Human kidney
Anatomy and Structure
The human kidney is a vital organ in the urinary system, responsible for filtering blood, removing waste, and balancing electrolytes. Each individual typically has two kidneys, located retroperitoneally on either side of the spine, just below the rib cage. The kidneys are bean-shaped, with a convex lateral surface and a concave medial surface where the renal hilum is located. This hilum serves as the entry and exit point for the renal artery, renal vein, and ureter.
Gross Anatomy
Each kidney measures approximately 11-14 cm in length, 6 cm in width, and 3 cm in thickness, weighing between 120-150 grams. The outermost layer is the renal capsule, a tough fibrous layer that provides protection. Beneath the capsule lies the renal cortex, a granular tissue that houses the glomeruli. The inner region, the renal medulla, contains the renal pyramids, which are cone-shaped tissues that converge into the renal pelvis. The renal pelvis is a funnel-shaped structure that collects urine and channels it into the ureter.
Microscopic Anatomy
The functional unit of the kidney is the nephron, with each kidney containing approximately one million nephrons. Nephrons are composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of the glomerulus, a network of capillaries, and Bowman's capsule, which encases the glomerulus. The renal tubule is divided into several segments: the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct. These structures work in concert to filter blood, reabsorb essential nutrients, and secrete waste products.
Physiology
The primary function of the kidneys is to maintain homeostasis by regulating the volume and composition of body fluids. This involves several key processes: filtration, reabsorption, secretion, and excretion.
Filtration
Filtration occurs in the glomerulus, where blood pressure forces water and solutes through the glomerular membrane into Bowman's capsule, forming the glomerular filtrate. This filtrate contains waste products, electrolytes, and small molecules, while larger proteins and cells remain in the bloodstream.
Reabsorption and Secretion
As the filtrate passes through the renal tubule, essential substances such as glucose, amino acids, and ions are reabsorbed into the bloodstream. This process is highly selective and involves active and passive transport mechanisms. Conversely, secretion involves the active transport of additional waste products and excess ions from the blood into the tubule, ensuring their removal from the body.
Excretion
The final urine, composed of waste products, excess ions, and water, is collected in the renal pelvis and transported to the bladder via the ureter. The kidneys excrete approximately 1-2 liters of urine daily, depending on fluid intake and other factors.
Regulation of Kidney Function
The kidneys are regulated by several mechanisms to ensure proper function and homeostasis.
Hormonal Regulation
The renin-angiotensin-aldosterone system (RAAS) plays a crucial role in regulating blood pressure and fluid balance. Renin, an enzyme produced by the kidneys, initiates a cascade that results in the production of angiotensin II, a potent vasoconstrictor. Angiotensin II stimulates the release of aldosterone from the adrenal cortex, promoting sodium and water reabsorption in the kidneys.
Another hormone, antidiuretic hormone (ADH), is released by the posterior pituitary gland in response to increased plasma osmolality. ADH enhances water reabsorption in the collecting ducts, concentrating the urine and conserving body water.
Autoregulation
The kidneys possess intrinsic mechanisms to maintain a constant renal blood flow and glomerular filtration rate (GFR) despite fluctuations in systemic blood pressure. This autoregulation is achieved through the myogenic response and tubuloglomerular feedback. The myogenic response involves the contraction or relaxation of afferent arterioles in response to changes in blood pressure, while tubuloglomerular feedback adjusts GFR based on the sodium chloride concentration in the distal tubule.
Pathophysiology
Kidney diseases can significantly impact the organ's ability to perform its functions, leading to systemic consequences.
Acute Kidney Injury
Acute kidney injury (AKI) is characterized by a rapid decline in kidney function, resulting in the accumulation of waste products and fluid imbalance. Causes of AKI include prerenal factors (e.g., hypovolemia), intrinsic renal damage (e.g., acute tubular necrosis), and postrenal obstruction (e.g., kidney stones).
Chronic Kidney Disease
Chronic kidney disease (CKD) is a progressive condition characterized by the gradual loss of kidney function over time. Common causes include diabetes mellitus, hypertension, and glomerulonephritis. CKD can lead to end-stage renal disease (ESRD), necessitating dialysis or kidney transplantation.
Glomerular Diseases
Glomerular diseases, such as glomerulonephritis and nephrotic syndrome, involve damage to the glomeruli, affecting filtration and leading to proteinuria, hematuria, and reduced GFR. These conditions can result from autoimmune disorders, infections, or genetic factors.
Clinical Significance
The kidneys play a pivotal role in maintaining overall health, and their dysfunction can have widespread effects.
Diagnostic Evaluation
Evaluation of kidney function involves several tests, including serum creatinine, blood urea nitrogen (BUN), and estimated glomerular filtration rate (eGFR). Urinalysis can provide additional information on urine composition and the presence of abnormalities such as proteinuria or hematuria.
Treatment and Management
Management of kidney diseases depends on the underlying cause and severity. Treatment options may include lifestyle modifications, pharmacotherapy, and renal replacement therapies such as dialysis or transplantation. Early detection and intervention are crucial in preventing the progression of kidney disease.