The kidney is a major organ in the urinary system, responsible for excusing waste products and maintaining the electrolytes in the body.

A two-branched rod-shaped structure with blue and red bean-shaped structures on either side

Illustration of the kidney with blood vessels

The urinary system consists of various organs such as kidneys, the bladder, urethra, renal pelvis, and ureters. These organs function together to excrete metabolic wastes, regulate the balance of water and electrolytes, maintain the pH balance and regulate hormonal secretions. Each kidney contains nephrons that function as the filtration system. Nephrons filter the blood by removing the waste products stored in the bladder and then excreted through urine. The structure of the kidneys is different across various animals, including primates and non-primates. Since the kidneys are one of the life-sustaining organs, any abnormalities or damage to the kidneys will lead to serious health risks if left untreated and can be fatal. In humans, the kidneys can be affected by various means, such as diabetes, high blood pressure, increased calcium intake, congenital diseases, increased intake of medications, etc. Detecting kidney issues will provide an opportunity for early treatment, thus avoiding the risk of kidney failure.

An Overview of the Kidney

The kidneys are located behind the intestine below the ribcage and on either side of the spine, protected by muscle, fat, and ribs. A pair of kidneys are bean-shaped organs as big as a fist, about 4 to 5 inches long. Typically, male kidneys (125–175g) are larger than females’ (115–155g). At the top of the kidney, the adrenal gland is present and responsible for the secretion of the erythropoietin hormone. The kidney is enclosed by a fibrous capsule made up of dense and irregular connective tissue that aids in maintaining the kidney shape and protecting it. The capsule is covered by a renal fat pad, a layer of adipose tissue responsible for absorbing any shock.

Brown bean-shaped structure with red, blue, and yellow-colored extensions

Detailed diagrammatic representation of kidney

The kidney consists of three regions: the cortex (outer renal), the medulla (inner renal), and the renal pelvis. The cortex is granular due to the nephrons (the kidney’s functional unit). Around 15% of nephrons have long loops of Henle extending into the inner renal medulla and are called juxtamedullary nephrons. In contrast, nephrons with a short Henle loop that does not go below the cortex are called cortical nephrons. The medulla contains several pyramid-shaped tissue masses called renal pyramids that have spaces between them called renal columns, through which the blood vessels pass. The tip of the renal pyramids is the papilla pointing towards the renal pelvis. The renal pelvis is connected to the ureter, which is present outside the kidney. Two or three extensions from the renal pelvis called major calyces are further branched into minor calyces that connect the renal pyramids to the ureter. The ureter is connected to the bladder, which stores the urine. The heart’s renal artery and renal vein are branched into tiny capillaries inside the kidney, present between the cortex and medulla.

Kidney functions 

Physiological functions of the kidney are associated with mechanisms of the respiratory and cardiovascular systems. The kidneys’ primary function is to excrete metabolic waste products such as uric acid and urea from the body. These wastes and excess H+ ions (hydrogen ions) are filtered out of the blood in addition to water, thus excreted from the body as urine. The removal of H+ ions will make the blood less acidic. The kidneys also assist in regulating the water balance by controlling the blood pressure and blood volume. The renal system is responsible for altering water retention, thus slowly maintaining the blood pressure and blood volume in the normal range (blood pressure homeostasis). 

 Furthermore, blood volume is controlled by maintaining salt balances such as calcium, potassium, and sodium, the electrolytes in the blood. The pH of the blood is balanced by controlling the H+ ions and bicarbonate ions (HCO3) in urine, hence making the blood less acidic or basic. The normal pH range in the blood is 7.35-7.45. Adrenal glands present on the top of both the kidneys are responsible for secreting hormones such as erythropoietin (EPO), which causes an increase in RBC production based on the decreased oxygen level in the tissues.

Kidney Mechanism 

A nephron is a functional unit of the kidney responsible for filtering the blood and metabolizing nutrients. Each kidney contains about one million nephrons with their internal structures. The blood entering the body enters the renal corpuscle (Malpighian body), which includes the glomerulus, where the protein from the blood is absorbed. The remaining fluid is passed through Bowman’s capsule into the renal tubules. This tubule consists of tubes beginning after the Bowman’s capsule finally reaches the collecting duct. The renal tubules consist of a proximal convoluted tubule that reabsorbs sodium, water, and glucose into the blood. The loop of Henle reabsorbs sodium, potassium, and chloride back into the blood. Finally, the distal convoluted tubule reabsorbs more sodium into the blood while taking potassium and acid (H+ ions). At the end of this tubule, the fluid is high in urea, the end product of protein metabolism released through urine.

Blue and red coiled structure around a yellow coiled tube

Detailed diagrammatic representation of a nephron

The glomerulus and the convoluted tubules are found in the renal cortex, part of the kidney. In comparison, the loop of Henle and the renal pyramids are found in the renal medulla region of the kidney. Nephrons are located in the renal pyramids. After the reabsorption of nutrients moves away from the kidney into the collecting duct, the fluid finally exits the nephron into the renal pelvis. The renal pelvis transports this fluid into the urinary bladder via the ureter. The beginning of the renal pelvis contains calyces that collect fluid containing waste and move it to the bladder. Hilum, present at the inner edge of the kidney, has a renal artery (which brings oxygenated blood from the heart to the kidney) and a renal vein (which takes filtered blood from the kidney to the heart).

Diseases and Treatments

Numerous diseases occur in the urinary system and the kidneys. Some of the common conditions associated with the kidneys are discussed below.

Red-colored two bean-shaped structures behind a body

Illustration of the location of the kidney

  • Kidney stones

High mineral or salt deposits such as calcium phosphate, calcium oxalate, or uric acid are kidney stones’ primary causes. Also known as renal stones, they are solid mineral or salt deposits formed either inside the kidneys, ureter, or the bladder. Significant symptoms of kidney stones include groin or lower abdominal pain when urinating, bloody urine, fever with chills (in case of infection), foul-smelling urine, and frequent urination. Small kidney stones pass on their own through urination and pain medication, while bigger stones are treated with shock wave lithotripsy, ureteroscopy, and cystoscopy.

  • Chronic kidney disease

The kidneys are damaged slowly over a long time and are often divided based on the severity of the damage sustained by the kidney. One of the most common causes of chronic kidney disease is diabetes and hypertension (high blood pressure). During the early stages, the symptoms do not appear. However, the onset of symptoms is often seen when the kidney suffers significant damage. Common symptoms include swollen feet, legs, or face; chest pain; weight loss; fatigue; dry, itchy skin; muscle cramps; etc. Based on the cause, this disease can be treated. However, chronic kidney disease can often not be cured.

  • Kidney cysts

The kidney cyst is a sac-like structure that contains fluid. The cause of kidney cysts is unknown; however, they may be caused by kidney failure that leads to fluid accumulation in small areas, thus forming a cyst. This disease is different from polycystic kidney disease, a genetic condition. Usually, the symptoms are not often observed for kidney cysts until the cyst ruptures, causing bleeding or increasing size, thereby pressing against the nerves. This will lead to severe pain in the back or abdomen, increased urination, bloody urine, and fever. The treatment includes rupturing and draining the fluid in the cyst or surgically removing the cysts.

  • Glomerulonephritis

Glomerulonephritis is a disease caused by the inflammation of glomeruli (tiny filters) that can be acute or chronic. This can be caused by lupus or diabetes. Prolonged and severe glomerular inflammation can injure the kidneys. Depending on whether it is acute or chronic glomerulonephritis, the symptoms can vary. A urine test can detect any indication of defects associated with the kidneys. Some symptoms include hypertension, decreased urination, fatigue, muscle cramps, edema (retention of fluid causing swollen feet, hands, abdomen, and face), proteinuria (bubble urine), hematuria (pink urine), etc. At the onset of these symptoms, seek an appointment with a doctor to get it checked. Reduced protein and salt-based diets, water pills (diuretics) to decrease the swelling, and dialysis to purify the blood are the few treatments for this disease.

  • Acute kidney failure 

Acute kidney failure is caused by any injury sustained to the kidney, such as burns, severe dehydration, uncontrolled bleeding, or excessive heavy medications. This disease is also known as acute kidney injury. The kidney sustains damage within a short time that affects other organs such as the heart, brain, and lungs. Some individuals are asymptomatic, while the common signs include swollen hands and feet, puffy eyes, decreased urine volume, fatigue, chest pain, etc. This disease is mainly treated with medications to control salt levels (like potassium, calcium, etc.) and dialysis to remove toxins from the blood.

Asha Rachel Thomas


Tags: Kidney - The Urinary System

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