How do kidneys work?
Depending on your weight, you have 4-6 litres of blood circulating in your body. This blood is transported to your kidneys by the renal arteries. Every day approximately 1,500 litres of blood pass through your kidneys and are treated, thanks to roughly one million nephrons.
The nephrons consist of small filters called glomeruli that filter a fluid of water, salts and waste materials from our blood. Protein and blood cells remain in the blood. The filtered fluid (primary urine) is transported in small tubules. There, we have a particular type of cells (tubular cells) that bring water and salts, such as sodium, calcium, phosphate and magnesium, back into the blood. What remains is excreted as final urine.
How much salts are absorbed through the tubular cells depends on the blood pressure and on the concentration of particular hormones that affect these cells. Thus, the kidney regulates our water and salt balance. Conversely, our kidneys also affect our blood pressure (for example, when the blood pressure decreases, more water and sodium is fed back into our blood).
Our kidneys also produce the hormone erythropoietin which stimulates the production of red blood cells.