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).