EXCRETION - BASICS
The amino acids
which are transported to different tissues
along with other food stuffs are involved in 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 out side 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 least toxic uric acid.
- They excrete uric acid as solid pellets for conservation of water.
- The animals which 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 some times it is called urino-genital system.
- The excretory system of rabbit consists of a pair of kidneys, a pair of ureters, an urinary bladder, an urino-genital canal and the urino-genital aperture.
Structure of Kidneys
- The kidneys are two dark red bodies present on either side of vertebral column towards anterior end of the peritoneal cavity, outside the coelom.
- The kidney of the right side is placed slightly anterior than the left kidney.
- The kidneys are attached to the dorsal body wall and are covered ventrally by 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 posterioly and opens into urinary bladder.
- At the emergence of ureter, it becomes dilated to form a funnel - like structure called 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.
- Each pyramid of the kidney possess 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. Malphigian capsule and
ii. 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 capillary
net work of afferent arteriole
which is a branch of renal artery
. The Convoluted duct behind the Malphigian capsule is divided into three regions namely
i. Proximal convoluted part present in the cortex
ii. Loop of Henfe - situated in the medulla and
iii. distal convoluted part that re-enters the cortex.
- The proximal convoluted tubule originates from Bowman's capsule and forms few coils in the cortex.
- Then it proceeds downwards into a descending limb.
- The descending limb then opens into ascending limb which together form a hair pin-like structure called, Henle's loop in the medulla.
- The ascending limb is continued into distal convoluted tubule.
- The distal convoluted part opens into collecting tubule which pass through medulla and converge to form a conical process, the pyramid projecting into pelvis.
- Thus the cortex consists of mostly malphigian capsule while the medulla consists of loops of Henle, parts of convoluted tubules and collecting tubule.
Blood supply to nephron
- The blood is supplied to Bowman's cup by afferent arteriole of renal artery.
- The blood from the glomerules is collected by efferent arteriole.
- The efferent arteriole after leaving the Malphigian capsule divides into a net work of capillaries so as 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 rabbit, main source of blood supply to the kidney is renal artery.
- The ureters originating from the two kidneys open into urinary bladder.
- The urinary bladder in female rabbit opens into urinogenital canal or vestibule.
- It is formed by the junction of neck of bladder with vagina.
- The vestibule opens out by vulva.
- The urinary bladder in male opens into urethra present in penis.
- The urethra or urinogenital duct opens out at the tip of penis.
- The urinogenital canal both in male and female transmits the germ cells and urine.
- The ureters, urinary bladder and urethra are made up of smooth muscle fibres.
- The ureters penetrate the bladder obliquely that prevents the reflex of urine to the kidneys.
- The opening of urinary bladder into the urethra or urinogenital canal is guarded by sphincters which keep the urethra closed except during urination.