glomerular filtration, tubular reabsorption/secretion, clearance

substances filtered in glomerulus

• water

• low-molecular weight substances

  • ½ of calcium ions

substances not filtered in glomerulus

• cells

• proteins (albumins, globulins)

• protein-bound substances (1/2 of cacium ions, fatty acids)

forces involved in filtration

• favouring filtration

  • glomerular capillary blood pressure - 60 mmHg

• opposing filtration

  • fluid pressure in Bowman’s space - 15 mmHg

  • osmotic force due to protein in plasma (oncotic pressure) - 29 mmHg

• net glomerular filtration pressure = 16 mmHg

glomerular filtration rate

volume of fluid filtered form the glomeruli into Bowman’s space per unit time

regulation of glomerular filtration rate

• net filtration pressure

• membrane permeability

• surface area available for filtration

filtered load

• total amount of any freely filtered substance per unit time

• GFR x plasma concentration of substance

types of tubular reabsorption

• paracellular (between cells)

• transcellular (across tubular epithelial cells)

substances reabsorbed in the tubules

• water - 99% reabsorbed

• sodium - 99.5% reabsorbed

• glucose - 100% reabsorbed

• urea - 44% reabsorbed

• potassium - net 86.1% reabsorbed (also secreted)

tubular reabsorption

• filtered loads are enormous, generally greater than the amounts of the substance in the body

• reabsorption of waste products is relatively incomplete

• reabsorption of most useful plasma components is relatively complete

• reabsorption of some substances are not regulated, while others are highly regulated

reabsorption by diffusion

often across the tight junctions connecting tubular epithelial cells

urea reabsorption in proximal tubule

• urea is freely filtered at glomerulus

• water reabsorption occurs in proximal tubule

• urea concentration in tubular fluid becomes higher

• urea diffuses into interstitial fluid and peritubular capillaries

reabsorption by mediated transport

• occurs across tubular epithelial cells

• requires the participation of transport proteins in the plasma membrane of tubular cells

• usually coupled to reabsorption of sodium

transport maximum (Tm)

tubule cannot reabsorb a substance anymore when the membrane transport proteins become saturated

transport maximum in diabetes mellitus

• plasma concentration of glucose becomes very high

• filtered load of glucose exceeds capacity of tubules to reabsorb glucose (Tm is exceeded)

• glucose appears in urine (glucosuria)

tubular secretion

• moves substances from peritubular capillaries into the tubular lumen

• mediated by diffusion and transcellular mediated transport

• most important substances secreted are hydrogen ion and potassium

• usually coupled to reabsorption of sodium

role of proximal tubule

• reabsorbs most filtered water and solutes

• major site of secretion for various solutes (except K⁺)

role of Henle’s loop

reabsorbs relatively large quantities of major ions (less water)

role of distal tubule and collecting duct

• volume of water and masses of solutes reaching here are relatively small

• fine-tuning of concentration

• determines the final amounts excreted in urine by adjusting rates of reabsorption and secretion

• most homeostatic controls are exerted here

clearance

volume of plasma from which a substance is completely removed by the kidneys per unit time

clearance formula

• mass of substance excreted per unit time / plasma concentration of substance

• (urine concentration of substance x urine volume of substance per unit time) / plasma concentration of substance

formula for mass of substance excreted per unit time

urine concentration of substance x urine volume of per unit time

inulin clearance

• polysaccharide that is administered intravenously

• freely filtered at glomerulus, but not reabsorbed, secreted, or metabolized by the tubule

• clearance of inulin (C_IN) is equal to volume of plasma originally filtered (GFR)

• most accurate marker of GFR

creatinine clearance

• waste product produced by muscle

• filtered freely at glomerulus, but not reabsorbed or metabolized by the tubule

• is secreted at the tubule, but amount is small

• used as a clinical marker for GFR

clearance vs GFR dynamics

• clearance of a substance > GFR → secreted at the tubule

• clearance of a substance < GFR → reabsorbed at the tubule