1] are the functional units of the kidneys that form urine.

The functional units of the kidneys that form urine are called nephrons. Here's a step-by-step explanation of how they work:

1. Blood enters the kidney through the renal artery, which splits into many smaller blood vessels (capillaries) within the kidney.

2. The smallest blood vessels are intertwined with the nephrons, which are the functional units of the kidney. Each kidney contains about a million nephrons.

3. Each nephron consists of a glomerulus and a tubule. The glomerulus is a tiny ball-shaped structure composed of capillary blood vessels. The tubule, a tube-shaped structure, is where the urine is formed.

4. The process of urine formation begins in the glomerulus. The blood pressure in the glomerulus causes water, salt, glucose and other substances to be filtered out from the blood into the Bowman's capsule, the beginning of the tubule.

5. These substances, known as the glomerular filtrate, then pass through the tubule. Along the tubule, some substances like glucose, certain ions, and water are reabsorbed back into the blood.

6. The remaining substances that are not reabsorbed form the urine. This includes wastes like urea and creatinine, and excess substances like water and salts.

7. The urine then drains into the renal pelvis, from where it is transported through the ureter to the bladder for storage until it is excreted from the body through the urethra.