The kidneys are two reddish-brown, bean-shaped organs located in the retroperitoneal space on either side of the spine, just below the rib cage. These organs receive a substantial portion of the body’s total blood output, acting as the central processing unit for the circulatory system. Their primary function is to continuously filter the blood, removing wastes and excess water while regulating the body’s internal environment.
The Functional Blueprint
The kidney is encased in a fibrous capsule and divided into two main regions: the outer renal cortex and the inner renal medulla. The renal cortex is the outer layer where initial blood filtration occurs. The renal medulla contains cone-shaped structures called renal pyramids.
The pyramids drain urine into cup-like structures called minor calyces, which merge to form major calyces. These calyces empty into the renal pelvis, a funnel-shaped structure connecting to the ureter. The ureter then carries the urine to the bladder.
At a microscopic level, the work of the kidney is performed by the nephron, which is considered the functional unit of the organ. Each kidney contains approximately one million nephrons that span both the cortex and the medulla. A nephron begins with the renal corpuscle, consisting of the glomerulus and Bowman’s capsule, located entirely within the cortex.
The renal corpuscle is attached to a long, winding renal tubule that extends deep into the medulla before looping back toward the cortex, eventually connecting to a collecting duct. The glomerulus is a tuft of capillaries where filtration begins, and the surrounding tubule is where the filtered fluid is processed and refined.
Waste Removal and Fluid Refinement
The formation of urine and the cleansing of blood are accomplished through three distinct, sequential processes within the nephron. The first step is glomerular filtration, a non-selective process that occurs as blood pressure forces water and small solutes from the glomerular capillaries into Bowman’s capsule. The filtering membrane is highly permeable to water, ions, glucose, and waste products like urea, but it prevents the passage of large components such as blood cells and most proteins.
This initial filtration produces a large volume of fluid, which is essentially plasma minus the proteins. The volume of this filtrate makes the second step, tubular reabsorption, necessary. Reabsorption is the process of selectively moving substances the body needs, such as water, glucose, amino acids, and bicarbonate, from the tubule back into the peritubular capillaries and the bloodstream.
Most of this reclamation occurs in the proximal convoluted tubule due to its extensive surface area and specialized transport mechanisms. Glucose and amino acids are completely reabsorbed under normal conditions, while the reabsorption of sodium and water continues throughout the nephron segments. This process ensures that only a small fraction of the initial filtrate becomes final urine.
The final step is tubular secretion, which is the active transport of substances from the blood in the peritubular capillaries directly into the tubular fluid. This is a mechanism for actively removing additional waste products, excess ions, or foreign compounds, such as certain drugs, that were not filtered initially. Secretion of hydrogen ions and potassium ions is particularly important for maintaining the body’s overall acid-base and electrolyte balance.
Maintaining Internal Balance
Beyond waste removal, the kidney maintains the stable internal environment of the body, a state known as homeostasis. A major component of this regulation is the precise management of extracellular fluid volume and osmolality. The nephron adjusts the amount of water and sodium it reabsorbs to ensure that the volume of circulating blood remains adequate.
The kidney achieves this fine-tuning by producing either concentrated or dilute urine, depending on the body’s hydration status. When the body needs to conserve water, the collecting ducts become highly permeable to water, allowing it to move back into the bloodstream. Conversely, when there is excess fluid, the ducts remain less permeable, resulting in the excretion of dilute urine.
Electrolyte balance is controlled, with the kidney adjusting the excretion or conservation of ions like sodium, potassium, and calcium. Sodium reabsorption is linked to water retention, and its control is a mechanism for regulating blood pressure and fluid volume. Potassium and calcium levels are also monitored, with the kidney adjusting their transport in the distal nephron segments in response to hormonal signals.
The kidneys work with the lungs to maintain acid-base balance, keeping the blood pH around 7.4. They regulate this balance by managing the body’s bicarbonate buffer system. When blood becomes too acidic, the kidneys reabsorb filtered bicarbonate back into the blood and generate new bicarbonate molecules. They simultaneously secrete hydrogen ions into the tubular fluid, often binding them to ammonia to be excreted as ammonium.
Hormonal Regulation
The kidneys function as endocrine organs by both producing hormones and serving as a target for hormones from other glands. One hormone secreted by the kidney is renin, an enzyme that initiates the renin-angiotensin system (RAS). Renin is released by specialized cells in response to low blood pressure or low salt levels.
The RAS cascade ultimately leads to the production of angiotensin II, a vasoconstrictor that immediately raises blood pressure, and stimulates the release of aldosterone. Aldosterone then acts on the kidney tubules to promote the reabsorption of sodium and water, which increases blood volume and raises blood pressure over the longer term. This system is a mechanism for systemic blood pressure control.
The kidney also produces erythropoietin (EPO), a hormone that targets the bone marrow. EPO is released in response to hypoxia, or low oxygen levels in the renal circulation, and it stimulates the production of red blood cells. This mechanism directly links kidney function to the body’s ability to transport oxygen effectively.
A final endocrine function involves the activation of Vitamin D, which is necessary for bone health and calcium regulation. The kidney converts the inactive form of Vitamin D into its active form, calcitriol. Calcitriol then acts to promote the absorption of calcium from the intestine and regulate calcium and phosphate levels in the blood.