The heart and monitoring heart function



Maths time

Heart structure

Heart rate during exercise

Interpreting heart function data


Gas exchange in large multicellular organisms is achieved by organs which have a large surface area and so are able to successfully provide the substances the organism needs in order to survive. In humans this is achieved by the lungs. But how does the oxygen acquired by the lungs actually reach every single cell of the body? A network of sorts is needed to do that. Many bigger and smaller tubes would come in handy. They would form like a… circulatory system. Oh wait, that’s precisely what mammals have: a circulatory system made of arteries, veins, capillaries, etc.

Plants, too, have a vascular (tubular) system. It is made of xylems and phloems. Yes, complicated names which you will love to learn about in the following topic (Transport systems in plants).

The key thing is that this circulation of a large amount of substances via a system of transportation is called mass flow, hence mass transport. Just more technical terms for you to learn, which describe something that really couldn’t get any simpler. Water, gases chillin’ through tubes in an organism.

Mass transport is required to fulfil the demands of a high basal metabolic rate arising from being multicellular and hence having a lower surface area to volume ratio.

Organisms exchange substances and heat with their environment all the time, and this possibility is crucial to survival. The specific way in which this is achieved is very tightly related to the shape and structure of the specific organism, as well as its environment. For example, unicellular organisms are so small that molecules such as oxygen and water can readily diffuse in and out via the membrane, due to the short diffusion pathway. Could this be achieved by a human, or even a bee? No – they are simply too big.

Two properties are important to consider here: the volume of an organism, and the surface area of an organism. The volume is what determines the amount of substances which need exchanging, while the surface area determines the amount which can be exchanged.

Surface area describes the number of cells in direct contact with the environment. Volume describes the space occupied by all metabolically active cells.

Key principle: as the size of an organism increases, the surface area to volume ratio decreases.

That might seem hard to really understand. Why use a ratio in the first place? Well, the ratio shows the relationship between surface area and volume ratio, i.e. how similar or dissimilar are they?


Maths Time

Small cube

Surface area = 12 x 6 = 6

Volume = 13 = 1

Surface area : volume = 6:1 = 6.00

Big cube

Surface area = 22 x 6 = 24

Volume = 23 = 8

Surface area : volume = 24:8 = 3:1 = 3.00

3 is smaller than 6, so as the cube/organism gets larger, the surface area to volume ratio decreases.

What this basically means is that the larger an organism gets, the less surface area is available to serve its increasing needs due to its increasing volume.


Heart structure

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