A little introduction!
I know this Science section is primarily concerned with human biology, but I love this topic so much I’ve decided to make an executive decision and bend the rules just a bit! So, this essay focuses on gas exchange in the lungs, but also touches on their structural similarity to trees, and how design and function are conserved across nature. I also love this picture of leaves and wanted an excuse to put it on my website...
Gas exchange & the lungs
Let's get down to work. The basic principle of gas exchange in the human body is that oxygen is brought into the bloodstream for transportation around the body, and carbon dioxide is expelled. The reason for this is that oxygen is necessary for the efficient production of energy from carbon-based compounds (carbohydrate sugars such as glucose) within cells, while carbon dioxide is a potentially hazardous waste by-product of the formation of this energy. The respiratory system makes all of this possible, and consists of the lungs and various muscles and nervous system components - including parts of the brain - that control inhalation and exhalation. The lungs are composed of richly-vascularized spongy tissue that is divided into lobes, mainly consisting of a network of airways terminating in tiny structures called alveoli. This is where gas exchange occurs.
The diagram below shows an alveolus in more detail. Each structure is surrounded by a dense blood capillary network - the thin wall of the alveolus allows the diffusion of oxygen INTO the blood vessels (and therefore, around the body) and carbon dioxide FROM the blood vessels (and therefore, expulsion from the body via the lungs):
Gas exchange in the tree
Gas exchange in trees, on the other hand, is a little more complex, as it involves two separate but complementary events. These are:
[Note: as an aside, both sugar and starch are categories of carbohydrate, and are used to generate energy in humans and plants; the term 'sugar' is generally used when referring to human aerobic respiration, while 'starch' is the plant equivalent. For the sake of simplicity, the diagram directly below refers to 'sugar' only.]
- Respiratory gas exchange, which is similar to the process in humans whereby oxygen is brought into the organism and carbon dioxide is removed, and cells in the tree consume plant carbohydrates (for example, starch) to produce energy; and
- Photosynthesis, which is the reverse of this. Here, sunlight (energy) is combined with carbon dioxide and water to produce carbohydrates and oxygen. This is, without a doubt, the most important biochemical reaction on the planet and provides us with the oxygen we need in order to stay alive.
[Note: as an aside, both sugar and starch are categories of carbohydrate, and are used to generate energy in humans and plants; the term 'sugar' is generally used when referring to human aerobic respiration, while 'starch' is the plant equivalent. For the sake of simplicity, the diagram directly below refers to 'sugar' only.]
The diagram underneath shows the mechanics of photosynthesis. All plants contain pigments (these are what give leaves their characteristic colouring; the most well-known being the green pigment, chlorophyll) that are capable of absorbing energy from sunlight:
Generally, photosynthesis and the production of oxygen occur during daylight, and respiration and the consumption of oxygen takes place in the dark [this is why you should never keep plants in your bedroom at night!]. The amount of oxygen produced is greater than the amount consumed leading to a net surplus, thereby maintaining the optimum atmospheric levels that are necessary to sustain life on earth.
Conservation of structural/functional motifs across nature
This is all wonderful in its own right, but what has always fascinated me is the similarity between the physical make-up of trees and the anatomical structure of human lungs: both are composed of a central trunk or trachea, which divides into increasingly smaller branches, before terminating in effector structures that are ideally suited to gas exchange across their surfaces (leaves or alveoli). This conservation of a structural pattern or motif across nature is not unique, and highlights the underlying connectivity between apparently disparate organisms from entirely different realms. The deceptively simple exchange of oxygen for carbon dioxide in the alveoli represents one of the most powerful physiological processes in our bodies, and without this exchange we would die within minutes; similarly, trees provide us with this oxygen and utilize our carbon dioxide waste in order to do so. The beautiful flow and synchronicity of this inter-relationship goes completely unnoticed for the most part, but it is the foundation of all existence and deserves our recognition and appreciation… so YAY! for gas exchange!