Types of Circulation (Types of circulatory system) in animals
Note:
The two types of animal circulation systems are:
- Open Circulatory Systems
(Circulatory fluid: hemolymph, also called haemolymph) - Closed Circulatory Systems
(Circulatory fluid: blood - which is contained within the structure of the heart and blood vessels through which the blood is conveyed around the body)
Two types of closed circulation systems are:
- Single Circulatory Systems e.g. in fish
- Double Circulatory Systems e.g. in mammals
Organisms need to move (transport) many different types of substances to wherever they are needed within the organism. This is important in order for cells to breathe and to receive energy, instructions, and materials for maintenance and cell functions. Animals also need to be able to remove from their bodies unwanted substances, such as waste products e.g. carbon dioxide.
Singled-celled (unicellular) organisms can move all the materials they need, which are also referred to as 'particles', by diffusion across their cell membrane and within the cytoplasm of the cell. Other very small organisms (microorganisms) can also move substances via diffusion and other types of movement across biological membranes. *Larger animals need a transport system or transport systems to convey respiratory gases, absorbed food material, waste products of metabolism and other substances around the body
What are the main types of circulation systems
(circulatory systems) in animals?
Not all animals have blood circulation systems e.g. some microorganisms such as unicellular amoeba don't need any circulation system. Many larger animals, even tiny insects, have circulation systems that move fluid around their bodies as a means of transporting supplies and waste products to/from cells (dissolved in the fluid).
There are two types of fluid circulation systems in animals.
(These circulation systems are also called transport systems and mass transport systems, the term 'mass transport' referring to the fact that something, i.e. the circulatory fluid and all the solutes dissolved within it, is being moved in volume. This can be compared with lots of people travelling by train as opposed to making their own way via different routes and modes of transport.)
The 2 types of circulation systems in animals are:
- Open Circulatory Systems (Open Circulation Systems)
e.g. in insects, where there may be some vessels but the circulatory fluid - 'hemolymph', also written 'haemolymph' - flows out of the vessels that form the circulatory system and bathes tissues directly. That is, instead of the blood capillaries in closed circulatory systems (see below), the main vessels in open circulatory systems open into open sinuses. - Closed Circulatory Systems (Closed Circulation Systems)
e.g. in vertebrates such as fish and mammals, blood is contained within the structure of the heart and blood vessels that have a range of sizes and structures, including arteries, arterioles, capillaries, venules and veins. Larger blood vessels move blood around the body, taking it from organ to organ, through the blood system, which is also called the vascular system. Substances pass between the blood and the tissues it supplies by moving through the thin walls of the smallest blood vessels, which are called capillaries.
Are all the circulatory systems in animals 'blood circulation systems' ?
Closed circulatory systems in animals are blood circulation systems.
The circulatory fluid in animals that have open circulatory systems is called 'hemolymph' or 'haemolymph' rather than 'blood' because in those animals there is no distinction between the 'blood' and the 'interstitial fluid' in animals that have closed circulatory systems - hemolymph being the combination of the equivalent of those fluids. Hemolymph has distinctive characteristics that differ from blood e.g. it contains hemocyanin, a copper-based protein that turns blue when oxygenated, as opposed to the iron-based hemoglobin in vertebrate red blood cells.
Also, in larger animals including humans, blood circulation is not the only circulation system - at least, blood is not the only fluid circulating. The human lymphatic system includes a network of lymph nodes and lymphatic vessels that contain a fluid called lymph. The lymphatic system - also called simply the lymph system, is not a closed system. (The lymphatic system is sometimes described in conjunction with the blood system, or it may be studied as part of the immune system.)
More about Closed Blood Circulation Systems:
The 3 three components of closed circulatory systems:
All closed blood circulatory systems include 3 essential parts (three components), specifically:
- Blood
- Blood Vessels
- Heart
As stated above, the blood (a fluid) is contained within a network of blood vessels, of which there are several different types according to their size and position within the blood circulation system. In order to perform the essential functions of blood, the blood must move around the blood system.
Movement of blood around the network of blood vessels happens because the heart pumps the blood through the heart itself and therefore onwards through the closed network of blood vessels which, together with the blood and the heart, form the closed circulatory blood system. 
Common to all closed circulatory systems
However, there are different types of closed blood circulation systems and also different structures of hearts (blood pumps) in animals with different types of blood circulation systems.
2 Types of closed blood circulation systems:
There are two types of closed blood circulation systems.
They are:
- Single Circulation Systems (Single Blood Circulation)
- Double Circulation Systems (Double Blood Circulation)
Notes about each of these follow below.
1.0 Single Circulatory System
Fish have single circulatory systems in which blood passes through the heart only once each time it completes a full circuit around the fish's body, including through its gills and all other organs and tissues.
- Blood flow rate and blood pressure fall when blood leaves a fish's gills (blood is oxygenated as it passes through fishes' gills - just as adult human blood is oxygenated as it passes through the lungs).
- The low blood pressure in the single circulatory systems present in fish is insufficient for efficient kidney function in mammals.
2.0 Double Circulatory System
Mammals have double circulatory systems - meaning that blood passes through the heart twice in order to complete a single complete circuit around the whole body, including through the lungs and all other parts (incl. organs and tissues) of the body.
Double circulatory systems include two circuits of blood flowing to and from the heart. They are:
- Pulmonary circulation is the circuit by which blood flows from the heart to the lungs, then back to the heart.
- Systemic circulation is the circuit by which blood leaves the left ventricle (LV) of the heart via the aorta, passes through the organs and tissues of the body (except for the lungs), then returns to the heart.
Separation of the blood circulation system into these two circuits enables blood at sufficient velocity (flow rate) and sufficient (high enough) blood pressure to efficiently transport essential substances to the cells of active mammals. This is necessary for the healthy functioning of their tissues & organs.
In order to pass all the way around a double blood circulatory system e.g. as in humans, blood must complete both the pulmonary circulation and the systemic circulation circuits, therefore it passes through the heart twice.
See also simple descriptions of the structure and functions of the human heart.
