Notes on Renal Processes

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Excretion, Filtration, Reabsorption, and Secretion in Kidneys:

renal excretion filtration reabsorption secretion

Renal Calculations:

• Amount excreted = (amount filtered - amount reabsorbed) + amount secreted.

• Excretion = disposed in urine.

• Filtration = 100% blood is filtered through glomerular capillaries into the Bowman's capsule, and most is reabsorbed back into blood through the peritubular capillaries through to the renal vein; an energy requiring process.

• Tubular secretion = amount that is excreted directly through the peritubular capillaries without being filtered through. Eg., H+ ions for pH balance.

Factors that affect filtration:

Hydrostatic pressure of glomerular capillaries:

• Promotes filtration.

Oncotic pressure of plasma:

• Opposes filtration.

Hydrostatic pressure in Bowman's Capsule:

• Opposes filtration.

Protein or oncotic pressure in capsule:

• This is zero.

Charge on Filtering Membrane:

• The filtering membrane is negatively charged; this inhibits the filtering of anions (negative). Protein is filtered if this charge is removed.

Bowman's Capsule:

• Concentrate entering the Bowman's capsule has the same concentration as plasma minus proteins.

Free Renal Filtration:

• 20% of substance that enters the kidney is freely filtered.

Filtration Fraction:

• Filtration fraction = FF = GFR/(Renal Plasma Flow).

Glomerular Filtration Rate:

• GFR is determined by the glomerular capillary pressure; direct proportionality.

Filtration Fraction:

• FF determined by renal plasma flow; direct proportionality.

Effect of Sympathetic NS:

• Vasoconstriction; decreased GFR; increased FF; increased reabsorption.

Effect of Angiotensin II:

• No effect on GFR; maybe minimal decrease.

Glucose Filtration Dynamics:

• At low plasma levels, glucose filtration = glucose reabsorption.

• As glucose plasma concentration rises in plasma to ~ 2mg/ml, glucose excretion curve enters the region of splay and begins to increase.

• As glucose plasma concentration rises in plasma to ~ 2mg/ml, glucose absorption curve enters the region of splay and begins level off.

Transport Maximum (Tm) Systems:

• Entire filtered load is reabsorbed until carriers are saturated; the excess is then excreted.

• Eg., Glucose.

Time-Gradient System:

• Eg., Sodium.

• A constant percentage of sodium is actively absorbed, that is, 2/3rd of all filtered sodium is reabsorbed.

• Oxygen consumption in the kidney is directly proportional to Na reabsorption and GFR.

• Na reabsorption is the main metabolic process in the kidney.

p-Aminohippuric Acid (PAH):

• Transport maximum system.

• Most is excreted through peritubular capillaries (80%) without being filtered through glomerular capillaries (20%).

• There is no reabsorption.

• Secretion = 4x greater than filtration.

• 80% continues to be secreted until nephron carriers become saturated.

Filtration vs Excretion:

Filtration = excretion:

• Insulin, mannitol.

Filtration greater than excretion:

• Glucose, sodium, urea.

Filtration less than excretion:

• PAH, creatinine.

Additional Reading:

Basic Nephrology

1. Renal Control in Acid-Base Balance
2. Renal Processes
3. Renal Clearance
4. Regional Transport
5. What are the indications for hemodialysis?

Related Topics

1. Kidney Disorders
2. Histology of the Urinary System
3. Acid-Base Disturbance: Acidotic or Alkalotic? [Size: 427 kB; Format: PDF]
4. Abdominal Examination for Internal Medicine

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