11.1 Renal anatomy and nephron function

The kidneys are vital organs that filter blood, remove waste, and maintain body balance. They contain millions of tiny structures called nephrons, which are responsible for urine production. Understanding kidney anatomy is crucial for grasping how our bodies regulate fluids and eliminate toxins.

Nephrons have several key parts, each with a specific job. The glomerulus filters blood, while the tubules reabsorb important substances and secrete waste. This process allows the kidneys to fine-tune our body's internal environment, keeping us healthy and functioning properly.

Renal Anatomy

Kidney structure and function

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• Kidneys are bean-shaped organs located in the upper abdominal cavity on either side of the spine
• Primary function is to filter blood, remove waste products (urea, creatinine), excess water, and maintain homeostasis of electrolytes and pH
• Receive blood supply from renal arteries and return filtered blood to circulation via renal veins
• Produce urine which is transported to the bladder for storage and eventual elimination via ureters

Nephron as the functional unit

• Nephrons are the microscopic functional units of the kidney responsible for blood filtration and urine production
• Each kidney contains approximately 1 million nephrons
• Nephrons consist of a renal corpuscle (glomerulus and Bowman's capsule) and a renal tubule (proximal tubule, loop of Henle, distal tubule, and collecting duct)
• Blood enters the nephron through the afferent arteriole, is filtered in the glomerulus, and exits through the efferent arteriole

Renal corpuscle components

• Glomerulus is a network of capillaries surrounded by Bowman's capsule where blood filtration occurs
• Bowman's capsule is a cup-shaped structure that surrounds the glomerulus and collects the filtrate (ultrafiltrate) from the blood
• Filtrate moves from the Bowman's capsule into the proximal tubule for further processing
• Collecting ducts receive urine from multiple nephrons and transport it to the renal pelvis and eventually the ureter (minor and major calyces)

Nephron Segments

Proximal tubule

• Proximal tubule is the first segment of the renal tubule following the Bowman's capsule
• Responsible for reabsorbing approximately 65% of the filtrate, including glucose, amino acids, and electrolytes (sodium, potassium, chloride)
• Actively secretes organic acids, bases, and drugs from the peritubular capillaries into the tubular lumen
• Characterized by a brush border of microvilli to increase surface area for efficient reabsorption and secretion

Loop of Henle

• Loop of Henle is a U-shaped segment of the nephron that extends into the medulla and consists of descending and ascending limbs
• Descending limb is permeable to water but not solutes, allowing for concentration of the filtrate
• Ascending limb is impermeable to water but actively pumps out sodium, chloride, and potassium, contributing to the concentration gradient in the medulla
• Countercurrent multiplication system in the loop of Henle helps to concentrate urine and maintain medullary osmotic gradient (sodium gradient)

Distal tubule and collecting duct

• Distal tubule is the segment following the loop of Henle and is responsible for fine-tuning urine composition
• Regulates sodium, potassium, and calcium reabsorption under the influence of hormones (aldosterone, parathyroid hormone)
• Collecting duct receives urine from multiple nephrons and is the final site of water reabsorption under the control of antidiuretic hormone (ADH or vasopressin)
• Urine concentration is adjusted in the collecting duct before it is transported to the renal pelvis and ureter for storage in the bladder (urine osmolality ranges from 50-1200 mOsm/kg)

Nephron Functions

Filtration in the glomerulus

• Filtration is the first step in urine formation, occurring in the glomerulus
• Blood pressure forces fluid and solutes out of the glomerular capillaries and into the Bowman's capsule
• Glomerular filtration barrier (endothelial cells, basement membrane, podocytes) allows passage of water, glucose, amino acids, and small proteins while restricting cells and large proteins
• Glomerular filtration rate (GFR) is approximately 125 mL/min in a healthy adult, with 180 L of filtrate produced daily

Reabsorption along the tubule

• Reabsorption is the process by which essential nutrients, electrolytes, and water are selectively transported from the tubular lumen back into the peritubular capillaries
• Occurs primarily in the proximal tubule (65% of filtrate reabsorbed) and to a lesser extent in the loop of Henle, distal tubule, and collecting duct
• Glucose and amino acids are completely reabsorbed in the proximal tubule via secondary active transport (sodium-glucose cotransporter SGLT, sodium-amino acid cotransporter)
• Sodium reabsorption drives the reabsorption of other solutes and water through various transport mechanisms (Na+/K+ ATPase, Na+/H+ exchanger, sodium-chloride cotransporter)

Secretion into the tubular lumen

• Secretion is the process by which substances are actively transported from the peritubular capillaries into the tubular lumen
• Occurs primarily in the proximal tubule and distal tubule
• Helps to eliminate waste products, excess potassium, and regulate blood pH
• Organic acids (uric acid), bases (creatinine), and drugs (penicillin) are secreted into the tubular lumen for excretion in the urine
• Hydrogen ions (H+) are secreted into the tubular lumen to maintain blood pH and contribute to the acidification of urine (pH range 4.5-8.0)