Functional Renal Anatomy and the Glomerulus

Scanning Electron Micrograph (SEM) of the Glomerular Capillary Wall

This scanning electron micrograph provides a different perspective on the organization of the filtration barrier. -Within the lumen of the capillary the *fenestrated endothelium* can be observed -The basement membrane is difficult to see but the elaborate branching of the podocyte foot processes reveals just how tightly covered the surface of the capillary endothelium is.

Decreased Glomerular Capillary Colloid Osmotic Pressure

This slide diagrammatically illustrates what would happen to the net filtration pressure in the case of decreased glomerular capillary colloid osmotic pressure. -*Liver disease or a nephrotic syndrome* (loss of protein in the urine) would be the most likely causes of decreased glomerular capillary colloid osmotic pressure.

Increased Glomerular Capillary Colloid Osmotic Pressure

This slide diagrammatically illustrates what would happen to the net filtration pressure in the case of increased glomerular capillary colloid osmotic pressure. -Increased glomerular capillary colloid osmotic pressure might result from a *hypersecreting tumor.*

Increased Glomerular Capillary Hydrostatic Pressure

This slide diagrammatically illustrates what would happen to the net filtration pressure in the case of increased glomerular capillary hydrostatic pressure. -*Hypertension* is the primary source of INCREASED glomerular capillary hydrostatic pressure. look at powerpoint for animation!!!

SEM of the Fenestrated Endothelium

A close up of the fenestrated endothelium. - Modeling studies suggest two populations of pores. -A large population of SMALL diameter pores that allows for the bulk of filtration and a small population of LARGE diameter pores that account for the presence of a small amount of larger solutes (proteins).

Three Layers of the Glomerular Filtration Barrier are.... Endothelium, Basement Membrane, and Podocytes

*Endothelium, Basement Membrane, Podocytes* -Note the presence of negative charges on the endothelium, basement membrane and podocyte foot processes. -These charges contribute to the ability of this filtration barrier to RESTRICT the movement of proteins into the ultra filtrate. >Heparin sulfate at basement membrane which repel negative charges of proteins ->Filtrate is LESS negative

Components of the Nephron and the Collecting Duct System

The primary functional unit in the kidney is the *nephron.* -There are approximately 1 million nephrons in a human kidney >The number DECREASES with age. -Each nephron consists of a glomerulus comprised of the blood supply (afferent arteriole, glomerular capillary bed, and efferent arteriole) and the Bowman's capsule which collects the *ultrafiltrate*. -The ultrafiltrate is sequentially processed through the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. -There are two types of nephrons based on the location of the glomerulus and the length of the loop of Henle: Juxtamedullary Nephrons and Cortical Nephrons

Decreased Glomerular Capillary Hydrostatic Pressure

This slide diagrammatically illustrates what would happen to the net filtration pressure in the case of decreased glomerular capillary hydrostatic pressure. -*Blood loss or dehydration* are common sources of decreases in glomerular capillary hydrostatic pressure.

A 45-year-old female is diagnosed with unilateral obstructive uropathy caused by the presence of a stone in one of the ureters. Which of the following Starling force changes is most likely responsible for the decrease in net filtration pressure in this patient's affected kidney?

a. ↓ Bowman's space hydrostatic pressure b. ↓ Bowman's space osmotic pressure c. ↓ glomerular capillary hydrostatic pressure d. ↓ glomerular capillary osmotic pressure e. ↑ Bowman's space hydrostatic pressure f. ↑ Bowman's space osmotic pressure g. ↑ glomerular capillary hydrostatic pressure h. ↑ glomerular capillary osmotic pressure ---- e. -> blockage of flow so increase in hydrostatic pressure in bowman's space so less filtration

An 11-yo girl presents with profound weakness and fatigue. Laboratory studies reveal severe hypophosphatemia, glucosuria, aminoaciduria, and metabolic acidosis. Which of the following defects most likely explains this clinical disorder?

A) Loss of function mutation of NPT2 B) Loss of function mutation of NHE-3 C) Gain of function mutation in GLUT-2 D) Loss of function mutation in Na/K ATPase E) Gain of function mutation in NBC-1 --- The findings of severe hypophosphatemia, glucosuria, aminoaciduria, and metabolic acidosis suggest that the defect is unlikely to be associated with a specific transport pathway. A loss of function mutation in the Na/K ATPase would have a general impact on tubular transport processes which might be expected to yield the constellation of findings reported.

An investigator develops a novel drug which specifically blocks the organic anion transporter in the proximal convoluted tubule. The most likely effect of this reduction in transporter activity would be:

A) an increase in organic cation transporter activity B) a decrease in sodium reabsorption C) a reduction in the furosemide mediated block of NKCC D) acidification of the tubular fluid ----- Furosemide acts from the luminal side by blocking the chloride binding site on NKCC. Some furosemide is filtered but the majority enters the tubule via the Organic Anion Transporter (OAT) transporter. C

Most of the filtered sodium ions are reabsorbed from the:

A) cortical collecting duct B) early distal convoluted tubule C) medullary collecting duct D) proximal convoluted tubule E) thick ascending limb of the loop of Henle --- PCT: Approximately two thirds of the filtered sodium is reabsorbed in the proximal convoluted tubule.

Where in the nephron is the tubular fluid always isotonic with respect to plasma, irrespective of the hydration state of the subject?

A) cortical collecting duct B) early distal tubule C) medullary collecting duct D) proximal tubule E) thick ascending limb of the loop of Henle -- Water and solute reabsorption is alway isotonic with respect to plasma in the proximal convoluted tubule. (D)

TEM of the Glomerular Capillary Wall

Mutations to proteins contributing to the filtration membrane have been shown to ALTER renal function. -Mutations in genes that encode podocyte proteins result in a fundamental CHANGE in the architecture of the podocyte slit which leads to *protein losses in the urine (proteinuria).* -For example, nephrin mutations were first described in families with congenital nephrotic syndrome (proteinuria > 3.5 g/dL of the Finnish type). -Pod: podocyte foot process -GBM: glomerular basement membrane -GEnC: glomerular endothelial cell -The asterisks are located just above the filtration slit proteins (slit diaphram) which can be seen as a thin line on this EM micrograph.

The glomerulus

The interface between the circulatory system and the renal filtration system occurs at the *glomerulus.* -Blood supplied by the afferent arteriole courses through the glomerular capillaries where it encounters the filtration membrane composed of the capillary endothelium, the basement membrane and the *podocyte foot processes.* >Podocytes: Visceral epithelial cells of Bowman's Capsule that wrap around the capillaries of the glomerulus. -The FILTERED blood exits through the EFFERENT arteriole. -A net positive filtration pressure forces ultrafiltrate from the capillary into Bowman's space and subsequently into the proximal tubule. -The juxtaglomerular apparatus (JGA) is located at the junction of the afferent and efferent arterioles and the specialized region of the DISTAL convoluted tubule containing the *macula densa cells.* >Macula Densa Cells: Function in sampling sodium concentration of the tubular fluid (ultra filtrate) -The JGA is involved in regulating FLOW RATE through the tubule in order optimize REABSORPTION. -Dynamic regulation of myocytes in the afferent and efferent arterioles, mesangial cells and podocyte foot processes also serve to regulate the rate of filtration. -Note the myocytes encircling the efferent arteriole.

Transmission Electron Micrograph (TEM) of Normal Human Glomerular Capillary

This electron micrograph is a cross section through a glomerular capillary. -The capillary endothelium, basement membrane, and podocyte foot processes are evident. -The asterisk indicates region where the mesangial cells are in close association with the capillary endothelium. >Mesangial cells: Phagocytosis, contraction (regulate blood flow), structural support, secretion

Juxtamedullary Nephrons vs. Cortical Nephrons

-*Juxtamedullary nephrons* are identified by a glomerulus located deeper in the cortical region and a long loop of Henle that extends down into the INNER medulla. -*The glomeruli of cortical nephrons* are more superifically located in the cortical region and have a short loop of Henle that does NOT extend into the inner medullary region (only unto outer medulla). -The unique anatomy of these two nephron types allow them to produce urine of DIFFERENT compositions which allows the kidney to generate urine ranging from *dilute to concentrated.*

Relationship Between Solute Charge and Filterability

-Negative charges on the filtration membrane RESTRICT movement of negatively charges substances into the ultrafiltrate. -Using dextrans over a range of sizes that were decorated with either negative, positive or neutrally charged side groups the relationship between size and charge on filtration was explored. -These experiments demonstrated that at any molecular radius the filterability was always *Negative<Neutral<Positive* -The molecular weight and molecular radius of a substance affect it's filtration. >As the radius and molecular weight increase there is a DECREASE in filtration as reflected by the filtrate/plasma concentration ratio. -Inulin is a polysaccharide with a molecular weight of 5,000 daltons which is FREELY filtered. - Myoglobin (M) and hemoglobin (H) are both negatively charged and this is reflected by their plotted location below the neutral curve. -*Myoglobin* is 17,000 daltons protein that is relatively FREELY filtered but about 25% of the myoglobin passing through the glomerular is NOT filtered. -*Hemoglobin and serum albumin* are 68,000 and 66,000 dalton proteins. >Only 0.1% to 1% of these larger proteins are FILTERED as they pass through the glomerulus. >The lower filtration of the slightly smaller serum albumin is due to it's LARGER molecular RADIUS

An investigator measures the glomerular pressures listed below. What is the net ultrafiltration pressure? HPGC = 39 mm HgHPBS = 15 mm HgCOPGC = 20 mm HgCOPBS = 0 mm Hg

1. 2 mm Hg 2. -2 mm Hg 3. 4 mm Hg 4. -4 mm Hg 5. 8 mm Hg 6. -8 mm Hg --- Use equation!!! 3

The structures indicated by the asterisks in the figure below are:

1. Afferent arterioles 2. Glomeruli 3. Glomerular capillaries 4. Bowman's space 5. Efferent arterioles -------- Blood supply of kidney with polymerization of tissue -Filling blood supply-> LUMEN of the glomerular capillaries (3) -> This figure digested away the bowman's capsule and other structures

Three solutes each with a molecular radius of 30Å differ only in their net charge. What is their relative filterability through the glomerular capillary wall?

1. Positive > Neutral > Negative 2. Positive > Negative > Neutral 3. Neutral > Positive > Negative 4. Neutral > Negative > Positive 5. Negative > Neutral > Positive 6. Negative > Positive > Neutral --- must be one that can filter most to least; negative filters least then neutral then positive so 1

A 12 yo girl was treated with oral penicillin for a sore throat. As a supplement to the penicillin she also received probenecid. Probenecid would be expected to have which of the following effects on the renal handling of penicillin? Probenecid would:

A) inhibit reabsorption of penicillin B) inhibit secretion of penicillin C) stimulate reabsorption of penicillin D) stimulate secretion of penicillin E) inhibit the metabolism of penicillin --- Penicillin is secreted into the tubules by the organic acid transporter. Probenecid blocks the organic acid transporter thus blocking penicillin secretion. (B)

An investigator injects a test drug intravenously into a rat and measures the Starling forces shown in the table below. The normal net filtration pressure for the control population of rats in this study is +10 mm Hg. What is the net ultrafiltration pressure in the treatment group of rats?

A. +60 mm Hg B. + 40 mm Hg C. +35 mm Hg D. -35 mm Hg E. - 40 mmHg F. -60 mm Hg ---- B. 75-10=65-25=40

An investigator injects a test drug intravenously into a rat and measures the Starling forces shown in the table below. The normal net filtration pressure for the control population of rats in this study is +10 mm Hg. Which of the following disorders most likely underlies the altered net filtration pressure in the drug treated group

A. Dehydration B. Hypertension C. Liver Damage D. Ureter obstruction --- B

Functional Organization of the Kidney

Arteries: Renal artery-> Segmental arteries-> Interlobar arteries-> Arcuate arteries-> Interlobular arteries-> Afferent Arteries (to the nephron)-> Glomerulus-> Efferent arteriole-> Peritubular capilaries or Vasa Recta Veins: Peritbular Capillaries-> Venules-> Interlobular veins-> Arcuate Veins -> Interlobar Veins -> Renal Vein

Pressure Profiles Along a Skeletal Muscle Capillary (Skeletal muscle capillary)

In a systemic capillary there is a significant *DROP in the capillary hydrostatic pressure* from the arterial end to the venous end. -*Capillary colloid osmotic pressure remains relatively constant.* -As a consequence, filtration occurs at the ARTERIAL end when capillary hydrostatic pressure is HIGH but absorption of fluid back into the capillary occurs when the capillary hydrostatic pressure FALLS below the capillary colloid osmotic pressure. -The point of no filtration is referred to as the filtration "pressure equilibrium."

Pressure Profiles Along a Skeletal Muscle Capillary (Glomerular capillary)

In contrast to the drop in capillary hydrostatic pressure seen across systemic capillary beds there is very LITTLE drop in the capillary hydrostatic pressure in a glomerular capillary bed. -Also note that the initial pressure is higher. -Because of the higher and constant glomerular capillary hydrostatic pressure there is NEVER a point when the hydrostatic pressure is below the colloid osmotic pressure and *filtration occurs along the ENTIRE length of the capillary bed*. -Also note that glomerular capillary colloid osmotic pressure INCREASES from the afferent arteriolar end to the efferent arteriolar end of the capillary bed. >This increase is of SIGNIFICANCE in the peritubular capillary bed where it *promotes REABSORPTION.*

Renal Pathology of Clinical Podocyte Disorders

Numerous alterations to the filtration barrier have been shown to contribute to renal disease. (A) Light microscopy image of a normal glomerulus, Jones methenamine silver (JMS) stain (B) Electron micrograph of a capillary loop from a normal glomerulus. -Arrow heads point to regularly arranged intact foot processes. (C) Extensive effacement of foot processes (arrowheads) in minimal change disease. Spiral arrows point to microvillus transformation of podocytes

Composition of Glomerular Filtrate

Organic molecules: glucose, amino acids Nitrogenous waste: Urea, uric acid, creatinine Ions: sodium, potassium, chloride Water: Main component filtered -The concentration of each constituent of the glomerular filtrate is approximately equal to it's plasma concentration. >Anything found in plasma that is below a MW of about 5,000 daltons will be FILTERED and be found in the ultrafiltrate at a concentration identical to it's concentration in the plasma. -NOTE: The absence of large molecular weight substances and smaller proteins are variably filtered depending on their size and molecular radius.

The Glomerular Basement Membrane contains...

THREE LAYERS -*Lamina rara externa* which is adjacent to podocyte processes contains heparan sulfate which blocks by charge (- charges) -Lamina densa dark is the central zone with type 4 collagen and laminin blocks by SIZE (Molecular Weight > 5800kDa) -Lamina rara interna is adjacent to endothelial cells uses heparan sulfate to block by charge

pressure Profiles Along a Glomerular Capillary

The 4 Starling forces can be used to calculate the net filtration pressure as described by the equation in this figure. -A positive net filtration pressure FAVORS filtration into Bowman's space -A negative net filtration pressure indicates flow from Bowman's space INTO the glomerular capillary -A net filtration pressure of zero would indicate NO NET filtration.

Starling Forces Affecting Net Filtration Pressure (Systemic Capillary)

The 4 Starling forces contributing to fluid movement in a systemic capillary are: -*Capillary hydrostatic pressure* (Blood Pressure): PROMOTES filtration -*Interstitial space hydrostatic pressure*: OPPOSES filtration -*Capillary colloid osmotic pressure*: OPPOSES filtration -*Interstitial space colloid osmotic pressure*: FAVORS filtration *The balance of the Starling hydrostatic and oncotic pressures results in the movement of fluid from the glomerular capillaries to Bowman's space forming the ultrafiltrate* *The hydrostatic pressure in the glomerular capillaries (BP) is higher than systemic capillaries and the capillary wall is LEAKIER leading the formation of large volumes of ultrafiltrate.*