Avian Urinary System
by Sherri Carpenter
Spring 2003
Kidney Form and Function
The urinary system is the body’s waste removal system. The kidneys' main function is to process and remove wastes (created from cell metabolism) and excess ions from the blood, regulate blood volume and maintain electrolyte balance. The ureters carry the wastes down to the cloaca where they exit out of the body.
Most birds, with the exception of Ostriches and rheas, lack a urinary bladder. Birds excrete urate instead of the toxic urea as mammals do. Urea (which is very toxic), is flushed from the body with a great amount of water. Urate (non toxic), a waste product from nitrogenous metabolism, is largely insoluble and uses very little water to excrete. This is probably an adaptation for water conservation in dryer climates. (click on drawings for a larger view.)
Another reason birds excrete urate instead of urea has to do with egg laying. The embryo is sealed within the egg and can only release gaseous waste products before it hatches. All other metabolic wastes, including nitrogenous wastes remain within the egg until hatching. There would be little room in the egg for the large amount of water needed to release urea, whereas the insoluble urates can be stored harmlessly until it hatches.
Avian kidneys are considerably larger in comparison to those of humans and reptiles. They have three lobes, a cranial (top), middle and caudal (bottom). They are lacking the defined cortex, medulla or renal pelvis found in mammals.
Avian kidneys are divided into units called lobules. The lobules have a cortex (outer area) and medullary cone. Nephrons are located in the lobules and empty into the collecting ducts that surround the lobule.
The lumbosacral nerves (sciatic) run through the kidneys. If a tumor is present, it may push on the nerves and cause paralysis of one or both legs.
click on drawings for a larger view.
The nephrons, (millions of them) are the structural and functional units of the kidney found in the lobules. These consist of the glomerulus, which is a knot of capillaries, loop of Henle, the renal tubule and the peritubular capillaries. One end of the renal tubule surrounds the glomerulus with a very porus membrane; it is called the Bowman's capsule.
The glomerulus is both fed and drained from small arteries called arterioles and its blood pressure is very high. Water and solutes smaller than proteins are forced through the capillary walls and pores of the glomerular capsule into the renal tubule. This fluid is called, filtrate. Water, glucose, amino acids, and needed ions are transported out of the filtrate back into the capillary blood. The peritubular capillaries are low pressure, porous vessels that reabsorb the solutes and water and return them to the veins leaving the cortex. Substances such as hydrogen, potassium, and nitrogenous wastes move from the blood of the peritubular capillaries through the tubule cells and into the collecting ducts (located in the medullary cones) and transported as urate.
Birds have differing types of nephrons.
- A reptile type that has a small glomerulus and no loops, which is found entirely in the cortex.
- a mammal type, with its convoluted tubules within the cortex and its loop descending into the medullary cone, which is capable of concentrating urine.
The mammal type has a high filtration rate and is always on, while the reptile type has lower filtration rate and shuts down during heavy salt loads. This strategy conserves water and maximizes urine concentration during osmotic challenges.
The majority of uric acid is secreted by the tubules of the reptilian type nephrons. As soon as the uric acid starts to crystallize in the kidney, the microvillus in the proximal tubule encapsulates the crystal with a protein coat formulating a highly soluble colloidal particle. This particle transports the uric acid through the nephrons to the ureters and into the cloaca along with urine.
The cloaca consists of three compartments. The coprodeum, receives the faeces, the urodeum receives the ureters and the gonadal ducts and the proctodeum is the common discharge area before the vent.
Once in the cloaca, both urine and the uric acid particles are sent to the large intestine where microorganisms recover the protein coat of the uric acid particle and water and electrolytes in the urine can be reabsorbed and balanced. Any free amino acids and sugars are taken up by specific transporters from the large intestines. The rest is than sent back into the cloaca. Uric acid functions as a magnet to pick off free radicals. Studies have shown that reduction of uric acid in chickens increases the rate of tissue aging to a rate found in human diabetics.
Other kidney functions include a precursor for the synthesis of Vitamin D3 for the absorption of calcium. The kidneys response to excess circulating phosphorus triggers the parathyroid gland to secret a hormone to adjust and balance the concentration of calcium and phosphorus. The hormone does this by promoting phosphorus excretion in the urine and activation removal of calcium from bone. The production of red blood cells by bone marrow is stimulated by the erythropoietin, which is produced by the kidneys when red blood cells are in short supply and/or delivery of oxygen to the tissues are in inadequate.
Taking a regular look at your birds' droppings can help alert you if something is wrong. Knowing what the changes look like in consistency and color from eating fruits and veggies may aslso help you evaluate if something is really wrong. Using regular newspaper or plain paper makes it easier to evaluate the droppings.
There are three parts to a birds droppings.
- Feces are usually solid, come from the colon and are usually coiled (not always). Color is usually green or brownish from eating pellets, seeds or veggies but can turn red for strawberries or red pellets. Blueberries and blackberries will produce an almost black like feces.
- Urates are chalky white.
- Urine is clear.
Urates and urine can together create a cloudy white mix. It helps to know what your bird has been eating that day. Here is a general list of colors for possible serious problems.
- Green or Yellow Urates - Liver Disease, Anorexia
- Brown or Chocolate Urates - Lead Poisoning
- Red Urine or Urates - Internal Bleeding
- Increased Urates - Dehydration
- Increased Urine - Disease, Eating foods high in water content, Drinking alot
This is from an African Grey that is approximately 7 yrs. old. He is on Zoopreme non colored pellets and Exact handfeeding formula to keep his weight up. The handing feeding formula that Copper is on is the macaw one for the extra fat. He was tested and also given calcium but he kept losing weight so this is what my friend does to keep his weight up. He is 397 grams. Is not a good eater. Is on Haldol twice a day 7 cc's for feather picking and has pretty much feathered out. Name is Copper
This is from another African Grey I'm not sure of the age. Shadow eats rainbow chunky pellets macaw size and a bean mix that my friend makes with veggies and grains in it.
This is another one from Shadow.
These are from my Quaker, who is 9 1/2 yrs. He has a fatty lipoma on his chest and by his vent. He also has high cholesterol (was just to the vet to see if it has gone down, don't know the results yet) He eats sprouts, mixed greens, squash, broccoli, cauliflower, peppers, parakeet seed, flax seed and a few pellets as treats. He was eating sprouts and seed mix when he did these. 
This is from my cockatiel Cheech, who is 10 1/2 yrs. old. She eats mostly sprouts, foundation formula pellets and parakeet seed mix, bean mix and some veggies. She was eating sprout and seed mix when she did this. No health problems.