Fisiología Renal - Filtración glomerular y flujo sanguíneo renal (IG:@doctor.paiva)

Filtration Glomerular and Renal Blood Flow

Introduction to Renal Physiology

• The class introduces renal physiology, focusing on glomerular filtration and renal blood flow.

• Eduardo Paiva is the instructor, outlining the topics to be covered, including generalities of glomerular capillary membrane and determinants of filtration.

Anatomy of Renal Blood Vessels

• Discussion on the renal artery branching into interlobar arteries and arcuate arteries leading to afferent arterioles.

• A description of how these arterioles form glomerular capillaries where filtration occurs before returning through efferent arterioles.

Structure of Nephrons

• Overview of nephron structure: includes Bowman's capsule, proximal convoluted tubule, thin and thick segments, distal convoluted tubule, cortical collecting duct, and medullary collecting duct.

• Zooming in on the glomerulus reveals its three-layered capillary membrane: endothelium with fenestrations, a basement membrane, and an outer epithelial layer (podocytes).

Filtration Mechanism

• The unique structure of the glomerular basement membrane allows for significant water and small solute filtration while repelling negatively charged plasma proteins due to their similar charge.

• Size and electrical charge influence filtration capacity; larger molecules with negative charges filter less easily compared to positively charged ones.

Filtration Rates

• Glomerular filtration rate (GFR) is approximately 20% of renal plasma flow; about 125 ml/min or 180 liters/day are filtered from plasma.

• Of this filtered volume, around 124 ml is reabsorbed back into circulation while only 1 ml is excreted as urine.

Determinants of Glomerular Filtration

• GFR depends on two main factors: net filtration pressure (NFP), which combines hydrostatic pressures within capillaries against oncotic pressures from proteins.

• NFP calculation involves a hydrostatic pressure in glomeruli at 60 mmHg versus opposing pressures from oncotic forces (32 mmHg from proteins in plasma).

Summary of Pressures Affecting Filtration

Understanding Glomerular Filtration

Net Filtration Pressure and Glomerular Coefficient

• The net filtration pressure is measured at 10 mmHg, indicating a force that promotes fluid outflow from the capillaries.

• The glomerular coefficient is defined as the product of permeability and surface area of glomerular capillaries, estimated by dividing glomerular filtration rate (GFR) by net filtration pressure.

• The GFR is approximately 125 mL/min, significantly higher than other body capillaries which have a filtration coefficient of 0.01; kidneys filter 400 times more efficiently.

Factors Affecting Glomerular Filtration Rate

• An increase in the glomerular coefficient enhances GFR, while diseases like hypertension and diabetic nephropathy can reduce it by thickening membrane walls and decreasing functional capillary numbers.

• Increased hydrostatic pressure in Bowman's capsule reduces GFR; conversely, lower pressure increases it. Obstructions such as kidney stones can elevate this pressure.

Plasma Filtration Dynamics

• As blood passes from afferent to efferent arterioles, plasma filters but proteins do not; this leads to an increased protein concentration in efferent arterioles by up to 20%.

• The oncotic pressure in the afferent arteriole averages around 28 mmHg, rising to about 36 mmHg in the efferent arteriole due to plasma filtration dynamics.

Renal Blood Flow and Oncotic Pressure

• Lower renal blood flow results in a higher fraction of filtered plasma, increasing oncotic pressure which subsequently decreases GFR.

• Changes in hydrostatic pressures are crucial for physiological regulation of GFR; higher pressures lead to increased filtration rates.

Regulation Mechanisms of Filtration

• Hydrostatic pressure depends on renal artery pressure and resistance within afferent/efferent arterioles; vasoconstriction raises resistance leading to reduced GFR.

• Mild constriction of the efferent arteriole can raise hydrostatic pressure and increase GFR initially but excessive constriction will decrease overall flow and thus reduce filtration.

Understanding Glomerular Filtration Dynamics

The Role of Hydrostatic Pressure in Glomerular Filtration

• Increased hydrostatic pressure enhances glomerular filtration, particularly with mild constriction of the afferent arteriole.

• Severe constriction of the afferent arteriole can decrease blood flow, leading to increased osmotic pressure that may surpass hydrostatic pressure, thus reducing glomerular filtration.

• Factors such as diabetic and hypertensive nephropathies can reduce glomerular filtration by increasing Bowman’s capsule pressure due to urinary tract obstructions.

Regulation of Renal Blood Flow and Filtration

• Renal blood flow regulation is influenced by factors like angiotensin II levels and sympathetic nervous system activity, which affect arteriolar resistance.

• Hormones like endothelin also play a role in vasoconstriction, impacting glomerular filtration rates negatively.

Key Metrics in Renal Hemodynamics

• Normal renal arterial pressure is approximately 100 mmHg; capillary pressures within the glomerulus are around 60 mmHg.

• In a typical 70 kg male, renal blood flow averages 1,100 ml/min, constituting about 22% of cardiac output.

Sodium Reabsorption and Oxygen Consumption

• Renal oxygen consumption correlates directly with sodium reabsorption; higher sodium reabsorption leads to greater oxygen usage due to energy demands.

• A complete cessation of glomerular filtration results in a significant drop (to one-quarter normal levels) in renal oxygen consumption due to reduced sodium reabsorption.

Determinants of Renal Blood Flow

• The difference between hydrostatic pressures in the renal artery and vein divided by total vascular resistance determines renal blood flow.

• Most vascular resistance occurs at interlobar arteries and arterioles; increases here lead to decreased renal blood flow through vasoconstriction mechanisms.

Autoregulation Mechanism

Control de la Filtración y Flujo Sanguíneo en los Riñones

Estructura de los Riñones

• Los riñones se dividen en dos partes principales: corteza y médula, lo cual fue discutido previamente en la clase de anatomía fisiológica.

Flujo Sanguíneo Renal

• El flujo sanguíneo hacia la médula renal es proporcionado por las masas rectas y los vasos retos.

• Solo el 12% del flujo sanguíneo renal llega a la médula, lo que indica un flujo relativamente bajo.