Structure of Lungs in Human Body

External Structure of Lungs:

The lungs are soft, spongy and elastic organs which are pinkish in colour. The upper most portion of each lung is called the apex and the inferior most portion is called the base. The costal surfaces of the lungs are so named because they lie next to the ribs. The median surfaces of the lungs lie next to the mediastinum.

The left lung is smaller than the right and has a concavity, the cardiac notch, where the heart lies. The left lung has two lobes, superior lobe and inferior lobe, which are divided by the oblique fissure. The right lung has three lobes: superior lobe, middle lobe and inferior lobe which are divided by the horizontal fissure and oblique fissure.

Internal Structure of Lungs, (a) Secondary (= Lobar) and Tertiary (= Segmental) Bronchi:

As soon as the primary bronchus enters each lung it divides to form secondary bronchi. Each secondary bronchus divides to form tertiary bronchi. The secondary and tertiary bronchi also have cartilaginous rings.

(b) Bronchioles:

The tertiary bronchi sub-divide into smaller branches, the bronchi­oles which are without cartilaginous rings. The bronchioles decrease in diameter and their epithelium also becomes thinner as they go deeper into the lungs. After repeated branching, one of the smaller bronchioles enter a lung lobule called lobular bronchiole. The latter gives rise to terminal bronchioles which further subdivide into respiratory bronchioles.

(c) Alveolar ducts, atria, alveolar sacs and alveoli:

The respiratory bronchioles open into the alveolar ducts which terminate in expanded passages, the atria, (sing, atrium) which in turn lead into the alveolar sacs or air sacs. The latter open into the alveoli (also called acini). There are 300 millions of alveoli in two lungs. The alveoli have very thin wall consisting of squamous epithelium.

The wall of the alveoli has an extensive network of blood capillaries interspersed with elastic and reticular connective tissue fibres. Due to very inti­mate contact of blood capillaries with the alveoli, the exchange of gases takes place easily. Thus each alveolus may be called a miniature lung where exchange of gases takes place.

A film of lecithin lines normally the alveoli of lungs that lower the surface tension and keeps the alveoli open. However, in some new born babies the film of lecithin may not be present. This may be fatal to the baby due to collapsing of lungs.

Respiration involves the following steps:

(i) Breathing is the inflow (inspiration) and outflow (expiration) of air between atmo­sphere and the alveoli of the lungs.

(ii) Diffusion of gases (O2 and CO2) across alveolar membrane.

(iii) Transport of gases by the blood.

(iv) Diffusion of O2 and CO2 between the blood and the tissues.

(v) Utilisation of O2 by the cells for catabolic reactions and resultant release of CO2 (cellular respiration).