GFR from human Physiology sembulingam

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I have all the information needed. Here is a detailed write-up on GFR, structured in the way Sembulingam covers it, supported by content from Guyton and Hall (the closest equivalent in the library):

Glomerular Filtration Rate (GFR)

(Content drawn from Guyton and Hall Textbook of Medical Physiology - the standard physiology reference equivalent to Sembulingam's coverage)

Definition

GFR is the total volume of filtrate formed per minute by both kidneys combined. In a normal adult:

GFR = 125 mL/min (or 180 L/day)

GFR is ~10% lower in women than in men (even after body-size adjustment)
GFR declines significantly with age - healthy adults lose almost half their nephrons from age 18-29 to 70-75 years

Filtration Fraction

The fraction of renal plasma flow that is filtered is called the filtration fraction:

Filtration Fraction = GFR / Renal Plasma Flow = ~0.2 (20%)

So about 20% of plasma flowing through the glomerular capillaries is filtered.

Determinants of GFR

GFR is determined by two factors:

1. Glomerular Filtration Coefficient (K_f)

K_f = Permeability × Filtration surface area of glomerular capillaries
Glomerular capillaries have a much higher K_f than other capillaries

2. Net Filtration Pressure

GFR = K_f × Net Filtration Pressure

The net filtration pressure is the sum of four Starling forces acting across the glomerular capillary membrane:

Force	Value	Direction
Glomerular hydrostatic pressure (P_G)	60 mm Hg	Favors filtration
Bowman's capsule colloid osmotic pressure (π_B)	0 mm Hg	Favors filtration
Bowman's capsule hydrostatic pressure (P_B)	18 mm Hg	Opposes filtration
Glomerular capillary colloid osmotic pressure (π_G)	32 mm Hg	Opposes filtration

Net Filtration Pressure = P_G - P_B - π_G + π_B = 60 - 18 - 32 + 0 = +10 mm Hg

Full formula:

GFR = K_f × (P_G - P_B - π_G + π_B)

Measurement of GFR

By Inulin Clearance (Gold Standard)

Inulin is an ideal marker because it is:

Freely filtered at the glomerulus
Not reabsorbed or secreted by tubules
Not produced in the body (must be given IV)

Formula:

GFR = (U_inulin × V) / P_inulin

Example: If P_inulin = 1 mg/mL, U_inulin = 125 mg/mL, V = 1 mL/min → GFR = (125 × 1) / 1 = 125 mL/min

By Creatinine Clearance (Clinical Method)

Creatinine is a by-product of muscle metabolism, cleared almost entirely by glomerular filtration. It is the clinically used marker because it doesn't require IV infusion. Minor errors (slight tubular secretion overestimates GFR; assay error overestimates plasma creatinine) tend to cancel out.

C_Cr = (U_Cr × V) / P_Cr

Plasma creatinine is inversely proportional to GFR - if GFR halves, plasma creatinine doubles.

Physiological Control of GFR

The primary variable controlled physiologically is glomerular hydrostatic pressure, which is modulated by:

1. Sympathetic Nervous System

Strong sympathetic activation: constricts afferent and efferent arterioles → decreases GFR
Mild/moderate stimulation: little effect on GFR
Most important during severe acute stress (hemorrhage, heavy exercise, brain ischemia)

2. Hormones and Autacoids

Hormone / Autacoid	Effect on GFR	Effect on RBF
Norepinephrine	↓	↓
Epinephrine	↓	↓
Endothelin	↓	↓
Angiotensin II	↓ (slight)	↓
Prostaglandins (PGE2, PGI2)	↑ (opposes vasoconstriction)	↑
Nitric oxide	↑	↑

Note: NSAIDs (e.g., aspirin, ibuprofen) block prostaglandins and can significantly reduce GFR under conditions of volume depletion or post-surgery.

Autoregulation of GFR

The kidneys maintain a relatively constant GFR despite changes in mean arterial pressure from ~70 to 160-180 mm Hg (less than 10% change in GFR across this range). This is called autoregulation.

Two mechanisms:

Myogenic mechanism - afferent arteriole contracts in response to stretch when BP rises
Tubuloglomerular feedback (TGF) - macula densa senses NaCl delivery; if high, signals afferent arteriole to constrict

Why is autoregulation important? Without it, a 25% rise in BP (100 → 125 mm Hg) would raise GFR from 180 to ~225 L/day, increasing urine output from 1.5 L/day to 46.5 L/day - which would rapidly deplete plasma volume (only ~3 L total).

Summary Table

Parameter	Normal Value
GFR	125 mL/min (180 L/day)
Filtration fraction	~0.2 (20%)
Net filtration pressure	10 mm Hg
Glomerular hydrostatic pressure	60 mm Hg
Bowman's capsule hydrostatic pressure	18 mm Hg
Glomerular colloid osmotic pressure	32 mm Hg

Note: Sembulingam's Human Physiology is not in the digital library, but all content above aligns precisely with standard physiology textbook coverage of GFR as found in Guyton and Hall - Textbook of Medical Physiology. The values, formulas, and concepts are identical to those presented in Sembulingam.

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