Download Physiology of urinary system

Physiology of urinary system
Functions of the Urinary System
•Filters waste products from blood
metabolites, foreign substances (e.g. drugs..), ammonia, urea etc.
•Conserves valuable nutrients
e.g. glucose, amino acids are not lost from the urine
•Regulates ion levels of EC fluid
•Regulates EC fluid pH
•Regulates EC fluid volume
•Regulates RBC production
• Stores and excretes urine
•Produces and secretes hormones
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Parts of nephron
•The functional units of the kidneys
are called nephrons.
•Each kidney is made up of
millions of nephrons.
Two types of nephrons
Cortical nephrons
-~85% of all nephrons
-Are located in the cortex
Juxtamedullary nephrons
-Are closer (juxta = next to)
the renal medulla
-The loops of Henle extend
deep into the renal pyramids
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Process of Urine Formation
Urine formatio depends on :
- Glomerular filtration
- Tubular reabsorption of the substance from the tubular fluid into blood
- Tubular secretion of the substance from the blood into the tubular fluid
Filtrate Composition
Glomerular filtrate is produced from blood
The filtrate must pass through:
• Pores between endothelial cells of the
glomerular capillary
• Basement membrane
• Filtration slits formed by podocytes
Filtrate is similar to plasma except it is
essentially protein-free
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Forces Involved in
Glomerular Filtration
PGC
- glomerular capillary hydrostatic pressure
πGC
- oncotic pressure of blood
PBS
- Bowman's space hydrostatic pressure
πBS
- oncotic pressure of Bowman's space =0
Net Filtration Pressure =(PGC-PBS)+(πGC - πBS )
Measures of Glomerular Filtration Rate
(GFR)
The rate of glomerular filtration is a function of the
- net filtration pressure,
- the permeability of the filtration membrane,
- the surface area available for filtration.
The measured GFR reflects these factors, and the total number of functioning
nephrons.
Average GFR is 125 ml/min for a healthy human
GFR is directly measured by measuring the inulin clearance or by measuring
the creatinine clearance .
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Regulation of GFR
1. Renal autoregulation
renal autoregulation maintains a
nearly constant glomerular filtration rate
at rest autoregulation mechanisms prevail
2. Neural controls
the sympathetic nervous system will override renal autoregulation
under stress afferent arterioles constrict filtration is reduced
sympathetic nervous system stimulates renin secretion by the
juxtaglomerular cells
3. Hormonal mechanism
the renin-angiotensin system reduces GFR
atrial natriuretic peptide) increases GFR
3. Hormonal regulation of GFR
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Quantitative description of renal function
1. Extraction ratio (E)
E=
( Pa - Pv )
0<E<1
Pa
E: is the amount of compound entering the
kidney that got excreted into the final urine
Pa
Pv
Pa is the concentration in renal artery
Pv is the concentration in the renal vein
Quantitative description of renal function
2. Concept of clearance
U*V=P*C
->
C = U * V / P (ml / min)
C is the virtual volume of plasma cleared from the given
substance within 1 min
U = urinary concentration
V = diuresis (ml / min)
P = plasma concentration (Pa)
C = clearance (ml / min)
Pa
Pv
Pv can be ignored !
U*V
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Quantitative description of renal function
3. Relation of C to E
Mass of substances entering and
leaving the kidney within 1 min
(mg/min)
Pa * RPF
Pv * RPF
RPF=Renal Plasma Flow
RPF =
(U * V) / Pa
=
(Pa - Pv) / Pa
C
U*V
E
U = urinary concentration
V = diuresis (ml / min)
P = plasma concentration (Pa PV)
C = clearance (ml / min)
C = RPF * E
Quantitative description of renal function
4. Measurement of GFR
Amount excreted (E) = filtered (F) - reabsorbed (R) + secreted (S)
U*V
Pa * GFR
no tubular transport Æ U * V = Pa * GFR
e.g. inulin, creatinine
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Quantitative description of renal function
5. Tubular function
The main function of tubule is secretion and reabsorption
e.g. production of concentrated or diluted urine
reabsorption of glucose in proximal tubule
secretion of PAH
Quantitative description of renal function
6. Definitions
RBF (renal blood flow) = 1200 ml / min (20% of CO)
RPF (renal plasma flow) = 670 ml / min
RPF = RBF * (1 - Htc)
GFR (glomerular filtration rate) = 125 ml / min
FF (filtration fraction) = 0.2
FF = GFR / RPF
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Transport in the different parts of tubule
Transport in the distal tubule
This part performs final tuning of urine by active secretion or absorption
Aldosterone dependent movement of Na+ and K+
aldosterone production is stimulated by
hyperkalemia and angiotensin II
Secretion of K+ and H+ based on body pH
Water permeability is regulated by ADH (vasopressin)
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Effect of antidiuretic hormone (ADH)
ADH
- is released by posterior pituitary when osmoreceptors of
hypothalamus detect an increase in plasma osmolality.
- it restores the water permeability of collecting duct.
Control of volume and osmosis of urine
Building up of osmotic gradient
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Control of volume and osmosis of urine
Formation of concentrated urine
Control of volume and osmosis of urine
Formation of diluted urine
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Acid-base balance
• Importance of isohydria: enzymes are sensitive to pH
• Physiological pH: 7.38 - 7.42 (acidosis - alkalosis)
• Defense: immediate: buffer systems
long-term regulation:
respiration (10-15 min)
renal function (hours - days)
Acid-base balance
pH = pK + log
[salt]
[acid]
Buffer minimizes the change in pH when a base or an acid is added to the solution.
Buffers systems consist of a weak acid (releases H+ ions) and
a weak base (binds H+ ions)
Buffer systems
Intracellular buffer systems
Phosphate buffer
Proteins
Extracellular buffer systems
Phosphate buffer
Proteins
Carbonic acid bicarbonate buffer
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Acid-base balance
pH = pK + log
[HCO3-]
pCO2
Respiratory acidosis
pCO2 ×
e.g. hypoventillation
Respiratory alkalosis
pCO2 Ø
e.g. hyperventillation
Metabolic acidosis
HCO3- Ø
e.g. diarrhea
Metabolic alkalosis
HCO3- ×
e.g. vomitus
Micturition
active and passive incontinance
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Hemodialysis
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