On the land and in dry atmosphere, excess water in the body is removed by evaporationthrough the skin. Greater percentage of the carbon IV oxide is excreted by diffusion through theskin; the rest is removed through expiration through the mouth and the lungs.
Page | 69[BIO NOTE- SS2]The kidneys are located on the dorsal wall of the amphibian body. The urinary bladder found asa ventral outgrowth from the cloaca is not truly a urinary bladder as the Wolfian ducts do notopen into it. The fluid in the bladder is believed to be secreted by the blood vessels in thebladder and the opening and closing of the bladder is controlled by a sphincter. The bladder inessence acts as a reservoir holding water for use during dry periods. The nitrogenous waste inamphibians is urea extracted from the blood by ultra filtration. The Wolfian duct passes throughthe outer side of the kidneys and opens into the cloaca.Excretion in fishExcretion in fish is by the kidneys and gills. The kidneys lie along either side of the dorsal aorta.Each kidney is long and extends a reasonable length of the body, tapering anteriorly. In theanterior part of the kidney called the leyding’s gland, the vas deferens is found.The posterior part is truly the urinary part and from it, ducts lead into the ureter. The ureterruns into the small urino-genital sinus that opens into the cloaca. Carbon IV oxide is excretedthrough the gills. The main nitrogenous waste is urea and is excreted by the kidneys. Fish in aneffort to maintain the osmotic pressure of its body retains much of its urea in the body fluid andin some tissues. Most tissues of fish except the brain the blood produce urea while a portion ofthe kidney tubules absorb urea from the glomerular filtrate. The resulting urine consists mainlyof water and excess salt. Excess urea is excreted through the gills.
Page | 70[BIO NOTE- SS2]Excretion in other vertebrates All vertebrates have a pair of kidneys which are generally similar in structure and functions asthose of mammals.Structure of the mammalian kidneyThere is a pair of kidneys in each vertebrate. The kidneys form the urinary part. In mammals,the kidneys are bean-shaped and dark-red in colour. They are surrounded by fat deposits whichhold the kidneys in position in the abdominal cavity. In man, the right kidney is slightly lowerthan the left kidney. The longitudinal section of the kidney shows that it consists of two mainregions: a.An outer cortexb.Inner medullaAn expanded tube at the depression on one side of the kidney where the ureter enters thekidney is called the pelvis,and these projections are known as pyramids. Two branches of thedorsal aorta called the renal arteries carry blood to the kidneys. Two renal veins carry bloodfrom the kidneys into the inferior vena cava. Actual excretion is carried out by the uniferoustubules. Each tubule ends as a small rounded funnel called the Bowman’s capsule located inthe cortex. A knot of blood capillaries fits into the bowman’s capsule. This knot is called theglomerulus.