Structure of Kidney — Definition

Definition

The kidney is a vital organ in the human excretory system, responsible for filtering blood and producing urine. Imagine two bean-shaped organs, each about the size of a clenched fist, located on either side of your spine, just below the rib cage. These are your kidneys. They play a crucial role in maintaining your body's internal balance, a process called homeostasis.

Let's break down its structure simply. On the outside, each kidney is protected by a tough, fibrous capsule. If you were to cut a kidney in half lengthwise, you'd see two main regions: an outer, lighter region called the cortex and an inner, darker region called the medulla.

The cortex is where the initial filtering of blood happens. The medulla is characterized by cone-shaped structures called renal pyramids, which are responsible for concentrating urine. Between these pyramids, extensions of the cortex, known as renal columns of Bertin, dip down into the medulla.

Blood enters the kidney through the renal artery and leaves through the renal vein. The point where these vessels, along with the ureter (the tube carrying urine to the bladder), enter or exit is called the hilum. Inside the kidney, the hilum opens into a funnel-shaped space called the renal pelvis, which collects urine from the pyramids before it flows into the ureter.

The true functional units of the kidney are microscopic structures called nephrons. Each kidney contains about one million nephrons. A nephron is like a tiny, sophisticated filter and processing plant.

It starts with a cup-shaped structure called Bowman's capsule, which encloses a tuft of tiny blood capillaries called the glomerulus. This entire unit (glomerulus + Bowman's capsule) is known as the renal corpuscle, and it's where blood is first filtered.

The filtered fluid, called glomerular filtrate, then passes into a long, convoluted tubule. This tubule has several parts: the proximal convoluted tubule (PCT), the loop of Henle (which dips into the medulla), the distal convoluted tubule (DCT), and finally, the collecting duct.

As the filtrate travels through these parts, essential substances like water, glucose, and amino acids are reabsorbed back into the blood, while waste products are further secreted into the tubule, ultimately forming urine.

This intricate structure allows the kidney to efficiently clean the blood and regulate body fluids.