The excretory system in the domestic fowl consists of the two kidneys each with a ureter that carries the urine produced by the kidneys to the cloaca where it leaves the body. This system is a very important one and its failure will lead to death. The excretory system of the domestic fowl is of importance because of its function of eliminating metabolic wastes and the maintenance of the electrolyte and water balance. When the kidneys are diseased or damaged and unable to carry out their functions efficiently, the animal becomes debilitated and death often follows quite quickly. Its functions in the domestic fowl are:
i. Maintenance of the electrolyte balance
ii. Maintenance of the water balance
iii. Elimination of the wastes particularly nitrogen products of metabolism (but excepting carbon dioxide)
A renal tubule or nephron
The two kidneys of the domestic fowl each generally with three lobes are found immediately behind the lungs on each side of the vertebral column and closely associated with it. They are brownish in colour and their consistency is such that they are easily damaged during their removal. The straight and narrow ureter is a tube that leaves the medial border of each kidney and opens into the cloaca adjacent to the deferent duct of the male or the oviduct of the female. The ureter connects to many funnel shaped structures from each lobe of the kidney.
The kidney, on close inspection consists of two regions - the renal cortex and the renal medulla. A microscopic examination of a section of kidney will show that it consists of a large number of renal tubules or nephrons, each divided into cortical and medullary parts. The renal tubules extract the constituents of the urine from the blood that flows through the kidney.
A renal tubule or nephron is composed of:
· Renal corpuscle consisting of a close network of blood capillaries almost enclosed in a capsule - called Bowman’s Capsule. It is here that the necessary very close association with the blood occurs.
· Proximal convoluted (spiral like) segment leading from Bowman’s Capsule.
· Loop of straight tubule called the Loop of Henle.
· Distal convoluted segment.
· Collecting tubule that directs the urine into the ureter for elimination from the body.
The domestic fowl does not have a bladder found in mammals. The urine is in a thick pasty form with a very low water content but high in uric acid from nitrogen metabolism. Water in urine is re-absorbed before elimination for re-use by the bird. It is usually passed as a paste and is deposited as a whitish or cream cap on some faecal stools.
When the kidneys are not functioning as efficiently as normal, or sometimes when a very high protein diet is provided, there will be large quantities of uric acid in the blood and the system may be unable to cope. The kidney tubules are likely to swell with accumulated urate deposits, the white lines being clearly visible on the surface of the kidneys. The accumulation may lead to damage of the kidney cells leading to nephritis. The high concentration of uric acid in the blood may result in filtration through the capillary walls leading to visceral gout - a whitish deposit on the surface of many visceral organs. Infectious bronchitis may produce these effects.
