A LITTLE BACKGROUND

The human kidneys:
• are two bean-shaped organs, one on each side of the backbone.
• Represent about 0.5% of the total weight of the body,
• but receive 20-25% of the total arterial blood pumped by the heart.
• Each contains from one to two million nephrons


The nephron is a tube; closed at one end, open at the other. It consists of a:
• Bowman's capsule.  Located at the closed end, the wall of the nephron is pushed in forming a double-walled chamber.
• Glomerulus.  A capillary network within the Bowman's capsule. Blood leaving the glomerulus passes into a second capillary network (not shown in the figure) surrounding the
• Proximal convoluted tubule. Coiled and lined with cells carpeted with microvilli and stuffed with mitochondria.
• Loop of Henle.  It makes a hairpin turn and returns to the
• Distal convoluted tubule, which is also highly coiled and surrounded by capillaries.
• Collecting tubule. It leads to the pelvis of the kidney from where urine flows to the bladder and, periodically, on to the outside world.

Formation of Urine
The nephron makes urine by
• filtering the blood of its small molecules and ions and then
• reclaiming the needed amounts of useful materials.
• Surplus or waste molecules and ions are left to flow out as urine.
In 24 hours the kidneys reclaim
• ~1,300 g of NaCl
• ~400 g NaHCO3
• ~180 g glucose
• almost all of the180 liters of water that entered the tubules.

The Kidney and Homeostasis
While we think of the kidney as an organ of excretion, it is more than that. It does remove wastes, but it also removes normal components of the blood that are present in greater-than-normal concentrations. When excess water, sodium ions, calcium ions, and so on are present, the excess quickly passes out in the urine. On the other hand, the kidneys step up their reclamation of these same substances when they are present in the blood in less-than-normal amounts. Thus the kidney continuously regulates the chemical composition of the blood within narrow limits. The kidney is one of the major homeostatic devices of the body.

Hormones of the Kidneys
The human kidney is also an endocrine gland secreting two hormones:
• Erythropoietin (EPO)
• Calcitriol (1,25[OH]2 Vitamin D3), the active form of vitamin D
as well as the enzyme renin.

Ref: and for more information: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/K/Kidney.html