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.

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.

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.

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:

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.