Transportation of Food and Other Substances in Animals!
In very simple animals, materials are transported through diffusion.
In complex animals, there is a special transport system to carry oxygen, carbon dioxide, nutrients, waste products, food and various other substances from one part of the body to the other.
This transport system, also called the circulatory system, comprises a blood vascular system and a lymphatic system.
The blood vascular system has three components—blood, blood vessels and the heart. The lymphatic system includes lymph, lymph vessels and lymph nodes.
Blood—a Fluid Transport Medium:
Blood is a liquid connective tissue having two main components—plasma and blood corpuscles.
Plasma is the liquid part of the blood. It is made up of water with various substances dissolved in it. These include proteins, salts, glucose, nitrogenous compounds, and so on. In many invertebrates, plasma contains the respiratory pigment.
Corpuscles are cells floating in the plasma. Red blood cells, a type of corpuscle in vertebrates, contain a red-coloured respiratory pigment called haemoglobin.
Heart—a Pumping Organ:
The heart is a muscular pumping organ. It pumps blood that comes to it from other parts of the body through the circulatory system. It pumps deoxygenated blood to the lungs for oxygenation, and oxygenated blood to all parts of the body.
Control of the heart:
The invertebrate heart is generally fully controlled by the nervous system. The vertebrate heart is controlled by a pacemaker system made up of specialized cardiac muscles.
Chambers in the heart:
The heart is divided into chambers in order to prevent the mixing of oxygenated blood with deoxygenated blood. A complete vertical partition of the heart into left and right chambers ensures a complete separation of oxygenated and deoxygenated blood. This type of partition is seen in animals having a double circulation system, with two circuits.
These animals (e.g., mammals, birds and crocodiles) have lungs. In one circuit blood flows between the heart and the lungs, and in the second circuit it flows between the heart and the body. The heart has four chambers—two atria (often also called auricles) and two ventricles. Such a heart is called ‘double’ heart.
This type of complete separation into chambers provides an ample supply of oxygen to all parts of the body. These animals are quite active, so they have a high rate of respiration and require an efficient supply of oxygen. Birds and mammals, being warm-blooded, need to spend energy to regulate their body temperature. This also requires oxygen.
Some vertebrates do not use energy for temperature regulation. Their temperature fluctuates with that of the environment and we call them cold-blooded. In these animals (except crocodiles) there is some mixing of oxygenated and deoxygenated blood in the heart.
This does not harm the animals as their energy demands are not very high. In amphibians the heart is three-chambered, having two atria and a single ventricle. Such a heart is called transitional heart. In most reptiles there are two atria and an incompletely divided ventricle.
Fish have a single circulation system. Their heart is two-chambered, having one atrium and one ventricle. Such a heart is called ‘single’ heart. The fish heart receives and pumps only impure blood.
The impure blood goes to the gills for oxygenation, and from there it goes to different parts of the body. The impure blood returns to the heart for being pumped out to the gills. Therefore, the fish heart is also called venous heart.
Blood vessels:
In vertebrates the blood vessels are arteries, veins and capillaries. Arteries are more muscular, while veins are more elastic. Capillaries are made up of a single layer of squalors epithelium. In invertebrates the blood vessels are not properly distinguished as arteries and veins.