Each kidney has several distinct parts that work together to filter blood, balance fluid levels, and produce urine. About the size of a closed fist, an adult kidney typically measures 10 to 12 cm long, 5 to 7 cm wide, and weighs roughly 135 to 162 grams depending on sex. Inside, the kidney is organized into layers, each containing specialized structures that handle different stages of blood filtration.
Outer Structures: Capsule and Hilum
The entire kidney is wrapped in the renal capsule, a protective covering made of three layers of connective tissue and fat. This capsule shields the kidney from physical injury, holds it in place, and anchors it to surrounding tissue.
On the inner, concave edge of each kidney sits the renal hilum, a notch-like opening where structures enter and exit. From front to back, the hilum contains the renal vein, the renal artery, and the renal pelvis (the starting point of the ureter). All blood flowing into and out of the kidney, along with all urine draining from it, passes through this single gateway.
Renal Cortex
Just beneath the capsule lies the renal cortex, the kidney’s outer functional layer. This is where blood filtration begins. The cortex houses the starting portions of the nephrons, the kidney’s microscopic filtering units. Each kidney contains roughly 900,000 to 1 million nephrons, though individual kidneys can range anywhere from about 200,000 to over 2.5 million. The cortex also produces a hormone that signals your bone marrow to make red blood cells.
Renal Medulla
Deeper inside, the renal medulla forms the kidney’s inner region. It is organized into cone-shaped sections called renal pyramids, with their pointed tips (called papillae) facing inward toward the center of the kidney. Between the pyramids, extensions of cortex tissue called renal columns separate each cone from its neighbors.
The medulla is where much of the fine-tuning of urine happens. The looping portions of the nephrons dip down into these pyramids, concentrating or diluting urine as needed before it drains from the pyramid tips into cup-shaped collectors.
Calyces and the Renal Pelvis
Each pyramid tip empties urine into a small cup called a minor calyx. Several minor calyces merge into larger chambers called major calyces, and these in turn funnel into the renal pelvis, a broad, funnel-shaped basin at the center of the kidney. Specialized pacemaker cells in the walls of the minor calyces generate rhythmic contractions that push urine along, similar to how the heart’s pacemaker drives a heartbeat.
The renal pelvis narrows as it exits through the hilum and transitions into the ureter, the tube that carries urine down to the bladder. This junction sits roughly at the level of the second lumbar vertebra, near your lower back.
The Nephron: The Kidney’s Filtering Unit
The nephron is where the actual work of blood filtration takes place. Each one is a self-contained unit with two main parts: a tiny cluster of blood vessels called the glomerulus and a long, winding tubule that processes the filtered fluid into urine.
Glomerulus
Blood enters the nephron through a small artery and flows into the glomerulus, a tight ball of capillaries enclosed in a cup-shaped capsule. Here, water, salts, sugars, and waste products are pushed out of the blood through a three-layered filter. The first layer is the capillary wall itself, which has tiny windows (50 to 100 nanometers across) that let fluid pass through. The second is a thin basement membrane that acts as a size-and-charge barrier. The third consists of specialized cells called podocytes, whose finger-like extensions wrap around the capillaries and form the final sieve. Together, these layers freely pass water and small molecules while holding back blood cells and most proteins.
Proximal Convoluted Tubule
The filtered fluid, now called filtrate, flows into the proximal convoluted tubule, the first and busiest section of the tubule system. This segment reclaims about 60 to 65% of the water and sodium that was filtered, along with most of the potassium, phosphate, bicarbonate, glucose, and amino acids. Essentially, the kidney filters everything out first and then immediately takes back nearly all the nutrients it needs.
Loop of Henle
From there, the filtrate enters the loop of Henle, a hairpin-shaped tube that dips down into the medulla and back up again. It has three sections: a thin descending limb, a thin ascending limb, and a thick ascending limb. The descending limb allows water to flow out, concentrating the urine. The ascending limbs are waterproof but let sodium and other salts pass through, which dilutes the fluid again. This back-and-forth reabsorbs another 30 to 40% of sodium and is the key mechanism the kidney uses to produce either concentrated or dilute urine depending on how hydrated you are.
Distal Convoluted Tubule
The diluted fluid then reaches the distal convoluted tubule, which handles the final adjustments. This segment reabsorbs another 5 to 10% of filtered sodium and chloride while also secreting potassium. Like the ascending limb before it, it does not allow water through, so urine continues to become more dilute here.
Collecting Duct
Multiple nephrons drain into shared collecting ducts, which run through the renal pyramids toward the pyramid tips. The collecting duct makes the last decisions about urine composition: specialized cells fine-tune acid-base balance by adjusting hydrogen and bicarbonate levels. Whether the collecting duct reabsorbs water at this stage depends on hormonal signals, particularly one that responds to dehydration by making the duct walls more permeable to water.
Blood Supply
The kidneys receive blood through the renal arteries, which branch directly off the body’s main artery. Once inside the kidney, each renal artery divides into progressively smaller vessels until blood reaches the tiny arterioles that feed individual glomeruli. After filtration, blood doesn’t go straight back to a vein. Instead, it flows into a second set of capillaries that wrap around the nephron tubules. This arrangement lets the blood pick up everything the tubules reabsorb (water, glucose, salts) before draining into progressively larger veins and finally exiting through the renal vein.
This two-capillary-bed system is unusual in the body and is what makes the kidney so efficient at reclaiming useful substances from the filtrate while letting waste products stay in the urine.
The Juxtaglomerular Apparatus
Where the distal tubule loops back and touches the arteriole feeding its own glomerulus, a small cluster of specialized cells forms the juxtaglomerular apparatus. This structure has two key components: granular cells in the wall of the arteriole that produce renin (a hormone involved in raising blood pressure), and a patch of modified tubule cells called the macula densa that sense sodium concentration in the passing fluid. When sodium drops too low, the macula densa signals the granular cells to release renin, triggering a hormonal chain that raises blood pressure and prompts the kidneys to retain more sodium and water. This feedback loop gives the kidney direct, moment-to-moment control over blood pressure.