Excretory System of Rabbit

The amino acids which are transported to different tissues along with other food stuff are involved in the synthesis of essential proteins and some others for producing energy.

• The excess amino acids can not be stored in the animal.
• They undergo sequential degradation and are converted into Ammonia. Ammonia is the most toxic nitrogenous waste and causes harmful effects if accumulated in the body.
• Hence Ammonia has to be sent outside immediately from the body. Ammonia is easily soluble in water and sent outside by utilizing some amount of water available in the body.
• The aquatic animals mainly excrete Ammonia as their chief nitrogenous product. Hence they are called Ammonotelic animals.
• If plenty of water is not available, the partially terrestrial animals without deficiency of water convert Ammonia into less toxic urea.
• They excrete urea as their chief nitrogenous waste and are called ureotelic animals.
• The desert animals like lizards, snakes, and birds could not tolerate the loss of water along with nitrogenous waste. Such animals convert their nitrogenous waste materials into the least toxic uric acid.
• They excrete uric acid as solid pellets for the conservation of water.
• The animals that excrete uric acid as their chief nitrogenous waste are called uricotelic animals.
• The separation of nitrogenous waste materials from the blood and their expulsion is called excretion.
• In vertebrates the excretory system and reproductive system are closely associated with each other, hence times it is called the urogenital system.
• The excretory system of a rabbit consists of a pair of kidneys, a pair of ureters, a urinary bladder, a urinogenital canal, and the urinogenital aperture.

Structure of Kidneys

• The kidneys are two dark red bodies present on either side of the vertebral column towards the anterior end of the peritoneal cavity, outside the coelom.
• The kidney on the right side is placed slightly anterior to the left kidney.
• The kidneys are attached to the dorsal body wall and are covered ventrally by the peritoneum.
• Each kidney is bean-shaped with a median notch called hilus on the inner side.
• The kidney has a convex outer surface and a concave inner surface.
• An elongated ureter originates from the hilus that runs posteriorly and opens into the urinary bladder.
• At the emergence of the ureter, it becomes dilated to form a funnel-like structure called the pelvis.
• A renal artery and a renal vein pass into the kidneys through the hilus.
• The kidney is enclosed by a thin outer capsule made up of connective tissue.
• Anatomically the kidney is made up of two layers namely outer cortex and inner medulla.
• The medulla is formed of several conical lobes called pyramids. The pyramids project into cup-like structures called calyces.
• The cortex opens into the medulla in between the pyramids. These extensions are called renal columns of Bertini.

Nephron

• Each pyramid of the kidney possesses thousands of tubules called uriniferous tubules or nephrons.
• In each pyramid number of uriniferous tubules join together to form a common tubule called collecting tubule or collecting duct.
• Many collecting ducts open into a straight duct called the duct of Bellini that opens at the apex of the pyramid.

The nephron consists of (i) a Malphigian capsule and (ii) a Convoluted duct.

Malphigian capsule consists of (a) a cup-shaped structure called Bowman's capsule and (b) a mass of blood capillaries called Glomerulus. The glomerulus is formed by the capillary network of afferent arteriole which is a branch of the renal artery. The Convoluted duct behind the Malphigian capsule is divided into three regions namely:

1. The proximal convoluted part present in the cortex
2. The loop of Henfe is situated in the medulla.
3. The distal convoluted part that re-enters the cortex.

• The proximal convoluted tubule originates from Bowman's capsule and forms a few coils in the cortex.
• Then it proceeds downwards into a descending limb.
• The descending limb then opens into the ascending limb which together forms a hairpin-like structure called, Henle's loop in the medulla.
• The ascending limb is continued into the distal convoluted tubule.
• The distal convoluted part opens into the collecting tubule which passes through the medulla and converges to form a conical process, the pyramid projecting into the pelvis.
• Thus the cortex consists of mostly amphibian capsules while the medulla consists of loops of Henle, parts of convoluted tubules, and collecting tubules.

Blood Supply to Nephron

• The blood is supplied to Bowman's cup by the afferent arteriole of the renal artery.
• The blood from the glomerules is collected by efferent arteriole.
• The efferent arteriole after leaving the Malphigian capsule divides into a network of capillaries to distribute the blood to the remaining parts of the tubule.
• All these capillaries unite together to form a renal vein that leaves the kidneys.
• As the renal portal system is absent in rabbits, the main source of blood supply to the kidney is the renal artery.
• The ureters originating from the two kidneys open into the urinary bladder.
• The urinary bladder in female rabbits opens into the urinogenital canal or vestibule.
• It is formed by the junction of the neck of the bladder with vagina.
• The vestibule opens out by vulva.
• The urinary bladder in males opens into the urethra present in the penis.
• The urethra or urinogenital duct opens out at the tip of the penis.
• The urinogenital canal both in males and females transmits the germ cells and urine.
• The ureters, urinary bladder, and urethra are made up of smooth muscle fibers.
• The ureters penetrate the bladder obliquely which prevents the reflex of urine to the kidneys.
• The opening of the urinary bladder into the urethra or urinogenital canal is guarded by sphincters which keep the urethra closed except during urination.