The human kidneys continuously filter blood, removing waste products and producing urine. These bean-shaped organs, about the size of a clenched fist, also regulate fluid levels, balance electrolytes, and help control blood pressure by producing hormones. This role maintains overall bodily health and stable internal conditions.
Main Kidney Components
Each kidney is encased by a tough, fibrous renal capsule, providing structural support and protection. Layers of fat, including the perirenal fat capsule, surround it, anchoring the kidneys and offering shock absorption. The kidney’s internal structure divides into two main regions: the outer renal cortex and the inner renal medulla.
The renal cortex is the outermost layer, appearing granular due to microscopic filtering units. It receives over 90% of the kidney’s blood supply. Projections of the cortex, called renal columns, extend into the medulla, separating its structures.
Beneath the cortex, the renal medulla contains 8 to 18 cone-shaped masses called renal pyramids. The pointed ends of these pyramids, known as renal papillae, face towards the kidney’s center.
The renal pelvis is a funnel-shaped structure in the central region of the kidney. This area collects urine as it is produced. It branches into cup-like projections called calyces, which are the initial collecting points for urine from the renal papillae.
The Nephron and Its Role
The nephron is the microscopic functional unit of the kidney, filtering blood and forming urine. Each human kidney contains approximately one million to 1.5 million nephrons. Nephrons regulate the body’s internal environment through three processes: filtration, reabsorption, and secretion.
The nephron begins with the renal corpuscle, located in the renal cortex. This component consists of the glomerulus and Bowman’s capsule. The glomerulus is a cluster of tiny blood vessels where initial blood filtering occurs. Blood pressure forces water and small solutes from these capillaries into Bowman’s capsule.
Bowman’s capsule surrounds the glomerulus. The space within it, called Bowman’s space, collects the filtered fluid, known as glomerular filtrate. This filtrate then moves into the renal tubule.
The renal tubule processes the glomerular filtrate into urine. It has several distinct segments, each with specialized functions. The first segment is the proximal convoluted tubule (PCT). Here, reabsorption returns essential substances like glucose, amino acids, salts, and water back into the bloodstream. The PCT also secretes ions, such as hydrogen and ammonia, to help maintain the body’s acid-base balance.
Following the PCT, the filtrate enters the loop of Henle, which extends into the renal medulla. This loop establishes a concentration gradient within the kidney, important for concentrating urine. The descending limb is permeable to water, allowing water to exit the filtrate. The ascending limb is largely impermeable to water but actively transports ions like sodium and chloride out, creating a salty environment in the medulla.
Next, the filtrate reaches the distal convoluted tubule (DCT), located in the renal cortex. This segment adjusts electrolyte levels, including sodium, potassium, and calcium, and further adjusts blood pH by secreting hydrogen and potassium ions while reabsorbing bicarbonate. The final segment is the collecting duct, which receives filtrate from multiple nephrons. The collecting duct regulates the final water content of the urine under hormonal influence.
Kidney Blood Flow
The kidneys filter approximately 150 to 200 quarts of blood daily. Blood enters each kidney through a large vessel called the renal artery, which branches directly from the abdominal aorta.
Inside the kidney, the renal artery divides into smaller arteries, including segmental, interlobar, and arcuate arteries. These lead to interlobular arteries, which give rise to tiny afferent arterioles. These afferent arterioles supply blood to the glomeruli within each nephron, where initial filtration occurs.
After blood is filtered in the glomerulus, it flows into efferent arterioles. These then subdivide into a network of capillaries called peritubular capillaries, which surround the renal tubules. In juxtamedullary nephrons, these capillaries form a specialized network called the vasa recta, running alongside the loop of Henle. These peritubular capillaries facilitate the reabsorption of needed substances back into the bloodstream. The filtered blood exits the kidney through the renal vein, draining into the inferior vena cava.
Urine Pathway
Once urine is formed in the nephrons, it follows a specific pathway within the kidney before exiting. The collecting ducts, which receive processed filtrate from multiple nephrons, merge as they descend through the renal medulla. These ducts eventually open at the tips of the renal pyramids, called renal papillae.
From the renal papillae, urine flows into small, cup-shaped structures known as minor calyces. Each minor calyx collects urine from one renal papilla. Several minor calyces then converge to form larger structures called major calyces.
There are usually two to three major calyces in each kidney, which unite to form the renal pelvis. The renal pelvis is a large, funnel-shaped cavity, acting as the central collection point for all urine produced by the kidney. From the renal pelvis, urine exits the kidney and travels down the ureter, a tube that carries it to the bladder for storage.