The simpler animals such as cnidarians and Platyhelminths lack specialized systems for the transport and distribution of materials. The organisms in these groups possess a large surface area to volume ration and diffusion of gases over the whole body is surface is sufficient for theirs needs. Internally the distance that materials have to travel is again small enough for them to move by diffusion or Cytoplasmic streaming.

An organisms increase in size and complexity so the quantity of materials moving in and out of the body increase. The distance that materials have to travel within the body also increases, so that diffusion becomes inadequate as a means for their distribution. Some other organism to another is therefore necessary. This generally takes the form of a mass flow system. There are two circulatory systems which rely on mass flow in animals, namely the blood vascular system and the lymphatic system. A vascular system is one which contains fluid-filled vessels involved in transport.

General characteristics of a blood vascular system
The purpose of a blood vascular system is to provide mass flow of materials from one part of the body to another over distances where diffusion would be too slow. On reaching their destination the materials must be able to pass through the walls of the circulatory system into the organs or tissues. Likewise, materials produced by these structures must also be able to enter the circulatory system. In other words, exchange systems are linked to mass flow systems. Every blood system possesses three distinct characteristics:

Two distinct types of blood system are found in animals. They are the open and closed blood systems.
The open blood system:
Blood is pumped by the heart into an aorta which branches into a number of arteries. These open into a series of blood spaces collectively called the haemocoel. In other word, blood does not stay in the blood vessels, hence the terms ‘open’. Blood under low pressure moves slowly between the tissues, gradually percolating back into the heart through open ended veins. Distributions of blood to the tissues are poorly controlled.
The closed blood system (echinoderms, most cephalopod mollusks, annelids, vertebrates including humans).

Blood vessels are named according to their structures and function. Vessels conveying blood away from the heart are called arteries. These branch into smaller arteries called arterioles. The arterioles divide many times into microscopic capillaries which are located between the cells of nearly all the body tissues. It is here that exchange of materials between blood and tissues takes place.
Within the organ or tissues the capillaries reunite to form Venules which being the process of returning blood to the heart. The Venules join to form veins. Veins carry blood back to the heart.