Renal Physiology

Renal Physiology D. C. Mikulecky Faculty Mentoring Program Virginia Commonwealth Univ.

FUNCTIONS OF THE KIDNEY • Water balance • Electrolyte balance • Plasma volume • Acid-base balance • Osmolarity balance • Excretion • Hormone secretion

THE URINARY SYSTEM • Kidneys • Blood supply: Renal arteries and veins • Ureter • Urinary bladder • Urethra

THE NEPHRON IS THE FUNCTIONAL UNIT OF THE KIDNEY Bowman’s Capsule Proximal Convoluted Tubule Distal Convoluted Tubule Peritubular Capillaries Cortex Glomerulus Medulla Collecting Duct Artery Loop of Henle Vein

THREE BASIC RENAL PROCESSES • Glomerular Filtration: Filtering of blood into tubule forming the primitive urine • Tubular Reabsorption: Absorption of substances needed by body from tubule to blood • Tubular Secretion: Secretion of substances to be eliminated from the body into the tubule from the blood

BASIC RENAL PROCESSES Efferent Arteriole Afferent Arteriole Glomerulus GF TR Kidney Tubule Peritubular Capillary TA Urine Excreted

Glomerular Filtration • First step in urine formation • 180 liters/day filtered • Entire plasma volume filtered 65 times/day • Proteins not filtered

Forces Involved inGlomerular Filtration + Glomerular Capillary Blood Pressure 55 - Plasma Colloid Osmotic Pressure 30 - Bowman’s Capsule Hydrostatic Pressure 15 Net Filtration Pressure + 10

Tubular Reabsorption • Water: 99% reabsorbed • Sodium: 99.5% reabsorbed • Urea: 50% reabsobed • Phenol: 0% reabsorbed

Tubular Reabsorption • By passive diffusion • By primary active transport: Sodium • By secondary active transport: Sugars and Amino Acids

Tubular Reabsorption is a Function of the Epithelial Cells Making up the Tubule Plasma Lumen Cells

Sodium Reabsorption PUMP: Na/K ATPase Plasma Sodium Lumen Cells Potassium Chloride Water

Rennin-Angiotensin-Aldosterone System • Stimulates Sodium Reabsorption in distal and collecting tubules • Naturetic peptide inhibits • In absence of Aldosterone, 20mg of sodium/day may be excreted • Aldosterone can cause 99.5% retention

Rennin-Angiotensin-Aldosterone System Fall in NaCl, extracellular fluid volume, arterial blood pressure Helps Correct Adrenal Cortex Juxtaglomerular Apparatus Lungs Liver Renin + Converting Enzyme Angiotensin Angiotensin Angiotensin Aldosterone Increased Sodium Reabsorption

DIURETICS • ACE Inhibitors (Angiotensin Converting Enzyme): Cause loss of salt---> water follows • Atrial Naturetic Peptide (ANP) also inhibits sodium reabsorption • Osmotic diuretics: Are not reabsorbed

Glucose and Amino Acids are reabsorbed by secondary active transport • They are actively transported across the apical cell membranes of the epithelial cells • Their active transport depends on the sodium gradient across this membrane • All other steps are passive

GLUCOSE REABSORPTION HAS A TUBULAR MAXIMUM Glucose Reabsorbed mg/min Excreted Filtered Reabsorbed Renal threshold (300mg/100 ml) Plasma Concentration of Glucose

Tubular Secretion • Protons (acid/base balance) • Potassium • Organic ions

Potassium Secretion PUMP: Na/K ATPase Plasma Sodium Lumen Cells Potassium Chloride Water

DUAL CONTROL OF ALDOSTERONE SECRETION Fall in sodium ECF Volume Blood Pressure Increased Plasma Potassium Increased Aldosterone secretion Increased Tubular Potassium Secretion Increased Tubular Sodium Reabsorption Increased Urinary Potassium Secretion Fall in Urinary Sodium Excretion

Reabsorption in Proximal Tubule (Summary) • Glucose and Amino Acids • 67% of Filtered Sodium • Other Electrolytes • 65% of Filtered Water • 50% of Filtered Urea • All Filtered Potassium

Secretion in Proximal Tubule (Summary) • Variable Proton secretion for acid/base regulation • Organic Ion secretion

Reabsorption in Distal Tubule (Summary) • Variable Sodium controlled by Aldosterone • Chloride follows passively • Variable water controlled by vasopressin

Secretion in Distal Tubule (Summary) • Variable Proton for acid/base regulation • Variable Potassium controlled by aldosterone

Collecting Duct (Summary) • Variable water reabsorption controlled by vasopressin • Variable Proton secretion for acid/base balance

REGULATION OF URINE CONCENTRATION • Medullary countercurrent system • Vasopressin

Medullary countercurrent system • Osmotic gradient established by long loops of Henle • Descending limb • Ascending limb

Descending limb • Highly permeable to water • No active sodium transport

Ascending limb • Actively pumps sodium out of tubule to surrounding interstitial fluid • Impermeable to water

COUNTERCURRENT MAKESTHE OSMOTIC GRADIENT From Proximal Tubule To Distal Tubule Cortex Medulla 300 450 600 750 900 1050 1200 1200 100 250 400 550 700 850 1000 1000 300 450 600 750 900 1050 1200 1200 Active Sodium Transport Passive Water Transport Long Loop of Henle

THE OSMOTIC GRADIENT CONCENTRATES THE URINE WHEN VASOPRESSIN (ANTI DIURETIC HORMONE [ADH]) IS PRESENT From Distal Tubule Interstitial Fluid Cortex Medulla 300 400 550 700 850 1000 1100 1200 300 450 600 750 900 1050 1200 1200 Collecting Duct Pores Open Passive Water Flow

WHEN VASOPRESSIN (ANTI DIURETIC HORMONE [ADH]) IS ABSENT A DILUTE URINE IS PRODUCE From Distal Tubule Interstitial Fluid Cortex Medulla 100 100 100 100 100 100 100 100 300 450 600 750 900 1050 1200 1200 Collecting Duct Pores Closed No Water Flow Out of Duct

Renal Failure • Acute: Sudden onset, rapid reduction in urine output - usually reversible • Chronic: Progressive, not reversible • Up to 75% function can be lost before it is noticeable

THE URINARY BLADDER STORES THE URINE • Gravity and peristaltic contractions propel the urine along the ureter • Parasympathetic stimulation contracts the bladder and micturition results if the sphincters (internal and external urethral sphincters) relax • The external sphincter is under voluntary control

Bladder filling reflexively contracts the bladder Internal Sphincter mechanically opens Stretch receptors in bladder send inhibitory impulses to external sphincter Voluntary signals from cortex can override the reflex or allow it to take place Reflex and Voluntary Control of Micturition

Renal Physiology