Kidneys

1. Overview

The kidneys are a pair of vital, bean-shaped organs responsible for filtering blood, maintaining fluid and electrolyte balance, and excreting waste products in the form of urine. In addition to their excretory functions, the kidneys also play crucial roles in blood pressure regulation, red blood cell production, and calcium metabolism. Each kidney contains millions of nephrons—the functional filtration units—making them highly efficient and essential for homeostasis.

2. Location

The kidneys are located in the retroperitoneal space of the posterior abdominal wall, on either side of the vertebral column. They lie between the levels of the T12 to L3 vertebrae, with the right kidney slightly lower than the left due to the presence of the liver. Key spatial relationships include:

• Posteriorly: The diaphragm, psoas major, and quadratus lumborum muscles.

• Anteriorly (right): Liver, duodenum, and ascending colon.

• Anteriorly (left): Stomach, spleen, pancreas, and descending colon.

3. Structure

Each kidney is approximately 10–12 cm in length, 5–7 cm in width, and 150 grams in weight. Structural components include:

External Features:

• Renal capsule: A tough, fibrous outer layer.

• Renal hilum: Medial indentation for entry/exit of vessels, nerves, and ureter.

Internal Anatomy:

• Cortex: The outer region, containing glomeruli and convoluted tubules.

• Medulla: The inner region, organized into renal pyramids containing loops of Henle and collecting ducts.

• Renal columns: Extensions of cortical tissue between pyramids.

• Renal pelvis: Funnel-shaped chamber receiving urine from major calyces and leading to the ureter.

Vascular Supply:

• Renal artery → segmental → interlobar → arcuate → interlobular arteries

• Afferent arterioles → glomeruli → efferent arterioles → peritubular capillaries or vasa recta

• Renal vein: Drains into the inferior vena cava

4. Function

The kidneys perform multiple vital functions:

• Filtration: Remove waste products (urea, creatinine) from the bloodstream.

• Reabsorption: Recover water, electrolytes, and nutrients.

• Secretion: Eliminate excess ions and metabolic byproducts.

• Excretion: Produce and transport urine to the bladder via the ureters.

5. Physiological Role(s)

The kidneys are involved in numerous physiological processes beyond waste elimination:

• Fluid and Electrolyte Balance: Regulate plasma sodium, potassium, calcium, phosphate, and water content.

• Acid-Base Homeostasis: Reabsorb bicarbonate and secrete hydrogen ions to maintain pH.

• Blood Pressure Regulation: Via the renin-angiotensin-aldosterone system (RAAS) and sodium handling.

• Erythropoiesis: Secrete erythropoietin, a hormone stimulating red blood cell production in bone marrow.

• Vitamin D Activation: Convert calcidiol to calcitriol, the active form of vitamin D, regulating calcium and phosphate metabolism.

• Detoxification: Metabolize and excrete drugs and toxins.

6. Clinical Significance

Acute Kidney Injury (AKI)

A sudden decline in kidney function due to ischemia, toxins, or obstruction. Presents with reduced urine output, rising creatinine, and electrolyte imbalances. It is often reversible if treated promptly.

Chronic Kidney Disease (CKD)

A progressive, irreversible loss of nephrons, often due to diabetes, hypertension, or glomerulonephritis. CKD is staged based on glomerular filtration rate (GFR), and advanced stages require dialysis or kidney transplantation.

Hypertension

Both a cause and consequence of renal disease. Renal artery stenosis or dysregulated RAAS activity can lead to secondary hypertension. CKD exacerbates systemic blood pressure through sodium and fluid retention.

Urinary Tract Infections (UTIs) & Pyelonephritis

Bacterial infections ascending from the bladder can involve the kidneys, causing flank pain, fever, and systemic illness. Pyelonephritis requires antibiotic therapy to prevent scarring and long-term damage.

Kidney Stones (Nephrolithiasis)

Hard deposits of calcium, oxalate, or uric acid form in the renal pelvis or calyces, causing intense pain, hematuria, and possible obstruction.

Polycystic Kidney Disease (PKD)

A genetic disorder characterized by numerous fluid-filled cysts that enlarge the kidneys and impair function over time, often leading to end-stage renal disease.

Renal Cancer

Renal cell carcinoma is the most common kidney cancer in adults. Symptoms may include hematuria, flank mass, or paraneoplastic syndromes. Imaging and biopsy are essential for diagnosis.

Kidney Transplantation

Reserved for patients with end-stage renal disease. Requires immunosuppression and careful matching of HLA antigens. Transplantation can restore nearly normal renal function and quality of life.