Lecture #7: Physiology: Glomerular Filtration Rate, Renal Blood Flow and Clearance

What are the major learning goals of this renal physiology lecture?

Understand renal blood supply and filtration anatomy; relate GFR to vascular resistance, blood pressure, hydrostatic and oncotic pressures; explain autoregulation and vasoactive influences; and apply these principles to renal pathology and board-style questions.

Approximately how much blood flow do the kidneys receive?

About 1 L/min, representing approximately 20% of resting cardiac output despite the kidneys being only about 0.5% of body weight.

Where does most renal blood flow go?

Most blood remains in the cortex through the peritubular capillaries; only about 5–10% enters the medulla through the vasa recta.

Why is renal medullary blood flow relatively low?

Low medullary flow helps preserve the high medullary solute concentration needed to concentrate urine while the vasa recta provide O₂ and nutrients through countercurrent exchange.

Why is the renal medulla particularly vulnerable to injury?

The medulla receives relatively low blood flow and is highly susceptible to hypoxia and ischemic injury, including acute tubular necrosis.

What is the renal vascular sequence surrounding a glomerulus?

Renal arterial system → afferent arteriole → glomerular capillaries → efferent arteriole → peritubular capillaries or vasa recta → renal venous system.

Why is glomerular capillary pressure relatively high?

The glomerular capillaries lie between two high-resistance arterioles, the afferent and efferent arterioles, allowing maintenance and regulation of high glomerular hydrostatic pressure.

What relationship describes renal blood flow?

Renal blood flow follows Ohm's law: Flow = ΔP/R, meaning flow increases with the pressure gradient and decreases with vascular resistance.

What does Poiseuille's law predict about vascular resistance?

Resistance is proportional to vessel length and blood viscosity and inversely proportional to the fourth power of vessel radius; therefore, small changes in arteriolar radius cause large changes in renal blood flow.

Why are afferent and efferent arterioles important regulators of GFR?

Changes in their diameter strongly alter renal blood flow and glomerular capillary hydrostatic pressure, allowing sensitive control of filtration.

What happens with afferent arteriolar constriction?

Renal blood flow decreases, glomerular capillary hydrostatic pressure decreases, and GFR decreases.

What happens with afferent arteriolar dilation?

Renal blood flow increases, glomerular capillary hydrostatic pressure increases, and GFR increases.

What happens with moderate efferent arteriolar constriction?

Renal blood flow decreases while glomerular capillary hydrostatic pressure increases, tending to increase GFR and filtration fraction.

What happens with efferent arteriolar dilation?

Renal blood flow increases, but glomerular capillary hydrostatic pressure and GFR decrease because blood exits the glomerulus more easily.

What is renal plasma flow (RPF)?

The volume of cell-free plasma passing through the kidneys each minute; normal RPF is approximately 600–650 mL/min.

How are RBF and RPF related to hematocrit?

RPF = RBF × (1 − hematocrit), and RBF = RPF/(1 − hematocrit).

How does increased hematocrit affect renal blood flow?

Increased hematocrit increases blood viscosity and vascular resistance, which tends to decrease renal blood flow.

What substance is used to estimate effective renal plasma flow?

Para-aminohippuric acid (PAH), because it is almost completely cleared from plasma during one passage through the kidneys.

What is the PAH clearance equation?

eRPF ≈ PAH clearance = UPAH × V / PPAH, where U is urine concentration, V is urine flow rate, and P is plasma concentration.

What is glomerular filtration?

Movement of fluid and filterable solutes from the glomerular capillaries across the filtration barrier into Bowman's space.

What is glomerular filtrate?

The fluid remaining after plasma undergoes ultrafiltration across the glomerular filtration barrier into Bowman's space.

What is tubular reabsorption?

Movement of a substance that was initially filtered into the nephron back into the bloodstream.

What is tubular secretion?

Transfer of a substance from the blood into the nephron lumen so that it can ultimately be excreted.

What is excretion?

Removal of a substance from the body in urine; excretion = filtration − reabsorption + secretion.

What is renal clearance?

The rate at which a substance is removed from plasma by the kidneys and excreted into urine.

What is the renal clearance equation?

Cx = Ux × V / Px, where Ux is urine concentration of substance X, V is urine flow rate, and Px is its plasma concentration.

What is glomerular filtration rate (GFR)?

The volume of fluid filtered from glomerular capillaries into the renal tubules per unit time and an important overall indicator of renal filtration function.

What is the three-layer glomerular filtration barrier?

Fenestrated glomerular endothelium → glomerular basement membrane → visceral epithelial podocytes with foot processes and slit diaphragms.

What substances normally filter freely through the glomerulus?

Small molecules such as electrolytes, glucose, amino acids, urea, creatinine, and vitamins filter readily and have concentrations in filtrate similar to plasma.

What substances are normally restricted by the glomerular filtration barrier?

Blood cells and large negatively charged proteins are strongly restricted under normal physiologic conditions.

What is the role of the glomerular basement membrane in filtration selectivity?

The GBM contains type IV collagen, heparan sulfate, glycoproteins, and proteoglycans; its negative charge and structure restrict large negatively charged molecules and proteins.

What is the role of podocytes in filtration?

Podocyte foot processes form filtration slits and slit diaphragms that provide an additional size-selective barrier to filtration.

What is the filtration coefficient Kf?

Kf is the product of glomerular membrane water permeability and available filtration surface area; increasing either factor increases filtration capacity.

What are the major Starling forces controlling glomerular filtration?

Glomerular capillary hydrostatic pressure, Bowman's space hydrostatic pressure, glomerular capillary oncotic pressure, and Bowman's space oncotic pressure.

Which Starling force primarily favors glomerular filtration?

Glomerular capillary hydrostatic pressure, which pushes fluid from the capillary into Bowman's space.

Which forces normally oppose glomerular filtration?

Bowman's space hydrostatic pressure and glomerular capillary oncotic pressure oppose filtration.

What is the normal oncotic pressure in Bowman's space?

Approximately zero because very little protein normally enters the glomerular filtrate.

What is the equation for net filtration pressure?

Net filtration pressure = PGC − PBS − πGC + πBS; because πBS is normally approximately zero, it is often simplified to PGC − PBS − πGC.

Why does glomerular capillary oncotic pressure rise along the capillary?

Water is filtered while plasma proteins remain in the blood, progressively concentrating the proteins and increasing oncotic pressure.

What is filtration fraction?

The proportion of renal plasma flow that becomes glomerular filtrate: FF = GFR/RPF.

What is a normal filtration fraction?

Approximately 20%.

How does an isolated increase in GFR affect filtration fraction?

It increases filtration fraction if renal plasma flow remains unchanged.

How does an isolated increase in RPF affect filtration fraction?

It decreases filtration fraction if GFR remains unchanged.

What is filtered load?

The amount of a substance filtered into the nephron per unit time: Filtered load = GFR × plasma concentration of the substance.

What substance is the gold standard for measuring GFR?

Inulin because it is freely filtered and neither reabsorbed nor secreted, so its clearance equals GFR.

Why is creatinine commonly used instead of inulin?

Creatinine is an endogenous product that is practical to measure and is freely filtered, making its clearance a useful estimate of GFR.

Why does creatinine clearance slightly overestimate true GFR?

A small amount of creatinine is secreted by renal tubules, causing creatinine clearance to exceed true GFR by approximately 10–20%.

How can clearance be used to determine renal handling of a substance?

If Cx < GFR, the substance is net reabsorbed or not freely filtered; if Cx = GFR, it is filtered without net reabsorption or secretion; if Cx > GFR, it undergoes net secretion.

What is renal autoregulation?

The intrinsic ability of the kidneys to maintain relatively stable RBF and GFR despite changes in renal perfusion pressure.

What are the two major intrinsic mechanisms of renal autoregulation?

The myogenic response of the afferent arteriole and tubuloglomerular feedback involving the macula densa and juxtaglomerular apparatus.

How does the myogenic mechanism respond to increased blood pressure?

Increased pressure stretches the afferent arteriole, causing smooth-muscle constriction that decreases RBF and GFR back toward baseline.

How does tubuloglomerular feedback respond to increased NaCl delivery to the macula densa?

High NaCl signals excessive filtration, promoting ATP/thromboxane A₂-mediated afferent constriction and reducing RBF and GFR.

How does tubuloglomerular feedback respond to decreased NaCl delivery to the macula densa?

Low NaCl promotes prostaglandin signaling, renin release, RAAS activation, and afferent dilation to support RBF and GFR.

How does angiotensin II affect renal arterioles?

At low-to-moderate levels, angiotensin II preferentially constricts the efferent arteriole, helping maintain glomerular capillary hydrostatic pressure and GFR when renal perfusion falls.

What is the protective renal effect of prostaglandins?

Renal PGE₂ and PGI₂ preferentially dilate the afferent arteriole during stress or volume depletion, helping preserve renal blood flow and protecting the kidney from ischemia.

Why can NSAIDs reduce GFR?

NSAIDs inhibit COX-dependent prostaglandin production, removing prostaglandin-mediated afferent vasodilation and allowing afferent constriction, which reduces renal blood flow and GFR.

How can ACE inhibitors reduce GFR?

ACE inhibition decreases angiotensin II, reducing efferent arteriolar constriction; efferent dilation lowers glomerular capillary hydrostatic pressure and may decrease GFR.