A human kidney is a dark reddish-brown, bean-shaped organ roughly the size of your fist. Each one measures about 11 cm long (a little over 4 inches) and sits against the muscles of your upper back, tucked just below the ribcage. Most people have two, and they look nearly identical, though the left kidney is slightly larger than the right.
Size, Shape, and Color
The classic bean shape is unmistakable: convex on the outer edge, concave on the inner edge, with a smooth, glistening surface in a healthy adult. A study of 665 adults published in the American Journal of Roentgenology found median kidney lengths of 11.2 cm on the left and 10.9 cm on the right. The left kidney also has a slightly larger volume, around 146 cubic centimeters compared to 134 on the right. In living tissue, the color is a deep reddish-brown, reflecting the enormous blood supply these organs receive. About 20% of the blood your heart pumps with each beat flows directly to your kidneys.
The right kidney sits a little lower in the body than the left because the liver, which is large and heavy, pushes it down. Both kidneys rest behind the organs of the abdomen, pressed against the back body wall, which is why kidney pain is often felt in the lower back rather than the belly.
What the Surface Looks Like
In an adult, the outer surface is smooth and enclosed in a thin, tough membrane called the renal capsule. This capsule is a layer of fibrous connective tissue that fits tightly around the kidney like shrink wrap, holding the soft internal tissue in shape. Outside the capsule sits a thick cushion of fat, and beyond that, another envelope of connective tissue called Gerota’s fascia. Together, these three layers act as padding and anchor each kidney in place.
On the inner, concave edge of the bean shape, there’s an indentation called the hilum. This is the kidney’s doorway: the artery bringing blood in, the vein carrying filtered blood out, the tube (ureter) draining urine toward the bladder, and a network of nerves and lymph vessels all pass through this single notch. If you were looking at a kidney from the front, these structures are stacked from front to back in that order: artery, vein, then ureter.
What It Looks Like Inside
Slicing a kidney in half from top to bottom reveals two distinct zones of tissue and a central drainage chamber. The outer zone, called the cortex, is lighter in color, a pale reddish tone. The inner zone, the medulla, is darker reddish-brown. On an ultrasound, this contrast is easy to spot: the medulla appears darker than the cortex.
The medulla isn’t one solid mass. It’s organized into cone-shaped sections called pyramids, typically 9 to 12 of them, with their pointed tips facing inward. Each pyramid tip drains into a small cup-shaped collector. These small cups merge into 3 or 4 larger cups, which then funnel into a wider chamber called the renal pelvis. The pelvis narrows into the ureter and carries urine down to the bladder. Think of it like a system of funnels nested inside each other, all converging toward a single exit pipe.
How Kidneys Look at the Microscopic Level
Zoom in far enough and each kidney is made up of about one million tiny filtering units called nephrons. Each nephron has two main parts: a microscopic filter (a tiny ball of blood vessels) and a long, winding tube. Blood enters the filter under pressure, and water, salts, sugars, and waste products get pushed through into the tube. As the fluid travels through the tube’s twists and loops, the kidney reclaims what the body still needs and lets the rest continue on as urine.
These nephrons are spread across both the cortex and the medulla. The filters sit in the cortex, which is why that outer layer has such a rich blood supply and lighter color. The tubes loop down into the darker medulla, where the chemistry of concentrating urine takes place. This architecture is what gives the two zones their distinct appearance when you cut the kidney open.
How Kidney Appearance Changes With Age
A fetal or newborn kidney looks noticeably different from an adult one. The surface isn’t smooth. Instead, it has a bumpy, segmented look, with visible grooves marking the boundaries between the kidney’s internal lobes. This lobulated appearance develops around 18 weeks of pregnancy and stays visible until a child is roughly 3 to 5 years old. After that, the cortex fills in and the surface smooths out into the sleek bean shape adults have. In a small percentage of adults (roughly 0.5% to 4%), the lobulated pattern persists, but it’s considered a harmless variation, not a sign of disease.
Aging and chronic illness can also change what a kidney looks like. Kidneys affected by long-term high blood pressure or diabetes tend to shrink and develop a rougher, more granular surface. Cysts, which are fluid-filled sacs, become increasingly common with age and show up as smooth, round bulges on or within the kidney tissue. A healthy kidney in a young adult, by contrast, is plump, smooth, and uniformly colored.