Many people who care for reptiles will at some point hear or read about the renal portal system. It is usually cited as the reason why drugs are administered only in the front half or two-thirds of a reptile's body. But what does this really mean and does it really matter? The answers lie in the methods reptiles have evolved to excrete waste and to conserve water. A little background anatomy and physiology, and a few definitions are important for further discussion.
Snakes have two kidneys, located about three-quarters of the way down the body, one slightly ahead of the other. They consist of segments connected in a row. Urine is collected in ureters and flows to the urodeum, a division of the cloaca. Snakes have no urinary bladder.
Many lizard species have two bean-shaped kidneys, located within the pelvis. Most species have a bladder, and unlike mammals, lizards may be able to modify the composition of urine within the bladder. This likely serves as a method of water conservation.
Chelonia (tortoises, terrapins and turtles) have paired kidneys on the underside of the carapace, or top shell. Urine drains through ureters to the cloaca, where it can be excreted or passed to a large urinary bladder to be stored (until it is excreted through the cloaca at a later point in time). In most species of chelonia, the cloaca, colon and urinary bladder can absorb water from the urine. Although it is not well understood, the bladder is so large that it likely plays a role in water storage.
In animals, the movement of blood through the body is organized into a system of arteries, which carry blood away from the heart, and veins, which carry it to the heart. A portal system is really a subunit, of sorts, within the circulatory system. A "portal system" may be defined as a vein that has a network of capillaries at either end. (Capillaries are the smallest blood vessels.)
In reptiles, a renal (kidney) portal system exists. From the heart, blood moves through the aorta, the largest artery in the body. The paired kidney arteries branch out from the aorta and deliver blood to the kidneys. Within the kidney, the renal artery branches into tiny capillaries, which exchanges life-sustaining oxygen for waste products. (This pattern of blood flow is seen in mammals, amphibians, birds and reptiles.)
The renal portal system is a second route by which blood moves from the back half of the body through the kidneys before returning to the heart. This system is found in birds, amphibians, reptiles and fish.
All animals produce nitrogenous wastes as a result of cell metabolism. These waste products take the form of urea, ammonia or uric acid and rapidly reach toxic levels if not excreted from the body. To eliminate ammonia and urea, they need to dissolve in water – which produces urine. A lot of water is needed, which makes it a bad way to eliminate waste in arid conditions. However, uric acid (another form of nitrogen waste) doesn't dissolve in water. Getting rid of uric acid is a way to conserve water.
Reptile stools usually have three components. First, the feces are usually brown and come from the digestive tract. Feces are often evacuated at the same time as the waste from the kidneys. The waste from the kidneys typically appears in two forms: a semi-solid white, chalky material and a larger volume of clear liquid. The chalky material is urate, the excreted form of uric acid. This is the stuff that sticks to your car windscreen if you park under roosting birds. The clear liquid is true urine, and is the means by which urea and ammonia are excreted. Typically, well-hydrated pet reptiles will excrete urine, as well as urate. Without adequate access to water, the urate fraction would increase, and urine production would decrease, as a means of conserving fluid.
The production of urate is an extremely efficient evolutionary strategy that allows reptiles to survive in arid conditions, in which a mammal could not tolerate. (Please note, this does not mean that reptiles can survive without water, as even the excretion of uric acid, as well as other physiologic functions, requires some water.) Production of uric acid also helps to compensate for the fact that reptiles are unable to concentrate their urine, as mammals do. This is because reptiles lack a structure within the nephron (the functional filtering unit of the kidney) called the Loop of Henle. It is the presence of this structure that allows mammals and birds to concentrate their urine. A hormonal mechanism exists in vertebrates that controls blood flow to the kidney by constricting the diameter of the capillaries within the kidney.