The kidneys are two fist-sized organs found on either side of the spine in the retroperitoneal space, which is between the mid-to-lower back and posterior abdominal wall. They’re protected by the ribs and layers of fat and muscle. Their major function is to filter the blood, removing excess water and waste by way of the urine. When we’re at rest, our kidneys are making use of around 25% of our cardiac output.
The kidney’s outer area is called the renal cortex, and its inner section is called the medulla, which contains the renal pyramids; these structures help move urine along the excretory process.
Urine is made in structures known as nephrons, and bundled collection ducts called renal papillae move it from the nephrons to the calyces, an area they pass on their way to be excreted. The renal papillae, renal pyramids, and medulla are sectioned off by the renal columns–extensions of the kidneys which are made up of connective tissue and split up each kidney into 6-8 lobes. The renal columns also provide the vessels that enter and exit the kidney cortex a supportive framework.
Each kidney contains approximately one million nephrons, which are the kidney’s functional units. Each nephron contains a glomerulus (a filter), as well as a tubule. Nephrons function in a process that’s divided into two steps:
When blood flows into a nephron, it enters the glomerulus, which is comprised of a cluster of miniscule blood vessels. The glomerulus has thin walls, which permits wastes, fluid (largely water), and smaller molecules, to easily pass into the tubule, while larger molecules like blood cells and proteins remain in the blood vessel.
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