12-+Renal+and+Acid-Base+Physiology

Renal and Acid-Base Physiology

• Lecturer: Dr. R. Ahangari

• University: University of Central Florida, Orlando

• Textbook: Physiology by Linda Costanzo, Merck and The Merck Manuals

Functions of the Kidney

• Homeostasis: Maintains constancy of extracellular fluid (ECF) volume and osmolality by balancing Na+ and water intake and excretion.

• Electrolyte Regulation: Maintains stable extracellular potassium (K+) concentration and blood/cellular pH by adjusting H+ and bicarbonate (HCO3-) excretion.

• Nutrient Conservation: Conserves nutrients like glucose and amino acids while excreting metabolic waste products such as urea and uric acid.

• Hormonal Production: Produces hormones including angiotensin II, erythropoietin, and prostaglandins.

Kidney Function Processes

• Reabsorption: The majority of ultrafiltrate is reabsorbed back into the blood from the tubule wall.

• Excretion: Remaining fluid not reabsorbed is excreted as urine.

• Secretion: Certain urinary solvents enter the nephron from tubular cells via secretion.

Nephron Structure

• Nephron Composition: Consists of a glomerulus and renal tubule.

  • Glomerulus: Capillary network from an afferent arteriole.

  • Renal Tubule Segments:

    • Proximal tubule

    • Loop of Henle (Thin descending, Thin ascending, Thick ascending limbs)

    • Distal tubule

    • Collecting ducts

Blood Flow Through the Kidney

• Renal Blood Flow: Blood enters the kidney via the renal artery, branching into interlobar, arcuate, and cortical radial arteries.

  • Afferent arterioles supply blood to glomerular capillaries.

  • Efferent arterioles direct blood to peritubular capillaries surrounding nephrons.

  • Vasa Recta: Specialized peritubular capillaries in juxtamedullary nephrons optimize urine concentration.

Body Fluid Distribution

• Total Body Water (TBW): ~60% of body weight; highest in newborns and adult males, lowest in adult females with high adipose.

  • Plasma constitutes ¼ of the ECF, interstitial fluid constitutes ¾ of the ECF.

  • 60-40-20 Rule:

    • TBW = 60% of body weight

    • Intracellular Fluid (ICF) = 40% of body weight

    • Extracellular Fluid (ECF) = 20% of body weight

Glomerular Filtration

• Filtration Mechanism: Glomerular capillaries contain pores allowing water and dissolved solutes (not proteins) to pass to Bowman’s capsule, forming the glomerular filtrate.

• Glomerular Filtration Rate (GFR): Volume of filtrate produced by the kidneys per minute.

• Inulin Clearance: Measurement of GFR using inulin, which is entirely filtered and not reabsorbed or secreted.

  • GFR Formula:

    • GFR = (U inulin * V) / (P inulin)

    • U = urine concentration of inulin (mg/ml)

    • P = plasma concentration of inulin (mg/ml)

Renal Clearance

• Clearance Equation: Measures the volume of plasma cleared of a substance per unit time.

• Formula: C = UV / P

• Units: ml/min or ml/24hr

Reabsorption and Secretion Mechanisms

• Glucose Reabsorption:

  • Na+-glucose cotransport in the proximal tubule reabsorbs glucose.

  • Transport Maximum (Tmax): Reabsorption rate saturates above plasma glucose concentration of 350 mg/dl.

• NaCl Regulation:

  • Na+ is filtered and reabsorbed through the nephron, predominantly in the proximal tubule (67% reabsorption).

  • Glomerulo-tubular balance: Proportional reabsorption of Na+ and water.

Distal Tubule and Collecting Duct

• Late Distal Tubule:

  • Contains principal cells that reabsorb Na+ and H2O, secrete K+.

  • Hormonal Regulation:

    • Aldosterone: Increases Na+ reabsorption and K+ secretion.

    • Antidiuretic Hormone (ADH): Increases H2O permeability.

Potassium (K+) Regulation

• K+ Sources:

  • Shifts between intracellular and extracellular fluid impact plasma K+ levels (hyperkalemia and hypokalemia).

  • Urinary K+ excretion varies widely based on dietary intake, aldosterone, and acid-base status.

  • Renal Regulation: Filtration, reabsorption, and secretion of K+ in nephron.

  • Mechanism: Principal cells in the nephron secrete K+, influenced by factors such as dietary references and hormonal effects.

Acid-Base Balance

• Types of Acids Produced:

  • Volatile Acid: CO2 from aerobic metabolism, forms carbonic acid (H2CO3).

• Buffers: Prevent changes in pH, primarily HCO3- in the extracellular fluid and organic phosphates intracellularly.

  • External Buffers: Main is HCO3-, significant for urinary buffering.

Renal Acid-Base Regulation

• HCO3- Reabsorption: Primarily occurs in the proximal tubule.

  • Key biochemistry: H+ and HCO3- formed from CO2 and H2O; H+ secreted into the lumen, and HCO3- reabsorbed into the blood.

• Regulation: Dependent on factors such as filtered load and PCO2 levels.

Excretion of Fixed H+

• Mechanisms:

  • Titratable Acid: H+ combines with urinary buffers (e.g., phosphate) for excretion.

  • Ammonium (NH4+) Excretion: Substantial means of eliminating fixed H+.

Acid-Base Disorders

• Metabolic Acidosis:

  • Caused by overproduction of acid/loss of base leads to increased arterial H+.

  • Respiratory compensation via hyperventilation.

• Metabolic Alkalosis:

  • Caused by loss of fixed H+ or gain of base leads to decreased arterial H+.

  • Respiratory compensation via hypoventilation.