Meaning: circulatory system refers to the continuous movement or flow of the blood round the body involving the heart and the blood vessels.

Types of circulatory systems

Circulatory systems in animals can grouped into three major categories:
(a) Closed and opened circulatory systems,
(b) Single and double circulatory systems
(c) Pulmonary and systemic circulatory systems.

closed and opened circulatory systems

Closed and opened circulatory system: the closed circulatory systems are made up of blood vessels called arteries from the heart which branch many times into small units called capillaries but eventually join up with other vessels called veins that are connected to the heart. By this design blood is therefore always confined within the cells of the body. Some invertebrates like annelids and all the vertebrates have closed circulatory systems. In mammals, for example, there is no mixing of oxygenated and deoxygenated blood in the heart. While oxygenated blood is confined to the left part of the heart, deoxygenated blood is confined to the right side.

ii. Opened circulatory systems: in this system, the blood vessels lead out of the heart but end in blood spaces called heamocoels within the body cavity. In these spaces, the blood comes into the direct contact with the cells after which it is returned to the heart. Arthropods and some Molluscs have opened circulatory systems.

Single and double circulatory systems

(i) Single circulatory system: in double circulatory system, the blood only passes through the heart twice every time it makes one complete movement round the body. Each time the blood passes through the heart, it goes through a separate pathway. This system is common in fish which has only two-chambered heart–one auricle and one ventricle.
(ii) Double circulatory system: in double circulatory system, the blood passes through the heart twice every time it makes one complete movement round the body. Each time the blood passes through the heart, it goes through a separate pathway. This is found in mammals. The double circulation gives rise to the two pathways involved in double circulatory system called pulmonary and systemic circulation.

differences between diffusion and circulatory system

1. It occurs mainly in small organisms
2. It takes place at a slow rate.
3. It is less efficient.
4. It does not require the expenditure of energy
Circulatory system
1. It occurs in multicellular organisms.
2. It takes place at a faster rate.
3. It is more efficient.
4. It requires the expenditure of more energy.

differences between open system and close system

Open system
1. Blood bathes the major organ and tissues directly
2. There are no small vessels or capillaries connecting arteries with veins
3. It is found in some Molluscs arthropods and lower
4. Blood returns directly to the heart
5. The blood ends up in blood spaces [haemocoels] within the body
Closed system
1. Fluids with nutrients get to the body tissues from the capillaries as tissue fluids and lymph
2. Smaller blood capillaries unite the small arteries with the veins
3. It is found in annelids, vertebrates and some invertebrates
4. Blood returns to the heart through the veins.
5. Blood flows via the arteries, veins and capillaries

Pulmonary and systemic circulation

1. Pulmonary circulation: the pulmonary circulation involves the movement of blood between the heart and the lungs. In other words, it involves the movement of blood from the heart to the lungs for oxygenation.
2. Systemic circulation: the systemic circulation involves the movement of blood between the heart and all other part of the body besides the lungs. In one complete circulation, blood from any part of the body enters the heart for the first time, it is then sent to the lungs for oxygenation. From the lungs, it is brought back to the heart for the second time before it can be redistributed to all parts of the body. This is the sequence involve in double circulation
Pulmonary and systemic circulation can be represented mathematically by linear equation as: pulmonary circulation= heart + lungs
Systemic circulation= heart + body

The appearance of the heart twice in the two equations represents the double circulation of blood in animals.
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2. Biology as an enquiry in science
3. Branches of biology
4. Processes of methods of science
5. Usefulness of science
6. Living and non-living things
7. Characteristics of living things
8. Differences between plants and animals
9. Organization of life
10. Complexity of organization in higher organisms
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13. Kingdom fungi
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16. Cell as a living unit of an organism
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19. Similarity and differences between plant and animal cell
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22. Plasmolysis
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25. Faccidity
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27. Feeding
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37. Joint
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