).
- The bone acts as a large calcium reservoir and source.
- Parathyroid hormone (PTH) regulates calcium reabsorption:
  - Stimulates bone resorption.
  - Stimulates vitamin D3 activation to increase intestinal calcium absorption.
  - Directly increases renal tubular calcium reabsorption.
Phosphate excretion: Proximal tubules reabsorb 75-80% of filtered phosphate.
- Distal tubules reabsorb 10%, with smaller amounts in other segments.
- Approximately 10% of filtered phosphate is excreted in urine.
- Increased plasma PTH decreases tubular phosphate reabsorption, resulting in more phosphate excretion.
Magnesium excretion: Proximal tubule reabsorbs approximately 25% of filtered magnesium, with 65% reabsorbed in the loop of Henle.
- A small amount (typically <5%) is reabsorbed in the distal and collecting tubules.
- Magnesium excretion is increased by higher extracellular magnesium concentrations, extracellular volume expansion, and higher extracellular calcium levels.

Urinary System: Fluid Balance & Transport

Renal mechanisms regulate extracellular fluid (ECF) homeostasis through various hormonal and other adjustments.
When intra-renal compensation fails, systemic mechanisms regulate salt and water balance (including changes in blood pressure, circulating hormones like renin-angiotensin-aldosterone system, ADH, ANP, and changes in sympathetic nervous system activity).
Urine transport from kidney to bladder:
- Urine follows the same composition as fluid from collecting ducts.
- Peristaltic contractions in the ureters propel the urine into the renal pelvis and then to the bladder.
- The detrusor muscle (smooth muscle in bladder walls) helps force urine towards the urethra.
- Both sympathetic and parasympathetic nerves modulate ureter contractions.
- Bladder distention triggers a spinal reflex, leading to bladder contraction.
Micturition reflex: Bladder distension stimulates stretch receptors in the bladder walls, triggering smooth muscle contractions of the bladder and relaxation of the internal and external sphincters. Inhibitory impulses can come from the cerebral cortex and prevent micturition until bladder volume is significant
- Various factors affect the rate of urine excretion: voluntary control and higher center inhibition, bladder pressure, muscle tone, and spinal reflex.

Pathological Terms

UTI: Urinary tract infection
Nephritis: Kidney inflammation
Hydronephrosis: Renal pelvis dilation
Anuria: No urine output
Dysuria: Painful urination
Enuresis: Lack of bladder control
Oliguria: Scanty urine output
Polyuria: Excessive urine output
Incontinence: Involuntary urine discharge
Diuretic: Increases urine output
Antidiuretic: Decreases urine output

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Description

Explore how the urinary system maintains homeostasis by regulating essential electrolytes such as potassium, calcium, phosphate, and magnesium. Learn about the mechanisms of reabsorption and secretion in the kidneys, including the roles of different tubules and hormonal influences like aldosterone.