Respiratory System Anatomy and Physiology

Anatomy & Physiology |

The respiratory system is situated in the thorax, and is responsible for gaseous exchange between the circulatory system and the outside world. Air is taken in via the upper airways (the nasal cavity, pharynx and larynx) through the lower airways (trachea, primary bronchi and bronchial tree) and into the small bronchioles and alveoli within the lung tissue.

The lungs are divided into lobes; The left lung is composed of the upper lobe, the lower lobe and the lingula (a small remnant next to the apex of the heart), the right lung is composed of the upper, the middle and the lower lobes.

The Nose

The uppermost portion of the human respiratory system, the nose is a hollow air passage that functions in breathing and in the sense of smell. The nasal cavity moistens and warms incoming air, while small hairs and mucus filter out harmful particles and microorganisms. This illustration depicts the interior of the human nose. The prominent structure between the eyes that serves as the entrance to the respiratory tract and contains the olfactory organ. It provides air for respiration, serves the sense of smell, conditions the air by filtering, warming, and moistening it, and cleans itself of foreign debris extracted from inhalations.

The Trachea, Bronchi Aviolar Ducts and Avioli

The trachea (windpipe) divides into two main bronchi (also mainstem bronchi), the left and the right, at the level of the sternal angle at the anatomical point known as the carina. The right main bronchus is wider, shorter, and more vertical than the left main bronchus. The right main bronchus subdivides into three lobar bronchi while the left main bronchus divides into two. The lobar bronchi divide into tertiary bronchi, also known as segmental bronchi, each of which supplies a bronchopulmonary segment. A bronchopulmonary segment is a division of a lung that is separated from the rest of the lung by a connective tissue septum.. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments. There are ten segments per lung, but due to anatomic development, several segmental bronchi in the left lung fuse, giving rise to eight. The segmental bronchi divide into many primary bronchioles which divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which go on to divide into 2 to 11 alveolar ducts. There are 5 or 6 alveolar sacs associated with each alveolar duct. The alveolus is the basic anatomical unit of gas exchange in the lung.

There is hyaline cartilage present in the bronchi, present as irregular rings in the larger bronchi (and not as regular as in the trachea), and as small plates and islands in the smaller bronchi. Smooth muscle is present continuously around the bronchi.

In the mediastinum, at the level of the fifth thoracic vertebra, the trachea divides into the right and left primary bronchi. The bronchi branch into smaller and smaller passageways until they terminate in tiny air sacs called alveoli.

The cartilage and mucous membrane of the primary bronchi are similar to that in the trachea. As the branching continues through the bronchial tree, the amount of hyaline cartilage in the walls decreases until it is absent in the smallest bronchioles. As the cartilage decreases, the amount of smooth muscle increases. The mucous membrane also undergoes a transition from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium.

The alveolar ducts and alveoli consist primarily of simple squamous epithelium, which permits rapid diffusion of oxygen and carbon dioxide. Exchange of gases between the air in the lungs and the blood in the capillaries occurs across the walls of the alveolar ducts and alveoli.

The Lungs

The lungs constitute the largest organ in the respiratory system. They play an important role in respiration, or the process of providing the body with oxygen and releasing carbon dioxide. The lungs expand and contract up to 20 times per minute taking in and disposing of those gases.

Air that is breathed in is filled with oxygen and goes to the trachea, which branches off into one of two bronchi. Each bronchus enters a lung. There are two lungs, one on each side of the breastbone and protected by the ribs. Each lung is made up of lobes, or sections. There are three lobes in the right lung and two lobes in the left one. The lungs are cone shaped and made of elastic, spongy tissue. Within the lungs, the bronchi branch out into minute pathways that go through the lung tissue. The pathways are called bronchioles, and they end at microscopic air sacs called alveoli. The alveoli are surrounded by capillaries and provide oxygen for the blood in these vessels. The oxygenated blood is then pumped by the heart throughout the body. The alveoli also take in carbon dioxide, which is then exhaled from the body.

Inhaling is due to contractions of the diaphragm and of muscles between the ribs. Exhaling results from relaxation of those muscles. Each lung is surrounded by a two-layered membrane, or the pleura, that under normal circumstances has a very, very small amount of fluid between the layers. The fluid allows the membranes to easily slide over each other during breathing.

Mechanics of Breathing

To take a breath in, the external intercostal muscles contract, moving the ribcage up and out. The diaphragm moves down at the same time, creating negative pressure within the thorax. The lungs are held to the thoracic wall by the pleural membranes, and so expand outwards as well. This creates negative pressure within the lungs, and so air rushes in through the upper and lower airways.

Expiration is mainly due to the natural elasticity of the lungs, which tend to collapse if they are not held against the thoracic wall. This is the mechanism behind lung collapse if there is air in the pleural space (pneumothorax).

Physiology of Gas Exchange

Each branch of the bronchial tree eventually sub-divides to form very narrow terminal bronchioles, which terminate in the alveoli. There are many millions of alveoli in each lung, and these are the areas responsible for gaseous exchange, presenting a massive surface area for exchange to occur over.

Each alveolus is very closely associated with a network of capillaries containing deoxygenated blood from the pulmonary artery. The capillary and alveolar walls are very thin, allowing rapid exchange of gases by passive diffusion along concentration gradients.

CO2 moves into the alveolus as the concentration is much lower in the alveolus than in the blood, and O2 moves out of the alveolus as the continuous flow of blood through the capillaries prevents saturation of the blood with O2 and allows maximal transfer across the membrane.