Renal Physiology and Homeostatic Mechanisms Explained

Homeostatic Mechanisms of Plasma Potassium

Dietary potassium intake can vary from 50 mEq/day to 150 mEq/day. To maintain potassium balance, urinary excretion must equal dietary intake. The renal mechanisms that allow this variability are known as the external balance of potassium.

Bladder Dysfunction Following Spinal Injury

A patient with a T12 spinal cord injury presents with paralysis, loss of sensation below T12, and urinary incontinence. Studies show spontaneous contraction of the detrusor muscle during bladder filling.

• Explanation: The parasympathetic system remains active, but the patient lacks voluntary control over the external sphincter.

Glomerular Filtration and Starling Forces

Glomerular ultrafiltration is governed by Starling forces, which determine fluid passage from the glomerular capillaries into Bowman's space.

Key Pressures

• P_GC: Glomerular capillary hydrostatic pressure
• P_BS: Bowman's space hydrostatic pressure
• π_GC: Glomerular capillary oncotic pressure
• π_BS: Bowman's space oncotic pressure

a) Pressures favoring and opposing filtration

• Favoring: P_GC and π_BS
• Opposing: P_BS and π_GC

b) Why is GFR higher than systemic capillary filtration?

The glomerular capillaries are fenestrated, allowing for a significantly higher filtration rate.

c) Factors affecting Glomerular Filtration Rate (GFR)

• Decrease in afferent arteriolar resistance: Increases GFR.
• Increased efferent arteriolar resistance: Increases GFR.
• Drop in blood pressure (e.g., severe bleeding): Decreases GFR.
• Hypoalbuminemia: Increases filtration.
• Acute urinary tract obstruction (e.g., renal lithiasis): Decreases filtration.

Hypertensive Patient Evaluation

In a hypertensive patient presenting with fatigue and a supine blood pressure of 160/100 mmHg, electrolyte concentrations in the blood must be carefully assessed for potential imbalances.