My Articles no 1

                                                Gaseous Exchange

Cellular respiration is the process in which the C-H bonds in food are broken by oxidation reduction reaction and the energy is transformed into ATP. In aerobic respiration, oxygen is used and there is complete oxidation of the food material. Carbon dioxide and water are also produced in this process.

Organisms get the oxygen, needed for cellular respiration, from their environment and provide it to their cells. The carbon dioxide produced during cellular respiration is taken out of the cells and ultimately from the body. Taking in oxygen and giving out of carbon dioxide is termed as gaseous exchange.

The term breathing is used for the process through which animals take air in their bodies to get oxygen from it and then give out the air for getting rid of carbon dioxide. Thus breathing and respiration are not synonymous. Respiration involves the mechanical and the bio-chemical processes whereas breathing is only the mechanical or physical process of exchange of gases.

Gaseous Exchange in Plants

Plants have no organs or systems for the exchange of gases with the environment. Every cell of the plant body exchanges gases with the environment by its own.

The leaves and young stems have stomata in their epidermis. The gaseous exchange occurs through these stomata. The inner cells of leaves (mesophyll) and stems also have air spaces among them, which help in the exchange of gases.

Leaf cells face two situations. During the daytime when the mesophyll cells of leaves are carrying out photosynthesis and respiration side by side, the oxygen produced in photosynthesis is utilized in cellular respiration. Similarly the carbon dioxide produced during cellular respiration is utilized in photosynthesis. However, during night when there is no photosynthesis occurring, the leaf cells get oxygen from the environment and release carbon dioxide through stomata. 

In woody stems and mature roots, the entire surface is covered by bark which is impervious to gases or water. However, there are certain pores in the layer of bark. These are called the lenticels.

The lenticels allow air to pass through them.

Gases diffuse in and out of the general surface of the young roots. The gases are found in the soil surrounding the roots.

The aquatic plants get the oxygen dissolved in water and release carbon dioxide in the water.

Gases Exchange in Human

In human and other higher animals the exchange of gases is carried out by the respiratory system. We can divide the respiratory system in two parts i.e. the air passageway and the lungs.

The Air Passageway

The air passageway consists of the parts through which the outside air comes in the lungs and after the exchange of gases it goes out. This passage of air consists of the following parts.

The nose encloses the nasal cavity. It opens to the outside through the openings called the nostrils. The nasal cavity is divided into two portions by a wall. Each portion is lined by fine hairs and mucous which filter the dust particles from the air. The mucous also moistens and warms the incoming air and keeps its temperature nearly equal to that of the body.

The nasal cavity opens into the pharynx by means of two small openings called internal nostrils. Pharynx is a muscular passage and is common to both food and air. It extends to the opening called internal nostrils. Pharynx is a muscular passage and is common to both food and air. It extends to the opening of the oesophagus and the larynx. The air goes from the pharynx into the larynx. We know that glottis is a narrow opening at the floor of pharynx which leads into larynx.

The larynx is a box, made of cartilage. It is present between pharynx and trachea. It is also called the voice box. Two pairs of fibrous bands called vocal cords are stretched across the larynx. The vocal cords vibrate when the air passes through them. This vibration produces sounds.

Larynx continues to the trachea, which is also called the windpipe. It is about 12 cm long tube which lies in front of the oesophagus. There are C-shaped cartilaginous rings in the wall of trachea. The cartilages keep the trachea from collapsing even when there is no air in it.

On entering the chest cavity, the trachea divides into two smaller tubes called bronchi (singular: bronchus). The bronchi also have cartilaginous plates in their walls. Each bronchus enters into the lungs of its side and then divides into smaller branches.

The bronchi continue dividing in the lungs until they make several fine tubes called bronchioles. The bronchioles progressively lose cartilages as they become narrower. The bronchioles end as fine tubules called the alveolar ducts. Each alveolar duct opens into a cluster of pouches called alveoli. The alveoli form the respiratory surface in human body. Each alveolus is a sac-like structure lined by a single layer of epithelial cells. It is bound on the outside by a network of capillaries.

The Lungs

All the alveoli on one side constitute a lung. There is a pair of lungs in the thoracic cavity. The chest wall is made up of 12 pairs of ribs and the rib muscles called intercostal muscles. A thick muscular structure, called diaphragm, is present below the lungs.

The left lungs are slightly smaller and have two lobes and the right lung is bigger with three lobes. They are spongy and elastic organs. The lungs also have blood vessels that are the branches of the pulmonary arteries and veins. Each lung is enclosed by two membranes called the outer pleural membrane and the inner pleural membrane. The membranes enclose a fluid which provides lubrication for the free expanding and contracting of the lungs.

The Mechanism of Breathing

The physical movements associated with the gaseous exchange are called breathing. There are two phases of breathing i.e. inhalation and exhalation.

1.       Inspiration or In halation

During inspiration, the rib muscles contract and ribs are raised. At the same time the dome-shaped diaphragm contracts and is lowered. These movements increase the area of the thoracic cavity, which reduces the pressure on lungs. As a result, the lungs expand and the air pressure within them also decreases. The air from outside rushes into the lungs to equalize the pressure on both sides.

2.       Expiration or Exhalation

After the gaseous exchange in the lungs, the impure air is expelled out in exhalation. The rib muscles relax bringing the ribs back to the original position. The diaphragm muscles also relax and it gets irs raised dome shape. This reduces the space in the chest cavity and increases the pressure on lungs. The lungs contract and the air is expelled out of them.

Human breathe 16-20 times per minute in normal circumstances i.e. at rest. The rate of breathing is controlled by the respiratory centre in the brain. The respiratory centre is sensitive to the concentration of carbon dioxide in the blood. When we do exercise or some hard job our muscle cells carryout cellular respiration at greater rate. It results in the production of more carbon dioxide which is released in the blood. This greater  than normal concentration of carbon dioxide stimulates the respiratory centre of brain. The respiratory centre sends messages to the rib muscles and diaphragm to increase the rate of breathing so that the excess carbon dioxide present in blood can be removed out of body. During exercise or other hard physical works the breathing rate may increase up to 30-40 times per minute.

Respiratory Disorders

                There are a number of respiratory disorders which affect people. The percentage of such disorders is particularly high in Pakistan. It is due to the more concentration of air pollutants not only in the urban but also in the rural atmosphere. Some of the important respiratory disorders are described next.

1.       Bronchitis

Bronchitis is the inflammation of the bronchi or bronchioles. It results in excessive secretions of mucus into the tubes, leading to the swelling of tubular walls and narrowing of tubes. It is caused by viruses, bacteria or exposure to chemical irritants (e.g. tobacco smoke).

There are two major types of bronchitis i.e. acute and chronic. The acute bronchitis usually lasts about two weeks and patients recover with no permanent damage to the bronchi or bronchioles. In chronic bronchitis, the bronchi develop chronic inflammation. It usually lasts for three months to two years.

Symptoms of bronchitis include a cough, mild wheezing, fever, chills and shortness of breath (especially when doing hard job).

2.       Emphysema

Emphysema is the destruction of the walls of the alveoli. It results in larger sacs but with less surface area for gaseous exchange. As lung tissue breaks down, the lungs do not come back to their original shape after exhalation. So air cannot be pushed out and is trapped in the lungs.

The symptoms of emphysema include shortness of breadth, fatigue, recurrent respiratory infections and weight loss. By the time the symptoms of emphysema appear, the patient has usually lost 50% to 70% of his / her lung tissue. The level of oxygen in blood may get so low that it causes serious complications.

3.       Pneumonia

Pneumonia is an infection of lungs. If this infections affects both lungs then, it is called double pneumonia. The most common cause of pneumonia is a bacterium, streptococcus pneumonia. Some viral (influenza virus) and fungal infections may also lead to pneumonia.

When the causative organisms enter the alveoli, they settle there and grow in number. They break the lung tissues and the area becomes filled with fluid and pus. The symptoms of pneumonia include a cold that is followed by a high fever, shivering, and a cough with sputum production. Patient may become short breath. The patient’s skin colour may change and become dusky or purplish. It is due to poor oxygenation of blood.

Vaccines are available to prevent pneumonia caused by S. pneumonia. Antibiotics are used in the treatment of this type of pneumonia.

4.       Asthma

Asthma is a form of allergy, in which there is inflammation of the bronchi, more mucous production and narrowing of the airways. In asthma patients, the bronchi and bronchioles become sensitive to different allergens (allergy causing factors) e.g. dust, smoke, perfumes, pollens etc. when exposed to any of such allergens, the sensitive airways show immediate and excessive response of constriction. In this condition, the patient feels difficulty in breathing.

The symptoms of asthma vary from person to person. The major symptoms  include shortness of breath (especially with exertion or at night), wheezing (whistling sound when breathing out), cough and chest tightnesss.

The chemicals with ability to dilate the bronchi and bronchioles are used in the treatment of asthma. Such medicine is given in the form of inhalers.

5.       Lung Cancer

Lung cancer is a disease of uncontrolled cell divisions in the tissues of the lung. The cells continue to divide without any control and form tumours. The cellular growth may also invade adjacent tissues beyond the lungs. The most common symptoms are shortness of breath, coughing (including coughing up blood) and weight loss.

The main causes of any cancer include carcinogens (such as those in cigarette smoke), ionizing radiation and viral infection. Smoking is the main cause of lung cancer. This risk of lung cancer is significantly lower in non smokers. Cigarette smoke contains over 50 known carcinogens.

Passive smoking (the inhalation of smoke from another’s smoking) is also a cause of lung cancer. The smoke from the burning end of cigarette is more dangerous than the smoke from the filter end.

Eliminating tobacco smoking is a primary goal in the prevention of lung cancer. The World Health Organization has called for government to stop tobacco advertising to prevent young people from taking up smoking.

Bad Effects of Smoking

Smoking is harmful due to the chemicals in cigarettes and smoke. Tobacco smoke contains over 4,000 different chemical s, out of which at least 50 are carcinogens and many are poisonous.

Many people think that lung cancer is the only smoking-related disease and it is the number one cause of death among smokers. But it is not right. Cigarette smoke affects the body from head to toe. Smokers have a much higher risk of developing a number of life threatening diseases.

Smoking may also lead to the cancers in kidneys, oral cavity, larynx, breast, bladder and pancreas etc. many chemicals in tobacco smoke damage the air passageway, which leads to emphysema and other respiratory disorders.

Smoking als has effects on the circulatory system. The carbon monoxide present in tobacco smoke lessens the oxygen-carrying capacity of haemoglobin.  Many other chemicals in smoke increase the production of blood platelets. When platelets are more than the normal numbers, they make the blood viscous and it can lead to arteriosclerosis.

Smokers are at greater risk of developing infections, particularly in the lungs. For example, smoking increases the risk of tuberculosis by two to four times, and of pneumonia by four times.

Smoking is also responsible for weakening and staining the teeth. Tooth loss is 2 in 3 times higher in smokers than in non-smokers.