Urinary System

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Chapter 25 (Glomerular Filtration)

Formation of urine in the kidney involves what three main processes

filtration, reabsorption and secretion

During filtration, large quantities of water and solutes

pass through the filtration membrane from the blood into the glomerular capsule

The formation of urine begins with filtration, during which fluid and

small solutes flow from the glomerulus to the capsular space

As filtrate passes through the tubules, specific substances

are reabsorbed back into the blood of the peritubular capillaries

Some solutes are removed from the blood of the peritubular

capillaries and secreted by the tubular cells into the filtrate

What force drives filtration in the glomerulus

hydrostatic pressure (blood pressure)

The filtration membrane is composed of what three layers

fenestrated glomerular endothelium, basement membrane and filtration slits

Glomerular filtration membrane

keeps blood cells and proteins out of the urine passage ways

Glomerular filtration is a process of bulk flow driven by

hydrostatic pressure of the blood

Water and small solute particles are

forced through the filtration membrane, while large proteins and blood cells are excluded

The fluid collecting in the capsular space is called

glomerular filtrate

The liquid (water) and solutes that have passed through a filter are known as


Glomerular filtrate is the liquid (water) and solutes that have

passed through the filtration membrane and entered the renal tubule

Glomerular filtrate is composed of what common components

water, ions (sodium, potassium, chloride), nitrogenous waste (urea, uric acid, creatinine), organic molecules (glucose & amino acids)

The concentration of each of the substances in the glomerular filtrate is similar

to its concentration in plasma

How might the contents of the filtrate be altered if the filtration membrane is damaged or destroyed

increased protein, producing a condition called "proteinuria"

How might the contents of the filtrate be altered if the filtration membrane is damaged or destroyed

presence of blood cells; depending on the amount of damage---this condition is called "hematuria"

Hydrostatic pressure

the pressure of fluid within a system or container

Glomerular hydrostatic pressure promotes filtration is measured at


Capsular hydrostatic pressure opposes filtration and is measured at


Glomerular osmotic pressure opposes filtration and is measured at


Osmotic pressure

the force of concentrated dissolved solutes to attract pure water when these solutes are separated from the water by a selectively permeable membrane

Net filtration pressure formula

glomerular hydrostatic pressure - (capsular hydrostatic pressure + glomerular osmotic pressure)


glomerular filtration rate

Glomerular filtration rate (GFR)

the total amount of filtrate formed by the kidneys per minute

In normal kidneys, the GFR is approximately

125 ml/minute or nearly 180 L in 24 hours (180,000ml) or nearly a 50 gallon barrels worth

Fluctuations in net filtration pressure

alter the GFR

Prolonged changes in the GFR will alter

water and solute excretion

Auto-regulation mechanisms counteract

changes in GFR


the automatic adjustment of blood flow within a localized region in response to its current tissue requirements

Under normal conditions

-systemic blood pressure is 120mmHg
-afferent arteriole diameter is normal
-glomerular hydrostatic pressure is normal -net filtration rate is normal (125ml/min)

Under conditions of mild exercise

-systemic blood pressure is 140mmHg
-afferent arteriole diameter is normal
-glomerular hydrostatic pressure is increased
-net filtration rate is increased (146ml/min)

Vascular smooth muscle cells contract when stretched,

reducing blood flow

Vascular smooth muscle cells relax and dilate when not stretched,

increasing blood flow

Myogenic mechanism (GFR regulation mechanism) in auto-regulation is when

vascular smooth muscle cells contract when stretched thus reducing blood flow and relax and dilate when not stretched thus increasing blood flow

When pressure against the vessel wall is reduced, the vessel dilates; changes in blood pressure therefore,

directly affect the constriction or dilation of the arteriole, and so glomerular blood flow

What are the two GFR regulation mechanisms

myogenic and tubuloglomerular mechanisms and sympathetic control

Tubuloglomerular mechanism

a secondary regulatory mechanism is the sensitivity of the macula densa cells of the juxtaglomerular apparatus to the filtrate of NaCl concentration, proportional to the rate of filtrate flow in the terminal portion of the ascending loop of Henle

In response to a high filtrate flow rate, macula densa cells release

vasoconstrictor chemicals that decrease the diameter of the afferent arteriole

The result of the release of vasoconstrictor chemicals is

-GFR decreases
-tubular filtrate flow slows
-reabsorption of sodium chloride ions increases

In response to a low filtrate flow rate, the macula densa cells release

-less vasoconstrictor chemicals
-signal the granular cells to release renin


the enzyme released by the juxtaglomerular cells; it converts angiotensinogen into angiotensinogen 1

Glomerular filtrate is formed by

filtration of water and small solutes through the filtration membrane

Net filtration pressure is the glomerular hydrostatic pressure minus

the opposing forces of capsular hydrostatic pressure plus glomerular osmotic pressure

Blood pressure and flow in the nephron is monitored and controlled by

renal auto-regulation mechanisms in order to maintain a relatively steady glomerular filtration rate (GFR)

During periods of severe blood loss, sympathetic

stimulation overrides the renal auto-regulatory mechanisms to shunt the blood to other critical areas

Extrinsic controls are

hormonal and neural

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