The blood-air barrier in the alveoli is 50 times thinner than a sheet of trace paper. This thinness facilitates faster oxygenation.
Lungs form the central organs of the respiratory system and facilitate the exchange of gases along with the associated airways and blood vessels. In addition, different parts of the lungs are also involved in certain non-respiratory functions, including certain homeostatic mechanisms as well as immune processes.
Human lungs are located in the thoracic cavity or chest and are enclosed within the rib cage. The two lungs are situated on either sides of the heart and are pinkish in color, especially at a young age. Exposure to the atmosphere and polluted air eventually gives rise to mottled patches, which tint the lungs gray in color. At the floor of the thoracic cavity lies the thoracic diaphragm which facilitates breathing.
Given below is a labeled diagram of the human lungs followed by a brief account of the different parts of the lungs and their functions.
Each lung is enclosed inside a sac called pleura, which is a double-membrane structure formed by a smooth membrane called serous membrane. The outer membrane of this structure is called parietal pleura and is attached to the chest wall, whereas the inner membrane is called the visceral pleura, and it covers the lungs as well as the associated structures. The space between the two membranes is called pleural cavity.
Each lung is divided into anatomical and functional segments called lobes through partitions called interlobar fissures. The right lung comprises three lobes: superior lobe, middle lobe, and inferior lobe. Horizontal fissure is the anatomical partition that separates the superior and middle lobes, whereas an oblique fissure separates the middle and inferior lobes.
The left lung is slightly smaller than the right, and is divided into two lobes by an oblique fissure. These two lobes are similar to the superior and inferior lobes of the right lung. The middle lobe is not present in the left lung.
Such a partition between the lobes provides protection from mechanical damage and also prevents the spread of an infection. As a result, if one lobe or a part of it is damaged, infected, or its functionality is compromised due to some local aberration, the other lobes can continue to function normally.
The trachea or windpipe is the major structure that connects the nasal and oral cavities to the lungs. The trachea bifurcates into main branches called bronchi, which enter into the two lungs. The bronchi are made up of hyaline cartilage and smooth muscles.
The left and right bronchi also differ in their dimensions, with the right one being wider than the left. The right bronchus branches out into three secondary bronchi, and the left bronchus gives rise to two secondary bronchi. The secondary bronchi segment into tertiary bronchi, which further give rise to bronchioles. Along with the branching, the content of hyaline cartilage decreases, reducing to none in the bronchioles, while that of smooth muscle increases.
Each tertiary bronchi gives rise to distinct respiratory units called bronchopulmonary segments which have their own set of bronchioles, alveoli, blood vessels, and lymphatic vessels. The trachea, bronchi, and the subsequent branches form the airways that facilitate the entry and exit of air from the lungs.
The bronchioles end into tiny sacs called alveoli, which are the site for gaseous exchange between lungs and blood. The alveoli are thin-walled, inflatable sacs that are arranged in clusters. The walls of the alveoli are made up of:
(i) Type I alveolar cells that form the structural base.
(ii) Type II alveolar cells that secrete surfactants, which reduce the surface tension at the air-water interface.
In addition, immune cells called macrophages are also present in the alveoli in order to engulf and destroy pathogens and foreign debris. The alveolar walls have extremely minute pores called pores of Kohn, which enable the flow of air from one alveolus to another.
Each alveolus is surrounded by a network of capillaries that transport blood to the alveoli, for oxygenation. There's a very thin space present between the walls of the alveoli and those of the capillaries. This interstitial space is called the blood-air barrier, and it is merely 0.5 µm thick.
The rate of diffusion of gases is directly proportional to the surface area and inversely proportional to the distance of diffusion. The alveoli provide both these conditions. They provide a large surface area within a compact space and reduce the distance of diffusion through an extremely thin blood-air barrier.
Print the diagram given below, and label the different parts of the human lungs. You may even share it with your friends and discuss the functions of each part.
Click on the image to print it.