To survive, all cells need a constant supply of oxygen. They use this oxygen to produce energy, producing carbon dioxide as a waste product. Air contains 21% oxygen and single celled and small organisms can survive by taking in oxygen directly from the air by diffusion across their cell membranes. Larger animals, including humans, cannot meet the metabolic needs of their millions of cells by diffusion through the outer layer of cells. Because of this we have a specialised respiratory system to obtain oxygen, deliver it to all of the cells of the body and remove carbon dioxide from them.
The human respiratory system consists of the lungs, structures leading from the external environment to the lungs, the diaphragm and the muscles of the thoracic cage.
The nose and mouth are where air first enters the body. Behind the small external nose protruding from the face is a large nasal cavity.
The nostrils have a dense network of hairs which filter out any particles in the air. The walls of the nasal cavity are covered in a mucous membrane with a rich blood supply. As the air passes over the mucous membrane, it is warmed and moistened and any remaining airborne particles get stuck to it. From the nasal cavity air passes backwards and downwards into the pharynx.
The mouth or buccal cavity has a roof formed by the hard and soft palate; in front it is closed by the lips, and to the sides by the muscles of the cheek. The tongue fills the mouth and forms its floor.
The hard palate is formed by the palatine and maxillae bones and the soft palate is muscular and hangs from the back of the hard palate, separating the mouth and pharynx. The buccal cavity is lined with a mucous membrane. When food is swallowed, the soft palate moves backwards to block the nasal cavity, so that food cannot enter and block the airway.
The pharynx is a muscular tube lined with mucous membrane that joins the nasal and buccal cavities with the oesophagus and larynx. It consists of three parts, the nasopharynx, oropharynx and laryngopharynx. The nasopharynx is situated behind the nasal cavities, the oropharynx behind the buccal cavity and the laryngopharynx behind the larynx.
The larynx or voice box is situated in the neck below the hyoid bone and is continuous inferiorly with the trachea. It allows air in and out of the lungs as well as being specialised for voice production.
It consists of 5 cartilages, thyroid, cricoid, epiglottis and two arytenoid cartilages. The thyroid cartilage is the largest cartilage and formed by two flat plates joined anteriorly in the mid line to form the laryngeal prominence, more commonly know as the Adam's apple.
The trachea is a tube composed of cartilages and membranes that allows air to pass from the pharynx into the lungs via the principle bronchi.
The trachea begins at the level of C6 below the cricoid cartilage of the larynx. It descends through the thorax, where it divides at the level of the T4 into right and left principle bronchi, which enter the right and left lungs respectively.
It consists of c-shaped cartilages anteriorly united by a fibroelastic membrane composed of collagen and elastin. Posteriorly there is a gap in the cartilages across which lies the trachealis muscle. The last tracheal cartilage (carina) is made up of a complete ring of cartilage. Inside, the trachea is lined with a specialised mucous membrane containing cilia which beat upwards to transport the mucous, along with any inhaled particles, out of the lungs where they can be swallowed and neutralized in the stomach.
At the level of T5 the right and left principal bronchi emerge as a division of the trachea. They have a similar structure to the trachea with incomplete rings of cartilage anteriorly united by a fibroelastic membrane. They travel obliquely and enter each lung through the hilum where they divide further into smaller lobar bronchi to each of the lungs' lobes. Each lobar bronchus branches further into segmental bronchi to each of the segments of the lungs. These bronchi continue to branch into smaller and smaller bronchi thereby forming the bronchial tree.
The bronchioles are the very last branches of the bronchial tree. They have no cartilage and are composed of a fibroelastic membrane and smooth muscle. The smallest bronchioles are known as terminal bronchioles and branch to form numerous alveolar ducts.
The alveolar ducts lead into the alveoli sacs and then into the individual alveoli where the gaseous exchange takes place. There are hundreds of millions of alveoli, providing a large surface area for the diffusion of gases. Alveoli are tiny thin walled air sacs with a rich blood supply. They are just one cell thick, (flattened epithelium) as are the capillaries that surround them, meaning that the gases diffuse across a distance of only 2 cells thick to gain access to the blood stream.
The internal surface of an alveolus is covered with a moist film allowing the oxygen from the air to dissolve on it, and a surfactant, rich in phospholipids and proteins which prevents it from collapsing when we breath out. Each alveolus contains a large number of macrophages that phagocytose particles and debris and kill bacteria that have entered the lungs and have been trapped on the moist walls.
The lungs are situated in the thoracic cavity, forming most of its contents except for the mediastinum. Each of the cone-shaped lungs is suspended in a pleural cavity either side of the heart and is connected to the mediastinum by its root.
Each lung has a concave base below which sits on top of the diaphragm and a pointed apex above which projects above the clavicle. Due to the shape and positioning of the heart, the two lungs differ slightly in shape and size, with the left lung being smaller than the right.
The lungs are organised into lobes, with the left having two lobes (superior and inferior) and the right having three lobes (superior, middle, and inferior). Each lung is further separated by connective tissue septa, thus forming pyramid-shaped bronchopulmonary segments.
The lungs are supplied with deoxygenated blood via the pulmonary arteries and the oxygenated blood is removed by the pulmonary veins. The lung tissue itself is supplied with oxygenated blood via the bronchial arteries, the bronchial veins removing deoxygenated blood. The principle bronchi, the pulmonary and bronchial arteries and veins all enter or leave the lungs at the hilum (root).
The left lung has two lobes separated by an oblique fissure and are supplied with air by the superior and inferior lobar bronchi. It is further divided into ten bronchopulmonary segments that are supplied with air by segmental bronchi (tertiary bronchi).
The left lung has several major landmarks;
The right lung has three lobes separated by an oblique fissure and a horizontal fissure and are supplied with air by the superior and inferior lobar bronchi. It is further divided into ten bronchopulmonary segments that are supplied with air by segmental bronchi (tertiary bronchi).
The right lung has several major landmarks;
The lungs are covered with a double sheet of thin membrane called pleura. Each membrane is a closed sac with a lung invaginated into it. This creates two layers over the surface of the lungs; the visceral and parietal layers that are continuous with each other at the hilum.
The visceral pleura is the innermost layer and adheres closely to the surface of the lungs and into the interlobar fissures and cannot be separated from the lung surface.
The parietal pleural is the outer layer and lines the thoracic wall, the diaphragm, and the structures within the mediastinum. The space between the two membranes is know as the pleural cavity, however in a normal person this is only a potential space as both layers are in close contact and slide over each other, with the aid of pleural fluid, during normal respiration.
Functions of the pleura and pleural fluid;
The flow of air in and out of the alveoli is called ventilation. The lungs are not muscular and cannot ventilate themselves but must rely on the coordinated actions of the diaphragm and surrounding muscles.
Inspiration is the flow of air into the lungs. When you take a breath in, your diaphragm contracts, flattening the dome and pulling it downwards, whilst the external intercostal muscles contract to pull the ribs upwards and outwards. This increases the volume of the thorax and lungs and decreases the air pressure inside. To equalize the pressure air flows into and fills the lungs/alveoli.
Expiration is the flow of air out of the lungs and under normal conditions is a passive process. When you breathe out your diaphragm relaxes, curving the dome and moving it upwards. This decreases the volume of the thorax and lungs and increases the pressure inside. To equalize the pressure air flows out of the lungs/alveoli while the elasticity of the lungs themselves cause the lungs to recoil back into a neutral position.
When you forcibly suck in or blow out air, other muscles are brought into play. In forced inspiration the pectoralis minor, sternocleidomastoid and erector spinae muscles all contract to pull the thoracic cavity up and outwards, thereby expanding its size as much as possible. In forced expiration the abdominal muscles (internal and external oblique, transversus abdominis and rectus abdominis) as well as the internal intercostals contract to pull the thoracic cavity down and inwards and pushing the diaphragm further upwards, thereby decreasing the size of the thoracic cavity and pushing as much air out as possible.
To understand gas exchange you must have a general understanding of diffusion and concentrations gradients.
Diffusion is the movement of particles from a substance with a higher concentration to a substance with a lower concentration down a concentration gradient. Molecules such as oxygen and carbon dioxide are lipid soluble and are therefore able to pass easily through the cells phospholipid outer membranes by simple diffusion.
Oxygen and carbon dioxide concentrations are measured in terms of its partial pressure (pO2).
Oxygen is delivered into the blood by using the relative differences in oxygen concentration (pO2) in the cells and blood. The epithelial cells of the alveolus have a low oxygen concentration and so when oxygen dissolves on the mucous it flows down the concentration gradient into the cell. The cells of the capillary walls surrounding the alveoli also have a low concentration and the oxygen moves into these cells. The blood inside the capillaries is deoxygenated blood and so oxygen again moves down the concentration gradient from capillary cells into the blood where it is loaded onto the red blood cells (erythrocytes).
The oxygen concentration is now higher in the blood than in the cells of the body. The oxygen is delivered to the tissues by moving into those cells with lower oxygen content.
Haemoglobin is a pigment found in red blood cells which increases their ability to carry oxygen molecules by as much as 70%. A single haemoglobin molecule binds to 4 molecules of oxygen. Within each red blood cell there are about 250 million molecules of haemoglobin.
The excretion of the waste product carbon dioxide is a little more complex than the transport of oxygen but it still relies on the use of concentration gradients.
Cells that have been metabolising have a high concentration of carbon dioxide compared to the cells of the capillary walls. Carbon dioxide therefore travels down the concentration gradient into the cells of the capillary wall and then into the blood. Once in the blood, the majority of carbon dioxide (70%) enters the red blood cells where it is combined with water to create bicarbonate. This keeps the carbon dioxide concentrations low and allows even move carbon dioxide to diffuse into the blood. The bicarbonate is then released into the blood plasma in which it travels to the lungs.
When the blood reaches the walls of the alveoli capillaries, the bicarbonate is taken back into the red blood cell where it is combined with hydrogen to form carbonic acid. Carbonic acid breaks down to form carbon dioxide and water, which diffuses into the lungs where it can be exhaled.
The concentration gradients of oxygen and carbon dioxide are maintained across the respiratory surface by the blood flow through the capillaries on one side and by airflow on the other side.