Urine Formation & Renal Function

Main kidney functions?

• Filter blood

• Regulate volume/osmolarity

• Maintain pH

• Excrete waste

Normal blood osmolarity?

~300 mOsm/L

Max urine concentration?

~1200 mOsm/L

Dilute urine definition?

<300 mOsm/L

Proximal Convoluted Tubule (PCT)What happens here?

• Reabsorption of solutes (Na⁺, glucose, etc.)

• Water follows (osmosis)

Descending Loop of Henle, Permeability?

• Permeable to water

• NOT permeable to solutes

Effect?

Filtrate becomes more concentrated

Ascending Loop of Henle (THICK LIMB)Permeability?

• Impermeable to water

• Pumps out ions (Na⁺-K⁺-2Cl⁻ symporter)

Effect?

Filtrate becomes dilute

Distal Convoluted Tubule (DCT) function:

Further solute reabsorption (no water)

Collecting Duct key regulator

ADH

countercurrent mechanicm, What creates medullary osmotic gradient?

Ascending loop pumping out ions

Why is this important?

Allows water reabsorption in collecting duct

Function of ADH

Increases water reabsorption

Mechanism of ADH

Inserts aquaporin-2 channels in collecting duct

With ADH, urine is

Concentrated

Without ADH urine is

Dilute

Trigger of the RAAs system

Low blood pressure

RAAS system pathway

Angiotensinogen → Renin → Angiotensin I → ACE → Angiotensin II

Effects of Angiotensin II?

• Vasoconstriction

• ↑Blood pressure

• ↑Aldosterone

Function of aldosterone?

• ↑Na⁺ reabsorption

• Water follows → ↑blood volume

How does furosemide work?

Inhibits Na⁺-K⁺-2Cl⁻ symporter in ascending loop → ↓gradient → ↑urine

Normal urine volume?

1–2 L/day

Normal urine pH?

~6

What is BUN?

Urea nitrogen → ↑ when kidney function ↓

What is creatinine?

Muscle waste → indicator of kidney function

Formula?

Clearance = (U × V) / P, U = urine concentration

• V = urine flow rate

• P = plasma concentration

Interpretation?

• High clearance → high excretion

• Low clearance → reabsorption

Substance fully reabsorbed?

Glucose → clearance = 0

Substance secreted?

Penicillin → high clearance

What is GFR

Rate of filtration into glomerulus

Normal GFR?

~120 mL/min

Best substance for GFR measurement?

Inulin (ideal), creatinine (clinical)

When is dialysis needed?

GFR ~15 mL/min

Why is loop length important?

Longer loop → stronger gradient → more water reabsorption

Why does ADH need the gradient?

Water only leaves if surrounding interstitium is hyperosmotic

What happens in water intoxication?

Blood becomes too dilute → dangerous ↓osmolarity