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Urinary System:

Physiology & Urine Formation

I). Volume of blood filtered

(cardiac output is the amount of blood pumped out of the ventricle each minute)


bottle of water

a regular bottle of water contains about 590 ml

Not all of the filtered material is excreted.

Normal urine output = 0.5ml/kg body weight/hr

 30 to 40 ml per hour for an adult


720 ml to 960 ml per day from

180 liters of water per day of filtered fluid (304.6  water  bottles per day)

results in  

1.21 to 1.62 water bottles of urine per day


II). General function and structure of the urinary system

A). Excretion of Metabolic Wastes

1). Urea

2). Creatinine

3). Uric Acid

B). Water-Salt Balance

Blood volume is associated with salt volume.

The greater the blood volume the greater the blood pressure.

Removing water lowers blood pressure

The excretion of hypertonic urine (more concentrated than blood) is dependent on

1) Reabsorption of water

2).Reabsorption of salt

C). Acid-Base Balance

The kidneys control this by excreting H+ ions and reabsorbing HCO3 (bicarbonate).


if plasma pH is low (acidic).


H+ secretion in the urine

and HCO3¯ reabsorption back to the plasma


 thus urine becomes more acidic,

and the plasma more alkaline.

if plasma pH is high (alkaline):


H+ secretion in the urine

and HCO3¯ reabsorption in to the plasma


thus urine becomes more alkaline,

and the plasma more acidic.

Ammonia (NH3) buffers the H+ in urine.

NH3 + H+-----------------> NH4

D). Hormonal Function


E).  Chemical Contents


III).  Nephron Structure

A).  Glomerulus

B).  Glomerular Capsule

C).  Renal Tubule

  • proximal convoluted tubule
  • loop of Henle
  • distal convoluted tubule

D).  Collecting Duct


nephron structure


IV). Capillary beds associated with filtration

A).  Glomerulus:  


Blood pressure inside of the glomerulus is very high.


Because of differences in the resistance between the afferent and efferent arterioles.


Forces the fluids and some solids out of the blood into the glomerular capsule.


B). Peritubular capillary network:

The blood pressure is lower and these are very porous and easily absorb solutes and water

 capillary loops called Vasa recta that surround the loop of Henle


V). Juxtaglomerular Apparatus

Regulates the rate of filtration and systemic (body) blood pressure

Afferent arteriole contains mechanoreceptors

Distal convoluted tubule contains chemoreceptors


VI). Summary of Stages of filtration

Urine formation requires:

renal filtration

A). Glomerular Filtration (small molecules enter tubule)

(hydrostatic pressure or pressure gradient)


Due to differences in pressure water, small molecules move from the glomerulus (capillaries) into the glomerular capsule.

B). Tubular Reabsorption  (many molecules are reabsorbed from the nephron into the capillary

(diffusion, facilitated diffusion, osmosis, and active transport)

i.e.  Glucose is actively reabsorbed with transport carriers.

If the carriers are overwhelmed glucose appears in the urine indicating diabetes

3). Tubular Secretion (substances are actively added to tubule)

Substances are actively removed from blood and added to tubular fluid

(active transport)

ie.  H+, creatinine, and some drugs are moved by active transport from the blood into the distal convoluted tubule.

VII). Glomerular Filtration

filtration pressures

Passive (no energy required) and nonselective and driven by differences in hydrostatic pressure form the glomerular capillaries and the glomerular capsule.

A). Filtration membrane

Allows for passage of all molecules smaller than proteins thus the concentrations of these molecules on both side of the barrier are equal.

B). Net Filtration Pressure

(pressure going out of the glomerulus into the tubule) -  (pressure pushing out of the tubule into the glomerulus)


= Glomerular Hydrostatic Pressure (55mm Hg)

(blood pressure)

-(Glomerular Osmotic Pressure + Capsular Hydrostatic Pressure)

caused by proteins in the plasma + pressure in the capsule

30mmHg 15mmHg

C). Glomerular Filtration Rate (GFR)

Affected by:

1). Total filtration surface area

2). Membrane permeability

3). Net Filtration Pressure

(as NFP goes up so does the GFR)

D). Control of glomerular filtration

1). Intrinsic Controls: Control from the renal system

Reabsorption is dependent on rate of filtration

The rate can be controlled by regulating the diameter of the afferent arteriole

The rate can be controlled by regulating the diameter of the afferent arteriole.

filtrate formation

i.  Myogenic Mechanism:  Vascular smooth muscle contracts in response to stretch and dilates in response to low pressure.

ii.  Tubuloglomerlar feedback mechanism

Directed by the cells in the juxtoglomerular apparatus.

Diameter of blood vessel and glomerular filtration rate



Large diameter


Increase volume

Results in a high net pressure

And a fast GFR (rate)



there is no time to reabsorb

Results in a high solute concentration


high osmotic pressure at the juxtaglomerular apparatus


vasoconstrictors will be released

Small diameter


lowers volume

Results in a low net pressure

And a slow GFR (rate)



there is too much time to reabsorb

Results in a low solute concentration


low osmotic pressure at the juxtaglomerular apparatus


vasodilators will be released

iii). Renin-Angiotensin mechanism

In response to drop in systemic blood pressure or

Activation of the juxtoglomerular cells in the distal tubule

Or activation of the juxtoglomerular cells by the sympathetic nervous system


Osmoreceptors release renin,

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Renin acts on angiotensinogen,

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Creating angiotensin I

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which becomes angiotensin II

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angiotensin II is a vasoconstrictor

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blood pressure in the entire body  rises




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stimulates the adrenal cortex to release aldosterone

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aldosterone increases reabsorption of more Na+

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Water follows the Na+ so more water is reabsorbed and blood pressure increases

2). Extrinsic controls (outside the renal system)

Renal autoregulation in can be overcome during periods of extreme stress.

Sympathetic nerve fibers stimulate

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the adrenal medulla

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which releases epinephrine

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causing vasoconstriction in the afferent arterioles

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which inhibits filtrate formation.

IX). Tubular Reabsorption

tubular resbsorption

Pathway by which substances are transported


tubular fluid

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through the epithelium (of the tube)

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into the interstitial fluid

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then diffuse into the peritubular capillaries

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and back into the blood stream


A. Mechanisms of tubular reabsorption

1.  Diffusion, Facilitated Diffusion, and Osmosis

Substances move across their electrochemical (concentration) gradient

2. Active Transport

i). electrochemical gradient

This supports the movement of negatively charged ions like Cl¯ and HCO3¯

ii). osmotic gradient

This allows for the movement of water into the peritubular capillaries

3.  Co- transport


Carriers are specific and limited in number.

That creates a transport maximum

(Tm  mg/minute)

for every solute.

When the carriers are exceeded the solute is excreted in the urine.

4. Endocytosis

B). Reabsorption in different sections of the tubule.

1). Glomerular capsule:

Only functions in glomerular filtration.

2). Proximal Convoluted Tubule.

  • cations (+ ions) (cotransport with Na+)
urine formation

3). Loop of Henle

i). Permeability of water

 Water can leave the descending limb but not the ascending limb

ii). Permeability of Na+

 Na+ cannot leave the descending limb but can the ascending limb


This difference in permeability results in concentrating the urine.


 iii). Countercurrent Mechanism

a).  Fluid moving down the descending limb creates a current that is counter to the fluid moving up the ascending limb.

b).  There is an osmotic gradient created by movement of NaCl out of the ascending limb of the loop of Henle.

c).  Every time Na+ is actively removed from the ascending limb; water in the descending limb is pulled out because of osmosis

d).  The descending limb is impermeable to solutes but freely permeable to water.

The osmolality (concentration of solutes) increases in the interstitial fluids or tissues of the medulla.

Thus water is ‘driven’ from the filtrates through osmosis.

e). So more Na+ that is actively removed the more water is pulled out. This creates a counter current that keeps on multiplying.

f). Meanwhile the peritubular capillaries & vastus rectus pick up and remove the water but the Na+ is not picked up as readily so the Na+ concentration keeps building.

g). The interstitial fluid (tissues) become hypertonic, but the filtrate becomes hypotonic.

(filtrate loses salt it becomes increasingly dilute)


As the collecting duct passes through the gradient in the medullary region urea diffuses out.

Increasing the gradient.

Water diffuses out too concentrating the urine.

iv. Fluid Concentrations


4). Reabsorption in the Distal Convoluted Tubule

Water reabsorption here is dependent on hormones.

Aldosterone: promotes the excretion of potassium (K+) and the reabsorption of sodium ions (Na+).

Aldosterone mediated active transport of Na+, which cotransports H+, K+, HCO3-, and Cl-

Atrial Natriuretic Hormone (ANH): promotes the excretion of Na+ and so is water which decreases blood volume and blood pressure.

Antidiuretic Hormone.

Diuretics: agents that increase the flow of urine

 5). Collecting Duct

Urea in response to the concentration gradient.

Water in response to ADH.

IX). Tubular Secretion

Reabsorption in reverse.

H+, K+, Creatinine, ammonium ion, various drugs move from the blood through the tubular wall and into the filtrate.

This occurs in the proximal and distal convoluted tubules and in the upper cortical part of the collecting du

X). Hormones affecting renal function

ADH Feedback

aldosterone feedback

A). ADH: functions to concentrate urine.

Increase water reabsorption in the collecting duct by opening water channels.

B). Aldosterone:

Increases the reabsorption of Na+

and thus water.

Affects blood pressure

ANP Feedback C). Atrial Natriuretic Peptide

Released from the heart in response to high blood pressure.

Suppresses aldosterone, ADH, and renin.










bladders urine formation

XI). Urination (micturition)

A). As the bladder fills with urine, sensory impulses go to the spinal cord and then the brain.

B). The brain can override the urge to urinate.

C). When urination occurs motor nerve impulses cause the bladder to contract and the internal and external sphincters to open. until 200 additional ml are collected. After 200 ml of urine have collected afferent impulses send impulses to the brain.

D). The decision to empty the bladder is made in the inferofrontal region of the brain.

E). Voiding reflexes are initiated