Transportation in Unicellular Organisms:In unicellular organisms, transportation of substances happens through diffusion and osmosis. Gases move in and out of the cell by diffusion. Other substances move by osmosis.
CIRCULATORY SYSTEM IN HUMANS
Diffusion and osmosis can result in the transportation of substances to short distances only. For bigger and complex organisms, there is a need of a more complex system for transportation of substances.
The circulatory system in humans is composed of three main components, viz. heart, blood vessels and blood.
BLOOD
Blood is a type of tissue which is responsible for transportation of substances. Blood works as the carrier of various substances. Following are the main components of blood.
HEART
The heart is a small muscular organ which is responsible for pumping the blood. The human heart has four chambers, viz. the right auricle, the right ventricle, the left auricle and the left ventricle. The upper chambers are called auricle or atrium. The lower chambers are called ventricle. The following flowchart shows the movement of blood through the heart (the blue colour shows deoxygenated blood and the red colour shows oxygenated blood):
From the body → Right Auricle → Right Ventricle → Pulmonary Artery → Lungs → Pulmonary Vein → Left Auricle → Left Ventricle → To the body
EXCRETION Various activities always go on inside the body of a living being. These activities are collectively called metabolism. Many harmful substances are created during metabolic activities. These substances can prove lethal if not removed from the body in time. Removal of waste from the body is called excretion.
Excretory System
The human excretory system is composed of a pair of kidneys, two tubes; called ureter and a urinary bladder.
KIDNEY: Kidneys are bean-shaped. They work like filters. Blood; laden with waste materials enters the kidney. The waste is filtered from the blood and the purified blood is sent to the normal circulation. The waste; along with water; is transferred to the urinary bladder through the ureters. The content of the ureter is called urine. Urine contains 95% water, 2.5% urea and 2.5% other wastes. Urine is expelled out from time to time.
Type of waste and mode of excretion: Protein is made up of nitrogen. Metabolism of protein creates nitrogenous wastes in the body. The nitrogenous waste is the main waste in animals. The nitrogenous waste takes different forms in different animals. These are; ammonia, urea and uric acid. Based on the type of nitrogenous waste, animals can be divided into following categories:
AMMONOTELIC: Ammonia is the main nitrogenous waste in these animals. A lot of water is required for removal of ammonia. Ammonotelism is present in aquatic animals, e.g. fish, frogs, etc.
UREOTELISM: Urea is the main nitrogenous waste in these animals. Less water is required for removal of urea. Ureotelism is present in mammals.
URECOTELISM: Uric acid is the main nitrogenous waste in these animals. Removal of uric acid requires a negligible amount of water. Uricotelism is present in reptiles and aves.
Note: Some of the wastes are removed along with sweat. Carbon dioxide is an important waste which is removed through the lungs.