Kidney Anatomy

Major Structures and Blood Vessels

A frontal section through the kidney reveals an outer, lighter-colored region known as the renal cortex and an inner, darker-colored region referred to as the renal medulla. Surrounding the renal cortex is the renal capsule, a tough, fibrous layer of connective tissue that helps contain, support, and protect the kidney’s soft inner tissues.

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Hilum and Sinus

Situated along the medial border of each kidney is a fissure known as the renal hilum. This fissure serves as the primary entry and exit point for the ureter, renal nerves, and vascular structures. Internally, this opening expands into the renal sinus, a fatty compartment that houses the major nerves and vessels, as well as the branching network of the renal calyces.

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Lobes and Columns

Each kidney contains approximately 6–8 renal lobes, which can be observed without a microscope. A renal lobe contains thousands of microscopic tubules called nephrons, which are the primary functional units of the kidney.

Because of its shape, the medullary portion of a renal lobe is called a renal pyramid. A pyramid’s base (wide end) lies adjacent to the renal cortex, and the apex (narrow end) extends into the kidney’s medial indentation. A pyramid appears darker and somewhat striated because it contains a high concentration of straight, parallel tubules called collecting ducts, which transport urine to a lobe’s apex.

A renal column extends between and separates adjacent renal lobes. The columns primarily consist of connective tissue and provide a supportive framework for blood vessels entering and exiting the cortex.

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Papilla, Calyces, and Pelvis

The renal calyces are chambers within the kidney through which urine passes as it travels to the ureter. A minor calyx surrounds the apex of the renal pyramids. Urine formed in the kidney passes through a renal papilla at the apex into a minor calyx. Two or three minor calyces converge to create a major calyx, through which urine passes before continuing through the renal pelvis into the ureter. The smooth muscles in the walls of the calyces and renal pelvis rhythmically contract and relax (peristalsis) to propel urine into the ureter.

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Major Arteries

Oxygenated blood enters the kidney from the abdominal aorta via the renal artery. At the renal sinus, the renal artery divides into segmental arteries, which then branch into interlobar arteries that pass through the renal columns toward the renal cortex. At the bases of the renal pyramids, the interlobar arteries branch into arcuate arteries that extend along the arches between the pyramids and the cortex. Smaller vessels branch from the arcuate arteries, circulating blood throughout the renal lobes.

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Major Veins

The arcuate veins carry deoxygenated blood from smaller vessels within the renal lobes to the interlobar veins, which traverse the renal columns and enter the renal sinus and hilum, where they converge into larger trunks. Blood leaves the kidney via the renal vein, which connects to the inferior vena cava.

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Assessment

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References and Attributions

NIH – “Anatomy, Abdomen and Pelvis: Kidneys.”

OpenStax College, Anatomy and Physiology – “Gross Anatomy of the Kidney.” (Access for free at – https://openstax.org/books/anatomy-and-physiology/pages/1-introduction)

Wikipedia – “Kidney.”

Wikipedia – “Nephron.”