https://youtube.com/watch?v=73ei6YD0VnM%3Fstart%3D78

Circulatory system is defined as continuous movement or flow of blood round the body which involves the heart  and blood vessels.

TYPES OF CIRCULATION

1. Closed circulation
2. Open circulation

iii.   Single circulation

1. Double circulation
2. Pulmonary circulation
3. Systemic circulation.

CLOSED CIRCULATION:   is a system made up of blood vessels called arteries from the heart which branch many times into a smaller unit called capillaries but eventually join up with other vessels called veins that is connected to the heart. Some invertebrate like annelids (earthworm) and all vertebrates have closed circulatory system.

OPEN CIRCULATORY: is a type of circulatory system where the blood vessels lead out of the heart end up in a blood space called HAEMOCOEL. In this space, the blood comes into direct contact with the cell after, which it is returned to the heart. Arthropods and some Mollusc use open circulation.

SINGLE CIRCULATION: is defined as a type of circulatory system in which the blood passes through the heart once every time the blood makes one complete circulation e.g. fish which has only two chambers heart i.e one auricle and one ventricle.

DOUBLE CIRCULATION: is a type of blood circulation in which the blood passes through the heart twice every time the blood makes one complete movement round the body. Each time the blood passes through the heart, it goes through a separate path way. This types of circulation system is found in mammals. Double circulation give rise to two pathways namely Pulmonary and Systemic circulation.

PULMONARY CIRCULATION: This is the movement of blood between the heart and the lungs for oxygenation.

IMPORTANCE OF PULMONARY CIRCULATION

1. Pulmonary circulation prevents mixing of deoxygenated blood with oxygenated blood.
2. It makes oxygen available in the life ventricles for system circulation.
3. It checks accumulation of water and enhances renewal of blood.
4. It prevents deoxygenated blood from entering the systemic and coronary circulation.

MECHANISM OF PULMONARY CIRCULATION

The superior vena cava returns blood from the head and the upper limbs into the right auricle. Blood from the lower limbs and the abdomen is brought to the right auricle by the inferior vena cava. The right auricle contract and the blood is forced through the tricuspid valves into the right ventricle. The right ventricle contact forcing the blood into the pulmonary artery. The pulmonary artery is divided into numerous arterioles which in turn are subdivided into microscopic capillaries in the lungs. With the thin walls of the capillaries and the moist nature of individual cells of the lungs, exchange of gases, nutrients and waste take place between the blood and the cell of the lungs. The capillaries unite to form VENULES. The veinules in turn from the pulmonary vein that bring back the oxygenated blood into the left auricle of the heart.

SYSTEMIC CIRCULATION

It is the movement of  blood between the heart and all part of the body apart from the lungs. When oxygenated blood arrive at the left auricle from the lungs. The left auricle relaxes and the left ventricle contrasts. Blood is forced up into   the AORTA, then to all part of the body. Blood rich in oxygen and other nutrients is taken to all living cells in the body. The aorta is the main artery   leaving the heart, it forms anorthic arch and give out three branches before running down the truck.

BLOOD VESSELS AND THE HEART

The blood vessels are the tubes through which the bloods flows away or to the heart.

ARTERIES:- Arteries receive blood directly  from the heart. To withstand the high pressure of blood that flow in them, they have thick wall which are muscular and elastic. These  walls also enable arteries constrict or dilate and thus regulate the amount of blood flowing in them.

VEIN  carries blood to back heart. The blood in them is at low pressure. Thus, vein is not as thick, muscular, or elastic as that of an  artery of about the same size. Most veins have valves inside them to prevent  backflow  of blood.

CAPILARIES- capillaries are microscopic blood vessels form a network linking arteriole to venules. The blood in the capillaries at the arteriole end is at higher pressure that at the venule ends. The tiny wall of the capillaries make it easy for oxygen, nutrient and waste product to diffuse between the cell and blood

Difference between artery and vein

1. It has thick  muscular wall	It has thickness muscular wall
2. It has elastic wall	Its wall is not elastic
3. It carries blood away from the heart	It carries blood to the heat
4. All arteries carry oxygenated except pulmonary artery	All veins carry deoxygenated blood except pulmonary vein
5.The blood in the artery is pink or bright in colour	The blood in the vein is dark red in colour
6. Blood at high pressure and flow fast.	The blood pressure is low and flows slowly
7.Valves are absent except semi- lunar valves near the heart	Valves are present
8. Arteries are deeply located in the muscle	Veins are  superficially located in the muscle
9. It has small lumen.	It has large lumen.