physiology 2 files/physiology 2/1-tubular... · 2018-07-02 · *all substances that are reabsorbed...
TRANSCRIPT
URINARY SYSTEM
Physiology 2
Presented by: Dr. Shaimaa Nasr Amin
Lecturer of Medical Physiology
General Education Program
Tubular processing of the
Glomerular Filtration
As glomerular filtrate enters the renal
tubule (now called tubular fluid), it flows
through the proximal tubule, loop of
Henle ,distal tubule, collecting tubule and
finally collecting duct.
*Result of tubular handling:
1- Volume is decreased .
2- Composition altered by the process of
reabsorption and secretion.
U/P ratio Concentration in plasma
(P)
Concentration in Urine
(U)
Substance
0 100 0 1-Glucose
(mg/dl)
0.6 150 90 Na+
(mEq/L)
60 15 900 Urea
(mEq/L)
150 1 150 Creatinine
(mg/dl)
Tubular reabsorption It involves:
1-Transport of substance across the tubular epithelium
into renal interstitial fluid.
2-Transport from the interstitial fluid into peritubular
capillaries.
Tubular Secretion *Transport of substances from the blood in peritubular capillaries
into the renal tubule.
Urinary excretion rate=
Filtration rate –reabsorption rate
+Secretion rate
Type of transport across the tubylar
epithelium
1-Transcellular
2-Paracellular
Mechanism of tubular
transport
Active transport
Passive transport
Pinocytosis
Active -1Transport
A-Primary active
Transport
B-Secondary active
transport
a- Co-transport
b- Counter transport
1-Active transport
It’s against concentration or electrical gradient
A-Primary active transport:
-Energy derived from direct hydrolysis of ATP by membrane
bound ATPase.
e.g. Na+ reabasorption across proximal tubular epithelium.
B-Secodary active transport:
-Energy NOT derived directly from ATP or from high energy
phosphate sources.
-Types:
a) Co-transport.
b) Countertransport.
a)Co-transport:
2 substances bind to specific carrier molecule and are co
transported together across the membrane one down its
electrochemical gradient and the other substance against its
chemical gradient.
-e.g. Secondary active transport of glucose
b) Countertransport:
The reabsorption of one substance is linked to secretion of
another.
e.g. Secondary active secretion of H+ into the tubule.
2-Passive transport a-Passive reabsorption of chloride.
b-Osmosis of water.
c-Passive reabsorption of urea.
a-Passive reabsorption of chloride:
-Through paracellular pathway
,following Na+ reabsorption.
b-Osmosis of water:
-After solute reabsorption out of the
tubule →↓their concentration inside
the tubule and ↑in the interstitium→
concentration gradient→osmosis of
water
c-Passive reabsorbtion of urea:
As water is reabsorbed from the tubule→ ↑urea concentration in
the tubular lumen→ concentration gradient favoring reabsorption
of urea.
*About 50% of the filtered urea is passively reabsorbed and the
remainder pass in urine.
3-Pinocytosis -Active transport for
reabsorption of proteins and
peptides in the proximal
convoluted tubule.
Tubular Transport maximum
*For many actively transported substances there is a
maximum rate at which each can be transported ,due to
saturation of the carrier system.
*The maximum rate that can be achieved is termed transport
maximum (Tm ) for the substance is expressed as mg/min.
Solutes that exhibit Tm-limited reabsorption:
*Glucose ,amino acid ,phosphate, sulphate.
Solutes with Tm-limited
secretion:
*Para-aminohippuric acid PAH
*Penicillin
Gradient time transport *All substances that are reabsorbed by diffusion ,transport by
this type is termed gradient –time transport
*It is determined by:
1-The electrochemical gradient for the substance across the
membrane.
2-The time that the fluid containing the substance remains within
the tubule which depends on tubular flow rate.
*Some actively transported substances
obeys the gradient-time transport
e.g. Na+ reabsorption by the proximal
tubule,as it is determined by:
1-Concentration of Na+ in the proximal
tubule.
2-The rate of flow (the slower the flow rate
of the tubular fluid, the greater the %of
Na+ that can be reabsorbed).
Absorption by the peritubular capillaries *Fluids and electrolytes are reabsorbed from the renal
interstitium into the peritubular capillaries by bulk flow as
peritubular capillaries behave like venous end of the
capillary.
Forces that act across peritubular
capillaries
2-Forces that oppose
reabsorption:
a) Hydrostatic
pressure inside the
peritubular capillaries
(13mmHg).
b) Colloid osmotic
pressure of proteins
in renal
interstitium(15mmHg).
1-Forces that favour
reabsorption:
a) Colloid osmotic
pressure of
peritubular
capillary(32mmHg).
b) Hydrostatic
pressure in renal
interstitium(6mmHg).