This document is part of a series that explores various aspects of the human body. The document aims to start you on the path of exploration, if you have not already started, and will not provide complete information. Look to challenge and supplement information in this document as appropriate.
In this document, let us look at the interactions of two parts- the air around us and the human body.
There is air all around us and we seem to take in air and throw it out at regular intervals, often without being aware that we are doing this. Air is there all around us. We may not be able to see it but we can definitely feel it.
Air maybe considered as an invisible mix of gases that surround the earth (and thus all that inhabit the earth as well). The major component of air is nitrogen (78%) and oxygen (21%).Air also has a mix of other gases and water vapour. It also has particles like dust and pollutants.
The human body, rather many bodies, inhabit the earth. For now, you can think of the human body as several parts that work in sync with each other and carry out several functions. Some parts provide a generalized function, some parts provide very specialized functions.
A good place to start maybe to consider how and why the human body interacts with air.
By definition, an interaction implies communication between or a reaction between two or more people or things, or acting in such a way as to have an effect on each other. There is a “give” and “take”.
If we take the definition at its literal value, there has to be a give and take between the human body and air if they are interacting. Air has something the human body needs, the human body has something air needs, and they exchange with each other. That is a very simplistic view of the interaction but a good place to move forward from.
What does the human body need from air?
All human bodies, irrespective of size, shape, sex, colour, race, ethnicity, locality and any other demographic are made up of cells. A mass of cells that connect together (and get a name in the bargain “Jack, Daniels, John, XYZ, ABC, etc etc etc) to perform various functions.
Let us go down the basic element of the body, then.
What does air have that the cells in the human body need?
To answer that, we need to first look at what the cell does. Why is the cell important within the context of a human body?
Cells are the basic units that carry out the processes of life (yeah, you got that right, process is not just a word that your teacher or boss hurls at you, process is there in life as well).
Cells need energy to carry out the processes of life that it is supposed to carry out. Cells in the human body get this energy by breaking down food molecules to release the chemical energy they contain. This is known as cell respiration and takes place in all cells of the human body.
The release of energy from food takes place through a process called as oxidation. Oxygen is used to oxidise food and carbon dioxide is produced as a by-product of this process. The main food oxidised is glucose, which is a sugar. Glucose has stored chemical energy that can then be converted into other forms of energy that the cell can use. The chemical energy thus generated can be used by the cell for
- Movement, by contraction of muscles
- Repair and regeneration (cell division) or building large molecules (proteins)
- Active transport of molecules and ions
In the cells of the human body, glucose combines with oxygen to produce carbon dioxide and water. This process is known as aerobic respiration because it uses oxygen. The aerobic steps in the process take place in the mitochondria of the cells.
This is a good place for you to research on energy (the different forms of energy and its production and expenditure), molecules and ions, and anaerobic respiration.
Cell respiration gives out energy. The energy is used to sustain the processes the cell has to maintain for life. Thus, the energy produced is used up.
How do cells transfer the energy produced to the processes that need it?
They do this with the help of adenosine triphosphate (ATP), which is found in all cells.
ATP is made up of adenosine (an organic molecule) attached to three phosphate (inorganic) groups. In the cell, ATP is broken down to lose a phosphate and become adenosine diphosphate (ADP). When this happens, energy is released and made available for the processes that require and demand energy.
We now see that ATP is broken down into ADP. This means that we need a steady supply of ATP (especially since it is found in all cells and is an essential component of the “energy protocol”)
ATP is made from the processes of cell respiration, using energy from the oxidised glucose to add a phosphate back onto ADP.
Thus, there is a cycle of production of energy, transfer and utilization of energy (ATP to ADP), and further replenishment in the cell (ADP to ATP). A loop that plays on and on until something interrupts it.
Consider a real life example (that plays out among adults and children) to illustrate the energy protocol in cells.
Consider a container of cookies as the energy. The mouth is where the processes that use energy happen.
There is no fun if the cookies remain in the container (unused energy). We need to transfer the cookies from the container to the mouth to sustain the processes.
This transfer happens when we exhibit good behaviour (ATP). When we exhibit good behaviour, we get permission to transfer the cookies from the container to our mouth.
Once the cookie enters the mouth, the previous good behaviour does not count anymore (L ). The good is removed and all that is left is behaviour (ADP).
To get more cookies from the container into our mouth, we need to convert the behaviour (ADP) again into Good behaviour (ATP).
We can see that oxygen is an essential part of this “energy protocol” within the cells.
Oxygen is used within the cell and gets converted to carbon dioxide. There has to be a continuous supply of oxygen into the cell, which translates to the cell having to continuously shed out the carbon dioxide it produces (to have enough space within the cell for oxygen). This happens through a process of diffusion across the cell wall. This is a good place for you to research on the structure and functions of the human cell. Pay attention to the cell wall.
The cells need a steady and continuous supply of oxygen. The blood carries the oxygen to the cells. The blood receives the oxygen from the air. Oxygen from the blood diffuses into the cells and carbon dioxide from the cells diffuse back into the blood. The blood gives up the carbon dioxide back into the air. This exchange of oxygen and carbon dioxide between the human body and the air takes place in the lungs.
If human bodies are continuously taking in oxygen, then air needs a steady supply of oxygen. This is where the exchange of carbon dioxide between the human body and air assumes significance. The carbon dioxide thrown out into the air gets absorbed by leaves of plants and trees which uses it for its sustenance (through photosynthesis). Oxygen is produced as a by-product and expelled by plants into the air. Thus, plants use the waste product expelled by humans (carbon dioxide) to sustain life and humans use the waste product (oxygen) expelled by plants to sustain life. Air serves as the middleman enabling this exchange to happen.
We now have an idea why the human body interacts with air. It is time to have a look at how this interaction takes place.
In very simple terms, the human body has to take in air with oxygen and expel air with carbon dioxide. Or, Air has to find a way to move in and out of the human body.
As we noted earlier, the human body has several parts and it makes use of a specialized system to deal with the gas exchange. For ease, this is known as the gas exchange system (earlier also known as the respiratory system). Let us explore the structure of this system to see how it is optimized for gas exchange.
The lungs, where the gas exchange happens, are enclosed in the chest or the thorax. The neck (that supports the head) is above the thorax and the abdomen (with the digestive system) is below the thorax. The thorax also contains the heart.
The thorax, thus, has two very important organs within it. The lungs and the heart. Needless to say, these internal organs have to be protected from injury. Hence the thorax has a mix of bones, cartilage and muscles that help to protect the internal organs from injury. The thorax also needs to be separated from the digestive system. The lungs and heart contract and expand, and the digestive system has peristaltic movements that can mimic expansion and contractions. The human body was designed such that organs and system (the lungs, heart and digestive systems) have enough space to expand and contract without impeding each other or intruding into the space of each other. To this end, the thorax is separated from the abdomen by a muscular sheet of tissue called the diaphragm.
The thoracic wall is made of ribs. Each rib is connected to the next (or separated from each other) by the “intercostal” muscles (costa= ribs, inter=between). The diaphragm is dome shaped with a fibrous “roof” and muscular edges forming the walls.
We breathe in air through our nose or mouth (we will discuss the nose and mouth in detail in a separate section as they serve multiple purposes (multitask) and focus now on the specialized structures for gas exchange).
The air we breathe in passes through a specialized pipe called as the windpipe or the trachea. The trachea then splits into two smaller pipes (one for each lung) called the bronchi. The walls of the trachea and the bronchus contain ring shaped cartilage that support the airways and keep them open when we breathe in air.
The trachea has a C shaped cartilage, in other words the cartilage does not completely encircle the trachea. Why is the cartilage C shaped in the trachea? The trachea lies adjacent to the food pipe or oesophagus. Food passes through the digestive system in waves. In the oesophagus, food passes as lumps (chewed in the mouth or gulped down as lumps) and not as squashed smoothies! The gaps in the tracheal cartilage allow lumps of food to pass easily down the food pipe without getting stuck by the tracheal cartilage. Thus, it allows for some expansion of the food pipe pushing the tracheal side a little bit inwards. This will not be possible if the tracheal cartilage rings were complete. The oesophagus separates from the traches where the bronchus forms and hence the bronchus has complete rings of cartilage. At this location, there will be difficulty in breathing if the food pipe intrudes into the space of the bronchus (as the airway or bronchus has become smaller).
Each bronchus then divides into smaller and smaller tubes called the bronchioles and eventually end at microscopic air sacs (air filled spaces bounded by membranes) called as alveoli. Gas exchange takes place in the alveoli.
We can now see that the structure is designed to take in a lot of air through larger cavities (the nose and mouth) and then progressively becomes smaller so that only air reaches the point where gas exchange takes place (the alveolar sacs). At the alveolar sac level, only gas exchange and air exists. Keep this in mind.
If air keeps moving in and out of the lungs, it is natural to expect that the lungs expand and contract as air moves in and out. We have seen earlier, briefly, that there is space for the lungs to expand and contract. We have seen that the heart also occupies space with the lungs in the thorax. We have seen there is a thoracic wall made of muscles and ribs and a diaphragm below.
The expansion of the lungs (and contraction) has a structural element to it. If the lungs expand too much without any structural limit, it may constantly rub against the heart and the thoracic wall. Not good for anyone! The constant rub against the heart may damage both the heart and the lungs, and the constant rub of the softer lungs against the harder muscles and ribs can damage the lungs.
The human body was thus designed to ensure that the lungs lie within sacs (membranes) that cover it and limit the physical expansion such that it does not rub against the heart or the walls of the thorax. There are two membranes (pleural membranes) that cover the lungs. These membranes make a continuous envelope around the lungs forming an airtight seal (like how the airlines seal those big suitcases). The two membranes can still rub against each other, right? So, the human body was further designed to take into account movement, expansion, contraction, friction and physical attributes like that. The two membranes are separated by a space that is filled with fluid, called the pleural fluid. The fluid acts as a lubricant so that the structural integrity of the pleural membranes is maintained.
We have seen the structural elements of the gas exchange system. How does it ensure that only air reaches the alveolar sacs?
We have seen that air contains dust and other smaller pollutants as well. How are these prevented from reaching the alveolar sacs? In other words, how are the airways kept clean?
The human body was designed to take into consideration this element as well. The decreasing size of the airways was one way to achieve this. Larger particles get stuck in the nose and mouth and are expelled. They cannot physically pass through the smaller spaces.
Additionally, the trachea and larger airways are lined with a layer of cells that produce mucus, a sticky liquid. The mucus (as yucky as it looks) actually serves a very useful purpose. It traps particles of dirt and bacteria that are breathed in. Other cells are covered with tiny hair like structures called cilia (like the bristles of a brush) that beat backwards and forwards sweeping the mucus and trapped particles out into the mouth and nose. These prevent such particles from entering the lungs where they may give rise to infections.
So how does air move in and out of the lungs? We have seen that the design of the human body has a well-planned out physical structure to aid the movement of air in and out of the lungs and enable gas exchange.
The movement of air in and out of the lungs (ventilation) is based on a difference of air pressure. Air moves from a place of high pressure to a place of low pressure. Ventilation depends on the fact that the thorax is an airtight cavity. When we breathe, we can change the volume of the thorax, and thus alter the pressure inside the air tight cavity. This causes air to move in and out of the lungs. Note that air does not move in and out of the lungs because they pull in and push out air physically. It is the difference in air pressure in the thoracic cavity that allows for air to move in and out.
How does the thoracic cavity change volume? Through the movement of the diaphragm and the thoracic wall (ribs and intercostal muscles).
When you breathe in deeply, the ribs move upwards and outwards. They are moved by the intercostal muscles. The outer intercostal muscles contract pulling the ribs up. At the same time, the muscles of the diaphragm contract pulling the diaphragm down into a flattened shape. The volume of the thoracic cavity is increased by both these muscle movements. As the volume increases, the pressure inside the thoracic cavity falls a bit. Now, there is less air pressure inside the thoracic cavity compared to outside. Air flows into the lungs.
What happens when you breathe out?
In shallow breaths, the elasticity of the lungs and the weight of the ribs acting downwards lead to a reduction of the thoracic volume (and consequently higher air pressure inside the thoracic cavity compared to outside) and air is expelled (exhalation).
In deeper breaths, the external and internal intercostal muscles contract pulling the ribs down and in. The muscles of the diaphragm relax and the diaphragm flattens. The volume of the thorax decreases and the pressure is raised slightly more than the atmospheric pressure. Air is forced out of the lungs.
Note that the difference in the air pressures between the external atmosphere and the thoracic cavity is not very much. If the difference is too much, there can be massive movement of air in and out causing injury. Only slight differences in pressure exist helping to regulate the air flow.
We have seen the elements of the physical design that help air move in and out, we have seen the differences in pressure that help the movement, we have seen the way the physical structure is designed such that only air reaches the actual point of gas exchange- the alveolar sacs.
We have mentioned earlier that blood carries the oxygen to the cells and the carbon dioxide from the cells. Thus, the exchange of gases is between air and blood at the alveolar sac level.
The alveoli must have a large structure that can bring air and blood close together over a large surface area for maximum or optimal exchange. The two lungs may have about 700,000,000 of the tiny air sacs giving a total surface area of 60m2. That is a large area not easily available in urban areas!
The alveoli are covered by small capillaries. Blood is pumped from the heart to the lungs and passes through the capillaries surrounding the alveoli. This blood carries carbon dioxide from the respiring cells. Around the lungs, the blood is separated from the air inside the alveolar sacs by only two cell layers- the cells making up the alveolar sac wall and the capillary wall. This distance is less than a thousandth of a millimetre.
Because the air in the alveolus has a higher concentration of oxygen, it diffuses into the capillaries across the two walls. Carbon dioxide diffuses from the blood into the alveolus through the two walls.
That, in essence, is the gas exchange system in the human body.
Areas for independent research
- Vital Capacity
- Residual Volume
- Control of Breathing rates
- Why is the heart placed with the lungs in the thorax? Why not elsewhere? Why not in the head or abdomen?
- Diseases or abnormalities of the gas exchange system. When you look at this, be sure to look at structural and functional implications. Connect the two. For example, what effect does smoking have on the structure of the gas exchange system? Why does that happen? What functional effects does it lead to?
- Consider smoking, emphysema, bronchitis, asthma, tuberculosis, pneumonia, and lung cancers and effects of air pollution.
- What happens if the pleural space is emptied of fluid or filled with more fluid or air? Connect structural and functional implications.
- This document has deliberately not been illustrated. Explore and find out illustrations that can embellish your understanding of this document and insert them appropriately.