Title: The Respiratory System

Key words: Lungs, trachea, bronchii, bronchioles, alveoli, gas exchange, respiration, pleura, cilia, goblet cells

Date: Aug 2000

Category: 6. The Body

Type: Article

Author: DJE Candlish

 

The Respiratory System

 

Introduction

The respiratory system has one overall function - to supply the cells of the body with oxygen and remove the carbon dioxide formed during cell respiration.

The respiratory system consists of two sections, upper and lower. The upper respiratory tract consists of the nasal cavities and pharynx while the lower contains the lungs. The trachea or windpipe connects the two.

Although not related to the function of the respiratory tract, the senses of smell and taste depend upon structures in the upper respiratory tract. These are called olfactory receptors. They can detect minute traces of chemicals in the air breathed in, which we interpret as smells or tastes.

The nasal cavities and sinuses

Behind the nostrils or nares lie the nasal cavities, which warm, moisten and filter air as it is first breathed in. The complex passages are lined with a mucous membrane of ciliated epithelium which traps dust particles and bacteria and removes them. The clean, warm air then passes through the pharynx and trachea down to the lungs.

Any inflammation or infection of the mucous membrane, such as the common cold, makes it swell. This causes nasal congestion and obstructs the flow of air through the nasal cavities. The person affected then has to breathe through their mouth until the congestion clears up or is treated with a decongestant.

The sinuses are hollow spaces in the skull which connect with the nasal cavities. Although not a part of the respiratory tract, they can also become inflamed and congested as a result of infection.

The pharynx

The pharynx lies behind the mouth and nasal cavities. It has two sections, the nasopharynx (upper) and oropharynx (lower). The nasopharynx is continuous with the nasal cavities. It contains the entrances to the Eustachian tubes, which connect the nasopharynx to the middle ear cavities to equalise air pressure in the ears. The oropharynx is shared by the respiratory and digestive tracts. Air passes through to the larynx and trachea and food passes through to the oesophagus.

 

The larynx

The larynx is a box shaped cartilage structure containing the vocal cords. The part of the larynx that projects forward at the front of the throat is known as the Adam's apple. Just above the larynx is a flap of tissue called the epiglottis, which closes the larynx off during swallowing, to prevent food entering the trachea.

The trachea

The trachea itself is a semi-rigid tube running from the larynx down to the lungs. It is reinforced by C-shaped bands of cartilage linked by fibrous muscular tissue. This gives the trachea strength without compromising the manoeuvrability of the head and neck. The lower end of the trachea divides into two branches, the bronchi.

 

The bronchi and bronchioles

Each bronchus supplies air to one lung. As the bronchi enter the lungs, they divide into progressively smaller tubes. The walls of the bronchi have three layers, reinforced by cartilage rings similar to those in the trachea. The outer wall is fibrous tissue, the middle layer is smooth muscle and the lining is mucosal tissue.

Beyond the bronchi, the smallest airways or bronchioles have no cartilage supports. Instead they have well developed layers of smooth muscle which can relax or contract to increase or decrease the airway diameter. As in the nasal cavities, the mucosa lining these airways has cilia and mucus secreting goblet cells to remove any dust or bacteria that reach this far in the air breathed in. Despite these protective measures, respiratory infections are very common.

The lungs

The most important component of the respiratory system, the lungs provide the oxygen needed by every cell in the body. The lungs are two spongy elastic organs that fill most of the chest space (thoracic cavity) around the heart. Both lungs are covered by a thin membrane, the visceral pleura, which slides on a film of fluid over the parietal pleura lining the thoracic cavity. This allows the lungs to expand and relax without friction during breathing.

Each lung is divided into sections or lobes. The right lung has three lobes, the left has two. Both lungs have an extensive network of blood vessels and lymphatic vessels, ducts and nodes. The smallest capillaries supply the tiny air sacs or alveoli at the end of the bronchioles. The alveoli are where gases are exchanged in the lungs. They have an enormous total surface area, due to their multi-lobed shape, to make gas exchange as efficient as possible.

 

Breathing and gas exchange

Breathing out is a passive process, in which the diaphragm and rib muscles relax, reducing the volume of the thoracic space and pushing the air from the lungs.

The alveolar walls are covered in a moist, thin mucus film to aid gas exchange. The concentration of oxygen in the blood flowing through the capillaries surrounding the alveoli is lower than in the alveoli when we have breathed in fresh air.

As a result, oxygen dissolves in the mucus film and diffuses rapidly through the alveolar wall into the capillaries and the blood plasma, before being picked up by red blood cells. These contain haemoglobin, a substance that combines with and releases oxygen very easily. The red cells transport the oxygen to every cell in the body.

Carbon dioxide, the waste product of respiration, is carried dissolved in the plasma and also in the red blood cells. When blood reaches the lungs from the tissues, the concentration of carbon dioxide is higher in the blood than in the alveoli, so it leaves the blood by diffusion into the alveoli.

Respiration

Cells require energy to function and this energy is provided by respiration. The mitochondria within cells use food substances provided by digestion to produce energy, in a process that uses oxygen and creates carbon dioxide. This chemical process is known as tissue respiration.

The process by which oxygen is transferred from red blood cells to tissue cells around the body and carbon dioxide leaves the tissue cells is known as internal respiration. External respiration is the process in which oxygen leaves the air in the alveoli and passes into the blood, with carbon dioxide going from the blood to the alveoli for removal from the body.