Circulatory System

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.

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The heart is made up of specialised muscle cells that contract voluntarily (True/ False)

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Multicellular organisms have a large surface area to volume ratio (True/ False)


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Multicellular organisms require circulatory systems due to their _____ surface area to volume ratio. Therefore they cannot rely on _____ alone like unicellular organisms (e.g. yeasts) for the exchange of substances. This is because their large _____ require substances to travel large ______ to enter. Subsequently, the surface for substances to enter proportionately______, ultimately becoming too _________if diffusion is the only means of exchanging substances .


Animals need a _____ because of their high metabolic rate necessary for generating sufficient energy (ATP). As animals are unable to ______ like plants, they have the capacity for _______ to obtain food. A biological pump is then essential for maximal _____ of the exchange of metabolic substances across cells.

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Double circulatory systems are found in organisms where oxygen demand is low (True/ False)

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Venules are a type of blood vessel (True/ False)


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The circulatory system produces hormones such as insulin (True/ False)


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Outline the four different functions of circulatory systems.

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The blood pressure in the systemic circulation is higher than in the pulmonary circulation (True/ False)


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Oxygenated and deoxygenated blood are mixed together to allow cells to receive as much oxygen as possible (True/ False)


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A single circulatory system has only ____ circulatory route that involves _____ sets of capillaries. The first set of capillaries facilitate the exchange of gases  _____ and ______,  whereas the second set is where the exchange of substances between the ______ and ______ occurs. Blood is said to travel _______ through the heart on one complete circuit. The single circulatory system is common in organisms where _____ ______ is ____, such as fish, echinoderms and earthworms.


On the other hand, a double circulatory system has ____circulatory routes. The ____ route carries ____ blood to the body, then _______ back to the heart after cellular gas exchange. Whereas the ______ route carries _______ blood to the lungs and back to the _____ upon oxygenation. Blood is said to travel _____ through the heart on one complete circuit. The double circulatory system is found in all _____- blooded animals and also _____.

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The circulatory system moves ammonia produced via deamination in the liver to the kidneys (True/ False)

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  • Immunology
  • Cell Biology
  • Mo

The heart is made up of specialised muscle cells that contract voluntarily (True/ False)

Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

Multicellular organisms have a large surface area to volume ratio (True/ False)


Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

Fill in the blanks.

Multicellular organisms require circulatory systems due to their _____ surface area to volume ratio. Therefore they cannot rely on _____ alone like unicellular organisms (e.g. yeasts) for the exchange of substances. This is because their large _____ require substances to travel large ______ to enter. Subsequently, the surface for substances to enter proportionately______, ultimately becoming too _________if diffusion is the only means of exchanging substances .


Animals need a _____ because of their high metabolic rate necessary for generating sufficient energy (ATP). As animals are unable to ______ like plants, they have the capacity for _______ to obtain food. A biological pump is then essential for maximal _____ of the exchange of metabolic substances across cells.

Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

Double circulatory systems are found in organisms where oxygen demand is low (True/ False)

Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

Venules are a type of blood vessel (True/ False)


Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

The circulatory system produces hormones such as insulin (True/ False)


Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

Outline the four different functions of circulatory systems.

Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

The blood pressure in the systemic circulation is higher than in the pulmonary circulation (True/ False)


Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

Oxygenated and deoxygenated blood are mixed together to allow cells to receive as much oxygen as possible (True/ False)


Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

Fill in the blanks.

A single circulatory system has only ____ circulatory route that involves _____ sets of capillaries. The first set of capillaries facilitate the exchange of gases  _____ and ______,  whereas the second set is where the exchange of substances between the ______ and ______ occurs. Blood is said to travel _______ through the heart on one complete circuit. The single circulatory system is common in organisms where _____ ______ is ____, such as fish, echinoderms and earthworms.


On the other hand, a double circulatory system has ____circulatory routes. The ____ route carries ____ blood to the body, then _______ back to the heart after cellular gas exchange. Whereas the ______ route carries _______ blood to the lungs and back to the _____ upon oxygenation. Blood is said to travel _____ through the heart on one complete circuit. The double circulatory system is found in all _____- blooded animals and also _____.

Show Answer
  • + Add tag
  • Immunology
  • Cell Biology
  • Mo

The circulatory system moves ammonia produced via deamination in the liver to the kidneys (True/ False)

Show Answer

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Table of contents

    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.

    BloodLymphVesselsHeart

    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 systemDouble 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.Present in warm-blooded animals (e.g., mammals and birds), also reptiles.

    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:

    1. 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.

    2. 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.

    Fill in the blanks.

    Multicellular organisms require circulatory systems due to their _____ surface area to volume ratio. Therefore they cannot rely on _____ alone like unicellular organisms (e.g. yeasts) for the exchange of substances. This is because their large _____ require substances to travel large ______ to enter. Subsequently, the surface for substances to enter proportionately______, ultimately becoming too _________if diffusion is the only means of exchanging substances .


    Animals need a _____ because of their high metabolic rate necessary for generating sufficient energy (ATP). As animals are unable to ______ like plants, they have the capacity for _______ to obtain food. A biological pump is then essential for maximal _____ of the exchange of metabolic substances across cells.

    Multicellular organisms require circulatory systems due to their small surface area to volume ratio. Therefore they cannot rely on diffusion alone like unicellular organisms (e.g. yeasts) for the exchange of substances. This is because their large sizes/volumes require substances to travel large distances to enter. Subsequently, the surface for substances to enter proportionately reduces, ultimately becoming too time-consuming if diffusion is the only means of exchanging substances.


    Animals need a heart because of the high metabolic rate necessary for generating sufficient energy (ATP). As animals are unable to photosynthesise like plants, they have the capacity for locomotion to obtain food. A biological pump is then essential for maximal efficiency of the exchange of metabolic substances across cells.


    Circulatory System Circulatory System
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    Frequently Asked Questions about 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 respiration
    • getting rid of metabolic waste products
    • transporting 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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