The kidneys are a pair of bean-shaped organs, each roughly the size of a human fist, located just beneath the rib cage on either side of the spine. These organs maintain the body’s internal stability, a process known as homeostasis. They continuously filter blood, removing waste and excess water while retaining necessary substances. This ensures a consistent internal environment for optimal physiological function. Kidney physiology examines how these organs manage fluid balance, regulate blood pressure, and maintain the blood’s chemical composition.
The Filtration Process
Kidneys purify blood using millions of microscopic filtering units called nephrons. Each kidney has about one million nephrons, which process blood and form urine. These units clean the blood through three steps: glomerular filtration, tubular reabsorption, and tubular secretion, removing waste while conserving vital substances.
Glomerular Filtration
The initial step, glomerular filtration, occurs within the renal corpuscle, a structure composed of a network of tiny blood vessels called the glomerulus, encased by a cup-shaped Bowman’s capsule. Blood enters the glomerulus under significant pressure, forcing water, small dissolved solutes, and waste products from the capillaries into Bowman’s capsule. This filtration is largely passive, driven by blood pressure within the glomerular capillaries. Blood cells and large proteins are retained in the bloodstream, as they are too large to pass through the filtration membrane.
Approximately one-fifth of the blood plasma entering the glomerulus is filtered daily, resulting in over 150 liters of fluid entering the renal tubules. This filtrate contains waste products and many needed substances, such as glucose, amino acids, and various ions. The remaining blood, along with retained proteins and blood cells, continues into a second set of capillaries surrounding the tubules.
Tubular Reabsorption
Following filtration, the filtrate flows into the renal tubule, where the body reclaims most beneficial substances through tubular reabsorption. Without it, the body would rapidly lose vast amounts of water, electrolytes, and nutrients. Cells lining the renal tubules transport essential substances, including nearly 99% of the filtered water, glucose, amino acids, vitamins, and ions like sodium, potassium, calcium, and bicarbonate, back into the bloodstream.
Reabsorption occurs along different segments of the renal tubule, with the proximal convoluted tubule being a primary site. Nearly all filtered glucose and amino acids are reabsorbed here, often against a concentration gradient, requiring cellular energy.
Tubular Secretion
The final step in urine formation is tubular secretion, which involves transporting additional waste, excess ions, and certain drugs from the blood into the filtrate within the renal tubules. This process removes substances not initially filtered or needing more efficient clearance. Substances commonly secreted include urea, creatinine, hydrogen ions, potassium ions, and some metabolic waste products.
Tubular secretion fine-tunes the blood’s chemical balance, particularly regulating acid-base balance by excreting excess hydrogen ions. This mechanism, often requiring energy, complements filtration and reabsorption. The remaining fluid and waste then proceed to the collecting ducts, ultimately forming urine.
Regulating Blood Pressure and Volume
The kidneys regulate blood pressure and fluid volume throughout the body. They continuously monitor changes in blood pressure and blood sodium concentration, detecting systemic fluid status. When blood pressure drops or sodium levels decrease, a hormonal cascade known as the Renin-Angiotensin-Aldosterone System (RAAS) is activated.
The process begins with specialized juxtaglomerular cells in the kidneys releasing renin into the bloodstream. This occurs in response to reduced blood flow or pressure within renal arteries. Renin then acts upon angiotensinogen, a protein produced by the liver, converting it into angiotensin I.
Angiotensin I, relatively inactive, encounters Angiotensin-Converting Enzyme (ACE) in the lungs, transforming it into angiotensin II. Angiotensin II raises blood pressure and volume. It constricts small arteries, increasing systemic vascular resistance and blood pressure.
Angiotensin II also stimulates the adrenal glands, located atop the kidneys, to release aldosterone, another hormone. Aldosterone acts on the renal tubules, increasing sodium and water reabsorption back into the bloodstream. This increases circulating blood volume.
Angiotensin II also prompts the pituitary gland to release antidiuretic hormone (ADH), promoting further water reabsorption. These combined actions restore blood volume and elevate blood pressure, maintaining circulatory stability.
Maintaining Chemical Balance
Beyond filtering waste and regulating blood pressure, the kidneys maintain the chemical balance of body fluids. They regulate electrolyte levels (minerals like sodium, potassium, and calcium). These electrolytes are vital for nerve signal transmission, muscle contraction, and fluid distribution.
The kidneys adjust ion excretion or reabsorption to maintain healthy concentrations. Sodium plays a primary role in fluid balance and nerve function; kidneys control its excretion to prevent imbalances. Potassium, an intracellular ion, is also managed for proper cell, nerve, and muscle function, especially heart rhythm.
Calcium, another kidney-regulated electrolyte, supports bone health, muscle contraction, and blood clotting. Kidneys work with hormones like parathyroid hormone to stabilize blood calcium levels, retaining or excreting it as needed. This fine-tuning of electrolyte levels supports cellular and systemic performance.
The kidneys also maintain the body’s acid-base (pH) balance, the equilibrium between acids and bases in the blood. Normal arterial blood pH is kept within a narrow range (7.35 to 7.45). Metabolic processes constantly produce acids; kidneys counteract this by excreting excess hydrogen ions or reabsorbing bicarbonate, a buffering agent, back into the blood. This dual action ensures stable blood pH, allowing enzymes and proteins to function correctly.
Hormone Production and Vitamin Activation
Beyond filtration and chemical regulation, kidneys function as endocrine organs, producing and activating hormones that influence body processes. These functions are distinct from blood-cleansing but are important for overall health. Examples include erythropoietin production and vitamin D activation.
Erythropoietin (EPO)
Erythropoietin (EPO) is a hormone produced by the kidneys. Specialized kidney cells monitor blood oxygen levels. When these cells detect low oxygen (hypoxia), they increase EPO release.
EPO travels to the bone marrow, where blood cells are made. It stimulates red blood cell production and maturation (erythropoiesis). New red blood cells are released into the bloodstream, increasing oxygen-carrying capacity and alleviating oxygen deficit. Without sufficient EPO, as in kidney disease, the body may not produce enough red blood cells, leading to anemia.
Vitamin D Activation
The kidneys activate vitamin D, which is ingested or produced in the skin from sun exposure. Inactive vitamin D undergoes conversions to become functional. The final activation step occurs in the kidneys.
Kidneys convert inactive vitamin D into its active form, calcitriol (1,25-dihydroxyvitamin D). Calcitriol is a hormone vital for calcium absorption from the digestive tract and maintaining healthy calcium and phosphate levels. This active vitamin D is important for strong bones and mineral balance.