How Our Kidneys Filter Urine

In this article we will see about how kidneys filter urine from our blood and the process of urination in detail.

The process of production of urine starts from the blood entry to the kidney. Blood enters into each kidney from two arteries such as the right and left renal arteries (both arteries in combination called aorta). The blood enters into the kidneys through the renal arteries through the hilum a depression inside the core of the kidney and the arteries branch into smaller and smaller arteries. The smallest arteries are called arterioles. These are situated all the way through the cortex.

Blood constantly move across these small arterioles. This flow is very crucial. Kidneys have their own special device to maintain this blood flow consistently. If blood pressure falls, the flow of blood is diminished. Kidney produces a hormone called renin and released it into the blood. This hormone stimulates the narrowing or contraction of arterioles and so blood pressure increased and blood flow in the kidneys is reinstated to customary.


Capillaries are the smaller blood vessels that are twisted and interlinked branches from each arteriole in the cortex of the kidney. The capillaries in collection form a small ball like structure called glomerulus. There are about one million glomeruli (singular-glomerulus) in the cortex of each kidney.

Kidneys manufacture urine by a process called filtration. Each microscopic glomerulus wall is thin enough to allow water, sugar, salt, urea, and other nitrogenous wastes like uric acid and creatinine to go away the bloodstream. These materials going away from each glomerulus are accumulated in a minuscule cup-like structure called Bowman's capsule, which encloses each glomerulus. The walls of the glomeruli check large substances like proteins and blood cells from filtering into the Bowman's capsules. These substances stay behind in the blood and normally do not come into view in urine.

Renal tubule is a long twisted tube attached to the Bowman's capsule. These tubules are covered by tiny capillaries. When the water, sugar, urea, salts and other nitrogenous wastes pass through these renal tubules, all of the sugar and most of the water and some salts go again to the bloodstream. This process is called re-absorption.

Through this process only vital materials like sugar, salts, water, and acids retain into the body. Some salts, acids, sugar, and water remain in the renal tubule after this process. The remaining material in the renal tubule called urine now contains 95% of water, 5% urea, creatinine, salts, and acids. These renal tubules are combined in a larger collecting tubule, and thousands of these collecting tubules show the way to the renal pelvis, a reservoir in the central area of the kidney. Calices (singular-calix or calyx) are the small, cup-like sections of the renal pelvis.

The renal pelvis tapered into the ureter. The ureter carries the urine to the urinary bladder where the urine is for the short term stored. The outlet area of the bladder is closed by sphincters that allow the urine to run off the bladder. When the bladder fills up, a pressure is situated on the bottom of the urethra, so that only we feel to urinate.