Remedial Class
DR. WA JEEHA BINTE SA JID
DEMONSTRATOR – I
PHYSIOLOGY
COMPOSITION OF BODY FLUIDS
40%
20%
75%
25%
GENERAL FUNCTIONS OF KIDNEYS AND
EXCRETORY SYSTEM
• Excretion of metabolic waste products and foreign chemicals
• Regulation of water and electrolyte balances
• Regulation of arterial pressure
• Regulation of acid-base balance
• Regulation of erythrocyte production
• Secretion, metabolism, and excretion of hormones
• Gluconeogenesis
Glomerular Filtration Rate and regulating
factors
Glomerular filtration rate (GFR) is the first step in the formation of urine.
It is primary measure of kidney function and a calculation of the volume of
fluid filtered from the kidney's glomerular capillaries into Bowman’s capsule.
Juxtaglomerular apparatus and Macula
Densa cells
Macula densa:
specialized region of the distal convoluted
tubule wall. Its primary role is to act as a sensor,
monitoring the concentration of (NaCl) in the
tubular fluid and the rate of fluid flow.
Juxtaglomerular cells (granular cells):
modified smooth muscle cells found primarily in
the wall of the afferent arteriole. They
synthesize, store, and secrete the
hormone Renin in response to low blood
pressure and sodium chloride concentration
sensed by the macula densa, or sympathetic
nerve stimulation.
TUBULOGLOMERULAR FEEDBACK VS
GLOMERULOTUBULAR BALANCE
TUBULOGLOMERULAR FEEDBACK
GLOMERULOTUBULAR BALANCE
A feedback mechanism by which the Macula Densa
senses NaCl concentration in the distal tubule and
adjusts GFR by altering afferent arteriolar tone.
Glomerulotubular balance is the intrinsic ability
of the proximal tubule to reabsorb a constant
fraction (~65%) of the filtered load of sodium
and water, despite changes in GFR.
Where does it occur? At the juxtaglomerular
apparatus (JGA), which includes:
If GFR increases:
1. Macula densa (in distal tubule)
More fluid is filtered.
2. Afferent arteriole
3. Juxtaglomerular cells
ALTERS GFR
Proximal tubule reabsorbs more in absolute
amount.
But the percentage reabsorbed remains
constant.
COUNTERCURRENT MECHANISM
Counter Current Multiplier
Counter Current Exchanger
Loop of Henle
Vasa Recta
Function: Actively creates a high-osmolarity
gradient in the medullary interstitium.
Process: As fluid moves down the
descending limb (water permeable) and up
the ascending limb (salt permeable, water
impermeable), NaCl is actively transported
out, increasing interstitial concentration.
Result: A single effect is multiplied, creating
a high osmotic gradient (up to 1200
mOsm/kg).
Uses ATP!
Function: Maintains the gradient created by
the multiplier.
Process: Blood vessels (vasa recta) loop
down and up alongside the Loop of Henle,
carrying blood in opposite directions to the
fluid flow.
Result: Solutes are picked up on the way
down and dropped off on the way up,
preventing the osmotic gradient from being
"washed away".
Doesn’t Use ATP!
MICTURITION REFLEX
Phase 1: Filling Phase (Storage)
Phase 2: Stretch Receptors Activated
As urine enters bladder:
When bladder fills (~300–400 mL):
Detrusor muscle relaxes (sympathetic
active)
Stretch receptors in bladder wall are
stimulated.
Internal sphincter contracts
Signals travel via pelvic nerves to:
Sacral spinal cord (S2–S4)
External sphincter contracts (voluntary)
Bladder can store ~300–400 mL
comfortably.
MICTURITION REFLEX
Phase 3: Spinal Reflex Activation
Phase 4: Role of Brain
Sacral spinal cord sends parasympathetic
signals back to bladder:
If socially appropriate, higher centers
(pontine micturition center) allow:
Detrusor contracts
• Inhibition of pudendal nerve
Internal sphincter relaxes
• External sphincter relaxes
This is the basic micturition reflex.
Urine flows If not appropriate Brain inhibits
reflex by:
• Keeping external sphincter contracted
• Suppressing parasympathetic activity
Hormones acting on nephron
Acid Base Imbalance
Renal Threshold
It is the specific concentration of a substance in the blood (plasma) at which the
kidneys can no longer reabsorb it completely, causing it to "spill over" into the
urine.
Under normal levels, the kidneys filter substances from the blood and then
reabsorb them in the proximal tubules. Once the blood concentration exceeds
the threshold, the transport proteins responsible for reabsorption become
saturated. This is known as the transport maximum.
Substance Variation: Every substance
has its own unique threshold. For
example, the threshold for glucose is
much higher than that for toxic
substances like urea.
In adults, the transport maximum for
glucose is 375mg/min.