Dibartola Chapter 2: Applied Renal Physiology

Major Functions of Each Portion of the Nephron Diagram

Renal Clearance

The volume of plasma that contains the amount of the substance excreted in the urine in 1 minute, the volume of plasma that must be filtered each minute to account for the amount of the substance appearing in the urine each minute under steady state conditions

Standard Renal Clearance Formula

• Standard clearance formula: U_xV/P_x

  • U_x - the concentration of the substance in urine

  • V - urine flow rate

  • P_x - the concentration of the substance in plasma

What are the three components of the glomerular capillary wall or filtration barrier?

Capillary endothelium

Basement membrane

Visceral epithelium

Structure of the Glomerulus

• Glomerulus consists of a capillary bed interposed between the afferent and efferent arterioles

• Capillary endothelium of the glomerulus is fenestrated by openings 50-100 nm in diameter

  • These openings exclude cells from the ultrafiltrate, but macromolecules are not restricted based on size

• Luminal surface of the endothelium is covered by negatively charged sialoglycoproteins that contribute to the charge selectivity of the filtration barrier

What is the glomerular basement membrane composed of?

Lamina rara interna on the endothelial side

Central lamina densa

Lamina rara externa on the epithelial side

Lamina Rara Interna and Lamina Rara Externa

Lamina rare interna and lamina rara externa contain polar noncollagenous proteins that contribute to the negative charge of the filtration barrier

Lamina Densa

Lamina densa contains nonpolar collagenous proteins that contribute primarily to the size selectivity of the filtration barrier

• Filtration barrier is permeable to molecules with effective molecular radii less than 2 nm and impermeable to those with radii greater than 4 nm

Visceral Epithelial Cells or Podocytes

• Visceral epithelial cells or podocytes constitute the outermost portion of the filtration barrier

  • Cover the glomerular basement membrane and glomerular capillaries on the urinary side of the barrier with their primary and interdigitating secondary foot processes

  • Filtration slits, 10-30 nm in width, are located between the secondary foot processes

  • Podocytes are phagocytic and may engulf macromolecules trapped by the filtration slits

  • Have a negatively charged sialoglycoprotein coat that contributes to the charge of selectivity of the filtration barrier

  • Believed to synthesize the glomerular basement membrane

Mesangium

• Not part of the filtration barrier

• Is a stabilizing core of tissue that forms an anchor for the glomerulus at the vascular pole and along the axes of the capillary lobules

• Mesangial cells are in contact with the basement membrane in areas where there is no capillary endothelium

• Extraglomerular mesangium fills the space between the macula densa and the glomerular arterioles and constitutes part of the juxtaglomerular apparatus (JGA)

• Mesangial cells contain microfilaments that can contract in response to specific hormones (e.g. angiotensin II), altering the surface area available for filtration

  • Also synthesize prostaglandins that contribute to renal vasodilation

• Contains macrophages that can clear filtration residues from the mesangial space by phagocytosis

What is the glomerular capillary wall selective for?

The glomerular capillary wall is a size-selective and charge-selective barrier to filtration

What gives the glomerular capillary wall its size selectivity?

Size selectivity primarily in the lamina densa of glomerular basement membrane

• Generally excludes molecules with radii greater than 4 nm

What gives the glomerular capillary wall its charge selectivity?

Charge selectivity resides in the negatively charged sialoglycoproteins (e.g. laminin and fibronectin) and peptidoglycans (e.g. heparan sulfate) of the capillary endothelium, lamina rara externa, and visceral epithelium

• Negatively charged macromolecules experience greater restriction to filtration than neutral ones

• Positively charged macromolecules experience less restriction to filtration than neutral ones

Glomerular Filtration Rate

Total filtration rate of both kidneys and represents the sum of hte single-nephron glomerular filtration rates (SNGFRs) of all nephrons

Superficial Cortical Nephrons

Short loops of Henle with little or no penetration into the renal medulla

Tend to excrete relatively more solute and water

Juxtamedullary Nephrons

Long loops of Henle that penetrate the inner medulla

Tend to conserve solute and water

Glomerular Ultrafiltrate

A protein-free ultrafiltrate of plasma containing water and all of the cyrstalloids of plasma in concentrations similar to those in plasma

Single-Nephron Glomerular Filtration Rate Formula

What accounts for the higher rate of filtration in the glomeular capillaries than in systemic capillaries?

• If average pressure are considered alone, net filtration pressure in the glomerulus is approximately 15 mmHg, which is similar to systemic capillaries

  • The fact that GFR is so much higher than the movement of fluid across systemic capillaries is explained by different values for K_f

    • Ultrafiltration constant, K_f, is dependent on the surface area available for filtration and the permeability per unit area of capillary to crystalloids and water

    • In the glomerulus, the surface area available for filtration is much greater than that found in the capillary beds of skeletal muscle and the unit permeability of the glomerular endothelium is more than 100 times that of skeletal muscle capillaries

  • The much higher value for K_f in glomerular capillaries than in systemic capillaries accounts for the much higher rate of filtration

  • K_f is not constant and can change as a result of disease and in response to hormones that cause mesangial cells to contract (e.g. angiotensin II)

Effect of Changes in Resistance in the Afferent Arterioles

Leads to parallel changes in GFR and renal blood flow

Effect of Changes in Resistance in the Efferent Arterioles

Lead to divergent changes in GFR and renal blood flow

Effect of Decreased Resistance in the Afferent Arteriole on Renal Blood Flow and GFR

Increased renal blood flow

Increased GFR

Effect of Increased Resistance in the Afferent Arteriole on Renal Blood Flow and GFR

Decreased renal blood flow

Decreased GFR

Effect of Decreased Resistance in the Efferent Arteriole on Renal Blood Flow and GFR

Increased renal blood flow

Decreased GFR

Effect of Increased Resistance in the Efferent Arteriole on Renal Blood Flow and GFR

Decreased renal blood flow

Increased GFR

Effect of Acetylcholine on the Afferent Arteriole

Relax

Effect of Nitric Oxide on the Afferent Arteriole

Relax

Effect of Dopamine on the Afferent and Efferent Arterioles

Relaxes both

Stimulation of dopaminergic receptors causes afferent and efferent vasodilation and increased renal blood flow with little change in GFR at low concentrations of dopamine

Effect of Bradykinin on the Afferent Arteriole

Relax

Effect of Prostacyclin on the Afferent Arteriole

Relax

Effect of Prostaglandin E2 on the Afferent Arteriole

Relax

Effect of Prostaglandin I2 on the Afferent Arteriole

Relax

Effect of Acetylcholine on the Efferent Arteriole

Relax

Effect of Nitric Oxide on the Efferent Arteriole

Relax

Effect of Bradykinin on the Efferent Arteriole

Relax

Effect of Prostacyclin on the Efferent Arteriole

Relax

Effect of Prostaglandin E2 on the Efferent Arteriole

No effect

Effect of Prostaglandin I2 on the Efferent Arteriole

Relax

Effect of Norepinephrine on the Afferent and Efferent Arterioles

Constricts both

Efferent arteriolar constriction usually predominates

• Renal blood flow decreases with minimal changes in GFR (i.e. filtration fraction increases)

Effect of Angiotensin II on the Afferent and Efferent Arterioles

Constricts both

Causes efferent more than afferent vasoconstriction

• Renal blood flow decreases with minimal changes in GFR (i.e. filtration fraction increases)

Effect of Endothelin on the Afferent Arteriole

Constrict

Effect of Thromboxane on the Afferent Arteriole

Constrict

Effect of Vasopressin on the Afferent Arteriole

No effect

Effect of Endothelin on the Efferent Arteriole

Constrict

Effect of Thromboxane on the Efferent Arteriole

Constrict

Effect of Vasopressin on the Efferent Arteriole

Constrict

Result of Norepinephrine, Angiotensin II, and ADH on Prostaglandins

• Norepinephrine, angiotensin II, and ADH cause vasoconstriction at the same time promoting the production of prostaglandins that cause vasodilation

  • These prostaglandins (PGE2 and PGI2) play an important role in maintaining RBF in hypovolemic states when angiotensin II and norepinephrine concentrations are increased

  • Effects of these prostaglandins are limited to the kidneys because they are rapidly metabolized in the pulmonary circulation

• NSAIDs that inhibit generation of prostaglandins by the COX pathway may cause renal ischemia and acute renal insufficiency in hypovolemic patients

Effect on Kinins on Renal Blood Flow

Locally produced kinins cause vasodilation and favor redistribution of renal blood flow to the inner cortical nephrons

Mass Balance Equation if there is a Substance that is Filtered by the Glomeruli but Neither Reabsorbed nor Secreted by the Tubules

Renal Clearance of a Substance that is Neither Reabsorbed nor Secreted

Equal to GFR

What is the laboratory standard for GFR determination?

Inulin clearance

What is used clinically to estimate GFR in the steady state?

Creatinine clearance

Creatinine Clearance

• Creatinine is produced endogenously and excreted primarily by glomerular filtration so its clearance can be used to estimate GFR in the steady state

  • Requirements for determination of endogenous creatinine clearance are an accurately timed urine sample (usually 24 hours), determination of the patient's body weight, and measurement of serum and urine creatinine concentrations

  • In the dog and cat, creatinine is filtered by the glomeruli and neither reabsorbed nor secreted by the tubules

  • Values for endogenous creatinine clearance in the dog and cat are approximately 2-5 mL/min/kg

    • Horse 1-2 mL/min/kg

Renal Clearance Equation

Clearance of a Substance Experiencing Net Reabsorption vs GFR

The clearance of a substance experiencing net reabsorption is less than GFR (T_x is negative)

Clearance of a Substance Experiencing Net Secretion vs GFR

The clearance of a substance experiencing net section is greater than GFR (T_x is positive)

Ratio of Clearance of a Substance to Inulin

• The ratio of the clearance of a substance to inulin clearance gives an indication of the net handling of that substance by the kidneys

  • If ratio is less than 1.0, the substance experiences net reabsorption

  • If greater than 1.0, it experiences net secretion

What % of cardiac output do the kidneys receive?

25% or more

What are the major sites of resistance in the kidneys?

Afferent and efferent arterioles, with an approximately 80-90% decrease in perfusion pressure across this region of the renal vasculature

What % of renal blood flow is directed to the renal cortex?

More than 90%

What % of renal blood flow is directed ot the outer medulla?

Less than 10%

What % of renal blood flow is directed to the inner medulla?

2-3%

Autoregulation

The intrinsic ability of an organ to maintain blood flow at a nearly constant rate despite change in arterial perfusion pressure

In the kidneys, between perfusion pressures of 80 and 180 mmHg, GFR and RBF vary less than 10%

Flow Equation

• Flow (Q) is equal to pressure (P) divided by resistance ( R )

  • Q = P/R

  • As pressure increases, flow can remain constant only if resistance increases proportionately

    • The site of this resistance change in the kidneys is the afferent arteriole

Effect of Anesthesia on Autoregulation

• Impaired by anesthesia in proportion to the depth of anesthesia

  • Afferent arterioles are maximally dilated at MAPs of 70-80 mmHg and at lower pressures, GFR declines linearly with RBF (autoregulation is lost)

What are the physiologic mechanisms that contribute to autoregulation?

Myogenic mechanism

Tubuloglomerular feedback

Myogenic Mechanism

• Based on the principle that smooth muscle tends to contract when stretched and relax when shortened

• As the afferent arteriole is stretched by increased perfusion pressure, it constricts, thus limiting transmission of this increased pressure to the glomerulus and minimizing any change in glomerular capillary hydrostatic pressure and SNGFR

• Represents a course control that operates with a delay of 1-2 seconds

Tubuloglomerular Feedback

• Represents a local intrarenal negative feedback mechanism for individual nephrons

• Increased sodium chloride concentration or transport in the distal tubule is sensed by the extraglomerular mesangial cells of the JGA as they monitor sodium chloride transport across the tubular cells of the macula densa

  • Transport of NaCl by the tubular cells of the macula densa requires functional NKCC2 (Na+, K+, 2 Cl cotransporter) and ROMK (potassium channel) in the luminal membranes and functional Na+, K+ ATPase in the basolateral membranes

  • Transcellular transport of NaCl causes generation of adenosine, which together with angiotensin II, causes afferent arteriolar constriction in the parent glomerulus

  • Afferent arteriolar constriction causes SNGFR to decrease, thus decreasing filtration and minimizing NaCl loss in the nephron

  • Effect occurs locally in the region of the juxtaglomerular interstitium

• Represents fine control that operates with a 10-12 second delay

Renal Plasma Flow Equation

• P_AX - renal arterial plasma concentration of x

• RPF_A - arterial renal plasma flow (RPF)

• P_Vx - renal venous plasma concentration of x

• RPF_V - venous RPF

• U_x - urine concentration of x

• V - urine flow

• If we chose a substance that is completely removed from the blood in one pass through the kidneys, P_Vx is zero and RPF = U_x V /P_AX

• If the substance x is not metabolized and is not excreted by any organ other than the kidneys, its concentration in any peripheral vessel equals P_AX

What is PAH clearance an estimate of?

Effective renal plasma flow

Fick Principle

The amount of a substance (V) removed by an organ is equal to the blood flow to the organ (Q) times the arteriovenous concentration difference of the substance in question (C_A-C_V)

Renal Blood Flow Equation

Filtration Fraction

The fraction of plasma flowing through the kidneys that is filtered into the Bowman space

Reabsorption

In the kidneys, refers to movement of water and solutes from the tubular lumen to the peritubular interstitium

Secretion

In the kidney, refers to movement of water and solutes from the peritubular interstitium to the tubular lumen

What do the luminal membranes of the kidney separate?

The cytoplasm of the tubular cell from the tubular fluid

What do the basolateral membranes of the kidney separate?

The cytoplasm of the tubular cell from the lateral intercellular space and peritubular interstitium

Transmembrane Potential Difference (PD)

The electrical PD between the outside and inside of the cell

Usually -60 to-70 mV (cell interior negative)

Transepithelial or Transtubular Potential Difference

The electrical PD between the tubular lumen and the peritubular interstitium, algebraic sum of the transmembrane PD between the tubular lumen and cell cytoplasm, and the transmembrane PD between the peritubular interstitium and cell cytoplasm

Only a few mV

Potential Difference Between the Tubular Lumen and Peritubular Lumen in the Early Proximal Tubule

The tubular lumen is a few mV negative relative to the peritubular lumen

Potential Difference Between the Tubular Lumen and Peritubular Interstitium in the Later Proximal Tubule

The tubular lumen is a few mV positive relative to the peritubular interstitium

Transepithelial Potential Difference in the Thick Ascending Loop of Henle

Lumen positive

Transepithelial Potential Difference in the Distal Tubule

Lumen negative

Paracellular Route

Movement of solutes and water between cells (i.e. from the tubular lumen to the lateral intercellular space across tight junctions connecting epithelial cells)

• Allows movement of ions and large, nonpolar solutes by passive diffusion and solvent drag

• Electrochemical, hydrostatic, and oncotic gradients are important driving forces for reabsorption by the paracellular route

• Accounts for only 1% of the surface area available for reabsorption and 5-10% of water transport

Transcellular Route

Movement of solutes and water through the cytoplasm of the tubular cells

• Accounts for 99% of the available surface area and 90-95% of water transport

• Both passive and active transport processes occur by this route

• All active transport processes must occur by this route

Leaky Epithelial Cell Junctions

• Proximal tubules

• Do not generate large transepithelial concentration gradients, exhibit small transepithelial PD, and have high water permeability

Tight Epithelial Cell Junctions

• Distal convoluted tubules, collecting ducts

• Can generate large transepithelial concentration gradients, exhibit a large transepithelial PD, and have low basal water permeability

  

Water in the Proximal Tubule

In the proximal tubule, water follows solute reabsorption osmotically and is said to occur isosmotically (reabsorbed fluid has the same osmolality as extracellular fluid)

What fraction of water and solute reabsorption occurs in the proximal tubule?

2/3

What % of glucose, amino acids, and bicarbonate are reabsorbed in the early proximal tubules?

Almost 99% of glucose and amino acids and 90% or more of bicarbonate

What are the four types of transport processes that contribute to renal tubular reabsorption?

Passive diffusion

Facilitated diffusion

Primary active transport

Secondary active transport

Passive Diffusion

• Movement of a substance across a membrane as a result of random molecular motion

• Simple diffusion can take place directly through the lipid bilayer of the cell membrane and through hydrophilic protein channels embedded in the cell membrane

• Simple diffusion requires no expenditure of metabolic energy

• Rate of transfer of solute is dependent on the permeability characteristics of the membrane, the electrochemical gradient, and the hydrostatic pressure across the membrane

• Rate of diffusion is linearly related to the concentration of the diffusing solute

• No maximal rate of transfer (V_max)

• Not a saturable process because a carrier is not involved

Facilitated Diffusion

• Movement of a substance across a membrane down its electrochemical gradient after binding with a specific carrier protein in the membrane

• The carrier protein binds the substance to be transported at one side of the cell membrane, undergoes an conformational change that causes translocation of the substance across the cell membrane, and releases the substance on the other side of the membrane

• Saturable process characterized by a maximal rate of transfer (V_max) because a carrier is involved

• The carrier has structural specificity and affinity for the substance transported and the process is subject to competitive inhibition

• Does not directly require metabolic energy

• Transfer may occur in either direction across the membrane

Primary Active Transport

• Movement of a substance across a membrane in combination with a carrier protein but against the electrochemical gradient

• Requires metabolic energy, supplied by hydrolysis of ATP

• Saturable process characterized by a V_max

• Subject to metabolic (e.g. cellular oxidative poisons) and competitive inhibition

• Examples

  • Na+, K+ ATPase in basolateral membranes of tubular cells throughout the nephron

  • H+ATPase in luminal membranes of tubular cells throughout the nephron

  • H+, K+ATPase in luminal membranes of a-intercalated cells in the collecting ducts

Secondary Active Transport

• Movement of two substances across a membrane after combination with a single carrier protein

• Uphill (against a concentration gradient) transport of one substance is linked to the downhill (down an electrochemical gradient) transport of another substance

• Saturable

• Demonstrates structural specificity and affinity of the carrier for the substances transported

• May be competitively inhibited

• Uphill transport occurs without direct input of metabolic energy, substance transported uphill said to experience secondary active transport

• Metabolic energy for secondary active transport at the luminal membranes comes from the primary active transport of sodium out of the tubular cell at the basolateral membrane by Na+, K+ATPase

Cotransport

Transported substances are moving in the same direction across the membrane

Countertransport

Transported substances are moving in opposite directions across the membrane

Pinocytosis

Uptake by cells of particles too large to diffuse through the cell membrane

Solvent Drag

Process whereby water moving across an epithelium by osmosis can drag dissolved solutes along with it

Morphology of the Proximal Tubule

• Brush border of the luminal surface of the proximal tubular cells consists of microvilli, which increase surface area, and lateral cellular interdigitations, which increase the surface area of the basolateral membranes

• Mitochondria supply energy via ATP for active transport

• The most proximal segments of the proximal tubule ultrastructurally are the most complex and suited for the mechanisms of solute transport

  • The morphologic complexity decreases along the length of the proximal tubule