Human Gas Exchange

Humans and most animals constantly need to breathe to survive. This article discusses how humans breathe and exchange gases with the environment. It also elaborates on some diseases of the respiratory system.

The arrangement of the human gas exchange system

Oxygen generates ATP in aerobic respiration and produces carbon dioxide as a by-product. Carbon dioxide needs to be removed as its buildup is toxic to cells. To function optimally, humans need to absorb large volumes of oxygen while removing large amounts of carbon dioxide from their blood.

Gas exchange is required because:

• The human body is composed of many living cells that undergo aerobic respiration.
• Humans are endothermic organisms, meaning they need to maintain a constant body temperature and have a high metabolic rate.

As a result, humans need a system capable of efficiently delivering oxygen to the body while removing carbon dioxide.

The human gas-exchange system consists of various organs and structures located in the chest cavity and is protected by the ribcage. They include:

• Trachea: The trachea is the primary airway that connects the mouth and the nasal cavity to the bronchi. C shaped cartilage rings that prevent the trachea from collapsing. Ciliated and mucus-producing epithelium line the trachea, sweeping microorganisms and dust particles away from the lungs.

• Lungs: Humans have two lungs. They are multi-lobed organs that form the centre of the respiratory system.

• Bronchi: Bronchi (singular: bronchus) are the two branches of the trachea to each lung. They have a similar structure to the trachea and are lined with ciliated mucosal epithelium. They become narrower and eventually connect to bronchioles.

• Bronchioles: Bronchioles are subdivisions of the bronchi that are lined with muscles. These muscles allow them to control the flow of air into the alveoli.

• Alveoli: Alveoli are the primary site of gas exchange. They are tiny air-sacs at the end of the bronchioles.

The mechanism of ventilation in gas exchange

For gas exchange to occur efficiently, the air is continuously moved in and out of the lung in a process called ventilation or breathing. The air movement is caused by the change of pressure inside the lungs created by the movement of the diaphragm and the intercostal muscles (located between the ribs).

Ventilation can be broken down into two phases: Inspiration (breathing in) and expiration (breathing out).

Inspiration

Inspiration is an active process that lowers the pressure inside the lung, causing the air to flow in. Here are the main events that occur during inspiration:

• The internal intercostal muscles relax.

• The external intercostal muscles contract to move the ribs upward and outwards.

• The diaphragm contracts and flattens.

Internal and external intercostal muscles act to increase the volume inside the thorax. Boyle’s law states that at a fixed temperature, the volume of gas is inversely proportional to the pressure exerted by the gas. Therefore, the increase in the volume of the thorax results in a drop in the pressure inside the lungs. Since the atmospheric pressure is higher than the pulmonary pressure, air enters the lungs.

Expiration

Quiet expiration is a passive process. The lungs and the thorax passively return to their original position after inspiration due to their natural elastic recoil. Expiration becomes active only when the demand for gas exchange is high, for example, during exercise or physical activities. The main events that occur during expiration are as follows:

• The external intercostal muscles relax.

• The internal intercostal muscles contract, moving the ribs downwards and inwards. (Only during active expiration)

• The diaphragm relaxes and so returns to its dome shape.

As a result, the lung volume decreases during expiration which causes the pulmonary pressure to exceed the atmospheric pressure, pushing the air out of the lungs.

Fig. 1 - The mechanism of muscle contractions during inspiration and expiration

Measuring pulmonary ventilation rate

The pulmonary ventilation rate can be measured by multiplying the tidal volume by the respiratory rate. Tidal volume and the respiratory rate are measured separately.

• Tidal volume: measured using a respirometer.
• Respiratory rate: number of breaths per minute counted.

$Pulmonaryventilationrate(d{m}^3.{min}^{-1})=tidalvolume(d{m}^3)\times respiratoryrate({min}^{-1})$

Gas exchange in the alveoli

The alveoli are tiny sacs in the lung that are in close contact with the blood. Collagenic and elastin fibres, which allow the air-sacs to stretch and expand during breathing, surround the alveoli. They are lined with a single cell layer epithelium (simple squamous epithelium) that allows fast exchange of gases between the air and the blood. The alveoli are surrounded by dense capillaries that are very narrow and lined with a single layer of endothelial cells.

Gas exchange in humans occurs at the epithelium of the lung alveoli. Exchange surfaces require specific features to allow efficient transfer of materials between organisms and the environments, and the alveoli are no exception. These features include:

Features of alveoli for an efficient gas exchange

Oxygen and carbon dioxide need to diffuse through the alveolar and capillary walls in opposite directions during the gas exchange.

The alveoli have very thin walls. They are lined with simple squamous epithelium that is only one cell thick. The blood capillaries surrounding the alveoli are also very thin, and their endothelium is one cell thick. Therefore, the gases have to diffuse over a short distance, increasing the rate and efficiency of gas exchange.

A single alveolus is tiny and has a small surface area on its own. But multiple alveoli collectively have a much larger surface area. There are approximately 300 million alveoli in each human lung with a collective surface area of 70${\mathrm{m}}^2$, about half the size of a tennis court! The alveoli are surrounded by networks of fine blood capillaries that collectively have a large surface area. This extensive surface area allows the gas exchange to occur rapidly.

The constant flow of blood through the alveolar capillaries and continuous air ventilation in and out of the lungs create and maintain a steep concentration gradient for gas exchange. This gradient for oxygen is from the alveoli to the blood, while the gradient for carbon dioxide is from the blood to the alveoli.

These features allow for rapid and efficient gas exchange to occur in the lungs.

The main lung diseases that impair the human gas exchange

Various disorders can affect the alveolar wall or obstruct the airways and impair the function of the lungs. These diseases include lung cancer, COPD, asthma and others.

Lung cancer

Cancer develops when mutations disrupt the control of cell replication, leading to unregulated cell divisions. Tumours in the lung form when oncogenes or tumour-suppressor genes in the bronchial epithelial cells mutate. As a result, these cells undergo rapid and unregulated cell replication and create a mass of abnormal and irregular cells.

The tumour eventually begins to disrupt the normal functioning of the lungs, such as by constricting the pulmonary arteries and veins. Malignant cancer cells also infiltrate the lymphatic system. They can then travel within the lymphatics and establish another tumour at another part of the body.

Patients with lung cancer often have symptoms such as a persistent cough, coughing up blood or increased mucus, sudden weight loss, and breathing difficulties.

COPD

Chronic obstructive pulmonary disease (COPD) refers to lung illnesses that include emphysema (shortness of breath) and chronic bronchitis.

COPD symptoms are shortness of breath, chest tightness, a persistent cough, wheezing when exercising or participating in any physical activity.

The ciliated epithelium of the trachea and bronchi of the lungs contain goblet cells that produce mucus. The cilia sweep up the dust and microorganisms trapped inside the mucus towards the throat and away from the lungs in healthy individuals. When these cilia become damaged or stop working, the mucus builds and causes narrowing of the airways.

Fig. 2 - Healthy and diseased alveoli in the lungs

Risk factors

Several specific risk factors increase the risk of developing COPD. These include:

• Smoking - Smoking is also known to increase the risk of developing lung cancer.

• Air pollution.

• Genetics (some people are genetically more likely to develop lung disease, while others are less likely).

• Infections - frequent chest infections increase the risk of developing COPD.

• Occupation - (working with harmful chemicals and gases also increases chances of developing COPD and lung cancer).

Asthma

During an asthma attack, the muscles lining the lungs’ airways constrict in response to anxiety or foreign particles. This causes narrowing of the airways and impairs breathing. People with asthma use inhalers that contain salbutamol. These inhalers cause the airway muscles to relax, resulting in the opening of the airways.

A triad of risk factors increases the risks of developing asthma in a person. This triad is called the atopy triad, and it includes:

1. Family history of asthma

2. Allergies such as hay fever

3. Eczema

Fig. 3 - The location of the lungs and airways in the body; B: a cross-section of a normal airway; C: a cross-section of an airway during asthma symptoms