BI 233 Renal Test

3 layers of tissue that surround each kidney

Renal capsule

Adipose capsule

Renal fascia

Internal anatomy of kidney

cortex, medulla, renal pyramids, renal papillae, renal columns, minor calyces, major calyces, renal pelvis

cortex

outer reddish area

medulla

deep to the cortex, reddish-brown

renal (medullary) pyramids

8-16 cone shaped structures

renal papillae

apex of pyramids

renal columns

extensions of the cortex that separates the pyramids

minor calyces

8-16, → major calyces

major calyces

2-3, → renal pelvis

renal artery

20-25% resting CO

blood supply for kidney

segmental artery → interlobar artery → arcuate artery → cortical radial artery/interlobular artery → afferent arterioles → glomerular capillaries → efferent arterioles → peritubular capillaries → interlobular vein → arcuate vein → interlobar vein → renal vein

microvasculature for kidney

afferent and efferent arterioles, peritubular capillaries

afferent arteriole

feeds the glomerulus and is larger in diameter than the efferent

efferent arteriole

drains the glomerulus and feeds into the peritubular capillary bed

peritubular capillaries

follows the course of the renal tubule and is adapted for reabsorption

vasa recta

vessels that trace the course of the longer juxtamedullary nephrons, peritubular capillary

nephron

The functional unit of the kidney

Each kidney contains about 1 million nephrons

The number of nephrons remains constant from birth

3 basic functions of the nephron

filtration: substances pass from the blood to the nephron

secretion: additional wastes and excess substances are added to filtrate

reabsorption: useful materials are returned to blood

parts of the nephron

renal corpuscle, renal tubule

renal corpuscle

glomerulus and glomerular capsule

glomerulus

tuft of looping capillaries arising from the afferent arteriole, consists of fenestrated endothelium which allows formation of filtrate via hydrostatic pressure, fenestrations restrict the passage of blood cells

basement membrane (negatively charged basal lamina) restricts passage of larger plasma proteins

glomerular capsule

aka Bowman’s capsule

parietal layer - simple squamous epithelium

visceral layer - overlies basement membrane and consists of specialized cells called podocytes

podocytes

interlocking pedicels of podocytes form the filtration slits (.006-.007 um)

albumin has diameter of 7.1 nm → <1% pass through

renal tubule

collects filtered blood

consists of proximal convolute tubule, loop of henle, distal convoluted tubule, collecting tubules

proximal convolute tubule

part of renal tubule, consists of simple cuboidal cells, apical microvilli, numerous mitochondria

primary function: reabsorption (ions, organic molecules, water, vitamins)

loop of henle

part of renal tubule

descending limb - thin segment: simple squamous epithelium, primary function: reabsorption of water

ascending limb - thin segment: simple squamous, thick: cuboidal, primary function: reabsorption of ions

distal convoluted tubule

part of renal tubule, made of simple cuboidal cells, primary function: secretion acids and ammonia and selective reabsorption of Na+ and water

late DCT and cortical collecting duct

principle cells - receptors for ADH and aldosterone

intercalated cells - plays role in blood pH regulation

collecting tubules

part of renal tubule, receive filtrate from several DCT, several collecting tubules converge to empty into larger papillary ducts, several papillary ducts empty into minor calyx → major calyx → renal pelvis

juxtaglomerular apparatus

the DCT makes contact with its own afferent and efferent arterial, endocrine structure that secretes the hormone erythropoietin and enzyme renin

contains macula densa

macula densa

osmoreceptor cells in the DCT that monitor osmolarity of filtrate

in response to decrease osmolarity of filtrate in the DCT the macula densa stimulates the JG cells to release renin (via decreased release of adenosine and closure of Ca2+ channels on JG cell membrane)

in response to increase osmolarity of filtrate in the DCT the macula densa stimulate the JG cells to inhibit renin (via increased release of adenosine and ATP and opening of Ca2+ channels on JG cell membrane)

juxtaglomerular cells

• specialized smooth muscle cells that act as mechanoreceptors

• monitor and regulate blood pressure and GFR

  • JG cells synthesis, store and release renin

  • increasing intracellular concentrations of cAMP leads to renin release

    • a decrease in intracellular calcium stimulates renin release

    • increasing intracellular calcium concentration inhibit cAMP and renin release

JG cells release renin in response to

1. decreased release of adenosine and ATP by the macula densa cells - in response to a decrease in tubule NaCl osmolarity

2. activation of B₁ receptors on JG cells

3. decreased stretch

   1. during stretch - mechanosensitive channels may initiate Ca²⁺ entry, contributing to membrane depolarization and opening of voltage sensitive L-type Ca²⁺ channels

   2. decreased stretch would close mechanosensitive Ca²⁺ channels and negate the inhibitory effect of Ca²⁺ on cAMP → increased renin

glomerular filtration

non-selective, passive process driven by hydrostatic pressure

filtrate must pass through 3 layers:

1. endothelial fenestrations

2. basement membrane (containing negatively charged glycoproteins)

3. filtration slits of the podocytes - filtrate only contains about 0.03% protein

filtrate vs plasma

filtrate is the same as plasma except it contains no significant amount of proteins

net filtration pressure

1. glomerular blood hydrostatic pressure

2. glomerular osmotic pressure

3. capsular hydrostatic pressure

glomerular blood hydrostatic pressure

force which drives H2O and solutes across the filtration membrane, ~55 mmHg (Avg HPc of capillaries at heart level = 35 mmHg)

Glomerular osmotic pressure

colloid pressure (the drawing pressure of suspended proteins), ~30 mmHg

capsular hydrostatic pressure

resistance to filtrate formation R/T capsule walls and fluid in the capsular space, ~15 mmHg

net filtration pressure calculation

NFP = GBHP - (GOP + CHP)

= 55 - (30 + 15)

= 10 mmHg

glomerular filtration rate

the amount of fluid filtered from the blood into Bowman’s capsule every minute

~115 mL/min in women (6.9L/hr, 165.6 L/day)

~125 in men (7.5 L/hr, 180L/day)

*only 1% total L/day filtrate actually leaves body as urine

*can vary in nephron to nephron depending on variable morphology

GFR calculation

the permeability constant of the filtration membrane (K_f) is ~12.5 mL/min per mmHg

thus GFR = K_f x NFP

= 12.5 mL/min per mmHg x 10 mmHg

=125 mL/min in men

GFR is determined by

1. surface area

2. permeability of filtration membrane

3. factors influencing net filtration pressure

GFR - Surface area

regulated by mesangial cells

relaxed → maximum surface area and high GFR

contracted → decreased surface area and low GFR

GFR permeability of the filtration membrane

very thin (0.1 um), in the absence of pathology permeability remains unchanged

GFR - NFP

anything that alters NFP alters GFR

ex: cirrhosis → hypoalbuminemia → decreased GOP

ex: renal calculi → ureteral obstruction → increased CHP

ex: nephrotic syndrome → proteinuria → decreased GOP and increased oncotic capsular pressure

regulation of glomerular filtrate

1. renal autoregulation

2. hormonal regulation of GFR

3. neural regulation of GFR

renal autoregulation

intrinsic ability of kidneys to maintain constant GBHP and GFR despite large changes in systemic arterial BP. GFR is nearly constant when mean arterial blood pressure is anywhere between 80-180 mmHg

myogenic and tubulo-glomerular mechanisms to do this

myogenic mechanism of renal autoregulation

tendency of vascular smooth muscle to contract when stretched, normalizes renal blood flow and GFR within seconds after BP changes occur

works well to control and normalize GBHP and GFR in relation to increases in mean arterial pressures

tubulo-glomerular mechanism of renal autoregulation

negative feedback system involving the macula densa cells of the JG apparatus, in response to sodium chloride concentrations in DCT the macula densa cells can:

1. increase or decrease release of vasoconstrictive paracrines (ATP and adenosine)

2. influence the rate of renin release from the JG cells

   1. ↑GFR → ↑NaCl in DCT at the macula densa

      1. Na+K + 2Cl- symporters on apical surface ↑ uptake of NaCl into macula densa cells → ↑ in Ca2+ and exocytosis of ATP and adenosine from the basolateral surface

      2. ATP binds to P2 receptors and adenosine (ADO) binds to A1 receptors on afferent smooth muscle cells. P2/A1 activation results in:

         1. A rise in intracellular Ca²⁺ → vasoconstriction of the afferent arteriole → ↓GFR

         2. Increased intracellular Ca²⁺ inhibits renin release by JG cells

   2. ↓GFR → ↓ NaCl in tubule fluid → ↓ uptake of NaCl into macula densa → ↓ in Ca2+ → ↓ exocytosis of ATP and adenosine from the basolateral surface → ↓ P2/A1 activation on VSM → ↓ intracellular Ca2+

      1. Vasodilation of afferent arteriole → ↑ GFR

      2. ↑ release of renin by JG cells.

hormonal regulation of GFR

a. renin - drop in GFR

b. ANP - raising GFR

Renin-angiotensin mechanism

Renin is released from the juxtaglomerular cells in response to:

1. increased frequency of renal sympathetic nerve impulses

   1. NE activates beta 1 receptors on JG cells → renin release

2. decrease stretch of the JG cells, related to decrease in blood volume or BP

   1. leads to decreased Ca2+ entry into JG cells and increased renin release

3. decreased NaCl osmolality past macula densa cells → decreased release of ATP and adenosine → decreased activation of receptors on JG cells and decreased Ca2+ entry

renin

released by juxtaglomerular cells → conversion of angiotensinogen → angiotensin I → angiotensin II by ACE (angiotensin converting enzyme)

ACE

synthesized by endothelial cells, as angiotensin I circulates through the lungs the concentration of ACE increases and the majority of angio I is converted to angio II

Angiotensin II effects

1. brief, but powerful vasoconstrictor of systemic arterioles, directly simulates VSM to constrict

   1. indirect mechanisms of vasoconstriction

      1. stimulates release of NE from sympathetic fibers

      2. stimulates catecholamine release from adrenal gland

      3. stimulates increased CNS sympathetic outflow

2. vasoconstriction of the efferent arteriole

   1. angio II constricts both the afferent and efferent arterioles, but preferentially increases efferent arteriole resistance to maintain GFR

      1. the efferent arteriole has smaller luminal diameter and angio II stimulated vasoconstriction produces a greater increases in resistance than at the afferent arteriole

      2. angio II stimulates iNOS. Nitric oxide is released from the afferent arteriole to act locally on the JG cells, thereby negating some vasoconstriction of the afferent arteriole

3. aldosterone release by the adrenal cortex

4. release of ADH (vasopressin)

5. stimulation of the thirst center

atrial natriuretic peptide (ANP)

29 amino acid peptide, synthesized, stored, and released by cardiac myocytes

signals for release:

1. atrial stretch - hypervolemia is the main factor governing release

2. neurohormonal stimuli - angiotensin II, endothelin stimulates release of ANP, activation of beta-receptors

release leads to:

1. increased GFR → increased diuresis

2. direct suppression of renin secretion

3. direct suppression of aldosterone release

4. direct suppression of vascular smooth muscle tone

5. inhibition of Na+ reabsorption in the PCT and cortical collecting duct

*ANP and B-type natriuretic peptide (BNP) levels increase with congestive heart failure (CHF) → in a client with dyspnea elevated BNP indicates CHF as a cause, rather than a pulmonary cause

neural regulation of GFR

• at rest sympathetic stimulation is minimal and renal vessels are dilated

• with moderate stimulation both afferent and efferent vasoconstrict to same degree

• *with increasing, near maximal, sympathetic stimulation NE activates alpha 1 receptors, which are plentiful, on smooth muscle fibers of the afferent arterioles → vasoconstriction

  • predominate constriction of the afferent over the efferent

    • shunts blood from kidneys and decreases GBHP and GFR

• when there is a drop in blood volume or pressure there is an increase in release of NE which results in

  • activation of alpha 1 receptor on VSM → increased PR

  • activation of beta 1 receptors on cardiac myocytes → increased SV

  • activation of beta 1 receptors on cardiac autorhythmic cells → increased HR

  • activation of beta 1 receptors on JG cells → increased renin release

tubular reabsorption

reclamation process

reabsorbed substances are reclaimed by

simple diffusion, osmosis, and carrier mediated - transport

facilitated diffusion

Carrier protein transfers along concentration gradient without expending energy e.g. Transfer of glucose and amino acids across the basolateral surface of tubule cell in PCT

active transport

Carrier protein transfers against concentration gradient using ATP e.g. Na+/K+ pumps in basolateral surface of tubule cells throughout the tubule

co-transport/symport

Two substrates are transported across cell membrane while bound to a carrier protein.

Movement always follows the concentration gradient of at least one of the molecules

e.g. transfer of glucose and amino acids across the apical surface of tubule cell in PCT

The kinetic energy of Na+ traveling down its concentration gradient is used to transport glucose or amino acids. Na+ binds to SGLT protein on apical surface. This binding results in conformational change that creates a high affinity binding site for glucose

Glucose binds to SGLT resultIng in second conformational change so that the SGLT opens to the cytosol. Na+ released and enters the cell → results in a decreased binding affinity for glucose

counter-transport/antiport

Similar to co -transport except transported ions are moving in opposite directions

e.g. Transfer of Na+ into and H+ out of tubule cells in PCT

Note: Secondary active transport uses the energy of a concentration gradient to move secondary ions across a membrane. The concentration gradient is previously established and maintained via active transport

Secondary active transport can involve either: Co-transport (symport) or Counter-transport (antiport)

active tubular reabsorption

• ATP dependent, occurs with substances moving against electrical-chemical gradients

• these substances generally pass through the apical surface by diffusion

• moves through the basolateral membrane via ATP dependent carrier (generally co-transported with Na+)

  • ex: glucose, amino acids, vitamins, ions, and lactate are all actively reabsorbed via secondary active transport

passive tubular reabsorption

• substances move along electro-chemical gradients via diffusion, facilitated transport, and osmosis

• much of this occurs along with the reabsorption of Na+

reabsorbed substances must pass through

1. luminal or apical surface of the tubule cells

2. basolateral membrane of the tubule cells

3. endothelial membranes

non-reabsorbed substances

• includes substances which are not reabsorbed at all or only partially reabsorbed

• substances include

  • waste products - creatinine, ammonia, organic acids and bases, urea

  • nitrogenous waste from metabolism of proteins

  • urea - even though 40-50% reabsorbed passively with solvent drag, thus partially reabsorbed, small enough to diffuse through membrane pores in PCT

  • uric acid

  • creatinine - metabolite of creatine phosphate found primarily in muscle, produced at a constant rate and excreted by the kidneys

why something might not be reabsorbed

1. lack carriers

2. are not lipid soluble

3. are too large

reabsorption of Na+ in the PCT

more Na+ passes through the filtration membrane than any other substance except H2O

75% reabsorption occurs in PCT at normal flow rates

active reabsorption of Na+ leads to:

1. osmosis of H2O (H2O follows Na+)

2. passive diffusion of anions (HCO3- and Cl-)

As H2O is reabsorbed from the tubules, it results in solvent drag, the drawing along of solutes like urea and fatty acids

Na+ reabsorption

can be reabsorbed everywhere in renal tubule

if aldosterone present can be reabsorbed in DCT and collecting tubule - otherwise impermeable to Na+

reabsorption permits secretion excess H+ and K+ ions

HCO3- reabsorption

not reabsorbed directly, rather HCO3- joins with H+ in tubule lumen, forms H2CO3 (carbonic acid)

H2CO3 → CO2 and H2O by carbonic anhydrase (CA) - on apical surface of PCT cells

CO2 diffuses into cell, intracellular CA reverses the reaction

HCO3- diffuses out of the cell

reabsorption of nutrients in the PCT

• normally 100% of filtered glucose, amino acids, lactic acid, and other metabolites are reabsorbed in the PCT, occurs by secondary active transport (symport with Na+)

• plasma proteins taken into tubule cell by pinocytosis and broken into AAs

  • diffuse out the basolateral surface and into the peritubular capillaries

• substances brought in by into PCT by symporters generally leave by facilitated diffusion through the basolateral membrane and then diffuse into peritubular capillaries

by the end of the PCT filtering has done

100% of filtered nutrients

80-90% filtered HCO3-

75-80% Na+ and H2O

50% Cl- and K+ reabsorbed

reabsorption additional cations, anions, H2O occurs in loop of Henle, DCT, and collecting tubules

Transport maximum

the transport rate at saturation point

• saturation is the maximal rate of transport that can occur when all available carriers are bound with substrate

• each type of transporter has a limit on how fast it can work measured in mg/min

• there are only so many symporter proteins that can only carry so much, so fast

• ex: glucose

  • Filtration glucose concentration (mg/min) is always equal to the plasma glucose concentration (mg/min)

  • Filtration does not exhibit saturation

  • Thus, the amount of glucose filtered into Bowman’s capsule is the same as the plasma concentration

Renal threshold

plasma concentration of a substance measured in mg/ml at which it begins to spill over into urine, i.e. Tm has been surpassed

ex: when the plasma concentration of glucose or aas is higher than usual, Tm may be surpassed and the excess spills over in the urine

clinical application: Diabetes mellitus: An increase in plasma glucose above 200mg/mL leads to an increased urine volume. Related to lack of insulin

Polyuria

for each solute ion that fails to be absorbed one H2O molecule remains in the urine

Polydipsia

increased thirst

Polyphagia

increased hunger

Reabsorption in the loop of Henle

Majority of active transport has occurred by the time filtrate reaches the loop of Henle - 40% K+, 25% Na+, Cl-, 15% H2O reabsorbed

Reabsorption in the DCT

1. diluting segment of DCT - Na+ Cl- ion symporter allows for sodium-chloride ion reabsorption

   1. Na+Cl- increases reabsorption of H2O

   2. symporter can be blocked by thiazide diuretics

2. late DCT - Na+ reabsorption requires aldosterone - aldosterone also increases excretion of K+ and H+ into urine, Na+ reabsorption increases reabsorption of Cl- and HCO3-, H2O (via osmosis in presence of ADH)

3. collecting tubule - 95% filtered solutes returned before reaching here, fine tuning of filtrate occurs here as active transport of cations is under hormonal control

   1. Na+ reabsorption requires aldosterone, aldosterone also increases excretion of K+ and H+ into urine

   2. water reabsorption requires ADH

aldosterone

a steroid hormone secreted by the zona glomerulosa of the adrenal cortex, acts on principle cells in the late DCT and collecting tubules to increase Na+ reabsorption

acts via gene activation and synthesis of

• Na+ leak channels which insert into apical surface

• K+ leak channels which insert into apical surface

• Na+/K+ ATPase which insert into basolateral surface

Is there Na+ reabsorption in the collecting tubules when there’s no aldosterone?

No

How aldosterone works

Enters principle cell, binds with a mineralocorticoid receptor protein in the cytoplasm which then travels to the nucleus to activate gene sites that form new mRNA. After about 45 min. specific proteins begin to appear in the tubule membrane, which is followed by an increase in Na+ absorption

Triggers for release of aldosterone

1. hyperkalemia (K+ enters TASK channels → increases cytosolic Ca2+ levels → steroidogenesis)

2. renin → angiotensinogen → angiotensin I → angiotensin II → aldosterone

3. stress → CRH → ACTH → release of aldosterone

potassium-sparing diuretics

aldosterone antagonists

1. Spironolactone: Binds to, and blocks the aldosterone receptor in the principle cell. Thus it interferes with the synthesis of the Na + channels

2. Amiloride: Does not interfere with the aldosterone receptor but rather:

   1. Blocks the newly synthesized Na + channel on the luminal surface of the tubule cell in the DCT and collecting duct

   2. Inhibits the NA + / K + ATPase on Basal surface of tubule cell

   3. Inhibits counter-transport of Na + and H +

Note: The potassium-sparing diuretics may produce hyperkalemia (>5.5 mEq/L)

ADH stimuli for release

1. plasma osmolarity: when serum osmolarity is high, osmoreceptors in hypothalamus signal release of ADH from neurohypophysis

   1. dehydration hemoconcentrates blood and raises the serum osmolarity

   2. once released ADH is carried in blood to the principle cells of the late DCT and cortical collecting duct

   3. ADH ligates the V2 receptors on the basolateral surface of principle cells and stimulates the insertion of aquaporin-2 (H2O channels) into the apical surface

      1. ADH activates the enzyme adenyl cyclase which activates cAMP

      2. cAMP diffuses to luminal side of cell and initiates the fusing of cytoplasmic vesicles containing aquaporins into the apical membrane

   4. the insertion of aquaporin-2 greatly increases H2O reabsorption in the late DCT, cortical collecting duct, and collecting duct

   5. when ADH is no longer present the vesicular structures detach from the luminal membrane, within 10-15 mins, return to the cytoplasm and the tubule cells return to their impermeable state

2. decrease in blood volume - drop in blood volume and blood pressure stimulate the release of ADH from the neurohypophysis

   1. at higher serum levels ADH ligates V1 receptors on vsm

   2. ligation of V1 receptors on vsm activates the same pathway as NE ligation of alpha 1 receptors, resulting in vasoconstriction of the VSM, and an increase in peripheral resistance

3. angio II: ligates AT1 receptors on the paraventricular nuclei and stimulates the release of ADH from the neurohypophysis

ADH release

normally released in small continuous bursts to control renal water reabsorption

of the 180L of H2O that passes into filtrate every day 160 L (90%) is reabsorbed by following Na+ and glucose

the last 20L (10%) occurs in the collecting tubule and is regulated by ADH

How much urine is excreted with no and maximal ADH?

maximal concentration ADH → 400-500 mL/day, very concentrated urine

absence ADH → up to 20L dilute urine per day

tubular secretion

formed urine not only consists of filtered substances but also secreted substances from the blood into the tubule lumen, functions to rid the body of certain substances and control pH

tubular secretion of H+

the body maintains a normal blood pH of 7.35 - 7.45 despite a continual production of more acids than bases by metabolic reactions

renal cells can increase blood pH by

1. secreting H+ ions into the filtrate

2. reabsorbing filtered HCO3- (which is the most important buffer of H+ ions in extracellular fluids)

3. producing new HCO3- to buffer H+ in the blood

mechanisms of urine dilution

normal concentration of glomerular filtrate as it enters the PCT is 300 milliosmoles per liter (mOsm) - mOsm represents concentration of particles namely NaCl in the filtrate

The thick limb of the ascending loop of Henle is almost impermeable to H2O but actively reabsorbs Na+ , Cl - and K+ from filtrate. Since ions but not H2O molecules leave the filtrate the concentration drops to about 100mOsm/L so that the filtrate leaving the ascending limb is more dilute than plasma

The normal osmolarity of fluid in almost all body parts is 300 mOsm/L except for the medulla of the kidney (from 300-1200-100)

Reasons:

1. Active transport of Na+ out of the thick ascending limb into the interstitium it is then carried downward by the downward flowing blood

2. Passive transport of ions along with Na+

3. Passive diffusion of urea

Countercurrent multiplier

Diffusion between two fluids moving in opposite directions

Countercurrent

PCT flows toward the renal pelvis; DCT flows toward the cortex

The thick ascending limb is relatively impermeable to H2O but there is strong active transport of sodium and chloride ions

Multiplier

Exchange increases as fluid movement continues

Countercurrent multiplier process

Na+ and Chloride ions are pumped of the thick ascending limb into interstitium

This increases the osmotic gradient in interstitial fluid around thin descending limb

Increases water flow out of the thin descending limb à increased solute concentration in ascending limb

Accelerates the transport of Na+ and Cl - out of the ascending limb

Urine concentration

When water intake is low the kidneys must still eliminate wastes and excess ions while conserving water

Excretion of concentrated urine depends on ADH

• In the presence of ADH water moves from collecting duct cells into interstitial fluid

• Thus, increasing the solute concentration of urine being formed à concentrated urine being excreted

ADH is released in response to:

• Increase in blood osmolarity

• Low blood volume and/or low blood pressure

• Angio II

Urine dilution

In the absence of ADH urine may be 4x more dilute than glomerular filtrate (65- 70 mOsm/L)

Micturition

Accumulation of 200 ml of urine → Activation of stretch receptors → Afferent signals to CNS → Efferent signals via parasympathetic splanchnic nerves → Contraction of the detrusor muscle → Relaxation of the internal sphincter(involuntary) → Urine in upper urethra → Urge to void - Voluntary (to void or not to void) → Relaxation of external urethral sphincter

Capacity is about 800ml