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Stages of Urine Formation

1. Glomerular Filtration - creates a plasma like filtrate of blood

2. Tubular Reabsorption- removes useful solutes from filtrate and returns them to the blood

3. Tubular Secretion- Removes wastes from blood

4. Water Conservation - Removes water from the urine and returns it to the blood


Glomerular Filtration

is capillary fluid exchange in which water and some solutes of the capillaries of the glomerulus into the capsular space of the nephron


Glomerular Filtrate

the fluid in the capsular space


Tubular Fluid

fluid from the proximal convoluted tubule through the distal convoluted tubule



fluid that enters the collecting ducts


Filration Membrane

1. Endothelial cell of glomerular capillary
2. Basement Membrane
3. Filration Slits


Filtration Pressure a Function of:

1. Blood Hydrostatic Pressure
2. Colloid Osmotic Pressure
3. Capsular Pressure

High BP in glomerulus makes kidneys vulnerable to hypertension


Regulation of glomerular Filration

GFR too high
- flow amount is too fast for renal tubes to absorb
- urine output rises, dehydration

GRF too low
- wastes are reabsorbed
- high levels of nitrogen containing compounds


GFR is controlled

by adjusting glomerular blood pressure from moment to moment

GFR control is achieved by three homeostatic mechanism
1. Renal autoregualtion
2. Sympathetic Control
3. Hormonal control


1. Renal Autoregulation

the ability of the nephron to adjust their own blood flow and GFR without external control

Two methods of renal autoregulation: Myogenic Mechanism and Tubuloglomerular feedback


A. Myogenic Mechanism

- tendency of smooth muscle to contract when stretched.


B. Tubuloglomerular Feedback

If GFR high
- filtrate contains more NaC;
snesed by Macula densa
Stimulates Juxtaglomerular cell (JG) cells to contact reducing GFR to normal

If GFR is low
macula relaxed afferent arterioles
blood flow increased and GFR rises


Regulation of Glomerular Filtration (2)

2. Sympathetic Control
- Sympathetic nerve fibers richly innnervate the renal blood vessels
- constricts the afferent arterioles


Regulation of Glomerular Filtration (3)

3. Hormonal Control
- JG (granular) cells secrete renin in response to low BP


Renin - Angiotensin - Aldosterone Mechanism

Is a system of hormones that help control BP and GFR


Stages of Angiostensin

1. Potent Vasoconstrictor -- raising BP

2. Constricts efferent arteriole raising GFR

3. Stimulates adrenal cortex to secrete aldosterone, promotes NA and H2O reabsorption

4. Stimulates posterior pituitary to secrete ADH which promotes H2O reabsorption

5. Stimulates thirst


The proximal convoluted Tubule

- reabsorbs 65% of glomerular filtrate, removes from blood then secretes them into the (tubular fluid) urine

- tubular fluid is hypertonic


Sodium Reabsorption

in tubular reabsorption, Na creates an osmotic and electrical gradient that drives the reabsorption of water and other solutes


The Transport Maxium

is the amount of solute that renal tubules can reabsorb is limited by the number of transport proteins in tubule cell's membrane


Transport maximum is reach when

transporters are saturated.

Each solute has its own transport maximum


Tubular Secretion

renal tubule extracts chemicals from capillary blood and secretes them into tubular fluid


Proposes of secretion in PCT and nephron loop:

1. Acid Base Balance
- secretion of varying proportion of hydrogen and bicarbonate ions help regulate pH of body fluids

2. Waste removal

3. Clearance of rugs and contaminants


The Nephron Lopp

Generates a salinity gradient that enables collecting duct to concentrate the urine and conserve water

- electrolyte reabsorption from filtrate

-Tubular fluid is very diluted as it enters distal convoluted tubule


The distal convoluted Tubule and Collecting duct

Fluid arriving in the DCT still contains 20% of H2O from glomerular (AKA still need to reabsorb water)

collecting duct is highly permeable to water

Regulated by hormones:
1. Aldosterone
2. Atrial Natriuretic Peptide
3. Antidirectic hormone


Aldosterone -

salt retaining hormone
secreted by adrenal cortex

Triggers for aldosterone secretion:
1. Low blood Na concentration
2. High Blood K concentration
3. Drop in BP


Functions of Aldosterone

stimulates reabsorption and NA and a secretion of K
net effects is that the body retains NaCl and water


Atrial Natriuretic Peptide (ANP)

Secretes by atrial myocardium of the heart in response to high blood pressure


Actions of ANP

1. Dilates afferent & constricts efferent arteriole

2. Inhibits renin and aldosterone secretion

3. Inhibits secretion of ADH

4. Inhibits NaCl reabsorption by collecting duct


Antidiuretic Hormones

Secreted in posterior pituitary in response to:
1. Dehydration
2. Loss of blood Volume
3. Rising Blood osmolarity

Action of ADH -- increases permeability of collecting duct to water


Water Conservation: The Collecting Duct (CD)

Begins in the cortex where it receives tubular fluid from several nephrons

Runs through medulla,and reabsorbs water, making urine up to 4 X more concentrated


Define Renal Clearance

the volume of blood plasma from which a particular waste is completely removed in 1 minute


Represents the net effect of three processes

1. Glomerular Filtration of the waste
2. Amount added by tubular secretion +
3.Amount removed by tubular reabsorption = renal clearance


Renal Clearance is determined by

collecting blood and urine samples, measuring the waste concentration in each, and measuring the rate of urine output

compare C to normal GFR of 126mL/min 48% is normal clearance for urea


Glomerular Filtration Rate

- measure GFR to asses kidney disease
- use inulin, a plant polysaccharide to determine GFR, neither reabsorbed or secreted
-Clinically GFR is estimated from creatinine excretion


Renal Clearance

1. Water Balance
2. Electrolyte Balance
3. Acid- Base Balance


Water balance

Total Body Water (TBW) 45-75% of body weight

Fluid Compartments:
1. 65% Intracellular Fluid (ICF)
2. 35% Extracellular Fluid (ECF)-- tissue, blood plasma, lymph, transcellular fluid.


Regulation of Fluid Intake

Governed mainly by thirst
osmoreceptors respond to Angll and rising osmolarity


Regulation of Fluid Outpu

- Variation in Urine volume
- Usually linked to adjustments in Na reabsorption

Dehydration decreases BV
Increase Na = increase osmolarity = ADH
Increase Blood Volume/increase NA = Inhibit ADH


Fluid Deficiency

A. Volume Depletion (hypocolemia)
- loss of water and sodium without replacement
- osmolarity remains normal

Dehydration (negative water balance)
- body eliminates more water than sodium
-Osmolarity of ECF rises


Where would fluid accumulate

tissue fluid


Profuse sweating

negative water balance


Electrolyte Balance

Important for:
1. Metabolism
2. Electrical potential
3. Body fluid osmolarity


Major Cation:
Major Anions:

Cations: Na, K, Ca, Mg, H
Anions: Cl, HCO3, P



above normal
below normal


Sodium Balance

one of principal ions responsible for resting membrane potentials
- principal cation of ECF (90%-95%)

Adult requires 0.5g/day
typical america 3-7g/day