These are two bean shaped kidneys in the body, one on either side,
located near the middle of the back behind the 13th rib. These 5 – 6 inch
long organs are responsible for processing waste products and filtering
the blood to ensure that the body is in a state of balance. The waste
comes from the normal breakdown from the food that is eaten.
It is essential that this waste is removed as it could damage the body.
Each kidney is joined to the aorta, which is the largest artery in the body
by a short renal artery, as they receive a huge blood supply.

Each kidney contains around a million nephrons, a tube which is closed
at one end, and open at the other. Inside the nephron is a tiny blood
vessel called a glomerulus. This vessel intertwines with a tiny tubule
where waste materials and water leave the blood and enters the urinary
system where it is turned into urine, to continue the journey to the
urethra. The main filtered substances consist of water, nitrogen
containing compounds, acids, salts and alkalis.

The cortex is the outer part of the kidney and is where the blood is
filtered. This process is called ultra-filtration or high pressure filtration
because it only works if the blood entering the kidney in the renal artery
is at high pressure.

The medulla is the inside part of the kidney and is where the amount of
salt and water in your urine is controlled. It is dark red in colour and
contains billions of loops of Henle’ which pumps sodium ions. ADH
stimulates the loops to work harder to pump more sodium ions, which
results in very concentrated urine, is produced.