Carbon dioxide, mineral salts, urea, creatinin, uric acid, bile pigments and excess water are the waste products in vertebrates.
The kidney is an organ unique to the vertebrates. It is the chief excretory unit in higher vertebrates but its major function in the lower vertebrates (fish) is osmoregulation. In fishes the nitrogenous waste are mainly eliminated through the gills.
In mammals. Skin has sweat glands which produce sweat for cooling the body in hot weather.
In some marine reptiles, external devices have evolved to deal with the excretion of excess salts. These salts excreting glands are similar to those of birds.
This sweat has some quantities of certain waste products like urea, salts, etc. so skin also acts for excretion to some extent. The lungs as you know are involved in elimination of CO2 and water. Some excretion also takes place through intestine as in the amphibians. Main excretory role in reptiles, birds and mammals is however played by a pair of kidneys and their ducts.
13.4.1 Human Urinary System
The human urinary system consists of a pair of kidneys with attached ureters. The ureters from both sides open into the bladder which opens into the urethra. Each kidney is a bean shaped organ enclosed in a fatty membrane. They are found attached to the dorsal body wall within the abdominal cavity, one on each side of the vertebral column, just above the waist-line. The right kidney lies slightly anterior to the left (fig. 13.5). The concave side of the kidney faces the vertebral column. The depression in the center of this surface is called hilus. It provides a place for the renal artery, renal vein and nerves to enter and leave the kidney.
The longitudinal section of a human kidney shows that it mainly consists of two regions. The outer dark red region is called cortex. The inner pale-red region is called medulla. The medulla consists of several cone shaped areas called pyramids.
The pyramids project into a funnel-like space present in center of concave surface of kidney called renal pelvis.
A tube, called ureter, originates from the renal pelvis (of each kidney). The ureters from both the kidneys open in a bladder, called as urinary bladder.
The urinary bladder opens to outside through a duct known as urethra.
Each kidney is composed of large number of tiny tubes – the urinary tubules or nephrons. Each nephron further consists of a cluster of blood capillaries called glomerulus and a long tubule called renal tubule. The renal tubule is closed at one end, which enlarges into a cup-shaped structure called Bowman ’s capsule. The Bowman’s capsule is surrounding the glomerulus and both are collectively called renal corpuscle. F 13.6 &13.7.
