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EXCRETORY SYSTEM
DR.Tazeen Hassan shah
M.Phil physiology
kidney
 Structural and functional unit of kidney is nephron
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glomerulus
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renal tubules:pct,loop of henle,dct,collecting
tubule,collecting ducts(250 each draining 4000
nephrons)
FUNCTIONS OF KIDNEY
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Removal of metabolic waste
Electrolyte balance
Water balance
Regulation of blood pressure
Regulation of acid-base balance
Erythropoiesis
Vit D activation
Release of renin
Gluconeogenesis
Release of kinins
VASCULAR BEDS OF KIDNEY
GLOMERULUS:60 mm of hg
PERITUBULAR CAPPILLARIES:13 mm of hg
TYPES OF NEPHRON
 Cortical nephron:present in renal cortex,short loop of
henle,that is surrounded by tortuous peritubular
cappillaries.
 Juxtra medullary nephron(30%):present near
corticomedullary junction,long loop of
henle,surrounded by straight pt cappillaries,k/as vasa
recta,concerned with concentration of urine.
URINE FORMATION
 GLOMERULAR FILTRATION
 SELECTIVE REABSORPTION
 ACTIVE SEREATION
GLOMERULAR FILTRATION
 125ml of plasma is filtered each min,or 180L/d
 Of which 178.5 lit are reabsorbed and 1.5 lit becomes
urine/day
 Whole plasma is filtered 60 times /day
SELECTIVE REABSORPTION
 As filtrate passes via renal tubules ,nutrionally
important subtances are reabsorbedto blood via
peritubular cappillaries,e.g:glucose,amino
acid,vitamins.
 Unwanted substances are not reabsorbed
eg:urea,creatinine
Active secreation
 Some substances are actively secreated into renal
tubules via peritubular cappillaries like H and K ions
EXCRETION
Substances not reabsorbed and those which are
secreated together with water are finally excreted via
kidney as urine.
GLOMERULAR FILTRATION
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Through high pressure bed of glomerulus
125ml/min or 180L/day
No cells are filtered,no proteins are filtered
Donnan’s effect:cat ions less and anions more filtered
Plasma bound substances are less filtered
FILTRATION MEMBRANE
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Surface area: 0.8 msq
Has three layers:
Endothelium: fenestrated .large pores.
Basement membrane:negatively charged
proteoglycans,these repel and oppose the filtration
of –vely charged plasma proteins.
 Epithelium:has podocyted with pores called slit
pores.
FILTRATION PRESSURE
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Hydrostatic pressure:60mmof hg
Plasma colloid osmotic pressure:32
Capsular pressure :18
Net filtration pressure:60-18-32=+10mm of hg
Filtration coefficient:125/10=12.5/min/mm of hg
Factors affecting GFR
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Increase bp….increase GFR
Inc gcp……inc GFR(glomerulonephritis)
Inc bcp……dec GFR(UT obstruction)
Dec filtration area…dec GFR
Afferent arteriolar constriction… dec GFR
Afferent arteriolar dilation…..inc GFR
Efferent arteriolar constriction…inc GFR…long term
dec GFR
 Sympathetic stimulation-dec GFR
 Angiotensin 2 …..dec GFR
FORCES FAVOURING FILTRATION
GHP=60mm of hg
bcop=0 mm of hg
FORCES OPPOSING FILTRATION
BHP=18mm of hg
GCOP=32mm of hg
AUTOREGULATION OF GFR
 OCCURS VIA JGA(JUXTRA GLOMERULAR
APPARATUS)
 MACULA DENSA:Proximal part of DCT,condensed
cells in close contact with aff and eff arteriole,act as
chemoreceptors and sense conc of Nacl reaching
DCT.
 JG CELLS:modified smooth muscle cells of afferent
arteriole,has renin containing granules,on signals
from macula densa release renin.
 LACIS CELL:present in mesangium,act by :
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afferent arterioler feedback mechanism
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efferent arterioler feedback mechanism
 When GFR dec less Nacl reaches macula densa in
dct,It causes 2 reactions:
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afferent arteriole dilation
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JG cells relese more renin
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more angiotensin 2 formed
 Constriction of efferent arteriole
 GFR inc toward normal