I). Volume of blood filtered

Kidneys are 1% of body weight and use 20-25% of the oxygen.

*¼ of the cardiac output is delivered to the kidneys each minute

*End result the entire plasma volume is filtered 60X a day or 180 liters of water 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

The excretion of hypertonic urine

1). Reabsorption of water

i). The excretion of hypertonic urine (more concentrated than blood) is dependent upon the reabsorption of water from the loop of Henle and the collecting duct.

ii). The loop of Henle is made up of a descending limb (down) and an ascending loop (up).

iii). NaCl passively diffuses out of the lower portion of the ascending limb and is actively transported out of the upper potion of the ascending limb.

iv). This creates an osmotic gradient

v). Because of the osmotic gradient water leaves the descending limb of the loop of Henle.

vi). Water also diffuses out of the collecting duct.

viii).  Hormone: ADH (Antidiuretic hormone) increases the amount of water absorbed. Alcohol and caffeine depress the affects of ADH.

2). Reabsorption of salt

99% of the Na+ that is filtered is returned to the blood

 

C). Acid-Base Balance

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

 reabsorb bicarbonate ions & excrete hydrogen ions if  acidic.

 do not reabsorb bicarbonate ions & do not excrete hydrogen ions if  basic.

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).  Tubule

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

D).  Collecting Duct

 

IV). Capillary beds associated with filtration

A). Glomerulus:

Differences in resistance of arterioles the blood pressure inside is very high.

 forces the solutes and fluids 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

afferent arteriol touches the distal convoluted tubule.

The afferent arteriole contains mechanoreceptors

The distal convoluted tubule contains chemoreceptors

VI). Summary of Stages of filtration

Urine formation requires:

A). Glomerular Filtration (small molecules enter tubule),

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

ii). Formed elements and blood cells are too big to pass through.

iii). Filtered products include:

• Water
• Nitrogenous wastes
• Nutrients
• Salts (ions)
• Glucose

B). Tubular Reabsorption  (many molecules are reabsorbed)

Occurs as molecules and ions are actively and passively reabsorbed from the nephron into the capillary network.

ie. Glucose is actively reabsorbed by being transported on carriers. If the carriers are overwhelmed glucose appears in the urine indicating diabetes.

Na+ is actively transported Cl- follows passively.

Reabsorbed Filtrates	Nonreabsorbed Filtrates
Most water	Some water
Nutrients	Nitrogenous waste
Required salts (ions)	Excess salts (ions)

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

The second way that substances are removed from blood and added to tubular fluid.

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

VII).  Mechanisms for Urine Formation

1). Glomerular filtration: small molecules enter tubule.

(hydrostatic pressure or pressure gradient)

2). Tubular reabsorption: Molecules are reabsorbed into the blood stream.

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

3). Tubular secretion: Substances are actively added to the tubular fluid.

(active transport)

VIII). Glomerular Filtration

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

A). Filtration membrane

Found between blood and capsule

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

However, there is a higher concentration of proteins

In the plasma then in the capsule.

B). Net Filtration Pressure

(pressure going in) –(pressure pushing out)

= 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)

Amount of fluid filtered from the blood into the capsule per minute.

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.

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.

The osmoreceptors in distal tubules respond to slowing flowing filtrate (thus decreased filtrate concentration) by releasing vasodilators to the afferent arterioles.

In response to fast filtrate rate and thus high solute concentration by releasing vasoconstrictors.

iii). Renin-Angiotensin mechanism

In response to drop in systemic blood pressure or

IX). Tubular Reabsorption

Concentrating of the filtrate by returning solutes and water to the blood stream.

Reabsorbtion capabilities and methods vary in each segment of the nephron.

A). Active Tubular Reabsorption

1). Cotransport

Most of these are cotransported by binding to the same as carrier complex as Na+.

So solute reabsorption is dependent on the ejection of Na+.

This includes glucose, amino acids, lactate, vitamins, and most ions

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.

( the Tm of glucose 375 mg/min glucose over this limit is excreted)

 2). Pinocytosis (endocytosis)

What few proteins pass the filter are digested into amino acids.

B). Passive Tubular Reabsorption

1). Process by which substances are transported out of the tubular fluid through the epithelium, into the interstitial fluid then diffuse into the peritubular capillaries and back into the blood stream.

2). Diffusion, Facilitated Diffusion, and Osmosis

3). Substances move across their electrochemical (concentration) gradient without the breakdown of ATP.

4). Positively charged Na+ ions are actively transported from the tubule into the peritubular capillaries creating:

i). An electrochemical gradient.

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

ii). An osmotic gradient

This allows for the movement of water into the peritubular capillaries

C). Reabsorption in different sections of the tubule.

1). Glomerular capsule:

Only functions in glomerular filtration.

2). Proximal Convoluted Tubule.

Responsible for:

• 65% of the Na+ reabsorption (active transport)

• Water (water follows Na+) (Osmosis, because of the solute gradient)
• *all of the glucose & amino acid reabsorption, (cotransport with Na+)

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

• anions (-ions) (passive diffusion across electrochemical gradient)
• Urea and lipid soluble solutes (passive diffusion across electrochemical gradient)

• Proteins (Endocytosis)

3). Loop of Henle

(GFR started at 125 ml/min and it is now 40ml/min)

i). The loop of Henle drops from the cortex into the renal medulla .

ii). Filtrate entering the loop of Henle is still isotonic with the blood plasma, (300 mOsm) it leaves the collecting duct significantly more concentrated (1200 mOsm)

iii). Permeability

Water cannot Water can leave the ascending limb descending limb of of the loop of Henle the loop of Henle

This difference in permeability results in concentrating the urine.

 iv). Countercurrent Mechanism

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

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

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

Thus water is ‘driven’ from the filtrates through osmosis

 c). The ascending limb is impermeable to water and actively transports NaCl into the interstial fluid.

This causes a strong osmotic gradient for water reabsorption.

The interstitial fluid becomes hypertonic, but the filtrate becomes hypotonic.

(filtrate loses salt it becomes increasingly dilute)

This gradient is what causes the water to move out of the filtrate in the descending loop

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.

VIII). 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

IX). Hormones affecting renal function

	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.
	C). Atrial Natriuretic Peptide Released from the heart in response to high blood pressure. Suppresses aldosterone, ADH, and renin.

X). 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.

D). after 200 ml of urine have collected afferent impulses send impulses to the brain.

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

F). Voiding reflexes are initiated