03: Glomerular Filtration and Blood Flow

How much of the plasma running through the glomerulus gets filtered out

20%

Types of nephrons

• Cortical

• Juxtamedullary

Which type of nephron is key for water retention and why

Juxtamedullary nephron; salty medulla helps pull water back out

Type of innervation in the kidney

S-ANS

Function of the S-ANS in the kidney

Constricts arterioles and inhibits cells, slowing down the process of urine production

How is micturition stimulated

Stretch receptors in the bladder

Bladders sphincters

Internal and external urethral sphincters

Nerves that provide input to the internal urethral sphincter

PS-ANS sacral nerves

Nerves that provide input to the external urethral sphincter

Pudendal nerve

Muscle that stimulates micturition following input from bladder stretch receptors

Detrusor muscle

Stimulus that increases the intensity of bladder contractions

Increased volume causes increased contraction intensity

Nerves that control the micturition reflex

PS-ANS nerves

How does the brain control micturition

Voluntary control of the external urethral sphincter

Atonic bladder pathogenesis

Damaged or destroyed PS-ANS fibers mean that there is no way to sense bladder fill → overflow incontinence

Automatic bladder pathogenesis

Loss of brain control → uncontrolled micturition reflex

Uninhibited neurogenic bladder pathogenesis

Loss of brain inhibition

Three steps of urine formation

1. Glomerular filtration

2. Tubular reabsorption

3. Tubular secretion

How does the glomerulus maintain enough hydrostatic pressure to filter plasma

Large arteriole in and small arteriole out creates enough hydrostatic pressure

Inputs that regulate glomerular filtration

• Kidney

• S-ANS

• Hormonal control

How does the kidney control how much filtration is done at the glomerulus

Changing the size of the afferent/efferent vessels will change the GFR

Where is most of the filtrate reabsorbed

From the tubules/loop of Henle into the peritubular capillaries

“Math” equation for kidney excretion

Filtration - reabsorption + secretion

T/F: if a substance is filtered, it is either totally reabsorbed or totally excreted

False, some substances are only partially reabsorbed

T/F: a substance can be filtered and secreted

True

Specific sugar used in testing to evaluate GFR

Inulin

Why is inulin used to measure GFR

It is entirely filtered out with no reabsorption or secretion

Is glucose reabsorbed, partially reabsorbed, or excreted entirely

100% reabsorbed, unless the BG is REALLY high

Is urea reabsorbed, partially reabsorbed, or excreted entirely

partially reabsorbed

How many times a day does all the plasma get filtered in a day

60x

Forces that determine GFR

• Hydrostatic pressure

• Osmotic pressure

• Capillary filtration coefficient (determined by the thickness of glomerular capillaries)

Properties of molecules that determine if they get filtered

Size and charge

How does molecular size relate to filtration

The higher the molecular weight of a molecule, the lower the filterability

How does molecular charge relate to filtration

Positively charged or neutral molecules are well filtered through the podocytes. Negatively charged molecules are repelled by the negative AAs on the podocytes and not filtered well

What does it mean if you find proteins in the urine

There is some disease that has destroyed podocytes and made the glomeruli more leaky

Forces in the glomerulus that favor filtration

• High glomerular capillary hydrostatic pressure

• Low bowman’s capsule osmotic pressure

Forces in the glomerulus that oppose filtration

• High glomerular capillary osmotic pressure

• Small amount of bowman’s capsule hydrostatic pressure

How does chronic diabetes cause glomerular injury

Chronic high GL is toxic to the glomeruli

As the blood moves through the capillaries, what happens to the hydrostatic and osmotic forces

Hydrostatic pressure in the blood decreases as water is filtered out, but osmotic pressure increases because the most numerous solutes (albumin!) don’t move with the water

Modifications to the arterioles that decrease GFR

Constriction of the afferent arteriole

Modifications to the arterioles that increase GFR

Constriction of the efferent arteriole

How does a urinary obstruction affect GFR

Backflow of urine will decrease GFR

How does osmotic pressure of glomerular blood affect GFR

The greater the osmotic pressure, the lower the GFR

How does blood pressure affect GFR

A decrease in blood pressure will result in a lower GFR (and vice versa)

How does S-ANS activity affect GFR

Increased S-ANS activity will result in constricted arterioles and a lower GFR

Controlling stimuli for arteriolar resistance

• Nervous

• Hormones

• Local sensors

Why does blood move slowly through the kidneys

There is a significant drop in BP as the blood enters the kidneys

T/F: an increase in BP will result in a proportionate increase in GFR

False, GFR is maintained at a pretty constant rate until the BP gets REALLY high

Why is autoregulation important for changing blood pressure

If GFR increased proportionally with spikes in blood pressure, there would be more urine output without a matching increase in reabsorption, and you would loose all the fluid in your body

Two hypotheses that explain autoregulation

• Myogenic hypothesis

• Tubuloglomerular feedback

Myogenic hypothesis of autoregulation

When BP spikes, the afferent arterioles in the kidney constrict to decrease blood flow, decreasing GFR to a normal amount

Tubuloglomerular feedback hypothesis of autoregulation

When BP drops, the filtrate moves slower and there is more time for Na⁺ reabsorption. The macula densa cells in the DCT sense the low sodium, interpreting it as a marker of low blood pressure, and this results in vasodilation of the afferent arterioles to increase the GFR back to normal

How do the macula densa cells control the efferent arteriolar pressure

The secrete renin in response to low GFR (low filtrate Na⁺) → Agt II → efferent vasoconstriction

How does protein ingestion affect GFR

Increases GFR