Structure and Function of Kidneys and Nephrons

Introduction

Key Concepts

Anatomy of the Kidneys

External Structure: The external surface of the kidney features a smooth cortex and a renal pelvis, the latter serving as a funnel for urine collection. The hilum is an indentation on the medial side of the kidney where blood vessels, nerves, and ureters enter and exit.

Internal Structure: Internally, the kidney is divided into three regions: the cortex, medulla, and renal pelvis. The cortex contains the glomeruli and convoluted tubules of nephrons, while the medulla consists of renal pyramids with collecting ducts. The renal pelvis collects urine from the nephrons and channels it into the ureter.

Nephron Structure

Renal Corpuscle: The renal corpuscle consists of the glomerulus and Bowman's capsule. The glomerulus is a network of capillaries where blood filtration begins, driven by blood pressure.

Renal Tubule: The renal tubule is divided into three main sections: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. These sections facilitate reabsorption and secretion processes essential for maintaining electrolyte balance and pH homeostasis.

Physiological Function of Kidneys

• Filtration: Blood enters the kidneys through the renal artery, which branches into arterioles supplying the glomeruli. Filtration occurs as water and small solutes pass through the glomerular membrane into Bowman's capsule, forming the filtrate.
• Reabsorption: As the filtrate moves through the renal tubule, essential substances such as glucose, amino acids, and ions are reabsorbed into the bloodstream through active and passive transport mechanisms.
• Secretion: Additional waste products and excess ions are secreted into the filtrate from the blood, aiding in the removal of substances like hydrogen ions and potassium.
• Excretion: The final product, urine, is collected in the collecting ducts, transported to the renal pelvis, and then funneled into the ureters for elimination from the body.
• Regulation of Blood Pressure: The kidneys regulate blood pressure through the renin-angiotensin-aldosterone system (RAAS), which adjusts blood vessel constriction and sodium retention.
• Electrolyte Balance: By selectively reabsorbing or secreting ions, the kidneys maintain optimal levels of electrolytes such as sodium, potassium, calcium, and phosphate.
• Acid-Base Balance: The kidneys regulate the body's pH by excreting hydrogen ions and reabsorbing bicarbonate ions, thus maintaining acid-base homeostasis.

Detailed Nephron Function

Proximal Convoluted Tubule (PCT): The PCT reabsorbs approximately 65% of the filtrate, including water, glucose, amino acids, and ions like sodium and chloride. This reabsorption occurs through both active and passive transport mechanisms, facilitated by transport proteins and carrier molecules.

Loop of Henle: The loop of Henle creates a concentration gradient in the renal medulla, enabling the kidneys to produce concentrated urine. The descending limb is permeable to water but not to ions, leading to water reabsorption. In contrast, the ascending limb is impermeable to water and actively transports sodium and chloride ions out of the filtrate, contributing to the hyperosmotic medullary environment.

Distal Convoluted Tubule (DCT): The DCT fine-tunes the filtrate by reabsorbing sodium and calcium ions under hormonal control (e.g., aldosterone and parathyroid hormone). It also secretes potassium and hydrogen ions, playing a crucial role in electrolyte and pH balance.

Collecting Duct: The collecting duct receives filtrate from multiple nephrons and reabsorbs additional water under the influence of antidiuretic hormone (ADH). This section also participates in the final adjustment of urine concentration and composition before it passes into the ducts that lead to the renal pelvis.

Regulatory Mechanisms

Renin-Angiotensin-Aldosterone System (RAAS): When blood pressure drops, the kidneys release renin, an enzyme that catalyzes the conversion of angiotensinogen to angiotensin I. Angiotensin-converting enzyme (ACE) then transforms angiotensin I into angiotensin II, a potent vasoconstrictor. Angiotensin II also stimulates the release of aldosterone from the adrenal cortex, promoting sodium and water reabsorption, thereby increasing blood volume and pressure.

Antidiuretic Hormone (ADH): ADH, released by the posterior pituitary gland in response to high plasma osmolality, enhances water reabsorption in the collecting ducts. This regulation conserves water and concentrates urine.

Erythropoietin (EPO) Production: In response to hypoxia, the kidneys secrete EPO, a hormone that stimulates erythropoiesis (red blood cell production) in the bone marrow, thereby enhancing the oxygen-carrying capacity of the blood.

Homeostatic Functions

• Fluid Balance: By adjusting the volume and concentration of urine, the kidneys regulate the body’s fluid balance, ensuring adequate hydration and maintaining blood volume.
• Electrolyte Balance: The precise reabsorption and secretion of ions maintain optimal concentrations of electrolytes essential for nerve function, muscle contraction, and overall cellular function.
• pH Regulation: Through the excretion of hydrogen ions and reabsorption of bicarbonate, the kidneys maintain the pH within the narrow range necessary for enzymatic activities and metabolic processes.
• Detoxification: By filtering blood and excreting metabolic waste products such as urea, creatinine, and uric acid, the kidneys prevent the accumulation of toxins that could disrupt physiological functions.

Pathophysiology: Kidney Diseases

• Chronic Kidney Disease (CKD): Characterized by a gradual loss of kidney function over time, often resulting from diabetes or hypertension. CKD impairs the kidneys' ability to filter blood effectively, leading to the accumulation of waste products.
• Acute Kidney Injury (AKI): A rapid decline in kidney function due to factors like severe dehydration, trauma, or exposure to nephrotoxic substances. AKI can be reversible with prompt treatment.
• Polycystic Kidney Disease (PKD): A genetic disorder where fluid-filled cysts develop in the kidneys, enlarging them and impairing function over time.
• Glomerulonephritis: Inflammation of the glomeruli, often caused by autoimmune diseases or infections, leading to impaired filtration and potential kidney failure.

Nephron Adaptations and Regeneration

• Adaptive Reabsorption: Nephrons can adjust the rate of reabsorption based on the body's needs, such as increasing glucose reabsorption during times of high intake.
• Renal Reserve: The kidneys have a significant functional reserve, allowing them to compensate for damage or loss of nephron function to maintain overall kidney performance.
• Regeneration: While mature nephrons have limited regenerative capacity, research is ongoing into nephron regeneration through stem cell therapy and tissue engineering to address kidney damage.

Comparison Table

Summary and Key Takeaways