The circulatory system is responsible for transporting substances (gases, nutrients, waste products) throughout the body. It is fundamental in connecting all the bodily systems and ensuring they function correctly.
Why is there a need for a circulatory system?
Consider the two questions in this section.
Can multicellular organisms rely on diffusion alone for transport and exchange of substances?
The answer is a NO, right?
The main reason is that the large sizes of multicellular organisms result in a small surface area to volume ratio. Substances need to travel large distances to get inside designated cells, tissues and organs. The surface that substances enter proportionately reduces. This becomes too time-consuming if the only means of the exchange of substances is diffusion.
In short, the larger an object, the smaller the surface area to volume ratio, as highlighted in the diagram below:
This is why multicellular organisms require circulatory systems (or ‘internal pipes’) to transport substances from one site to another.
Animals have hearts, but plants do not. Why is this?
Animals and plants are both multicellular organisms and have their own ‘piping systems’ (i.e. vessels). However, animals have a high metabolic rate that is necessary for generating sufficient energy (ATP). Since animals cannot photosynthesise, they have the capacity for locomotion to obtain food which requires a lot of energy. Therefore, a biological pump (i.e. the heart) is essential for maximising the exchange of metabolic substances across cells.
What are the functions of circulatory systems?
Circulatory systems are well-organised transport systems with pumps to keep fluid moving through them. Their relevant functions can be summarised as follows:
Supplying respiring cells with nutrients such as glucose absorbed from small intestines
Maintaining a constant supply of oxygen from lungs to cells undergoing aerobic respiration
Getting rid of metabolic waste products such as carbon dioxide in respiring tissues and transporting it back to the lungs
Besides the importance of circulatory systems in respiration, they also transport substances made from one part of the body to another, such as hormones made in the pancreas to muscle cells.
What are the different components of the circulatory system?
There are four components of the circulatory system, whose names and functions are described below.
Table 1. The main components of the circulatory system and their function.
| Lymph | Vessels | Heart |
Medium for specialised cells (e.g. red blood cells, white blood cells) to carry out their function | Made of tissue fluid to regulate the osmotic pressure in the body | Facilitates the movement of blood to specific tissues in the body. | A hollow, muscular organ that pumps blood. Made of specialised muscle cells that contract involuntarily without rest. |
Red blood cells play a role in the transport of oxygen and carbon dioxide. | Medium for specialised white blood cells (e.g. lymphocytes) to carry out their function | There are five different types of vessels (arteries, arterioles, capillaries, venules and veins) |
Also contains plasma where solutes (e.g., glucose) are dissolved and transported. |
What are the types of circulatory systems in multicellular organisms?
With a better understanding of the importance of circulatory systems, let’s go into more detail about the different types of circulatory systems out there. The examples focus on the types of circulatory systems in animals.
There are two main types of circulatory systems - the open circulatory system and the closed circulatory system. Below is a table to contrast their differences.
Table 2. Differences between open and closed circulatory systems.
Open circulatory system | Closed circulatory system |
No gas exchange. Haemolymph only transports food and waste products. | The exchange of substances happens instead via the walls of the blood vessels. As closed circulatory systems facilitate gas exchange, oxygen-carrying pigment is often present. |
Present in arthropods such as insects and most molluscs. | Present in echinoderms (e.g., starfish, sea urchins), cephalopod molluscs (e.g., squids), earthworms, and all vertebrates. |
‘Blood’ (haemolymph) leaks out of vessels into the cavities surrounding cells under low pressure (haemocoel), then re-enters the heart via an open-ended vessel. | ‘Leak free’ because blood is contained within tubes without coming into direct contact with cells, allows a continuous journey of blood out to the most distant parts of the body and back to the heart at high pressures. |
Both snails and squids are from the same mollusc phylum; however, they have evolved different circulatory systems. A squid has a closed circulatory system that creates a high-pressure blood flow, so when the squid is injured you would see black ink gushing out. A snail has an open circulatory system where the blood flow is slower due to the lower hydrostatic pressure. If you pick up a snail, you will notice that it feels squishy (this is from the lower pressure).
What are the types of closed circulatory systems?
Given how closed circulatory systems facilitate efficient blood flow, these circulatory systems are crucial in organisms with higher oxygen demand. For example, in warm-blooded animals with high metabolic rates, closed circulatory systems satisfy the need to remove waste products rapidly.
Similarly, there are two main types of closed circulatory systems. These include single and double circulatory systems. Below is a table that contrasts their differences:
Table 3. Single and double circulatory systems
| Single circulatory system | Double circulatory system |
Has only one circulatory route that involves two sets of capillaries: - First set - oxygen and carbon dioxide exchange.
- Second set - exchange of substances between the blood and cells.
| Has two different circulatory routes: - Systemic - carries oxygenated (oxygen-rich) blood to the body, then back to the heart after gas exchange.
- Pulmonary - carries deoxygenated (oxygen-poor) blood to the lungs, then back to the heart upon oxygenation.
|
Blood travels once through the heart on one complete ‘circuit.’ | Blood travels twice through the heart on one complete ‘circuit.’ |
Present in fish, echinoderms and earthworms where oxygen demand is low. | |
The structure of the human circulatory system
The human circulatory system is a closed double circulatory system consisting of both pulmonary and systemic circulation.
In pulmonary circulation, blood leaves the right ventricle via the pulmonary artery, enters the lung to get oxygenated, then is directed to the left atrium via the pulmonary vein. On the other hand, blood leaves the left ventricle to the rest of the body via the aorta, then returns to the right side of the heart in the vena cava in the systemic circulation.
What are the advantages of a double circulatory system?
There are two advantages of a double circulatory system:
Ensures that there is no mixing of blood - not only allows respiring cells to receive as much oxygen as possible but blood flow can also be directed more precisely to the organs that need most oxygen and nutrients.
Enables pressure differences - the systemic circulation has a higher pressure to receive oxygenated blood rapidly. The pulmonary circulation has a lower pressure to prevent damage to vessels and allow gas exchange.
Food for thought: I like to compare double circulatory systems to the water pipes in our homes, where there are separate pipes for clean and dirty water to prevent cross-contamination.
Circulatory System - Key takeaways
- Multicellular organisms need circulatory systems due to their small surface area to volume ratios. Animals require hearts for maximal efficiency of the exchange of metabolic substances across cells.
- The circulatory system plays a role in respiration and the transport of substances. It consists of four components - blood, lymph, vessels and heart.
- Animals have either open or closed circulatory systems. There are two types of closed circulatory systems - closed single and double circulatory systems. Humans have closed double circulatory systems.
- The advantages of closed circulatory systems include no mixing of blood and pressure differences enabled.
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