The document is a lecture on renal physiology and urine formation by Dr. Ayub Abdulcadir Sheikh. It contains 14 pages covering the following topics: - An introduction to the urinary system and nephron anatomy. - Nerve supply and mechanism of micturition. - Glomerular filtration rate and the glomerular filtration barrier. - Tubular reabsorption and secretion mechanisms along different parts of the nephron. - Formation of dilute and concentrated urine depending on water content and ADH levels. - Acidification of urine through intercalated cell function. - Control of extracellular fluid osmolarity and electrolyte

1. VS

2. Dr: Ayub Abdulcadir Sheikh:
 Postgraduate MBBS, at University of Somalia.
 Resident physician at Sureya Medical Center.
 A lecturer physiology at Frontier University.
Dedication:
To all my family especially may parents (Allah may bless you), also my student
in frontier university.

3. TABLE OF CONTENTS
1. Introduction to Urinary system ………………………......……………..……….……. 1
2. Nerve Supply of Urinary Bladder & Sphincter + Micturition ………..….…….…… 2
3. Glomerular Filtration Rate ……………………………………………………….....… 3
4. Tubular Reabsorption & Secretion ……………………………….……….……… 4 – 7
5. Formation of Dilute and Concentrated Urine …….….………………………...…..… 8
6. Acidification of Urine ……………………………………………………….….…….. 11
7. Control of Extracellular Fluid Osmolarity & Regulation of Electrolytes …… 12 – 14

4. Page | 1
The Kidneys: are 2 pairs of cone shaped structure.
 The Function of Renal system:
1. Excretion of metabolic waste products and foreign chemicals.
2. Regulation of water and electrolyte balances
3. Regulation of bodyfluid osmolality and electrolyte concentrations.
4. Regulation of arterial pressure.
5. Regulation of acid-base balance.
6. Regulation oferythrocyte production.
7. Secretion, metabolism, and excretion of hormones
Gluconeogenesis.
 Nephron:
A) The functional and structural
of the kidney.
B) Types of nephron:
 Cortical nephron.
 Juxtamedullary nephron.
C) Parts of nephron:
 Renal corpuscle.
 Renal tubules.
 The renal circulation:
1- The glomerular Capillaries (High hydrostatic pressure = about 60
mm Hg, causes rapid fluid filtration).
2- The peritubular capillaries (lower hydrostatic pressure = about 13
mm Hg) permits rapid fluid reabsorption).

5. Page | 2
 NERVE SUPPLY TO URINARY
BLADDER AND SPHINCTERS::
1. Sympathetic (Hypogastric) nerve - (Nerve of
Filling).
2. Parasympathetic (pelvic) nerve - (Nerve of
Empyting).
3. Somatic (Pudendal) nerve.
Micturition: is the process by which the urinary
bladder empties when it becomes filled.
 Mechanism Action of micturation (Uniration):
1. Increased extracellular fluid volume causes the kidney to excrete the water.
2. Then sympathetic nerve fiber causes relaxation of detrusor muscle of urinary bladder & constricts
the internal sphincter.
3. This leads the urine to flow a drop by drop until the bladder is fully filled with urine.
4. The pressure inside the bladder increased.
5. Firing the stretch receptor (parasympathetic sensory) in the bladder and generation of impulses.
6. Transmission of impulses to the spinal cord then to the brain.
7. The responses will stimulate both the parasympathetic motor to the bladder and somatic nerve
leads.
8. To contraction the detrusor muscle and relaxation of internal sphincter + relaxation of external
sphincter.
9. Now the urine will outside the body.
URINE FORMATION RESULTS FROM

6. Page | 3
 Glomerular filtration rate: Amount of plasma
filtered by glomerular capillaries into Bowman’s
capsule per minute.
 Normal GFR—125 mL/min (range 90–140mL/min)
or 180 L/day.
A high GFR is that it allows the kidneys to rapidly remove
waste products from the body that depend mainly on
glomerular filtration for their excretion.
 The high rate of glomerular filtration depends on:
1- A high rate of kidney blood flow.
2- The special properties of the glomerular capillary
membranes.
• Filtration fraction: is the fraction (portion) of the
renal plasma, which becomes the filtrate.
 Glomerular filtration
barrier or GLOMERULAR
CAPILLARY MEMBRANE:
1. Endothelium of the
capillary.
2. Basement membrane.
3. Epithelial cells of
podocytes.
 Negatively charged large molecules are filtered less.
 Positively charged molecules are filtered much more.
 Small molecular weights are more filtered.
 Autoregulation: is the intrinsic ability of an
organ to regulate its own bloodflow.
 Two mechanisms are involved in renal
autoregulation:
1. Myogenic response.
2. Tubuloglomerular feedback.
 Myogenic autoregulation:
1. Increased arterial blood pressure &increased GFR then
2. Vascular wall stretch causes
3. More calcium will enter vascular smooth muscle cell and
contraction.
4. Decreased blood flow& decreased GFR.

7. Page | 4
 Tubular reabsorption is highly selective.
 Glomerular filtration is non-selective.
 Tubular reabsorption plays a much
more important role than tubular
secretion.
 Pathways of tubular reabsorption:
1. Transcellular route.
2. Paracellular route.
3. Ultrafiltration(bulk flow).
 Factors that causes substance transportation:
1. Primary Active Transport: (Energy is ATP) example
“Na-KATPase, H ATPase, Ca ATPase.
2. Secondary Active Transport: ( Energy is concentration
gradient) example “Na-Glucose symport.
3. Pinocytosis: (Active transport) example “Reabsorption
of protein”.
4. Transport Maximum: a limit to the rate at which the
solute can be transported.
5. Gradient-time transport: time that the fluid containing
the substance remains within the tubule & its
electrochemical gradient.
 Secondary Active Secretion
into the Tubules.
 Example: (sodium-hydrogen
exchanger)

8. Page | 5
REABSORPTION AND SECRETION ALONG DIFFERENT
PARTS OF THE NEPHRON:
 Proximal Convoluted Tube:
 Substance that reabsorbed:
 Nutrients (Gkucose, Amino Acids).
 Electrolytes (Na, K, Ca, Cl, Po, Hco3).
 Water.
 Urea & Uric Acid.
 Substances that secreted:
1. Bile salts,Oxalates, Urate, Para-
aminohippuric acids.
2. Drugs.
3. Toxins.
 Loop of Henle:
 Thin descending segment.
 Thin ascending segment.
 Thick ascending segment.
 Thin descending segment:
 Substances that reabsorbed:
1. Highly reabsorbed water.
2. Moderate reabsorption of Na &
urea.
 Thin ascending segment:
 Low reabsorption of water &
Solutes.
 Thick ascending segment:
 Water does reabsorb this segment.
 Substances that reabsorbed:
 Highly and active reabsorption of Na, Cl, K, Ca,
Mg, Hco3.
 Has 4 mechanism for reabsorption of
substances:
A. Na-K ATPase.
B. 1Na, 2Cl, 1K cotransporter – X “Loop
diuretics”.
C. Na-H counter transporter.
D. Paracellular reabsorption of cations (Mg,
Ca, Na, K).
 Substances that secreted:
 H ions by Na-H conter transporter.

9. Page | 6
 Distal Convoluted Tube:
 2 portion:
1. Macula densa (Tubuloglomerular
feedback).
2. Highly convoluted tube.
 Types of convoluted tube:
A. Early distal convoluted tube.
B. Late distal convoluted tube.
 Collecting Tube:
 Has 2 portion:
 Cortical collecting tube.
 Medullary collecting tube.
Cortical collecting duct.
 Cortical collecting and distal convoluted
has 2 cells:
 Principle cells:
 Reabsorption of Na and water from the
lumen – Na channel blockers.
 Secretion of K ions into the lumen –
Aldesterone antagonists.
 Intercalated cells:
 Have 2 cells that regulate acid-base
balance.
 Type A:
 Secretion of H ions by “H ATPase, H-K
ATPase” and Reabsorption of HCO3.
 Type B:
 Secretion of HCO3 by “Cl-HCO3
exchanger”.
Medullary collecting ducts:
 Substances that reabsorbed:
1. Reabsorption of Na, Cl, Urea.
2. Reabsorption of water caused by
ADH.
3. Reabsorption of HCO3.
 Substances that secreted:
1. H ions against concentration
gradient.

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11. Page | 8
 Osmolarity of glomerular filtrate is same
as that of plasma and it is 300 mOsm/L.
 Dilute Urine = 50 – 90 mOsm/L.
 Concentrated Urine = 1,200 mOsm/L.
 Osmolarity of urine depends upon two factors:
1. Water content in the body.
2. Antidiuretic hormone (ADH).
 Formation of Dilute Urine:
 Increased body water causes (increased
ECF volume).
1. The osmolarity of fluid in proximal CT is
300 mOsm/L.
2. As fluid descends more its osmolarity
increased step by step due to more water
reabsorbed).
3. Now the fluid in the loop of henle is mush
concentrated up to 1,200 mOsm/L due to
more water reabsorbed.
4. As fluid ascends its osmolarity decreased
up to 100 mOsm/L in the distal CT, it’s
due to more of reabsorption NaCl and
other electrolytes.
5. In the absence of ADH hormone there is
active excretion of water and
reabsorption of additional NaCl.
6. Now the fluid osmolarity is 50-60
mOsm/L.
 Formation of Concentrated Urine:
 Decreased body water causes (decreased
ECF volume).
1. The osmolarity of fluid in proximal CT is 300
mOsm/L.
2. As fluid descends more its osmolarity
increased step by step due to more water
reabsorbed).
3. Now the fluid in the loop of henle is mush
concentrated up to 1,200 mOsm/L due to
more water reabsorbed.
4. As fluid ascends its osmolarity decreased up
to 100 mOsm/L in the distal CT, it’s due to
more of reabsorption NaCl and other
electrolytes.
5. In the presence of ADH hormone there is
active reabsorption of water and excretion
of NaCl.
6. Now the fluid osmolarity is 1200
mOsm/L.
7.
 FACTORS THAT CAUSES FORMATION OF
CONCENTRATED URINE:
A) Development and maintenance of medullary
gradient by countercurrent system.
Countercurrent system is subdivided into:
 Countercurrent multiplier formed by loop of
Henle.
1. Active reabsorption of sodium chloride.
2. Reabsorption of sodium from collecting duct.
3. Recirculation of urea.
 Countercurrent exchanger formed by vasa
recta.
B) Secretion of ADH.

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Control of Extracellular Fluid Osmolarity, and
Regulation of Electrolytes:
 Regulation ofextracellular fluid osmolarity and sodium
concentration are closely linked.
 Two primary systems are especially involved in regulating the
concentration of sodium and osmolarity of extracellular fluid:
1. The osmoreceptor-ADH system.
2. The thirst mechanism.

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