Function of the Kidney

1
Function of the Kidney

• Physics of Circulation and Respiration
• Maartje Dekker
• December 2004

2
Function of the Kidney

• Outline
• What does it do?
• How does it work?
• Dialysis machines

3
Function of the Kidney Function overview

• What does the kidney do?
• Remove undesirable substances from the blood
  plasma
• Toxins
• Metabolism wastes
• Excess of ingested water
• Excess of mineral salts
• Regulate the acidity of blood
• 25 of the blood flows through
• the kidneys (1,2 l/min)
• 130 ml/min is concentrated into
• ultra filtrate

4
Function of the Kidney Terminology

• Blood is filtered in the nephrons
• The cortex of each kidney contains 1,2 million
  nephrons
• The nephron consists of a renal corpuscle and a
  renal tubule
• The renal tubule consists of the convoluted
  tubule and the loop of Henle
• The main filter of the nephron is glomerulus
  which is located within the Bowman's capsule

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Function of the Kidney The glomerulus

• The glomerulus consists of a network of parallel
  capillaries.
• It is a filter made up from different layers
• Endothelium cells containing pores in their
  cytoplasm of radius 25-50nm
• The basal membrane
• Lamina rara interna
• Lamina densa
• Lamina rara externa
• Epithelium cells

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Function of the Kidney The basal membrane

• Flitration based on size
• All 3 laminae contain collagen
• Laminae rarae contain laminin molecules to bind
  the collagen to the endo-, epithelium cells
• The collagen network acts as a filter
• Molecules with 6 nm lt r lt 1,8 nm can pass the
  filter
• Filtration based on charge
• Laminae rarae contain heporan sulfate
  proteolycate which
• has a negative charge
• Cell membranes of endo-, epithelium cells are
  charged negatively
• Negatively charged proteins are hindered in their
  passing

7
Function of the Kidney The glomerulus

• Filtration depends on
• Plasma flow through glomerulus, Qa
• Hydrostatic pressure Pgc
• Colloid osmotic pressure at begin (pa ) and end
  (pe ) of filtration
• Filtration capacity of the capillaries, S
• Fluctuations in blood pressure have little effect
  due to auto regulation of the blood pressure in
  the glomular vessel
• Filtration stops if blood pressure drops below 36
  nmHg

8
Function of the Kidney Transport in the
tubular apparatus

• Filtrate produced per day 180 l
• Urine production 1,5 l
• Water reabsorbed in proximal tubulus (80)
• Active transport of minerals
• Water transport by osmotic pressure difference
• Urine concentrated in loop of Henle (6)
• Another 12-15 is reabsorbed in the distal
  collecting tubuli

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Function of the Kidney Damage of the glomerulus

• 3 different kinds of damage
• Inflammation of the kidney causing damage to the
  filter and a decrease of the flow through the
  glomerulus
• Loss of charge of the membrane resulting in a
  higher permeability for proteins
• Scar forming after inflammation causing loss of
  functional tissue and filtering surface
• and the adaptations made by the kidney
• Functional adaptation loss of nephrons results
  in a rise in blood pressure
• Arteriole resistance in the glomerulus decreases
• Qa increases
• Pgc (total filtration pressure) increases
• fn (filtration fraction) increases
• Structural adaptation hypertrophy of kidney
  tissue resulting in an increase in filtering
  surface
• Kf (permeability filtration capacity) remains
  the same or decreases causing kidney
  insufficiency

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Function of the Kidney Glomerulosclerosis

• Glomerulosclerosis the degeneration of
  capillary walls due to e.g. diabetes or subtotal
  nephrectomy ( clinically removal of part of the
  kidney)
• A sign of glomerulosclerosis is proteinuria
  (urine contains large amount of proteins)
• Glomerulosclerosis is caused by hyper filtration
  due to high blood pressure in the capillaries

11
Function of the Kidney Kidney insufficiency

• Kidney insufficiency may be caused by
• Nefritis (inflammation of the kidney)
• Nefricalcinosis (kidney stones in tubuli)
• Nefrocirrhosis (shrinking of the kidney)
• Solutions are
• Hemo dialysis outside of the body the blood is
  made to flow past a membrane. Differences in
  concentration and pressure make the waste pass
  through the membrane into the dialysis solution

• Perioneal dialysis the abdomen is filled with
  dialysis solution. Waste passes through the
  abdominal membrane (peritoneum) into the fluid
• Kidney transplantation

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Function of the Kidney Hemo dialysis machines

• Hemo dialysis machines contain a semi permeable
  membrane
• The membrane is non permeable for proteins and
  blood cells
• Transport of wastes takes place by osmotic and
  pressure difference
• A dialysis machine should have the following
  qualities
• Efficient removal of metabolic wastes
• Removal of excess water
• No damage to the blood
• No blood clotting inside machine
• Sterile
• Disposable

Dialysis solution is non-sterile. Therefore blood
pressure should be higher than dialysate pressure
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Function of the Kidney Hemo dialysis machines

• Types of hemo dialysis machines
• (Twin) coil type (1956) blood flows through
  celluloid tubes
• Dialysate flows in perpendicular direction
• Membrane surface 1,0 -1,5 m2
• Priming volume 120 -240 ml
• Dialysate is recycled
• Needs pump
• Plate type (1948) blood flows between two
  membranes Dialysate flows in opposite direction
• Membrane surface 1,0 -1,6 m2
• Priming volume 140 -300 ml
• Needs 300 l dialysate per dialysis
• Does not need pump
• Capillary type blood flows through hollow,
  celluloid fibres (13.500) inside a plastic tube
• Dialysate flows in the tube,outside the fibres
• Membrane surface 1,3 -2,5 m2
• Priming volume 100 -180 ml
• Needs pump