Oxygen Transport and the Lungs

This section explains oxygen transport and the lungs, including: the role of the lungs in respiration, the structure of the lungs, the process of the gas exchange, oxygen transport in blood and the role of haemoglobin in oxygen transport.

The Role of the Lungs in Respiration

The lungs are essential organs in the process of respiration, which involves the exchange of gases, primarily oxygen ($O_2$) and carbon dioxide ($CO_2$). The main function of the lungs is to take in oxygen from the air and remove carbon dioxide, a waste product of cellular respiration, from the body.

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Respiration occurs in two stages:

Breathing: The process of taking in oxygen and expelling carbon dioxide.

Gas Exchange: The movement of oxygen and carbon dioxide between the lungs and the blood.

The lungs are located in the chest, protected by the rib cage, and are separated from the rest of the body by a membrane called the diaphragm, which helps with breathing.

Structure of the Lungs

The lungs contain millions of tiny air sacs called alveoli (singular: alveolus), which are the sites of gas exchange. The structure of the lungs is adapted for efficient gas exchange:

• Alveoli: Small, balloon-like structures where oxygen is transferred to the blood, and carbon dioxide is removed. Each alveolus has a large surface area, which maximises the rate of gas exchange.
• Capillaries: Tiny blood vessels that surround the alveoli. They allow oxygen to diffuse into the blood and carbon dioxide to diffuse out of the blood.
• Thin Walls: The walls of the alveoli and capillaries are only one cell thick, which reduces the distance over which gases have to diffuse, making the process of gas exchange more efficient.
• Rich Blood Supply: The alveoli are surrounded by a dense network of capillaries to ensure that oxygen is rapidly transported away from the lungs to body tissues, and carbon dioxide is quickly removed.

The Process of Gas Exchange

Gas exchange in the lungs occurs by diffusion, the movement of gases from an area of high concentration to an area of low concentration. Here's how the process works:

Oxygen Inhalation: When you breathe in, air enters the lungs through the trachea, which divides into two bronchi (one for each lung). The bronchi branch into smaller tubes called bronchioles, which eventually lead to the alveoli.

Oxygen Diffuses into the Blood: In the alveoli, oxygen from the air moves through the thin alveolar walls into the surrounding capillaries. This happens because the concentration of oxygen in the alveoli is higher than in the blood.

Carbon Dioxide Diffuses into the Alveoli: At the same time, carbon dioxide (which is produced by cells during respiration) diffuses from the blood (where its concentration is higher) into the alveoli (where its concentration is lower) to be exhaled.

This process allows oxygen to enter the bloodstream and be transported to cells for use in cellular respiration, and it removes carbon dioxide, which would otherwise build up and become toxic.

Oxygen Transport in the Blood

Once oxygen enters the bloodstream, it binds to haemoglobin in the red blood cells to form oxyhaemoglobin. This process is essential because oxygen is not very soluble in plasma (the liquid part of the blood), so it needs to be carried by haemoglobin for efficient transport around the body.

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• Haemoglobin is a protein that can bind to oxygen molecules. In the lungs, where the oxygen concentration is high, haemoglobin binds to oxygen. In tissues where the oxygen concentration is low (due to cellular respiration), oxygen is released from haemoglobin.

The oxygen-rich blood is then carried by the circulatory system through the arteries to body tissues. As the blood reaches cells, oxygen is released from the haemoglobin and diffuses into the cells to be used in aerobic respiration, which produces energy for the cells.

Breathing Mechanism

Breathing is controlled by the respiratory system, which involves the movement of air in and out of the lungs. The two main phases of breathing are:

• Inhalation (breathing in): Air is drawn into the lungs. This happens when the diaphragm contracts and moves down, and the intercostal muscles (muscles between the ribs) contract, pulling the rib cage up and out. This increases the volume of the thoracic (chest) cavity and decreases the pressure inside, causing air to flow into the lungs.
• Exhalation (breathing out): Air is pushed out of the lungs. The diaphragm relaxes and moves upwards, while the intercostal muscles relax, causing the rib cage to move down and in. This decreases the volume of the thoracic cavity and increases the pressure inside, forcing air out of the lungs.

The Role of Haemoglobin in Oxygen Transport

As mentioned, haemoglobin is the protein in red blood cells that binds to oxygen. This is a reversible process, meaning haemoglobin can pick up oxygen in the lungs and release it in tissues. The ability of haemoglobin to bind with oxygen depends on the concentration of oxygen in the surroundings:

• In the lungs, where oxygen is abundant, haemoglobin binds with oxygen to form oxyhaemoglobin.
• In tissues, where oxygen is used for cellular respiration and the oxygen concentration is lower, haemoglobin releases oxygen to be used by cells.

The Oxygen Dissociation Curve

The relationship between the partial pressure of oxygen (the concentration of oxygen) and the amount of oxygen bound to haemoglobin can be illustrated by an oxygen dissociation curve. The curve shows that as the oxygen concentration increases (such as in the lungs), more oxygen binds to haemoglobin. Conversely, when the oxygen concentration decreases (such as in body tissues), haemoglobin releases oxygen.

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Breathing and Gas Exchange in Different Conditions

Several factors can affect the efficiency of gas exchange and oxygen transport in the body:

• Exercise: During physical activity, muscles require more oxygen. The body responds by increasing the rate and depth of breathing to bring in more oxygen. At the same time, the heart pumps more blood to deliver oxygen to muscles more rapidly.
• Altitude: At higher altitudes, the concentration of oxygen in the air is lower. This leads to reduced oxygen availability, making it harder for the body to obtain the necessary oxygen for cellular respiration. The body adapts over time by producing more red blood cells to carry more oxygen.
• Smoking: Smoking damages the lungs, leading to reduced surface area for gas exchange and impairing the ability to transport oxygen. Carbon monoxide, a harmful component of cigarette smoke, binds to haemoglobin more strongly than oxygen, reducing the amount of oxygen the blood can carry.
• Respiratory Diseases: Conditions like asthma, pneumonia, and chronic obstructive pulmonary disease (COPD) can limit airflow into the lungs and reduce the efficiency of gas exchange. These conditions often lead to difficulty in breathing, reduced oxygen intake, and a decreased ability to remove carbon dioxide.

The process of oxygen transport and gas exchange is vital for life. The lungs play a key role in obtaining oxygen from the environment, while the blood carries this oxygen to tissues and organs where it is used in respiration to produce energy. Proper functioning of the lungs and circulatory system is essential for maintaining cellular functions and overall health. Any disruptions to this system, such as respiratory diseases or lifestyle factors like smoking, can impact oxygen supply and overall well-being.