Kidney Function and ADH

• A 62-year-old woman with a unilateral nephrectomy has a GFR of 50 ml/min, urine flow rate of 2.0 ml/min, and plasma glucose of 200 mg/100 ml.
• Her kidney transport maximum for glucose is 150 mg/min.
• Her approximate rate of glucose excretion is 0 mg/min.
• To determine glucose excretion, you must find the difference between filtered glucose and the transport maximum.
• GFR (50 ml/min) multiplied by plasma glucose concentration (2 mg/ml) equals 100 mg/min of filtered glucose.
• 100 mg/min (filtered glucose) minus 150 mg/min (transport maximum) equals -50 mg/min.
• All glucose in the filtrate is reabsorbed, so there is 0 mg/min lost in excretion despite having only one kidney.

ADH and the Kidney

• ADH is released in a racing greyhound as it becomes dehydrated.
• ADH affects the late distal tubule and collecting duct of the nephron.
• ADH increases the permeability to water reabsorption in these areas.
• This action puts water back into circulation, reducing dehydration.

Glomerular Filtration Pressure

• Average hydrostatic pressure in glomerular capillaries: 60 mm Hg.
• Hydrostatic pressure in Bowman's space: 20 mm Hg.
• Average colloid osmotic pressure in glomerular capillaries: 35 mm Hg.
• Net pressure driving glomerular filtration is 5 mm Hg.
• Net pressure = Glomerular hydrostatic pressure – Colloid osmotic pressure – Bowman’s hydrostatic pressure.
• Net pressure = 60 mm Hg - 35 mm Hg - 20 mm Hg.

Filtration Fraction Calculation

• Glomerular filtration rate (GFR) is 150 mL/min.
• Renal plasma flow is 480 mL/min.
• Filtration fraction is 0.31.
• Filtration fraction is calculated by GFR/Renal plasma flow.
• Filtration fraction = 150/480.
• Glomerular hydrostatic pressure is irrelevant for this calculation.

Glomerular Filtration Rate (GFR)

• A 50% decrease in afferent arteriolar resistance would increase GFR.
• A 50% decrease in efferent arteriolar resistance would decrease hydrostatic pressure in the glomerulus.
• Decreased systemic blood pressure would decrease renal blood flow.
• Increased hydrostatic pressure in Bowman’s capsule would increase net fluid movement into the glomerulus against filtration.

Vasodilator Effects on Renal Function

• A vasodilator drug causes a 50% decrease in efferent arteriolar resistance.
• Arterial pressure remains unchanged.
• Renal blood flow increases.
• GFR decreases.
• Filtration fraction decreases.
• GFR falls without any change in arterial pressure.
• Filtration fraction (GFR/RBF) decreases due to the GFR decrease.

Micturition Reflex

• Urine distension activates stretch receptors in the bladder wall.
• Sensory neurons transmit signals to the sacrum of the spine via parasympathetic fibers.
• Parasympathetic neurons return to the bladder.
• This causes the detrusor muscle in the bladder wall to contract.
• Contraction puts pressure on the urine, forcing it into the proximal urethra, creating the urge to urinate.
• The detrusor muscle then relaxes, allowing more urine to flow into the bladder until the next reflex.
• Urination occurs if the urethral sphincter relaxes.

Aldosterone and Acid-Base Balance

• Excess H+ in the blood triggers increased aldosterone production.
• Aldosterone functions in the late distal tubule.
• Aldosterone activates the exchange of secreting H+ and reabsorbing HCO3- in the intercalated type A cells.
• The exchange helps the body eliminate excess H+ by reabsorbing HCO3- and secreting H+ into the filtrate for excretion in the urine.
• This is a specific example of secretion with regards to H+.