Lec 16 - Comparative Renal Anatomy

Tubular fluid

Filtrate after it leaves Bowman’s capsule

Tubular flow order

PCT → Loop of Henle → DCT → Collecting tubule → Collecting duct

Tubular reabsorption

Movement of substances from tubular fluid back to blood

Tubular secretion

Movement of substances from blood into tubular fluid

Excretion equation

Excretion = Filtration − Reabsorption + Secretion

How much glucose is reabsorbed?

~100% reabsorbed

Bicarbonate reabsorption

99.9% reabsorbed

Sodium reabsorption

~99.4% reabsorbed

Chloride reabsorption

~99.1% reabsorbed

Potassium reabsorption

~87.8% reabsorbed

Urea reabsorption

~50% reabsorbed

Creatinine reabsorption

0% (fully excreted)

Types of reabsorption

Active and passive

Water reabsorption mechanism

Osmosis following solute

Transcellular transport

Through cells

Paracellular transport

Between cells

Na+ transport mechanism

Active transport via Na+/K+ ATPase

Na+ importance

Drives most reabsorption

Cl- reabsorption

Passive diffusion

Urea permeability

Lower than chloride

Transport maximum (Tm)

Max rate of carrier-mediated transport

When Tm exceeded

Substance appears in urine

Proximal tubule (PCT)

Reabsorbs ~65% of filtrate

PCT reabsorbs

Na+, Cl-, HCO3-, K+, water, glucose, amino acids

PCT secretion

H+, organic acids, toxins

Glucose reabsorption mechanism

Na+ cotransport (SGLT)

Amino acid reabsorption

Na+ cotransport

Na+ countertransport

Exchanged with H+

Na+/K+ pump location

Basolateral membrane

Water reabsorption in PCT

Follows Na+ osmotically

PCT reabsorption type

Isosmotic

Hormone affecting PCT

Angiotensin II increases Na+ reabsorption

CO2 role in PCT

Helps buffer regulation via HCO3-

Passive ion reabsorption

Driven by gradients and permeability

PCT secretion examples

Bile salts, urate, drugs

Glucose threshold

Point where glucose appears in urine

Reason glucose appears before Tm

Transporters saturate unevenly

Proximal tubule concentration ratio (=1)

Same concentration as filtrate

Ratio <1

More reabsorbed than water

Ratio >1

Less reabsorbed than water

Loop of Henle

Concentrates urine via countercurrent system

Descending limb

Permeable to water

Ascending limb

Impermeable to water

Thin descending limb

Water reabsorption only

Thick ascending limb

Reabsorbs Na+, Cl-, K+

Ascending limb fluid

Hypo-osmotic

Water reabsorbed in descending limb

~50% of water

NaCl reabsorbed in ascending limb

~2/3

Countercurrent exchange

Maintains gradient

Countercurrent multiplication

Creates gradient

Distal tubule receives

~15–20% of filtrate

Distal tubule reabsorption

Variable and hormone-controlled

What type of transport is Na+ reabsorption in DCT?

Active transport

Aldosterone function

Increases Na+ reabsorption

Effect of aldosterone

Reduces Na+ loss in urine

ANP/BNP effect

Opposes aldosterone

Water reabsorption in DCT

Follows Na+

Ca2+ reabsorption

Controlled by PTH and calcitriol

K+ secretion in DCT

Exchange with Na+

Source of secreted K+

Peritubular fluid

H+ secretion in DCT

Exchange with Na+

H+ role

Regulates acid-base balance

H+ source

Carbonic acid dissociation

Aldosterone effect on H+

Increases secretion

Collecting duct

Final regulation of urine

Collecting duct receives

Fluid from multiple nephrons

Hormones controlling collecting duct

Aldosterone and ADH

ADH function

Increases water permeability

Aldosterone function

Increases Na+ pumps

Medullary collecting duct

Highly permeable to water

Medulla osmolarity

~4x greater than cortex

Na+ reabsorption in CD

Aldosterone-dependent

HCO3- reabsorption

Exchanged for Cl-

Urea reabsorption

Diffuses out of duct

H+ secretion in CD

Depends on pH

Low pH

Secrete H+

High pH

Secrete bicarbonate

Water balance goal

Maintain ECF osmolarity

Extracellular water determined by

Intake + renal excretion

Urine osmolarity range

~50 to 1400 mOsm/L

Dilute urine

Occurs with excess water

Concentrated urine

Occurs with dehydration

Water excretion independence

Independent of solute excretion

Filtrate reabsorbed before DCT

~85%

Effect of drinking water

Increased urine volume

Solute excretion

Remains relatively constant

Formation of dilute urine

NaCl reabsorption without water

Ascending limb role

Creates dilute tubular fluid

Late DCT/CD

Further dilute urine

ADH (vasopressin)

Controls water reabsorption

ADH release trigger

High ECF osmolarity

ADH target

DCT, collecting tubule, collecting duct

Aquaporins

Water channels inserted by ADH

Diuresis

Low ADH → more urine

Antidiuresis

High ADH → concentrated urine

Obligatory urine volume

Minimum urine needed to excrete solutes

Seawater example

Causes dehydration (too much solute)

Requirements for concentrated urine

High ADH + high medullary osmolarity

Medullary gradient

300 → 1200–1400 mOsm

Single effect

~200 mOsm gradient created