Fisiología Renal - Regulación de la reabsorción tubular (IG:@doctor.paiva)

Regulation of Renal Tubular Reabsorption

Introduction to Renal Physiology

• The session introduces the topic of renal tubular reabsorption, emphasizing its importance in nephrology.

• Key hormones involved in this process include aldosterone, vasopressin, and atrial natriuretic peptide, which play crucial roles in glomerular filtration and tubular reabsorption.

Glomerulotubular Balance

• The concept of glomerulotubular balance refers to the intrinsic ability of renal tubules to increase reabsorption in response to higher tubular loads.

• This balance is vital for preventing overload in distal segments of the renal tubule as it correlates with increased glomerular filtration rate (GFR).

Mechanisms Influencing Reabsorption

• The mechanisms behind glomerulotubular balance are not fully understood but are believed to involve changes in physical forces within the tubules and interstitial spaces.

• Reabsorption is defined by a net filtration coefficient that combines permeability, surface area, and various pressures acting on capillaries.

Forces Affecting Reabsorption

• Net reabsorption is influenced by hydrostatic pressure from peritubular capillaries and osmotic pressures from interstitial fluid.

• Hydrostatic pressure opposing reabsorption is approximately 13 mmHg while osmotic pressure from interstitial fluid stands at about 15 mmHg.

Calculating Net Reabsorptive Pressure

• Favorable forces for reabsorption include interstitial hydrostatic pressure (6 mmHg) and oncotic pressure from peritubular capillaries (32 mmHg).

• The net reabsorptive pressure can be calculated by subtracting opposing forces from favorable ones, resulting in a net value of 10 mmHg.

Regulation of Physical Forces

• Factors influencing peritubular capillary hydrostatic pressure include arterial blood pressure and resistance in afferent/efferent arterioles.

• An increase in arterial blood pressure raises hydrostatic pressure but decreases overall reabsorption due to increased opposing force.

Impact of Oncotic Pressure on Reabsorption

• Increased resistance or vasoconstriction leads to reduced flow towards peritubular capillaries, affecting hydrostatic pressures negatively.

• Oncotic pressures depend on systemic plasma protein levels; an increase enhances entry forces into capillaries thus promoting greater reabsorption rates.

Factors Influencing Capillary Reabsorption

Key Determinants of Reabsorption

• The filtration coefficient is crucial; a higher coefficient leads to greater reabsorption, while a lower coefficient results in reduced reabsorption.

• Increased capillary hydrostatic pressure, due to factors like arteriolar dilation or elevated blood pressure, decreases reabsorption.

• Conversely, decreased hydrostatic pressure enhances reabsorption; understanding these dynamics is essential for grasping fluid balance.

Effects of Interstitial Pressure

• Situations that elevate capillary hydrostatic pressure and decrease peritubular osmotic pressure hinder liquid and solute uptake from the interstitium into capillaries.

• An increase in interstitial fluid can dilute proteins, raising interstitial hydrostatic pressure and lowering osmotic pressure, which affects water and solute movement back into tubules through retro-diffusion.

Diuresis Mechanisms

• Diuresis by pressure occurs when arterial pressure rises, leading to increased water excretion. This is linked with sodium excretion as well.

• The relationship between increased arterial pressure and glomerular filtration rate (GFR) is noted; however, changes in GFR are minimal despite slight increases in blood flow.

Hormonal Control of Reabsorption

Role of Aldosterone

• Aldosterone is produced in the adrenal cortex and acts on distal tubule cells to enhance sodium and water reabsorption while promoting potassium secretion.

• Its production is stimulated by elevated plasma potassium levels or angiotensin II release from renin activity.

Angiotensin II Functionality

• Angiotensin II promotes sodium chloride and water reabsorption while facilitating hydrogen ion secretion. It also constricts arterioles to reduce capillary hydrostatic pressure favorably for absorption.

Antidiuretic Hormone (ADH)

Hormonal Regulation of Water and Electrolyte Balance

Antidiuretic Hormone (ADH) and Its Mechanism

• The effect of ADH is primarily water reabsorption in the kidneys, stimulated by increased plasma osmolarity.

• Alcohol inhibits ADH production, leading to increased urination and potential dehydration due to reduced water reabsorption.

• ADH binds to receptors that stimulate aquaporin production, enhancing water reabsorption at the luminal membrane.

Atrial Natriuretic Peptide (ANP)

• ANP is produced by atrial myocardial cells; it reduces sodium and water reabsorption in the distal tubule and collecting duct.

• This peptide is released in response to increased blood volume and pressure, helping counteract conditions like congestive heart failure.

• ANP plays a crucial role in managing blood volume by reducing congestion through its natriuretic effects.

Parathyroid Hormone (PTH)

• PTH acts on the proximal convoluted tubule and thick ascending limb of Henle's loop, decreasing phosphate reabsorption while increasing calcium absorption.

Sympathetic Nervous System Influence

• Activation of the sympathetic nervous system leads to sodium retention via vasoconstriction of renal arteries, affecting glomerular filtration rate.