8.2 Glomerular Filtration and Tubular Reabsorption

Your kidneys are amazing filters, constantly cleaning your blood. They do this through glomerular filtration, where tiny blood vessels sieve out waste and excess water. This process creates about 180 liters of filtrate daily!

But your body can't lose all that fluid. That's where tubular reabsorption comes in. Your kidneys reclaim most of the filtered water and important nutrients, fine-tuning what stays and what becomes urine.

Glomerular Filtration Process

Filtration Steps and Rate

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• Glomerular filtration is the first step in urine formation involves fluid filtering from the glomerular capillaries into Bowman's capsule
• The glomerular filtration rate (GFR) is the volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit of time averages about 125 mL/min or 180 L/day in a healthy adult

Factors Influencing GFR

• Factors influencing GFR include renal blood flow, afferent and efferent arteriolar resistance, plasma oncotic pressure, and Bowman's capsule hydrostatic pressure
• Starling forces, including hydrostatic and oncotic pressure gradients, determine the net filtration pressure across the glomerular capillaries
• Autoregulation maintains a relatively constant GFR despite changes in systemic blood pressure primarily through the myogenic mechanism and tubuloglomerular feedback
• Neural and hormonal factors, such as the sympathetic nervous system, renin-angiotensin-aldosterone system (RAAS), and atrial natriuretic peptide (ANP), can modulate GFR by altering renal blood flow and glomerular pressures

Glomerular Filtration Membrane

Membrane Composition

• The glomerular filtration membrane consists of three layers: the fenestrated endothelium of glomerular capillaries, the basement membrane, and the filtration slits between podocyte foot processes
• The fenestrated endothelium, with its pores, allows for high permeability to water and small solutes (glucose, amino acids, electrolytes)
• The basement membrane, composed of type IV collagen and negatively charged glycoproteins, contributes to the size and charge selectivity of the filtration barrier
• Podocytes, with their interdigitating foot processes and slit diaphragms, further restrict the passage of large molecules (proteins, blood cells) and maintain the integrity of the filtration barrier

Membrane Function and Pathology

• The filtration membrane acts as a selective barrier, allowing the passage of water, small solutes, and low molecular weight substances while restricting the filtration of larger molecules
• The size and charge selectivity of the membrane prevents the loss of essential proteins (albumin) and blood cells into the filtrate
• Disruption of the glomerular filtration membrane, as seen in certain glomerular diseases (nephrotic syndrome, glomerulonephritis), can lead to proteinuria and impaired filtration function

Tubular Reabsorption Substances

Water and Electrolytes

• Water: A large portion of the filtered water is reabsorbed along the nephron and collecting duct, with the majority reabsorbed in the proximal tubule and descending loop of Henle
• Sodium (Na+): Sodium is actively reabsorbed in the proximal tubule, thick ascending limb of the loop of Henle, distal convoluted tubule, and collecting duct
• Chloride (Cl-): Chloride is reabsorbed passively with sodium in the proximal tubule and actively in the thick ascending limb of the loop of Henle
• Potassium (K+): Potassium is reabsorbed in the proximal tubule and thick ascending limb of the loop of Henle, with secretion and reabsorption in the distal nephron segments for fine-tuning of potassium balance
• Calcium (Ca2+) and magnesium (Mg2+): These divalent cations are reabsorbed primarily in the thick ascending limb of the loop of Henle and distal convoluted tubule

Organic Solutes and Acid-Base Regulation

• Glucose: Glucose is completely reabsorbed in the proximal tubule through secondary active transport via sodium-glucose cotransporters (SGLTs)
• Amino acids: Amino acids are reabsorbed in the proximal tubule through secondary active transport mechanisms coupled with sodium gradients
• Bicarbonate (HCO3-): Bicarbonate is reabsorbed in the proximal tubule and collecting duct plays a crucial role in maintaining acid-base balance
• Urea: Urea is partially reabsorbed in the proximal tubule and inner medullary collecting duct contributes to the osmotic gradient in the medulla

Tubular Reabsorption Mechanisms

Transport Mechanisms

• Passive reabsorption: Substances move down their concentration or electrochemical gradients without energy expenditure occurs for water in the proximal tubule and descending loop of Henle
• Active reabsorption: Energy-requiring mechanisms, such as primary active transport (Na+/K+-ATPase) and secondary active transport (Na+-glucose cotransport), are utilized to move substances against their concentration gradients
• Solvent drag: The movement of water through osmosis can carry along dissolved solutes (electrolytes, small molecules), facilitating their reabsorption

Regulation of Reabsorption

• Hormonal regulation: Hormones like aldosterone, antidiuretic hormone (ADH), and parathyroid hormone (PTH) modulate tubular reabsorption of specific substances
  • Aldosterone increases sodium reabsorption and potassium secretion in the distal nephron
  • ADH increases water permeability in the collecting duct, promoting water reabsorption
  • PTH enhances calcium reabsorption in the distal nephron
• Neural regulation: Sympathetic nerve activity can alter renal blood flow and tubular reabsorption, particularly in the proximal tubule
• Local factors: Paracrine signaling molecules, such as prostaglandins and nitric oxide, can influence tubular transport processes (sodium reabsorption, water permeability)
• Peritubular capillary dynamics: The balance between hydrostatic and oncotic pressures in the peritubular capillaries affects the driving force for reabsorption, particularly in the proximal tubule