chapter 17 - Renal Physio

Describe the gross structure of the kidneys

2 Kidneys

2 Ureters

1 Urinary Bladder

1 Urethra

Describe the function of the kidneys

• Regulation of the ECF environment in the body through the formation of urine

• Regulation of the volume of blood plasma

• Regulation of the concentration of waste products in, electrolytes in and the pH of the plasma

Where are nephrons primarily located?

In the renal cortex

What are the major parts of a nephron?

Renal corpuscle:

• Glomerular capsule surrounding glomerulus

Renal tubule:

• Proximal convoluted tubule

• Nephron loop

• Distal convoluted tubule

How/where is urine carried?

Collecting ducts carry urine to a minor calyx through the renal medulla

What type of muscle surrounds the urethral sphincter?

Internal urethral sphincter (smooth muscle)

External urethral sphincter (skeletal muscle)

From where does blood enter the glomerulus?

From the afferent arteriole

How many nephrons are cortical nephrons?

85%

Trace the flow of blood through the kidneys

Interlobar arteries —> Arcuate arteries —> Cortical radiate arteries —> Afferent Arteriole —> Glomerulus —> Efferent Arteriole —> Peritubular Capillaries

Trace the flow of filtrate through the kidneys to the collecting duct

Afferent arteriole —> Glomerulus —> Proximal Tubule —> Descending limb —> Ascending limb —> Distal convoluted tubule —> Collecting Duct

Define Filtration

Movement of materials from the glomerulus into the glomerular capsule

Imagine you are cleaning your backpack and you empty everything from the pack onto the table.

Define Reabsorption

Movement of materials from the renal tubule into the surrounding peritubular capillaries

Imagine you are cleaning your backpack and you are replacing most of the contents of the backpack and returning them to the bag

Define Secretion

Movement of materials from the peritubular capillaries into the renal tubule

Imagine you are cleaning your backpack and you are determining what needs to be thrown away given life’s needs at the time

Define excretion

removal of urine from the body

What are the three structures that make up the filtration membran?

Glomerular wall, basement membrane, podocytes

What kind of particles can pass through the filtration membrane of the glomerulus

Small particles:

Water, glucose, ions, amino acids

Large particles are held back in the plasma and do not enter the filtrate: Proteins and cells

When does filtrate form?

When plasma passes through the filtration membrane in the glomerulus

Define GFR

Glomerular Filtration Rate: Volume of filtrate produced by both kidneys per minute

What can have an effect on GFR?

Vasodilation or vasoconstriction of the afferent arteriole affects GFR.

• This can occur by two mechanisms: Sympathetic nerve effects or Renal autoregulation

How does the SNS control the GFR?

The SNS stimulates constriction of the afferent arterioles which decreases GFR and decreases rate of urine formation.

• If blood pressure drops, we dilate the afferent arteriole

• If blood pressure rises, we constrict the afferent arteriole. This preserves blood volume and diverts blood to the muscles and heart.

What happens when blood pressure drops? What changes in the kidneys?

The sympathetic nerves active and send messages to vasoconstrict the afferent arterioles in the kidneys.

This lowers GFR

The low GFR the lowers Urine production

The low production leads to increased blood volume

How does renal autoregulation of the GFR work?

We maintain a relatively constant GFR even with changing BP.

• If blood pressure drops, we dilate the afferent arteriole

• If blood pressure rises, we constrict the afferent arteriole

What are the names of the two mechanisms for renal autoregulation?

Myogenic mechanism

Tubuloglomerular feedback

What is the myogenic mechanism that is used by renal autoregulation?

• Myogenic vasoconstriction of afferent arterioles in response to stretching

  of the vascular smooth muscle with higher pressure – keeps GFR low and

  protects glomerulus from too much pressure

Describe the tubuloglomerular feedback mechanism that is used by renal autoregulation?

Macula densa cells in juxtaglomerular apparatus detects high NaCl in distal convoluted tubule and signals to constrict the afferent arteriole to reduce filtration rate.

When there is high NaCl in the DCT, that means there is not enough time to reabsorb the NaCl in the PCT and it remains in the filtrate at high levels.

Where does 85% of reabsorption occur? What is reabsorbed?

Occurs in the Proximal Convoluted Tubule (PCT)

• 65% of H2O

• 65% of NaCl

• All glucose

• All amino acids

• Most ions

Fluid/filtrate leaving the PCT is isosmotic to the plasma in the afferent/efferent arterioles

What is reabsorption?

Is returning water and solutes to the blood that were filtered. ~99% of what is filtered is reabsorbed

Describe reabsorption in the Nephron Loop

Descending limb is permeable to water

• 25% of reabsorption of water occurs in the descending limb due to osmosis

Ascending limb is permeable to Na+

• 25% of reabsorption of Na+ occurs in the ascending limb

• Na+ is pumped out by active transport

What is the function of juxtamedullary nephron loops?

It creates the osmotic gradient in the medulla. At the medulla/cortex boundary, concentration is 300 mOsM.

As you move towards the papilla, the concentration increases.

If we did not have this concentration gradient, we could never concentrate urine beyond the concentration of blood.

The vasa recta keeps equilibrium of concentration

Explain the countercurrent multiplier/exchanger system

• The hairpin turn of the

  nephron loop and the

  surrounding vasa recta

  create a system where we

  keep the osmotic gradient

  in balance

• Na+ pumped out of

  ascending limb

• Water reabsorbed in

  descending limb by

  osmosis

• Vasa recta (blood vessels)

  capture any excess Na+ or

  water and preserve the

  gradient

What hormones are released when blood pressure is low

ADH and Aldosterone

What is released when blood pressure is high?

Atrial natriuretic peptide (ANP)

Reabsorption in the collecting duct and DCT is dependent on hormones:

Water: ADH

Na+: aldosterone

Ca2+: PTH

The DCT and collecting duct are not always permeable to water, only become permeable in the presence of ADH

What does the ADH do?

• Released when BP is low or when we are dehydrated

• ADH targets the collecting duct and DCT of the nephrons

  • Stimulates the formation of aquaporins in the membranes of the cells

  • Water is reabsorbed and urine formation decreases/urine concentration increases

  • ADH also stimulates urea secretion in collecting duct and reabsorption in nephron loop

    • This increases concentration in interstitial fluid outside nephron to draw more water due to osmosis

What role does urea play?

Water reabsorption increases when urea is reabsorbed out of collecting duct and secreted into Loop of Henle

What role does Aldosterone play?

• Produced in adrenal cortex

• Aldosterone stimulates Na+ reabsorption in the collecting duct when blood pressure is low/we are dehydrated

• Aldosterone activates Na+/K+ pumps in the collecting duct and DCT

• Will cause less urine production and increased concentration of urine

• Will also cause K+ secretion

How do collecting ducts adjust urine concentration?

Collecting ducts use the osmotic gradient to adjust urine concentration

When we are overhydrated we do not…

We do not secrete ADH.

Water remains in collecting duct and we have a high volume of dilute urine

When we are dehydrated…

We secrete ADH.

Water goes out into medulla, so we have a small volume of concentrated urine.

Note, the collecting duct also secretes urea to maintain the concentration gradient in the medulla. Urea goes back into the filtrate in the loop of Henle (urea recycling)

Define Renal Plasma Clearance

Volume of plasma from which a substance is completely removed in one minute by excretion in the urine.

EX. inulin which is filtered but neither reabsorbed nor secreted.

• Amount of inulin that enters the urine equals the amount that enters the glomerular filtrate

• Renal plasma clearance of inulin is qual to the GFR

How do we estimate GFR (eGFR)

Creatinine is filtered in the kidneys and not reabsorbed, slightly secreted by the tubules

Its plasma concentration, together with a person’s age, sex, and weight is used in equations to calculate an eGFR

Define: Transport Maximum Concept

• Carrier-mediated transport displays the property of saturation

• This means that when transported molecule is at high levels in the blood, all the carriers become occupied and the transport rate reaches a maximal value which is the transport maximum.

• When rate of glucose filtration is greater than the transport maximum of the carriers, the carriers become saturated and the excess glucose will continue through nephron and end up in urine

Urine flow in Kidneys:

Cortex (Nephrons) —> renal pyramids —> minor calyces —> major calyces —> renal pelvis —> ureter

What makes up the renal corpuscle?

glomerulus and bowman’s capsule

What are the layers of the Bowman capsule?

Outer parietal layer is made of simple squamous

Inner Visceral layer is made of podocytes

What cells make up the Juxtaglomerular apparatus (JG apparatus)

Macula densa cells and JG cells

What are the macula densa cells

Chemo receptors, detect Na+ content of fluid in DCT

Importance of JG cells

Make renin, can detect BP and fluid entering the……

Where does filtration occur?

All throughout until you get to the minor calyax

What do the pressures of the renal corpuscle do?

Outward pressures promote or enhance filtration (from glomerulus —> Bowman capsule)

Inward pressures limit or stop filtration (keeps fluids in glomerulus)

In regards to the tubuloglomerular feedback mechanism, what happens when GFR is too high?

NaCl content in the DCT is high

• Response to lower GFR would be to constrict the afferent arteriole

In regards to the tubuloglomerular feedback mechanism, what happens when GFR is too low?

NaCl content in the DCT is low

• Response to elevate GFR would be to dilate the afferent arteriole

Describe the RAAS mechanism

Renin is released when systemic BP is low

Renin converts angiotensinogen into angiotensin I

ACE converts angiotensin I into angiotensin II

• Aldosterone is released Na+ reabsorption increases

• ADH is released H2O reabsorption increases

• Vasoconstriction of vessels

These reduce urine volume and increase conc of urine. helps conserve blood volume

What does ANP do

decreases sodium reabsorption to lower BP

K+ secreation occurs in the DCT and collecting duct when ______

when aldosterone is released

In the PCT, H+ are seceted when pH of the blood is _____

too low (acidic)

What happens when we need to concentrate our urine?

Urea is reabsorbed and secreted

name of the smooth muscle in the bladder

Detrusor muscle is the smooth muscle in the wall of the bladder

When we choose to void, what do we relax

We relax the external sphincter

What are the inward pressures

Capsule hydrostatic pressure

Blood colloid osmotic pressure (osmotic pressure of glomerulus)

What are the outward pressures?

Glomerular hydrostatic pressure

Colloid osmotic pressure of capsule

These outwards pressures promote filtarate formation