Renal Physiology: Structure, Function, and Regulation of the Kidneys

Introduction to Renal Physiology

Overview of Renal Functions

The kidneys are essential organs responsible for regulating the composition and volume of blood plasma, maintaining homeostasis, and excreting metabolic waste products. They also play a role in hormone secretion and glucose synthesis.

• Regulation of Blood Volume and Pressure: By controlling water and sodium balance, kidneys maintain blood volume and pressure.

• Electrolyte Balance: Kidneys regulate the concentration of inorganic ions (e.g., Na+, K+, Cl-, HCO3-, PO43-).

• Acid-Base Balance: Kidneys maintain pH by excreting H+ and reabsorbing HCO3-.

• Excretion of Wastes: Urea, uric acid, creatinine, bilirubin, and foreign chemicals are eliminated.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources during fasting.

• Hormone Secretion: Erythropoietin (EPO), renin, and 1,25-dihydroxyvitamin D are produced by the kidneys.

Body Fluid Compartments and Osmosis

Distribution of Body Fluids

• About 60% of body weight is water: 40% intracellular, 20% extracellular (interstitial fluid and plasma).

• Kidneys regulate extracellular fluid (ECF), including plasma, interstitial fluid, and cerebrospinal fluid.

Ion Composition

• Extracellular: High in Na+, Cl-, HCO3-

• Intracellular: High in K+, PO43-

Osmosis and Osmolarity

• Osmosis: Net diffusion of water across a selectively permeable membrane from high to low water concentration (low to high osmolarity).

• Osmolarity: Number of solute particles per liter of solution (expressed in osmoles/L).

• Aquaporins: Specialized water channels facilitating rapid water movement across cell membranes.

• Osmotic Pressure: Pressure required to prevent osmosis.

Tonicity

• Isotonic: Equal osmolarity inside and outside the cell; no change in cell volume.

• Hypertonic: Higher osmolarity outside; cell shrinks as water leaves.

• Hypotonic: Lower osmolarity outside; cell swells as water enters.

Renal Anatomy and the Nephron

Gross Anatomy of the Urinary System

• Kidneys: Retroperitoneal organs with cortex (outer) and medulla (inner) regions.

• Ureters: Drain urine from kidneys to bladder.

• Bladder: Stores urine; innervated by autonomic nervous system.

• Urethra: Conducts urine out of the body (micturition).

Nephron Structure

• Renal Corpuscle: Consists of the glomerulus (capillary tuft) and Bowman's capsule.

• Renal Tubule: Includes proximal convoluted tubule, Loop of Henle (descending and ascending limbs), distal convoluted tubule, and collecting duct.

• Each kidney contains about 1 million nephrons.

Types of Nephrons

• Cortical Nephrons (85%): Located mainly in the cortex; short loops of Henle.

• Juxtamedullary Nephrons (15%): Located near the medulla; long loops of Henle; crucial for urine concentration.

Renal Blood Supply

• Blood enters via the renal artery, branches into afferent arterioles, then glomerular capillaries.

• Blood exits glomerulus via efferent arteriole, then peritubular capillaries (cortical nephrons) or vasa recta (juxtamedullary nephrons).

• Vasa recta maintain medullary osmotic gradient.

Basic Renal Processes

Filtration, Reabsorption, Secretion, Excretion

• Filtration: Movement of plasma from glomerular capillaries into Bowman's space.

• Reabsorption: Movement of substances from tubule back into blood.

• Secretion: Movement of substances from blood into tubule.

• Excretion: Removal of substances in urine.

Filtration Barrier

• Three layers: fenestrated endothelium, basement membrane, podocyte filtration slits.

• Permits passage of water, electrolytes, glucose, amino acids; restricts proteins and blood cells.

Starling Forces and Net Filtration Pressure

• Forces involved:

  • Glomerular capillary hydrostatic pressure (PGC): favors filtration (~60 mmHg)

  • Bowman's space hydrostatic pressure (PBS): opposes filtration (~15 mmHg)

  • Osmotic force due to plasma proteins (πGC): opposes filtration (~29 mmHg)

• Net Filtration Pressure:

  • $Net\ Filtration\ Pressure = P_{GC} - P_{BS} - \pi_{GC}$

  • Example: $60 - 15 - 29 = 16\ mmHg$

Glomerular Filtration Rate (GFR)

• Volume of fluid filtered per unit time (~125 mL/min or 180 L/day).

• GFR is regulated by blood pressure, arteriolar resistance, and surface area.

• Autoregulation maintains GFR over a wide range of blood pressures (80–180 mmHg).

Regulation of GFR

Arteriolar Resistance

• Constriction of afferent arteriole: Decreases GFR.

• Constriction of efferent arteriole: Increases GFR.

• Dilation of afferent arteriole: Increases GFR.

• Dilation of efferent arteriole: Decreases GFR.

Juxtaglomerular Apparatus (JGA)

• Composed of macula densa (distal tubule), juxtaglomerular (JG) cells (afferent arteriole), and mesangial cells.

• Macula densa senses NaCl load and flow; JG cells secrete renin; mesangial cells regulate filtration surface area.

• Tubuloglomerular feedback: Increased flow past macula densa leads to afferent arteriole constriction, reducing GFR.

Renal Handling of Substances

Filtered Load

• Amount of a substance filtered per day: $Filtered\ Load = GFR \times [Substance]_{plasma}$

• If excreted < filtered load: reabsorption occurred; if excreted > filtered load: secretion occurred.

Examples of Renal Handling

Substance	Filtered	Reabsorbed	Secreted	Excreted
Inulin	Yes	No	No	Yes (all filtered)
Creatinine	Yes	No	Slightly	Yes (slightly overestimates GFR)
Glucose	Yes	Completely	No	No (in healthy)
Electrolytes	Yes	Partially	No	Yes
Organic acids/bases	Yes	No	Yes	Yes (all in urine)

Reabsorption Mechanisms

• Sodium: Actively reabsorbed via Na+/K+ ATPase on basolateral membrane; passive entry on apical side.

• Glucose: Reabsorbed in proximal tubule by sodium-glucose cotransporter (SGLT) on apical side (secondary active transport), and GLUT transporter on basolateral side (facilitated diffusion).

• Transport Maximum (Tm): Maximum rate of reabsorption due to transporter saturation; excess glucose appears in urine (glucosuria) when Tm is exceeded.

• Urea: Reabsorbed by diffusion, dependent on water reabsorption.

Tubular Secretion

• Active process; substances (e.g., H+, K+, penicillin) secreted from peritubular capillaries into tubule.

Renal Clearance

• Volume of plasma from which a substance is completely removed per unit time.

• $Clearance = \frac{U_S \times V}{P_S}$ Where US = urine concentration, V = urine flow rate, PS = plasma concentration.

• Inulin clearance = GFR; glucose clearance = 0 (in healthy); creatinine clearance ≈ GFR.

Water and Sodium Regulation

Water Reabsorption

• Majority reabsorbed in proximal tubule (67%) and descending limb of Loop of Henle (15%) via aquaporins (AQP1).

• Collecting duct water reabsorption is regulated by antidiuretic hormone (ADH/vasopressin) via AQP2 channels.

• ADH increases water reabsorption, reducing urine volume (antidiuresis).

Countercurrent Mechanism

• Loop of Henle creates a hyperosmotic medullary interstitium via countercurrent multiplication.

• Descending limb: permeable to water, impermeable to solutes.

• Ascending limb: impermeable to water, actively reabsorbs NaCl.

• Vasa recta maintain the osmotic gradient via countercurrent exchange.

• Urea recycling contributes to medullary hyperosmolarity.

Hormonal Regulation

• ADH (Vasopressin): Released from posterior pituitary in response to increased plasma osmolarity or decreased blood volume; increases water reabsorption in collecting ducts.

• Aldosterone: Steroid hormone from adrenal cortex; increases Na+ reabsorption and K+ secretion in distal tubule and collecting duct; regulated by renin-angiotensin system and plasma K+ levels.

• Atrial Natriuretic Peptide (ANP): Released from atria in response to increased blood volume; inhibits Na+ reabsorption and aldosterone secretion, increases GFR.

Acid-Base Balance

pH Regulation

• Normal arterial plasma pH: 7.35–7.45.

• Acidosis: pH < 7.35; Alkalosis: pH > 7.45.

• Kidneys regulate H+ by excreting acid and reabsorbing/generating HCO3-.

Buffer Systems

• Bicarbonate (HCO3-): Main extracellular buffer.

• Phosphate and Proteins: Main intracellular buffers (e.g., hemoglobin).

Renal Handling of Acid-Base

• For each H+ excreted, one HCO3- is added to plasma.

• In acidosis: kidneys secrete more H+ and generate new HCO3-.

• In alkalosis: kidneys excrete more HCO3-.

• Mechanisms include secretion of H+ (bound to phosphate or as NH4+ from glutamine metabolism).

Disorders of Acid-Base Balance

Disorder	Cause	Compensation
Respiratory Acidosis	Hypoventilation, ↑CO2	Kidneys ↑ H+ excretion, ↑ HCO3- reabsorption
Respiratory Alkalosis	Hyperventilation, ↓CO2	Kidneys ↑ HCO3- excretion
Metabolic Acidosis	Diarrhea, diabetes, exercise	↑ Ventilation, ↑ H+ secretion
Metabolic Alkalosis	Vomiting	↓ Ventilation, ↑ HCO3- excretion

Summary Table: Key Renal Processes

Process	Location	Main Function
Filtration	Glomerulus	Plasma filtered into Bowman's space
Reabsorption	Proximal tubule, Loop of Henle, Distal tubule, Collecting duct	Recovery of water, ions, nutrients
Secretion	Proximal and distal tubules, Collecting duct	Removal of additional wastes (H+, K+)
Excretion	Collecting duct → Ureter → Bladder → Urethra	Elimination of urine

Key Equations

• Net Filtration Pressure: $Net\ Filtration\ Pressure = P_{GC} - P_{BS} - \pi_{GC}$

• Filtered Load: $Filtered\ Load = GFR \times [Substance]_{plasma}$

• Renal Clearance: $Clearance = \frac{U_S \times V}{P_S}$

Additional info:

• All values and mechanisms are based on healthy adult physiology; pathological states (e.g., diabetes mellitus, diabetes insipidus, proteinuria) illustrate deviations from normal function.

• Understanding the interplay between filtration, reabsorption, secretion, and excretion is essential for interpreting clinical tests of renal function.