Describe and explain how the human kidney is involved in excretion and how it regulates the concentration in blood?

Kidney is an organ in body of human beings and all other vertebrate animals that perform many vital functions including the production of urine.  Urine is the fluid carries various waste materials out of the body.

Human kidneys consist of three layers.  These layers are the cortex on the outside of the organ, the medulla, and the pelvis.  Blood flows into the medulla through the renal artery.  In the medulla and cortex, the renal artery branches into increasingly smaller arteries.  Each of these arteries ends in a blood filtration unit called a nephron.  Two kidneys, on average contain a total of about 2 million nephrons, filtering about 190 litres of blood daily.

Nephrons separate from the blood the wastes that the body cannot use. These waste substances, which include ammonia, urea, uric acid, and excess water, make up urine.  The urine passes into the pelvis layer of the kidney.  A tube called the ureter carries urine from each kidney into the urinary bladder.  Urine collects in the bladder until it passes out of the body through another tube, the urethra.

In addition to producing urine, the kidneys secrete a hormone called erythropoietin, which controls the production of red blood cells.  The kidneys also convert vitamin D from an inactive form to an active form, which is essential for normal bone development.  The kidneys also help maintain the blood pressure of the body by releasing an enzyme called renin.