Beneath a tough outer capsule, the kidney reveals an internal cortex, medulla, and central pelvis linked by calyces and papillae, all served by a network of renal arteries and veins, and roughly 1 to 1.5 million microscopic nephrons per kidney. Together these structures filter blood, balance fluids, and produce urine. Knowing where each layer sits makes the whole system easier to picture.
This walkthrough covers the structure of the kidney from outside to inside, then ties each layer to what it actually does.
The Kidney’s Outer Architecture and Protective Layers
Each kidney sits high in the back of the abdomen, tucked under the lower ribs, and weighs about 115 to 170 grams in adults. The organ’s classic bean shape comes from a convex lateral border that curves outward and a concave medial border that dips inward.
The hilum and the renal capsule
The dip on the inner border is called the hilum. That small opening is the doorway where the renal artery enters, the renal vein exits, and the ureter drops down toward the bladder. Wrapping the entire organ is the renal capsule, a thin but tough layer of fibrous tissue. The capsule keeps the kidney’s shape, shields it from friction against nearby structures, and holds the internal pressure needed for filtration.
Perirenal fat and surrounding support
A layer of perirenal fat surrounds the capsule and acts like packing foam, absorbing shocks and holding the kidney in place against the back wall of the abdomen. The adrenal gland sits like a small cap on top of each kidney, though it functions as a separate hormone-producing organ rather than a kidney component.
The adrenal gland sits on top of each kidney but functions as a separate endocrine organ, not a filter.
Three Main Regions: Cortex, Medulla, and Pelvis
Cutting a kidney in cross-section reveals three distinct regions, each with a clear job. From outside to inside, those regions are the cortex, the medulla, and the pelvis.
The renal cortex
Just beneath the capsule, the outermost functional layer forms a band called the renal cortex. It looks granular because it holds the bulk of the kidney’s working units, including the glomeruli and the coiled portions of the nephron tubules. Most of the actual blood filtration happens here, which is why the cortex is sometimes described as the functional heart of the kidney.
The renal medulla
Deeper inside, 8 to 18 cone-shaped renal pyramids make up the renal medulla. These pyramids point inward, and their striped appearance comes from straight tubule segments and collecting ducts running in parallel. The medulla concentrates the filtrate that the cortex produces, pulling water back into the body and leaving a more concentrated urine behind.
The renal pelvis
At the kidney’s center, a funnel-shaped chamber collects urine before it drains into the ureter. It collects urine from the calyces and channels it into the ureter, which then carries it down to the bladder for storage.
With urine now gathered in the pelvis, the focus shifts to the quieter architecture bridging cortex and pelvis.
Internal Structures Between Cortex and Pelvis
Between those three main regions sit several bridging structures that carry urine in the right direction. They sound similar, so it helps to think of them as a short staircase from cortex down to ureter.
- Renal columns of Bertin: extensions of cortex tissue that slide inward between adjacent pyramids, anchoring the cortex to the deeper layers.
- Renal papillae: the rounded tips of each pyramid, where newly concentrated urine drips out into the collecting system.
- Minor calyces: cup-like tubes that catch urine from one or more papillae.
- Major calyces: larger channels that merge several minor calyces before delivering urine into the renal pelvis.
| Structure | Location | Role |
|---|---|---|
| Renal columns of Bertin | Between pyramids | Anchor cortex inward |
| Renal papillae | Pyramid tips | Release urine |
| Minor calyces | Around papillae | Collect urine from papillae |
| Major calyces | Deeper in medulla | Merge into renal pelvis |
| Renal pelvis | Central cavity | Funnels urine to ureter |
Blood Supply and the Path of Filtration
The kidney processes about 180 liters of filtrate every day, which requires an unusually rich blood supply. Roughly 20 to 25 percent of the heart’s output at rest flows through the kidneys, even though they make up less than 1 percent of body weight.
Arterial pathway from aorta to nephron
The renal artery branches off the abdominal aorta and enters at the hilum. Inside, it splits into segmental arteries, then interlobar arteries that travel between the pyramids, then arcuate arteries that arch along the border between cortex and medulla, then interlobular arteries that fan outward into the cortex. Tiny afferent arterioles branch off these interlobular arteries and feed each nephron’s glomerulus. This branching ladder is what delivers blood to the actual filtration surface.
Venous return and filtration volume
After filtration, blood leaves the nephron through efferent arterioles, enters a network of peritubular capillaries, and drains into interlobular veins, arcuate veins, interlobar veins, and finally the renal vein, which empties into the inferior vena cava. The kidneys filter roughly 200 quarts of fluid every 24 hours, returning most of it to the bloodstream while excreting about 1 to 2 quarts as urine.
That massive daily filtration is made possible by roughly a million tiny processing units working in parallel.
The Nephron as the Functional Unit of the Kidney
Each kidney contains an estimated 1 to 1.5 million nephrons, the microscopic filters that do the actual work. Every drop of filtrate begins inside one of these structures.
The renal corpuscle
Every nephron begins at the renal corpuscle, a structure built from two distinct parts. The glomerulus is a tight knot of capillaries where fluid and small solutes are squeezed out of the blood. Bowman’s capsule is a double-walled cup that surrounds the glomerulus and catches the filtrate the moment it leaves the blood. Together, glomerulus plus Bowman’s capsule form the entry point for everything the kidney filters.
The renal tubule sequence
From Bowman’s capsule, filtrate flows through a tubule with four named segments, each adjusting the fluid in a different way:
- Proximal convoluted tubule: reabsorbs the bulk of water, sodium, glucose, and amino acids back into the blood.
- Loop of Henle: dips into the medulla and creates the salt gradient that lets the kidney concentrate urine.
- Distal convoluted tubule: fine-tunes sodium, calcium, and pH based on hormonal signals.
- Collecting duct: carries the finished fluid through the medulla and adjusts final water content before emptying into a papilla.
How Each Part Contributes to Urine Formation
The whole anatomy comes together in three coordinated steps: filtration at the corpuscle, adjustment along the tubule, and drainage through the collecting system.
Glomerular filtration
Pressure inside the glomerulus forces water and small dissolved solutes across a filtration barrier into Bowman’s capsule, while cells and large proteins stay behind in the blood. The result is roughly 180 liters of filtrate per day, a volume that seems impossibly large until you remember the body reclaims nearly all of it.
Tubular reabsorption and secretion
As filtrate travels the tubule, useful substances return to the bloodstream and waste products move in the opposite direction. This reabsorption and secretion step is what turns a crude filtrate into actual urine. By the time fluid reaches the collecting duct, about 99 percent of the filtered water and most of the filtered sodium have already been reclaimed.
Drainage through the collecting system
The collecting duct empties into a papilla, which drips urine into a minor calyx. Minor calyces merge into major calyces, which deliver the urine to the renal pelvis. From the pelvis, the ureter carries it down to the bladder, where it is stored until elimination through the urethra.
| Step | Location | What happens |
|---|---|---|
| Filtration | Glomerulus and Bowman’s capsule | Water and small solutes enter the tubule |
| Reabsorption | Proximal tubule and loop of Henle | Useful water and salts return to blood |
| Secretion | Distal tubule and collecting duct | Extra wastes move from blood into tubule |
| Drainage | Calyces, pelvis, ureter, bladder | Urine is stored and released |
About 99 percent of the water filtered by the glomerulus is reabsorbed before urine leaves the kidney.
FAQ
What are the main parts of the kidney?
A renal capsule, cortex, medulla with pyramids and papillae, columns of Bertin, calyces, pelvis, artery, vein, and nephrons each made of a corpuscle and tubule together form the kidney’s main anatomy. Together these structures filter blood and produce urine.
What is the function of the renal cortex?
Most glomeruli and nephron tubules sit in the renal cortex, making it the primary site of blood filtration. Damage to the cortex therefore tends to have a direct effect on overall kidney function.
What is the structure of a nephron?
A nephron is a renal corpuscle (a glomerulus wrapped in Bowman’s capsule) connected to a four-part tubule: the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and a collecting duct. Each segment adjusts the filtrate in a different way.
Where is the renal pelvis located?
At the central hilum, the major calyces merge into the renal pelvis, the chamber where the ureter begins. It is the funnel that channels finished urine down toward the bladder.
How do the kidneys filter blood?
Blood enters through the renal artery, branches down to each glomerulus, and is pressurized across a filtration barrier into Bowman’s capsule. Tubular segments then reclaim what the body needs and excrete what it does not, producing urine that drains through the calyces, pelvis, and ureter.
What is the difference between the renal cortex and medulla?
The cortex is the outer region where filtration starts and where most nephron parts sit. The medulla is the inner region of pyramids and collecting ducts that concentrates the filtrate into urine. They share a border but perform different steps.
