Kidney

The kidney is a vital organ in the human body, playing a crucial role in maintaining overall health. Shaped like a bean, each kidney is typically about the size of a fist. The kidneys are a part of the urinary system and are composed of various structures, including the cortex, medulla, and nephrons.

Location

The kidneys are located in the back of the abdominal cavity, on either side of the spine. They lie just below the rib cage, with the right kidney sitting slightly lower than the left to accommodate the liver.

Structure and Anatomy

The kidney is a complex organ with a unique structure, made up of various internal and external components that work together to support its vital functions. Below is a detailed description of its anatomy.

External Structure

• Shape and Size: The kidney has a bean-like shape. Each kidney is about 10 to 12 centimeters long, 5 to 7 centimeters wide, and 3 to 4 centimeters thick, weighing around 150 grams in an average adult.
• Capsule: Surrounding the kidney is a tough, fibrous renal capsule, which protects it from physical damage and infections. This capsule is covered by a layer of fat called the perinephric fat, which offers additional protection.
• Hilum: The medial concave border of the kidney features the renal hilum, which serves as the entry and exit point for the renal artery, renal vein, ureter, and nerves. It acts as the gateway to the internal structure of the kidney.

Internal Structure

• Cortex: The outer region of the kidney is known as the renal cortex. It appears granular and reddish-brown due to the presence of numerous glomeruli, blood vessels, and parts of the nephrons. The cortex extends into the inner kidney regions through renal columns, which separate the renal pyramids.
• Medulla: Below the cortex is the renal medulla, which consists of cone-shaped tissue structures called renal pyramids. Each kidney contains 8 to 18 pyramids. These pyramids have a striated appearance due to the presence of parallel bundles of nephrons’ loops (loop of Henle) and collecting ducts.
• Renal Pyramids: The renal pyramids are essential components of the medulla. Their base faces the cortex, while the apex, called the renal papilla, points toward the inner kidney. These pyramids drain urine into the minor calyces.

Collecting System

• Minor and Major Calyces: Urine produced in the nephrons is first collected in the minor calyces, small cup-like structures. Several minor calyces converge to form larger major calyces. There are usually two to three major calyces in each kidney.
• Renal Pelvis: The major calyces merge to form a funnel-shaped structure known as the renal pelvis, located in the hilum. The renal pelvis collects the urine and channels it into the ureter.

Nephron

• Bowman’s Capsule and Glomerulus: Each kidney contains around one million nephrons, which are the functional units of the kidney. At the beginning of the nephron is the Bowman’s capsule, which surrounds a network of capillaries called the glomerulus. Together, they form the renal corpuscle, where blood filtration occurs.
• Proximal Convoluted Tubule (PCT): From the Bowman’s capsule, the filtered fluid enters the proximal convoluted tubule, a twisted segment of the nephron located in the cortex.
• Loop of Henle: The loop of Henle dips into the medulla and consists of a descending limb and an ascending limb. It plays a key role in the concentration of urine.
• Distal Convoluted Tubule (DCT): After the loop of Henle, the fluid enters the distal convoluted tubule. This part of the nephron is located in the cortex and drains into the collecting ducts.
• Collecting Ducts: The collecting ducts run through the renal pyramids of the medulla, merging with others to deliver urine into the renal papillae.

Vascular Supply

• Renal Artery: The kidney receives blood through the renal artery, a direct branch of the abdominal aorta. Upon entering the kidney, the renal artery divides into smaller segmental arteries.
• Arcuate Arteries and Interlobular Arteries: These arteries branch further into the arcuate arteries, which arch over the base of the renal pyramids, and interlobular arteries, which extend into the cortex.
• Glomerular Capillaries: Within the nephron, blood flows into the glomerular capillaries for filtration. The filtered blood exits the glomerulus via the efferent arteriole and enters the peritubular capillaries that surround the nephron tubules.
• Renal Vein: After passing through the nephron and peritubular capillaries, deoxygenated blood exits the kidney through the renal vein, which drains into the inferior vena cava.

Function

The kidney plays a central role in maintaining the body’s homeostasis by regulating fluid balance, filtering waste, and ensuring the proper function of various metabolic processes. Below is a detailed exploration of its main functions:

Filtration of Blood

One of the primary roles of the kidney is to filter the blood and remove waste products. This process begins in the glomerulus, a network of capillaries located within the Bowman’s capsule in each nephron.

• Glomerular Filtration: Blood enters the glomerulus through the afferent arteriole and is filtered across the thin walls of the capillaries. Large molecules like proteins and blood cells remain in the bloodstream, while smaller molecules such as water, electrolytes, glucose, and waste products pass into the nephron as filtrate.
• Selective Reabsorption: After filtration, the kidney reabsorbs essential nutrients and molecules from the filtrate back into the bloodstream. This takes place primarily in the proximal convoluted tubule (PCT), where glucose, amino acids, and ions like sodium and chloride are reabsorbed into the peritubular capillaries.

Regulation of Fluid and Electrolyte Balance

The kidney is responsible for maintaining the balance of water and electrolytes in the body. This is vital for the proper functioning of cells and tissues.

• Water Balance: Through the processes of reabsorption and secretion, the kidney adjusts the amount of water that is retained or excreted. In the loop of Henle and collecting ducts, water is reabsorbed or allowed to pass into urine based on the body’s hydration levels. The hormone antidiuretic hormone (ADH) regulates this process by controlling the permeability of the collecting ducts to water.
• Electrolyte Balance: The kidney also regulates the levels of key electrolytes, such as sodium, potassium, calcium, and bicarbonate. The distal convoluted tubule (DCT) and collecting ducts adjust the reabsorption or excretion of these electrolytes to maintain their balance in the blood. Aldosterone, a hormone released by the adrenal glands, influences the reabsorption of sodium and the excretion of potassium in this region.

Excretion of Waste Products

The kidney eliminates various waste products from the body, ensuring the removal of harmful substances that accumulate from metabolic processes.

• Nitrogenous Wastes: Urea, a byproduct of protein metabolism, is filtered from the blood and excreted in the urine. Creatinine, a waste product from muscle metabolism, and uric acid, a breakdown product of purines, are also eliminated through the kidney.
• Toxins and Drugs: The kidney filters out other waste products, including excess drugs, toxins, and metabolic byproducts, ensuring they do not accumulate in the body. These are excreted into the urine during filtration and tubular secretion.

Acid-Base Balance

The kidney plays a crucial role in maintaining the body’s pH by regulating the concentration of hydrogen ions (H⁺) and bicarbonate (HCO₃⁻) in the blood.

• Secretion of Hydrogen Ions: In the distal convoluted tubule and collecting ducts, the kidney can secrete hydrogen ions into the urine, reducing the acidity of the blood. This process is vital in situations of acidosis, where blood pH drops below normal levels.
• Reabsorption of Bicarbonate: The kidney also helps maintain an alkaline environment by reabsorbing bicarbonate from the filtrate in the proximal convoluted tubule, which acts as a buffer to neutralize excess acids in the blood.

Regulation of Blood Pressure

The kidney helps regulate blood pressure by controlling blood volume and releasing hormones that affect vascular resistance.

• Renin-Angiotensin-Aldosterone System (RAAS): In response to low blood pressure or reduced blood flow to the kidney, specialized cells called the juxtaglomerular cells release the enzyme renin. Renin converts angiotensinogen into angiotensin I, which is further converted into angiotensin II, a potent vasoconstrictor. Angiotensin II narrows blood vessels and stimulates the release of aldosterone, increasing sodium and water reabsorption to raise blood volume and pressure.
• Blood Volume Regulation: By controlling the amount of water reabsorbed into the bloodstream, the kidney directly influences blood volume and, consequently, blood pressure. When blood pressure is too high, the kidney reduces water reabsorption and increases urine output to lower blood volume.

Hormone Production

The kidney is involved in the production and release of several hormones essential for bodily functions.

• Erythropoietin (EPO): The kidney produces erythropoietin, a hormone that stimulates the production of red blood cells in the bone marrow. When oxygen levels in the blood are low, the kidney releases EPO to ensure adequate oxygen-carrying capacity.
• Calcitriol (Active Vitamin D): The kidney converts inactive vitamin D into its active form, calcitriol, which is crucial for calcium absorption from the intestines and maintaining proper calcium levels in the blood.

Regulation of Red Blood Cell Production

Through the release of erythropoietin, the kidney monitors and regulates the oxygen-carrying capacity of the blood. When blood oxygen levels are low, such as in the case of anemia or chronic low oxygen levels, the kidney increases erythropoietin secretion, stimulating the bone marrow to produce more red blood cells.

Clinical Significance

The kidney’s proper functioning is vital for maintaining overall health. Its ability to filter waste, regulate blood pressure, balance electrolytes, and maintain fluid balance makes it central to the body’s homeostasis. When the kidneys are damaged or fail, it can lead to serious health conditions such as chronic kidney disease (CKD), acute kidney injury (AKI), kidney stones, and hypertension.

• Chronic Kidney Disease (CKD): A gradual loss of kidney function, often caused by diabetes, hypertension, or other underlying conditions. CKD can lead to the buildup of waste products in the blood and, if untreated, may progress to end-stage renal disease (ESRD), requiring dialysis or a kidney transplant.
• Acute Kidney Injury (AKI): A sudden reduction in kidney function, often reversible if diagnosed and treated promptly. Causes can include dehydration, infections, or medications.
• Kidney Stones: Crystals formed from minerals and salts in the urine can cause severe pain, block the urinary tract, and lead to infections or kidney damage.
• Hypertension: High blood pressure can result from or cause kidney damage. The kidney’s role in regulating blood pressure makes it vulnerable to complications from hypertension, potentially leading to further damage in a cycle of deterioration.