Your kidneys filter 180 litres a day and keep almost all of it
The kidneys are two reddish-brown, bean-shaped organs tucked against the back wall of the abdomen, each about 12 centimetres long. They take a fifth to a quarter of the heart’s output, filter roughly 180 litres of fluid a day, and hand almost all of it back to the blood.
The kidneys sit high in the abdomen on either side of the spine, outside the lining of the abdominal cavity. Because the liver takes up room on the right, the right kidney usually lies a little lower and is slightly smaller; one Danish study found median lengths of 11.2 cm on the left and 10.9 cm on the right. Their upper ends touch the liver and spleen, so both kidneys drop slightly with every breath in. At the indented hilum the renal artery enters and the renal vein and ureter leave, and the whole organ is wrapped in a tough capsule cushioned by layers of fat.
Inside, an outer cortex and inner medulla form eight to 18 cone-shaped lobes. Urine from each pyramid tip drains through minor and major calyces into the renal pelvis and on down the ureter. The working units are nephrons: a filtering corpuscle in the cortex followed by a tubule that dips into the medulla. Filtration holds back cells and large proteins, and of the ultrafiltrate made each day only 800 to 2,000 millilitres normally ends up as urine. More than half the water is reclaimed in the first stretch of tubule, as is all glucose at normal blood levels.
Along with removing urea and uric acid, the kidneys regulate fluid volume, blood pressure, electrolytes and acid–base balance, working with hormones such as renin, aldosterone and antidiuretic hormone. Almost 70 percent of our roughly 20,000 protein-coding genes are active in adult kidneys, and uromodulin, the most plentiful protein in urine, is the top kidney-specific one.
Chronic kidney disease affects an estimated 13.4 percent of people worldwide, and 5 to 7 million patients with kidney failure are thought to need dialysis or other replacement therapy. Nephrologists treat problems of kidney function, while urologists deal with structural ones such as stones, cysts and tumours.
Source: Kidney
