Human Excretory System

In humans, the excretory system is built around a small set of organs that together produce, carry, store and expel urine: a pair of kidneys, a pair of ureters, a single urinary bladder and a urethra.

The kidneys

The kidneys are the central organs of this system. They are reddish-brown, bean-shaped structures lying against the back inner wall of the abdominal cavity, positioned roughly between the last thoracic vertebra and the third lumbar vertebra. In an adult, each kidney is about 10 to 12 cm long, 5 to 7 cm wide and 2 to 3 cm thick, and weighs on average between 120 and 170 grams.

The inner, concave surface of each kidney carries a notch called the hilum. This is the gateway of the organ: the ureter leaves through it, and blood vessels and nerves enter through it. Just inside the hilum lies a broad, funnel-shaped chamber, the renal pelvis, whose finger-like extensions are called calyces.

Internal zones

• The whole kidney is wrapped in a tough outer capsule.
• Beneath the capsule the tissue is organised into two zones: an outer cortex and an inner medulla.
• The medulla is not uniform; it is broken into a few cone-shaped masses called medullary pyramids, whose tips project into the calyces.
• The cortical tissue does not stop at the boundary of the medulla -- it dips down between the pyramids as extensions known as renal columns or Columns of Bertini.

The nephron -- the functional unit

Each kidney contains close to one million tiny, complex tubular structures called nephrons. These are the true working units where urine is made. A nephron has two main parts: the glomerulus and the renal tubule.

The glomerulus is a compact tuft of capillaries. It is supplied by the afferent arteriole, a fine branch of the renal artery, and the blood leaving it is drained away by the efferent arteriole.

The renal tubule begins as a double-walled, cup-shaped structure called Bowman's capsule, which wraps around the glomerulus. The glomerulus together with Bowman's capsule is known as the Malpighian body or renal corpuscle. From here the tubule runs on through several distinct stretches:

• Proximal convoluted tubule (PCT): a highly coiled network continuing from Bowman's capsule.
• Henle's loop: a hairpin-shaped segment with a descending limb going down and an ascending limb coming back up.
• Distal convoluted tubule (DCT): another highly coiled region continuing from the ascending limb.
• Collecting duct: the DCTs of many nephrons empty into these straight tubes. Many collecting ducts then converge and open into the renal pelvis, passing through the medullary pyramids at the calyces.

Where the nephron sits, and two types of nephrons

The Malpighian corpuscle, the PCT and the DCT all lie in the cortex, while the loop of Henle dips down into the medulla. How far the loop reaches divides nephrons into two kinds:

• Cortical nephrons: the majority. Their loop of Henle is short and barely enters the medulla. …