Renal System: Anatomy and Function

• The renal system filters blood, reabsorbs necessary substances, and excretes waste as urine to maintain homeostasis.

Urinary System Overview

• The urinary system includes paired, bean-shaped organs.
• It is located along the back body wall below the diaphragm and vertebral column.
• The right kidney sits lower than the left to accommodate the liver on the right side.

Kidney Anatomy

• Renal arteries supply about 1/4 (1200 ml) of the heart's output to the kidneys per minute.
• Arterial and venous blood flow follow similar routes in the kidney.
• Nerves reach the kidney through sympathetic fibers from the renal plexus.

Kidney Functions

• Kidneys remove toxins, metabolic wastes, and excess ions from the blood.
• They regulate blood volume, chemical composition, and pH.
• The kidneys perform gluconeogenesis during prolonged fasting.

Endocrine Functions of the Kidney

• Kidneys secrete renin, which regulates blood pressure and kidney function.
• Erythropoietin is produced, which regulates RBC production.
• The kidneys activate vitamin D, critical for calcium homeostasis.

Nephron Structure

• Nephrons are structural and functional units that produce urine in the kidneys.
• There are about 1 million nephrons per kidney.
• Each nephron includes a renal corpuscle for filtration and a renal tubule for reabsorption and secretion.

Types of Nephrons

• Cortical nephrons comprise 80-85% of nephrons, located in the outer cortex with loops of Henle that penetrate only a small way into the medulla.
• Ascending limbs in cortical nephrons' loops of Henle have only a thick segment.
• Blood supply for cortical nephrons comes from peritubular capillaries originating from efferent arterioles.
• Juxtamedullary nephrons make up 15-20% of nephrons, located deep in the cortex near the medulla, featuring long loops of Henle extending into the medulla's deepest regions.
• Ascending limbs in juxtamedullary nephrons' loops of Henle feature both thin and think segments.
• Blood supply for juxtamedullary nephrons is from the vasa recta, branching off peritubular capillaries before forming peritubular venules.
• Distal convoluted tubules from multiple nephrons merge into a single collecting duct.
• Collecting ducts combine into hundreds of papillary ducts, draining into minor and major calyces, the renal pelvis, and ultimately the ureters.

Glomerular Filtration

• Glomerular filtration rate is 105-125 mL/min, producing fluid isotonic to blood.
• Filtered substances comprise water and blood solutes (except proteins), ions, glucose, amino acids, creatinine, and uric acid.

Filtration, Reabsorption, and Secretion

• 3 L of water in plasma results in 180 L filtered, 178-179 L reabsorbed and 1-2 L secreted.
• 200 g of protein in plasma results in 2 g filtered, 1.9 g reabsorbed and 0.1 g secreted.
• 3 g of glucose in plasma results in 162 g filtered, 162 g reabsorbed and 0 g secreted.
• 1 g of urea in plasma results in 40 g filtered, 24 g reabsorbed and 30 g secreted.
• 0.03 g of creatinine in plasma results in 1.6 g filtered, 0 g reabsorbed and 1.6 g secreted.

Glomerular Filtration Regulation

• Hormones regulate GFR: Angiotensin II and atrial natriuretic peptide (ANP).
• Angiotensin II is a vasoconstrictor, reducing GFR.
• ANP released due to increased blood pressure relaxes the glomerulus, increasing surface area for filtration and increasing GFR.

Reabsorption Pathways

• Paracellular reabsorption of water and solutes occurs between tubule cells.
• Transcellular reabsorption of water and solutes happens through the cells.

Sodium Movement

• The sodium-potassium pump ejects Na+ from renal tubule cells at the basolateral membrane, driving reabsorption.
• Absent Na+-K+ pumps on the apical membrane ensure one-way Na+ reabsorption.
• The pumps consume about 6% of the body's resting ATP.

Reabsorption in the Proximal Convoluted Tubule

• Na+-glucose symporters transports two Na+ ions and one glucose molecule from tubular fluid into the cells.
• Symporter entry occurs at the apical membrane, while glucose exits at the basolateral side by facilitated diffusion; phosphate, sulfate, amino acids, and lactic acid are also reabsorbed similarly.
• Sodium reabsorption promotes hydrogen secretion through secondary active transport, where sodium enters PCT cells via Na+-H+ antiporters, secreting H into urine.

Bicarbonate Reabsorption and Osmotic Gradients

• Carbonic anhydrase catalyzes CO2 and H2O into carbonic acid (H2CO3), which dissociates into H+ and HCO3-.
• Bicarbonate exits via facilitated diffusion into the blood with Na+.
• Solute reabsorption causes water osmosis into peritubular capillaries.

Reabsorption in Second Half of PCT

• Solutes (Cl-, K+, Ca2+, Mg2+, urea) passively reabsorbed due to electrochemical gradients.
• Gradients drive paracellular and transcellular reabsorption.
• Aquaporin-1 in PCT cell membranes enhances water permeability, aiding osmosis.

Reabsorption in the Thick Ascending Limb

• Symporters in the thick ascending limb cells move one Na+, one K+, and two Cl- from the tubular fluid.
• Sodium gets actively transported into the interstitial fluid and then diffuses into the vasa recta.
• Chloride exits through leakage channels in the basolateral membrane.
• Recycled potassium maintains the system's efficiency

Tubular Fluid and Cation Reabsorption

• K+ leakage into tubular fluid creates charge imbalance, making interstitial fluid more negative.
• This attracts Na+, K+, Ca2+, and Mg2+ for reabsorption via the paracellular route.

Reabsorption in the Distal Tubule and Collecting Duct

• Filtrate reabsorption is 90-95% complete by the end of the distal convoluted tubule.
• Principal and intercalated cells are present in the terminal distal convoluted tubule and collecting duct.
• Sodium enters principal cells through the apical membrane's leakage channels, then is actively transported out across the basolateral membrane.
• Sodium passively diffuses into peritubular capillaries.
• K+ pumps in Sodium-potassium continuously bring K+ into the cells and K+ leaves through channels, and K+ diffuses into the tubular fluid, which becomes a main source of K+ excreted in urine

Hormonal Control of Reabsorption

• Five hormones affect sodium, chloride, calcium, water reabsorption, and potassium secretion.
• The hormones (angiotensin II, antidiuretic hormone, aldosterone, atrial natriuretic peptide, and parathyroid hormone) maintain systemic blood flow, blood pressure, and renal blood.
• Antidiuretic hormone (ADH) is released by the posterior pituitary in response to low blood flow in this part of the brain.
• ADH affects facultative water reabsorption by increasing water permeability in principal cells.
• In the absence of ADH, the apical membranes of principal cells are almost impermeable to water.
• PTH stimulates cells in early distal convoluted tubules to increases reabsorb Ca2+ into the blood and inhibits phosphate reabsorption in proximal convoluted tubules.