The urogenital system:
The kidneys are located in the corresponding depressions on the ventral surface of the synsacrum and ilium (renal fossa).
Each kidney is divided
into three lobes, there is not a precise division between the cortex and the
medulla, and there are lots of calyces on each lobe. The renal portal system
is functional and of clinical importance. The external iliac veins and the
venous trunk of the last segments of the intestines turn into capillaries
as they enter the kidneys. After passing through the kidney parenchyma the
capillaries converge into efferent vessels, which drain into the caudal vena
cava. Administration of substances in the pelvic limb or via the chloaca is
not recommended because they pass directly through the kidneys and are excretated
without passing through the systemic circulation.
Urine is transported by the ureters to the chloaca (urodeum) because no urinary bladder exits.
Birds excrete semisolid urine (most of the water is absorbed in the urodeum) rich in uric acid. The testicles are intra-abdominal and situated next to the cranial aspect of the kidneys. In birds without an external sexual dimorphism endoscopy is necessary to ascertain the sex. There are two possible approaches (McLelland, 1992): over the sternal notch between the penultimate and the last rib, immediately ventral to the cranial border of the sartorius muscle; or over the triangle formed by the proximal end of the femur, the last rib and the cranial border of the pubis.
The epydidimis is located on the dorsomedial border of the testis and a deferent duct drains to the urodeum.
As testicles are intraabdominal organs the optimum temperature for spermatozoid production is achieved by the proximity of the air sacs which cool the testicles during forced inhalation. There are no accessory sex glands and the penis is rudimentary (penile papillae) except in some web-footed species (ducks) where it can be up to 8cm.
Only the left ovary and oviduct are developed in females.
However, in some raptors, the right ovary and oviduct are still functional. During sexual activity the ovary has a bunch like appearance, due to the numerous follicles on the surface.
The oocytes contained in the follicles quickly become visible, as they are wrapped in layers of vitelline yolk (future egg yolk). The oviduct has two functions: to ensure the egg progresses towards the cloaca, and to secrete the substances which will protect it from the environment. The avian oviduct can be divided into: infundibulum, magnum, isthmus, tubular part of the uterus, uterus and vagina.
The majority of the albumin is formed in the magnum and isthmus, whilst the formation of the shell membranes takes place in the ampulla and the formation of the shell in the uterus. The vagina is the last section of the oviduct in which the cuticle and specific pigment of the egg are formed: the opening into the cloaca (uroceo) is found next to the left ureter.