Circulatory System

Circulatory System

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The circulatory system is like a transportation network in your body. It moves gases, nutrients, and waste products to where they need to go. Without it, your body's different systems wouldn't work well together. That's why the circulatory system is so important – it keeps everything connected and functioning correctly. So, take care of your circulatory system by making healthy choices like eating a balanced diet, exercising regularly, and avoiding smoking. Your body will thank you for it! Remember, the circulatory system is the foundation of a healthy body.

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:

Surface area to volume ratio of a cube. As the surface area increases, the volume increases at a higher magnitude
Surface area to volume ratio of a cube. As the surface area increases, the volume increases at a higher magnitude

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?

It's interesting to know that animals and plants have their own 'piping systems'. These systems help transport substances such as gases, nutrients, and waste products throughout their bodies. Animals, having a high metabolic rate, require a lot of energy to generate enough ATP for their survival. Unlike plants, animals cannot photosynthesize so they need to move around to obtain food. This requires a significant amount of energy. That's why animals have a biological pump, which is the heart, to maximise the exchange of metabolic substances across cells. This ensures that every part of the body receives the necessary nutrients and oxygen it needs to function properly.

What are the functions of circulatory systems?

Circulatory systems play a crucial role in transporting fluids throughout the body. They have pumps that help keep the fluids moving. The main functions of these systems can be summarised as follows:

  • Supplying respiring cells with nutrients such as glucose absorbed from the small intestines
  • Maintaining a constant supply of oxygen from the 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

Apart from these critical roles in respiration, circulatory systems also transport substances made in one part of the body to another. For instance, hormones produced in the pancreas are transported to muscle cells through the circulatory system. This shows how important these systems are in maintaining the proper functioning of the body.

What are the different components of the circulatory system?

There are four components of the circulatory system, whose names and functions are described below.

The main components of the circulatory system and their function.

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.

Differences between open and closed circulatory systems
Differences between open and closed circulatory systems

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).

A squid
A common snail
A common snail

What are the types of closed circulatory systems?

Closed circulatory systems are crucial in organisms with high oxygen demands because they facilitate efficient blood flow. Warm-blooded animals with high metabolic rates require closed circulatory systems to quickly remove waste products. There are two main types of closed circulatory systems: single and double circulatory systems. The table below contrasts their differences:

FeatureSingle Circulatory SystemDouble Circulatory SystemHeartOne chamberTwo chambersBlood flowBlood flows through the heart once in each cycleBlood flows twice through the heart in each cycleOxygenationBlood is oxygenated in the gills or lungsBlood is oxygenated in the lungsExamplesFish, some amphibiansMammals, birds, reptiles

As the table shows, single circulatory systems have only one chamber in the heart, and blood flows through the heart once in each cycle. In contrast, double circulatory systems have two chambers in the heart, and blood flows twice through the heart in each cycle. In terms of oxygenation, blood is oxygenated in the gills or lungs in single circulatory systems, while in double circulatory systems, blood is oxygenated in the lungs. Examples of organisms with single circulatory systems include fish and some amphibians, while mammals, birds, and reptiles have double circulatory systems.

The structure of the human circulatory system

The human circulatory system is a closed double circulatory system consisting of both pulmonary and systemic circulation.

Circulatory system (closed) in humans


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?

Double circulatory systems offer two key advantages. Firstly, they ensure that there is no mixing of blood, which enables respiring cells to receive as much oxygen as possible. It also allows blood flow to be directed more precisely to the organs that need the most oxygen and nutrients. Secondly, double circulatory systems enable pressure differences. The systemic circulation has a higher pressure, which enables the rapid delivery of oxygenated blood, while the pulmonary circulation has a lower pressure to prevent damage to vessels and allow gas exchange.

To illustrate this point, we can compare double circulatory systems to the water pipes in our homes. In our homes, there are separate pipes for clean and dirty water to prevent cross-contamination. Similarly, in double circulatory systems, the separation of oxygenated and deoxygenated blood ensures that there is no mixing of blood and that oxygenated blood is delivered precisely where it is needed.

In summary, the circulatory system is crucial for multicellular organisms with small surface area to volume ratios. It plays a vital role in respiration and the transport of substances. The circulatory system consists of four components - blood, lymph, vessels, and heart. Animals have either open or closed circulatory systems, with closed systems being further divided into single and double circulatory systems. Humans have a closed double circulatory system. The advantages of closed circulatory systems include no mixing of blood and pressure differences enabled.

Circulatory System

How does the circulatory system work?

The circulatory system works by transporting nutrients, oxygen and waste products throughout the body with the help of the heart as a pump and blood vessels as routes of transportation.

What are the three types of circulatory systems?

The three types of circulatory systems include open, closed single and closed double circulatory systems.

What are the four main parts of the circulatory system?

Vessels, heart, blood and lymph.

What is the function of the circulatory system?

The circulatory system has four main functions:supplying respiring cells with nutrients such as glucose maintaining a constant supply of oxygen to cells undergoing aerobic respirationgetting rid of metabolic waste productstransporting hormones from the organ produced to the target site 

What organs are in the circulatory system?

The organs in the circulatory system include the heart, lungs, blood and lymphatic vessels.

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