Structure and function of organs in the mammalian lung system

Organs in the mammalian lung system include the conducting airways, lungs, and diaphragm (Figure 1). The conducting airways begin at the nasal cavity, followed by the pharynx (common name ‘throat’), the larynx (common name ‘voice box’), trachea (common name ‘windpipe’), bronchi, and terminate at the bronchioles.

Figure 1: The human lung sustem. Conducting airways are labelled in blue.

Conducting airways

Air enters the body through the nose, which is lined by a mucous membrane. This wet membrane humidifies and warms the air breathed in, to prevent drying out the rest of the system. Nose hairs filter the air breathed in, to prevent dirt entering the lungs, thus preventing infection. Air then passes the pharynx. 

The trachea is a tube about 12 cm long, and as thick as your thumb. The upper part of the trachea contains the larynx. The vocal cords are two bands of tissue that extend across the opening of the larynx. After passing the larynx, the air moves into the bronchi. 

The trachea and bronchi are reinforced by cartilage rings, to prevent their collapse when there is a decrease in pressure while breathing in. Cartilage rings also stops the trachea from being squashed when the person changes neck position. 

Trachea and bronchi are also lined with ciliated cells and mucous-producing cells (Figure 2). Cilia are cellular projections that can ‘beat’ or ‘move’. Cilia transport a mucous sheet (that’s collected dust and dirt) back up towards the pharynx where it is swallowed. This feature of the lung system is called the ‘mucociliary escalator’.

Figure 2: Electron microscope image of cilia on the trachea, bronchi, and bronchioles.

Bronchi branch into smaller and smaller bronchi, and the smallest bronchi branch into smaller and smaller tubes called bronchioles (Figure 3). Bronchioles are not reinforced by cartilage rings, because they are too narrow.

Figure 3: Branching pattern in airways.

Lungs and alveoli

The lungs are large, paired organs in the chest (also known as the ‘thoracic cavity’), and are protected by the ribs. The lungs contain the lower portion of the conducting airways (bronchi and bronchioles) as well as the alveoli, where gas exchange takes place. Alveoli are kept moist due to their internal structure.

Alveoli (plural) are air-filled sacs at the ends of bronchioles. They are surrounded by a network of thin-walled capillaries containing red blood cells. Only about 0.2 μm separate the air from red blood cells due to the extremely thin walls of the alveolus and capillary (Figure 4). All these factors are adaptations that improve exchange of O₂ and CO₂.

There are around 300 million alveoli per lung, filled with air, giving the lungs a sponge-like consistency. This organisation produces a very large surface area that is available for gas exchange.

Figure 4: Cells at the diffusion barrier.

Diaphragm and intercostal muscles

The bottom of the thoracic cavity is formed by the diaphragm. The diaphragm is a dome-shaped sheet of muscle that forms the floor of the thorax and the roof of the abdomen. Contraction of the muscle flattens the diaphragm. This increases the volume of the thoracic cavity, decreases pressure in the airways, and causing breathing in (inhalation). Passive relaxation of the muscle allows the diaphragm to lift back towards the thoracic cavity. This decreases the volume of the thoracic cavity, increasing pressure in the airways, causing breathing out (exhalation). Muscle contraction is physical, therefore energy is expended.

Movement of the diaphragm is responsible for 75% of bulk air flow. The other 25% of bulk air flow is due to contraction of intercostal muscles, which move the rib cage up and outwards causing inhalation, and down and inwards causing exhalation.

Figure 5:  A) Comparison of inhalation (left) and exhalation (right). During inhalation, the diaphragm flattens (B) and the ribs move up and out (C). While during exhalation, the diaphragm domes up (B) and the ribs move down and in (C).

Other adaptations

In order for diffusion of gases to occur, a concentration gradient is maintained through the process of breathing. Breathing maintains a high O₂ and low CO₂ concentration in the lungs. Breathing rate can increase and decrease according to demand. Breathing rate is controlled by the respiratory centre at the base of the brain. This respiratory centre sends signals to the diaphragm and intercostal muscles, telling them to contract. These signals are triggered by increasing levels of CO₂ in the blood. The higher the level of CO₂, the faster the breathing rate.

The air that organisms breathe contain dust, dirt, viral particles, and bacteria that can damage the lungs. The respiratory system has protective mechanisms to avoid damage. In the nasal cavity, hairs and mucus trap small particles, viruses, bacteria, dust, and dirt to prevent entry. If particulates make it beyond the nose or enter via the mouth, the bronchi and bronchioles contain several protective devices. The lungs produce mucus that traps particulates, and the mucociliary escalator transport this mucus out of the lung system. Lastly, immune cells called macrophages, patrol the alveoli for pathogens.