CARD 112 Lecture 20: Urine Formation

What are the four basic renal processes?

Filtration, reabsorption, secretion, excretion

Where does filtration occur?

Glomerular capillaries into Bowman’s space

Where does reabsorption occur?

From tubule lumen into peritubular capillaries

Where does secretion occur?

From peritubular capillaries into tubule lumen

What is excretion?

Elimination of substances from the body via urine

What determines the amount excreted?

Filtration – reabsorption + secretion

What vessel brings blood to the glomerulus?

Afferent arteriole

What vessel drains blood from the glomerulus?

Efferent arteriole

What structure receives filtrate first?

Bowman’s space

What structure receives reabsorbed substances?

Peritubular capillaries

T/F: Secretion moves substances from the tubule into the blood.

False: secretion moves substances from blood into the tubule.

T/F: Filtration is a passive process.

True (driven by pressure)

T/F: Excretion equals filtration minus reabsorption plus secretion.

True

T/F: Reabsorption always requires ATP.

False: some reabsorption is passive

Filtration occurs from glomerular capillaries into ______.

Bowman’s space

Reabsorption moves substances from the tubule into ______.

Peritubular capillaries

Secretion moves substances from blood into the ______.

tubule lumen

Excretion is the amount of a substance that leaves the body in ______.

urine

What percentage of cardiac output do the kidneys receive?

21%

How much blood flows to the kidneys per minute?

1 L

What percentage of plasma becomes filtrate?

20%

How much filtrate is produced per day?

180 L/day

How much urine is excreted per day?

1-2 L/day

What forces drive filtration?

Starling forces (pressure gradients)

What happens to the 80% of plasma that is not filtered?

It leaves via the efferent arteriole

What does high renal blood flow allow?

Precise control of body fluid composition

What is diminished glomerular filtration rate (GFR) a sign of?

Chronic kidney disease (CKD)

What type of molecules are freely filtered?

Water, ions, glucose, small solutes

T/F: Filtrate is identical to plasma except for proteins.

True

T/F: Only 1% of filtrate is excreted.

True

T/F: Filtration is driven by osmotic gradients only.

False: hydrostatic and oncotic pressures BOTH contribute.

T/F: A decrease in GFR is always harmless.

False: it may indicate chronic kidney disease (CKD)

Filtration is driven by ______ forces.

Starling

What percentage of plasma is filtered at the glomerulus?

20%

What percentage of filtered fluid is reabsorbed?

99%

What two factors determine GFR?

Filtration coefficient (Kf) and net filtration pressure (NFP)

What determines the filtration coefficient (Kf)?

Surface area and permeability

What determines net filtration pressure (NFP)?

Hydrostatic and colloid osmotic pressures

What happens to glomerular filtration rate (GFR) if surface area decreases?

GFR decreases

What happens to glomerular filtration rate (GFR) if permeability increases?

It increases

What two forces favor filtration in the glomerulus?

Glomerular hydrostatic pressure (PGC) and Bowman’s space oncotic pressure (πBS)

What two forces oppose filtration?

Bowman’s space hydrostatic pressure (PBS) and glomerular oncotic pressure (πGC)

What is the typical glomerular hydrostatic pressure (PGC)?

50 mmHg

What is the typical Bowman’s space hydrostatic pressure (PBS)?

10 mmHg

What is the typical glomerular oncotic pressure (πGC)?

25 mmHg

What is the approximate net filtration pressure (NFP)?

10-15 mmHg

What happens when glomerular hydrostatic pressure (PGC) > glomerular oncotic pressure (πGC)?

Filtration occurs

What happens when glomerular oncotic pressure (πGC) > glomerular hydrostatic pressure (PGC)?

Absorption occurs

What type of pressure draws fluid into capillaries?

Colloid osmotic pressure

What type of pressure pushes fluid out of capillaries?

Hydrostatic pressure

T/F: Bowman’s space normally contains proteins that contribute to oncotic pressure.

False: Bowman’s space oncotic pressure (πBS) is normally ZERO because proteins DO NOT filter.

T/F: Hydrostatic pressure in the glomerulus is higher than in systemic capillaries.

True

T/F: A decrease in πGC increases filtration.

True

T/F: Filtration stops when net filtration pressure (NFP) reaches zero.

True

Net filtration pressure is typically ______ to ______ mmHg.

10’ 15

What is the formula for net filtration pressure (NFP)?

(PGC + πBS) – (PBS + πGC)

(glomerular hydrostatic pressure + Bowman’s space oncotic pressure) – (Bowman’s space hydrostatic pressure + glomerular oncotic pressure)

What is the normal value of Bowman’s (πBS)?

0 mmHg

What happens to net filtration pressure (NFP) along the glomerular capillary?

It decreases as glomerular oncotic pressure (πGC) increases

What causes glomerular oncotic pressure (πGC) to increase along the capillary?

Protein concentration rises as fluid is filtered

What happens when filtration equilibrium is reached?

Filtration stops

What happens to glomerular hydrostatic pressure (PGC) along the glomerular capillary?

It remains relatively constant

What happens to Bowman’s space hydrostatic pressure (PBS) during obstruction?

PBS increases, reducing net filtration rate (NFP)

What happens to glomerular filtration rate (GFR) if glomerular oncotic pressure (πGC) increases?

GFR decreases

T/F: glomerular oncotic pressure (πGC) decreases along the glomerular capillary.

False: it increases as fluid is filtered.

T/F: glomerular hydrostatic pressure (PGC) remains high along the entire glomerular capillary.

True

T/F: Increasing Bowman’s space hydrostatic pressure (PBS) increases filtration.

False: it opposes filtration

T/F: Filtration equilibrium occurs when net filtration pressure (NFP) reaches zero.

True

Glomerular oncotic pressure (πGC) increases because proteins become more ______.

concentrated

What pressure remains constant along the glomerular capillary?

Glomerular hydrostatic pressure (PGC)

What pressure increases along the capillary?

Glomerular oncotic pressure (πGC)

At what point does filtration stop?

Filtration equilibrium

What pressure opposes filtration and rises to ~35 mmHg?

Bowman’s space hydrostatic pressure (PBS) + glomerular oncotic pressure (πGC)

What happens to renal blood flow (RBF) when the afferent arteriole constricts?

RBF decreases

What happens to glomerular hydrostatic pressure (PGC) when the afferent arteriole constricts?

PGC decreases

What happens to glomerular filtration rate (GFR) when the afferent arteriole dilates?

GFR increases

What happens to glomerular hydrostatic pressure (PGC) when the efferent arteriole constricts?

PGC increases

What happens to glomerular filtration rate (GFR) when the efferent arteriole dilates?

GFR decreases

What happens to renal blood flow (RBF) when the efferent arteriole constricts?

RBF decreases

What happens to renal blood flow (RBF) when efferent arteriole dilates?

RBF increases

What arteriole change increases both RBF and GFR?

Afferent arteriole dilation

What is the purpose of glomerular filtration rate (GFR) autoregulation?

To maintain a nearly constant GFR despite changes in blood pressure

Between what MAP range is glomerular filtrationr rate (GFR) autoregulated?

80–180 mmHg

What happens to glomerular filtration rate (GFR) when MAP rises above 180 mmHg?

GFR increases (autoregulation fails)

What happens to glomerular filtration rate (GFR) when MAP falls below 80 mmHg?

GFR decreases sharply

What cells detect stretch in the afferent arteriole?

Juxtaglomerular (JG) granular cells

What type of channels open when Juxtaglomerular (JG) cells stretch?

Stretch‑activated cation channels

What ion enters Juxtaglomerular (JG) cells during the myogenic response?

Ca2+

What is the effect of Ca²⁺ entry into Juxtaglomerular (JG) cells?

Vasoconstriction of the afferent arteriole

What happens to renal blood flow (RBF) when the afferent arteriole constricts?

RBF decreases

What happens to glomerular hydrostatic pressure (PGC) when the afferent arteriole constricts?

PGC decreases

T/F: Autoregulation keeps GFR constant between MAP 60–200 mmHg.

False: the range is 80–180 mmHg.

T/F: The myogenic response is intrinsic to vascular smooth muscle.

True

T/F: Stretch of the afferent arteriole causes vasoconstriction

True

T/F: Autoregulation prevents glomerular filtration rate (GFR) from changing during extreme blood pressure changes.

False: autoregulation FAILS outside 80–180 mmHg.

The myogenic response helps stabilize ______.

Glomerular filtration rate (GFR)

What structure senses pressure changes in the kidneys?

Afferent arteriole / juxtaglomerular (JG) cells

What structure senses NaCl delivery in TGF?

Macula densa

What transporter allows macula densa cells to sense NaCl?

Na⁺/K⁺/Cl⁻ cotransporter (NKCC2)

What happens to macula densa cells when NaCl delivery increases?

They depolarize

What ion enters macula densa cells during depolarization?

Ca²⁺