10.6 Renal Physiology & Osmoregulation

Nephron Microanatomy & Glomerular Filtration

The kidney regulates fluid volume, electrolyte balance, arterial blood pressure, plasma osmolarity, and acid-base homeostasis while excreting metabolic waste products (urea, uric acid, creatinine). The functional unit of the kidney is the Nephron ($\,\sim 1\text{ million per kidney}$).

                                  [GLOMERULUS]
                                       |
                           (Glomerular Filtration)
                                       v
                          [Proximal Convoluted Tubule]
                          - Reabsorbs 65% Na+/H2O, 100% Glucose/AA
                          - Secretes H+, Urea, NH4+, K+ ('HUNK')
                                       |
                                       v
                         [Descending Loop of Henle]
                         - Permeable to H2O / Impermeable to Solutes
                         - Concentrates tubular fluid (up to 1200 mOsm/L)
                                       |
                                       v
                     [Thick Ascending Loop of Henle (TAL)]
                     - Active NKCC2 Cotransporter
                     - Impermeable to H2O (Dilutes tubular fluid)
                     - Establishes Medullary Gradient
                                       |
                                       v
                          [Distal Convoluted Tubule]
                          - Hormone fine-tuning (Aldosterone/PTH)
                                       |
                                       v
                           [Collecting Duct System]
                           - ADH inserts AQP2 water channels
                           - Final urine concentration

The Glomerular Filtration Barrier

Filtration occurs at the Renal Corpuscle, which consists of the Glomerulus (a high-pressure capillary network supplied by the afferent arteriole and drained by the efferent arteriole) enclosed within Bowman's Capsule. The ultrafiltration barrier comprises three distinct layers:

1. Fenestrated Capillary Endothelium: Perforated by $70\text{--}100\text{ nm}$ pores; excludes cellular elements (RBCs, WBCs, platelets).
2. Glomerular Basement Membrane (GBM): Acellular meshwork of type IV collagen and negatively charged heparan sulfate proteoglycans. Filters based on size and charge, repelling negatively charged plasma proteins (e.g., albumin).
3. Podocyte Epithelium (Visceral Layer): Specialized epithelial cells extending interdigitating foot processes (pedicels) around capillaries. Gaps between pedicels form filtration slits bridged by nephrin proteins.

Starling Forces governing Glomerular Filtration Rate (GFR)

Glomerular Filtration Rate (GFR) is determined by hydrostatic and oncotic pressure gradients across the glomerular membrane:

$$\text{GFR} = K_f \cdot \left[ (P_{\text{GC}} - P_{\text{BS}}) - (\pi_{\text{GC}} - \pi_{\text{BS}}) \right]$$

Where:

• $K_f$ is the ultrafiltration coefficient (membrane permeability $\times$ surface area).
• $P_{\text{GC}}$ is Glomerular Capillary Hydrostatic Pressure ($\,\sim 55\text{ mmHg}$, favors filtration).
• $P_{\text{BS}}$ is Bowman's Space Hydrostatic Pressure ($\,\sim 15\text{ mmHg}$, opposes filtration).
• $\pi_{\text{GC}}$ is Glomerular Capillary Oncotic Pressure ($\,\sim 30\text{ mmHg}$, opposes filtration).
• $\pi_{\text{BS}}$ is Bowman's Space Oncotic Pressure ($\,\approx 0\text{ mmHg}$ in healthy protein-free filtrate).

Net Filtration Pressure (NFP) Calculation: $\text{NFP} = 55 - 15 - 30 = +10\text{ mmHg}$.

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Tubular Reabsorption & Secretion along the Nephron

As ultrafiltrate flows through the nephron tubules, its volume and composition are modified by tubular reabsorption (lumen $\rightarrow$ peritubular capillary) and secretion (peritubular capillary $\rightarrow$ lumen).

1. Proximal Convoluted Tubule (PCT)

• Bulk Reabsorption: Reabsorbs $\,\sim 65\%$ of filtered $\text{Na}^+$ and $\text{H}_2\text{O}$, and $100\%$ of filtered glucose and amino acids under normal physiological conditions.
• Mechanism of Glucose Reabsorption: Secondary active transport via SGLT2 (Sodium-Glucose Cotransporter 2) across the apical membrane, driven by the basolateral $\text{Na}^+/\text{K}^+$ ATPase gradient. Glucose then leaves the cell into interstitial fluid via GLUT2 facilitated diffusion.

  AAMC MCAT Trap: When blood glucose exceeds the renal threshold ($\,\sim 180\text{ mg/dL}$), SGLT2 transporters become fully saturated ($T_m$, transport maximum). Excess glucose remains in the tubular fluid, acting as an osmotic diuretic and causing glucosuria and polyuria (classic symptoms of untreated Diabetes Mellitus).

• Secretion: The PCT actively secretes hydrogen ions, urea, ammonia, potassium, and organic anions/drugs.

  Mnemonic: "Dump the HUNK" (Secreted in PCT: H+, Urea, NH4+, K+).

2. Loop of Henle & Countercurrent Multiplication

The Loop of Henle creates the renal hypertonic medullary interstitial gradient ($300\text{ mOsm/L}$ in cortex to $1200\text{ mOsm/L}$ in deep inner medulla):

• Descending Limb: Highly permeable to $\text{H}_2\text{O}$ (expresses constitutive Aquaporin-1 channels) but impermeable to ions/solutes. As fluid descends into the hypertonic medulla, water leaves by osmosis into the interstitium, concentrating tubular fluid up to $1200\text{ mOsm/L}$ at the hairpin bend.
• Thin & Thick Ascending Limb (TAL): Completely impermeable to $\text{H}_2\text{O}$. The Thick Ascending Limb actively transports solutes out of the lumen via the $\text{NKCC2}$ cotransporter ($\text{Na}^+/\text{K}^+/2\text{Cl}^-$). Pumping ions into the medullary interstitium without water dilutes the tubular fluid ($\,\sim 100\text{ mOsm/L}$ leaving TAL) while maintaining the hypertonic medullary gradient.

  Clinical Correlation: Loop diuretics (e.g., furosemide) inhibit the $\text{NKCC2}$ cotransporter in the TAL, disrupting the medullary gradient and causing profound diuresis.

3. Distal Convoluted Tubule (DCT) & Collecting Duct System

• Distal Convoluted Tubule (DCT): Reabsorbs $\text{Na}^+$ and $\text{Cl}^-$ via the thiazide-sensitive $\text{NCC}$ cotransporter. Site of calcium reabsorption regulated by Parathyroid Hormone (PTH).
• Collecting Duct: Final site for hormone-regulated fine-tuning of water, electrolyte, and acid-base balance.

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Endocrine Control of Blood Pressure & Volume

                                  [LOW BLOOD PRESSURE / PERFUSION]
                                                 |
                                                 v
                                   [JG Cells Release RENIN]
                                                 |
                                                 v
                                   [Angiotensinogen -> Ang I]
                                                 |
                                      (ACE in Lung Capillaries)
                                                 |
                                                 v
                                         [ANGIOTENSIN II]
                                                 |
         +---------------------------------------+---------------------------------------+
         |                                                                               |
         v                                                                               v
 [Systemic Vasoconstriction]                                                    [Adrenal Cortex]
  (Increases TPR & MAP)                                                                  |
                                                                                         v
                                                                                   [ALDOSTERONE]
                                                                                         |
                                                                                         v
                                                                             [Principal Cells in DCT/CD]
                                                                             - Up-regulates ENaC & Na+/K+ ATPase
                                                                             - Reabsorbs Na+ & H2O (Isotonic)
                                                                             - Secretes K+ & H+
                                                                             - Increases Volume & MAP
                                                                               (No change in Osmolarity)


                                  [HIGH PLASMA OSMOLARITY]
                                                 |
                                                 v
                                [Posterior Pituitary Secretes ADH]
                                                 |
                                                 v
                                    [Principal Cells in CD]
                                    - V2 Receptors / cAMP Signaling
                                    - Inserts Aquaporin-2 (AQP2) Channels
                                                 |
                                                 v
                                    [Water Reabsorption (Hypotonic)]
                                    - Increases Blood Volume & MAP
                                    - DECREASES Plasma Osmolarity

1. Renin-Angiotensin-Aldosterone System (RAAS)

• Trigger: Decreased renal perfusion pressure (detected by renal afferent arteriolar baroreceptors in Juxtaglomerular JG cells), decreased $\text{Na}^+$ delivery to the Macula Densa, or sympathetic $\beta_1$ stimulation.
• Cascade: JG cells release Renin into circulation. Renin cleaves plasma Angiotensinogen (synthesized by the liver) to produce Angiotensin I (Ang I). Angiotensin-Converting Enzyme (ACE), primarily located in pulmonary capillary endothelium, cleaves Ang I into Angiotensin II (Ang II).
• Angiotensin II Actions: Potent systemic arteriolar vasoconstrictor (elevates TPR), selectively constricts efferent arterioles to maintain GFR, stimulates PCT $\text{Na}^+/\text{H}^+$ exchange, and stimulates the adrenal cortex (zona glomerulosa) to secrete Aldosterone.

2. Aldosterone

• Mechanism: Hydrophobic steroid hormone that diffuses into principal cells of the DCT and Collecting Duct, binding intracellular mineralocorticoid receptors.
• Physiological Action: Increases gene expression and membrane insertion of apical ENaC (Epithelial Sodium Channels) and basolateral $\text{Na}^+/\text{K}^+$ ATPase pumps. Reabsorbs $\text{Na}^+$ from tubular fluid into plasma, with water following passively by osmosis.
• Net Effect: Increases blood volume and blood pressure without altering plasma osmolarity (isotonic volume expansion). Promotes renal excretion of $\text{K}^+$ and $\text{H}^+$.

3. Antidiuretic Hormone (ADH / Vasopressin)

• Mechanism: Nonapeptide hormone produced in hypothalamic nuclei and released from the posterior pituitary in response to elevated plasma osmolarity or low blood volume.
• Physiological Action: Binds $V_2$ G-protein-coupled receptors ($G_s$) on principal cells of the Collecting Duct. Activates adenylate cyclase to elevate cAMP, stimulating PKA to induce exocytosis of vesicles containing Aquaporin-2 (AQP2) water channels into the apical membrane.
• Net Effect: Increases renal water reabsorption, increases blood volume/pressure, and DECREASES plasma osmolarity (hypotonic fluid retention, highly concentrated urine).

4. Atrial Natriuretic Peptide (ANP)

• Mechanism: Peptide hormone released by cardiac atrial myocytes in response to atrial stretch (high blood volume).
• Physiological Action: Antagonizes RAAS and ADH. Dilates afferent arterioles and constricts efferent arterioles (increasing GFR), inhibits Renin and Aldosterone secretion, and blocks $\text{Na}^+$ reabsorption in collecting ducts.
• Net Effect: Promotes natriuresis ($\text{Na}^+$ excretion) and diuresis (water loss), reducing blood volume and blood pressure.

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Renal Acid-Base Regulation

The kidneys maintain arterial blood pH within the tight physiological range of $7.35\text{--}7.45$ through two primary mechanisms:

1. $\text{H}^+$ Secretion: $\alpha$-intercalated cells in the distal nephron secrete excess $\text{H}^+$ into the tubular lumen via primary active $\text{H}^+$-ATPase pumps and $\text{H}^+/\text{K}^+$-ATPase antiporters. Secreted $\text{H}^+$ is buffered in urine by titratable acids (phosphate, $\text{HPO}_4^{2-}$) and ammonium ($\text{NH}_4^+$).
2. $\text{HCO}_3^-$ Reabsorption & Generation: $\,\sim 80\%$ of filtered bicarbonate is reabsorbed in the PCT. For every $\text{H}^+$ secreted into the lumen, carbonic anhydrase facilitates the generation of one new $\text{HCO}_3^-$ ion that enters peritubular capillary blood across the basolateral membrane.