8.1 Kidney Structure and Nephron Function

The kidneys are vital organs that filter blood and produce urine. Their complex structure, from the outer cortex to the inner medulla, houses millions of nephrons. These microscopic units are the workhorses of the urinary system, performing filtration, reabsorption, and secretion.

Nephrons consist of a renal corpuscle and tubule system. They filter blood in the glomerulus, reabsorb essential substances in the proximal tubule, and concentrate urine in the loop of Henle. The distal tubule and collecting duct fine-tune urine composition based on the body's needs.

Kidney Anatomy and Function

Gross Anatomy

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• The kidney is a bean-shaped organ located in the retroperitoneal space of the abdominal cavity, with one on each side of the vertebral column
• The renal hilum is the concave medial surface of the kidney where the renal artery, renal vein, and ureter enter and exit the kidney
• The renal capsule is a thin, fibrous connective tissue layer that surrounds the kidney and provides protection and support

Internal Structures

• The renal cortex is the outer region of the kidney that contains the renal corpuscles and convoluted tubules of the nephrons
• The renal medulla is the inner region of the kidney that consists of renal pyramids, which contain the loops of Henle and collecting ducts
• The renal pelvis is a funnel-shaped structure that collects urine from the collecting ducts and leads to the ureter
• The renal artery supplies oxygenated blood to the kidney (afferent arterioles), while the renal vein drains deoxygenated blood from the kidney (efferent arterioles)

Nephron Structure and Function

Nephron Components

• The nephron is the basic structural and functional unit of the kidney, responsible for the formation of urine through filtration, reabsorption, and secretion
• Each nephron consists of a renal corpuscle (glomerulus and Bowman's capsule) and a renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct)
• The renal corpuscle filters blood, creating an ultrafiltrate that enters the renal tubule
• The proximal convoluted tubule reabsorbs essential nutrients (glucose, amino acids), ions (sodium, chloride), and water from the ultrafiltrate back into the bloodstream

Urine Concentration and Modification

• The loop of Henle creates a concentration gradient in the medulla, which is essential for the production of concentrated urine
• The descending limb of the loop of Henle is permeable to water, while the ascending limb is impermeable to water but actively transports ions (sodium, chloride) out of the tubule
• The distal convoluted tubule and collecting duct further modify the ultrafiltrate by reabsorbing or secreting specific substances (potassium, hydrogen ions, bicarbonate) based on the body's needs
• Hormones such as antidiuretic hormone (ADH) and aldosterone regulate water and electrolyte balance in the distal nephron segments

Filtration in the Renal Corpuscle

Glomerular Filtration Barrier

• The renal corpuscle consists of a cluster of capillaries called the glomerulus and a surrounding Bowman's capsule
• The glomerulus receives blood from an afferent arteriole and is drained by an efferent arteriole, creating a high-pressure system that promotes filtration
• The glomerular filtration barrier consists of the fenestrated endothelium of the glomerular capillaries, the basement membrane, and the filtration slits between the foot processes of podocytes
• The filtration barrier allows the passage of water, small molecules (urea, creatinine), and ions while preventing the filtration of larger molecules such as proteins (albumin) and blood cells (erythrocytes)

Filtration Forces

• The hydrostatic pressure in the glomerulus and the osmotic pressure of the blood are the main driving forces for filtration
• Glomerular hydrostatic pressure (60 mmHg) favors filtration, while the Bowman's capsule hydrostatic pressure (18 mmHg) opposes filtration
• The blood colloid osmotic pressure (28 mmHg) opposes filtration, as it tends to retain fluid within the capillaries
• The net filtration pressure (NFP) is the sum of these forces, typically around 10-14 mmHg, which drives the formation of the glomerular ultrafiltrate
• The ultrafiltrate formed in the Bowman's capsule has a composition similar to that of plasma, but without proteins and cellular components

Cortical vs Juxtamedullary Nephrons

Cortical Nephrons

• Cortical nephrons are located entirely within the renal cortex and constitute about 85% of the total nephron population
• The loops of Henle in cortical nephrons are shorter compared to those in juxtamedullary nephrons, typically extending only into the outer medulla
• Cortical nephrons play a more significant role in the regulation of electrolyte and acid-base balance, as they have a higher rate of tubular reabsorption and secretion
• The peritubular capillaries surrounding cortical nephrons have a lower osmolarity compared to the vasa recta, which allows for more efficient reabsorption of solutes and water

Juxtamedullary Nephrons

• Juxtamedullary nephrons are located near the boundary between the cortex and medulla, with their loops of Henle extending deep into the medulla (inner medulla)
• The longer loops of Henle in juxtamedullary nephrons allow for the creation of a more concentrated medullary interstitium, which is essential for the production of concentrated urine (urine osmolarity up to 1200 mOsm/L)
• Juxtamedullary nephrons have a greater capacity for urine concentration due to their longer loops of Henle and their proximity to the vasa recta, which maintain the medullary concentration gradient through countercurrent exchange
• The vasa recta are specialized capillaries that run parallel to the loops of Henle, helping to preserve the osmotic gradient in the medulla by removing excess solutes and water
• Juxtamedullary nephrons are more important for urine concentration and water conservation, especially during periods of dehydration or reduced fluid intake