RESPIRATORY SYSTEM

      All living cells need a constant supply of energy. Green plants change the sun's energy to chemical energy. In turn, the animal cells obtain nutrients from the plant's stored chemical energy. These cells then require oxygen to release the energy for their life processes. It is a chemical process takes place in the cell that use oxygen and gives out carbondıoxide is called cellular respiration.

      In the cellular respiration nutrients are broken down and energy is released. The end products of aerobic cellular respiration are energy, carbon dioxide and water.

        Food + Oxygen                                          Carbon dioxide +Water+ Energy   

    All organisms that carry on aerobic cellular respiration have the problem of obtaining oxygen from the environment and getting rid of carbon dioxide. The process by which a living organism exchanges oxygen and carbon dioxide with its environment is called respiration.

     

    The exchange of oxygen and carbon dioxide between an organism and its environment involves the passage of these gases through a boundary surface. The surface through which gas exchange takes place is called the respiratory surface. A respiratory surface must have the following characteristics:

(1)   It must be thin-walled so that diffusion across it occurs rapidly

(2)   It must be moist because the oxygen and carbon dioxide must be in solution

(3)   It must be in contact with an environmental source of oxygen

(4)   In most multicellular organisms, it must be in close contact with the system that transport dissolved materials to and from the cell of the organism.

   Gas exchange through the respiratory surface takes place by diffusion. The direction of the gas exchange is determined by the concentration gradients of the gases on each side of the respiratory surface. As oxygen is used up inside the organism's tissues, more oxygen diffuses in. When the carbon dioxide concentration builds up within the tissues, this gas diffuses out. The larger the area of the respiratory surface, the greater the amount of gas exchange that can occur over a given period of time.

     In protists and very small multicellular animals, the diffu­sion of respiratory gases can take place directly between the cells and the environment. In larger animals however, most of the body cells are not in contact with the outside environment, and. therefore, direct diffusion cannot serve as the mechanism of gas exchange. In addition, larger animals often have an outer protective layer, such as scales, feathers, or skin, that prevents any significant gas exchange. Therefore, large multicellular animals have their respiratory surfaces in specialized organs or systems.

Human Respiratory system

     The exchange of gases between the atmosphere and the blood is external respiration. This process occurs in the lungs. The exchange of gases between the blood or tissue fluid and the cells themselves is internal respiration.

External Respiration

        The organs involved in external respiration can be divided into two groups. One group includes the organs involved in the mechanics of breathing. They are the ribs, rib muscles, diaphragm, and abdominal muscles. The other group includes the passages though which air travels to get to the bloodstream. These are the nostrils, nasal passages, phar­ynx, trachea, bronchi, bronchial tubes, and air sacs.

      

The Nose and Nasal Passages

      The air enters the nose in two streams through two nostrils. From the nostrils, air enters the nasal passages. These passages lie above the mouth cavity. Long hairs at the opening of the nostrils prevent the entrance of foreign particles. The wall of the nasal passages are lined with a mucous membrane. These cells secrete mucus, a sticky fluid that trap bacteria, dust, and other particles in the air. The mucus also moistens the air. Just below the mucus membrane is a rich supply of the capillaries. As air passes through the nose, it is warmed by the blood in these capillaries. Thus, the nasal passages serve to filter, moisten, and warm inhaled air before it reaches the lung. Both the fil­tering and warming advantages are lost when you breathe through your mouth.

     

 The Trachea

       From the nasal passages, air goes through the pharynx and down the windpipe, or trachea. The upper end of the trachea is protected by a flap of cartilage. This flap is called the epiglottis. When you swallow, the epiglottis closes over the trachea. This prevents food from getting into the lungs. The upper end of the trachea holds the voice box, or larynx. This forms a lump on the outside of the neck called the Adam's apple. Vocal cords are lo­cated inside the larynx. Our vocal cords are used to make sounds. Rings of cartilage support the trachea to keep it open for the passage of air.

The trachea and its branches are lined with tiny hairs called cilia. The cilia are constantly moving. They carry inhaled dirt and foreign particles upward toward the mouth. This dirt is removed when you cough, sneeze, or clear your throat.

The Bronchi and Air Sacs

  The trachea divides at its lower end. It forms two branches called bronchi. One bronchus extends to each lung. Each bronchus divides and forms many small bronchial tubes. These divide again into even smaller bronchioles. The bronchioles end in air sacs. Each air sac is made of clusters of tiny sacs called alveoli. The walls of the alveoli, which are only one cell thick, are the respiratory surface. They are thin and moist and are surrounded by a rich network of capillaries. It is through these walls that the exchange of oxygen and carbon dioxide between blood and air occurs. It has been estimated that the lungs contain about 300 million alveoli, with a total surface area of about 70 square meters. This would be 40 times the surface area of the skin.

       Besides irritating the trachea and bronchi, smoking inter­feres with the uptake of oxygen in the air sacs. When cigarette smoke is inhaled, about one-third of the particles remain in the alveoli. Phagocytic cells called macrophages can slowly remove many of the particles. However, an excess of particles from smoking or from other sources of air pollution breaks down the walls of the air sacs and causes the formation of inelastic, scar like tissue. This greatly reduces the functional area of the respiratory surface and may lead to a disease called emphysema.

Phases of Human Respiration

In humans, respiration can be divided into four distinct phases.

1.      Breathing is the movement of air into and out of the lungs.

2.      External respiration is the exchange of oxygen and carbon dioxide between the air and the blood in the lungs.

3.      Circulation is the carrying of dissolved gases by the blood to and from the body cells.

4.      Internal respiration is the exchange of oxygen and carbon dioxide between the blood and the body cells.

Note that these stages of respiration are physical processes. They should not lie confused with cellular respiration, the chemical processes within the" cells by which nutrients are broken down and energy is released.

The Mechanics of Breathing

Do you remember the last time you ran to catch a bus or train? By the time you took your seat you probably were breathing heavily. Breathing is the movement of the air into and out of the lungs.

The lungs are spongy, air-filled sacs in the chest cavity. Breathing is caused by muscle action. The muscles are those between the ribs, and in the diaphragm and abdo­men.

The Movements of Breathing

      The lungs fill much of the body cavity from under the shoulders down to the diaphragm. This cavity is called the thoracic cavity. The lungs are covered by a double mem­brane called the pleural membrane. One membrane is attached to the surface of the lungs. The other covers the inside of the thoracic cavity. These membranes secrete a lubricating mucus. This lets the lungs slide freely in the chest during breathing.

Place your hands on the sides of your chest and take in a deep breath. This is called inspiration. Can you feel your chest cavity expand? During inspiration, three things hap­pen to expand your chest cavity:

1.      The rib muscles contract, pulling the ribs up and out.

2.      The muscles of the dome-shaped diaphragm contract. This straightens and lowers the diaphragm. This action enlarges the chest cavity from below.

3.      The abdominal muscles relax. This allows compression of the abdominal contents when the diaphragm lowers.

When the chest cavity is expanded, air pressure inside the thorax decreases. Air rushes into the lungs to equalize the pressure.

     Place your hands on your chest again and observe the changes when you force the air from your lungs. This is called expiration. During expiration four things happen to reduce the size of your chest cavity.

1.      The rib muscles relax. This allows the ribs to spring back.

2.      The diaphragm relaxes, rising to its original position.

3.      The abdominal muscles contact. This pushes the ab­dominal organs up against the diaphragm.

4.      Elastic fibers in your lungs shrink and help to force air out of the lungs.

At expiration, the decrease in size of the chest cavity increases the air pressure inside the cavity. Air rushes out of the lungs to equalize the pressure.

Control Of Breathing

    Looking at another person, count the number of inspirations for one minute. This is the res­piration rate. In humans, inspiration and expiration (the cycle) occur from 16 to 24 times a minute. The exact rate depends on physical activity, position, mood, and age. Nerves and chemicals control your breathing and the res­piration rate.

 Nerves from the lungs, diaphragm, and rib muscles lead to a respiratory control center. This center is located at the base of the brain. There are also specials structures in the aorta and several other large arteries that are sensitive to the concentrations of oxygen and carbon dioxide in the blood. These chemoreceptors send messages to the respiratory center. It controls the regular rhythm of breath­ing. The amount of carbon dioxide in the blood is detected directly by the breathing control center. If the carbon diox­ide concentration is high, the brain signals the diaphragm and rib muscles. They increase the breathing rate. This increased rate forces more carbon dioxide out through the lungs and breathing settles back to a normal rate.

        During heavy muscular exertion, lactic acid is produced as well as carbon dioxide. This increases the acidity of the blood. The increased acidity also stimulates the respiratory center of the brain and increases the rate of breathing.

      External respiration is the exchange of oxygen and carbon dioxide between the air and the blood in the lungs. After inhalation, the concentration of oxygen in the alveoli is higher than the con­centration of oxygen in the blood. Oxygen dissolves into the moist lining of the alveoli and diffuses from the region of higher concentration (the alveoli) to the region of lower con­centration (the blood). Independently, carbon dioxide diffuses in the opposite direction—out of the blood and into the al­veoli.

As the blood is pumped through the vessels of the body by the beating of the heart, oxygen-rich blood from the lungs is carried to the body tissues and oxygen-poor blood from the tissues is returned to the lungs.

Air Capacity of the Lungs

    Each time you inhale and exhale, only about 500 milliliters of air are exchanged. The maximum amount of air that you can move through your lungs is called the vital capacity. This is the total amount of air that moves through your lungs when you inhale and exhale as hard as you can. The vital capacity of the normal person is about 4,500 milliliters. A well-trained athlete may have a vital capacity of 6,500 mil­liliters.

Internal respiration

Internal respiration is the exchange of oxygen and carbon dioxide between the blood and the body cells. In the capil­laries of the body tissues, oxygen diffuses from the blood through the intercellular fluid to the body cells; carbon dioxide diffuses from the cells through the intercellular fluid into the blood. Each gas diffuses down a concentration gradi­ent, i.e., from a region of higher concentration to a region of lower concentration.

Gases can also dissolve in liquids.

This is why oxygen and carbon dioxide can be transported by the blood. The solubility of oxygen, carbon dioxide, and nitrogen varies. Temperature changes will also affect the amount of gas that can be dissolved in a liquid. Warm water will hold less dissolved gas than will cold water.

Gas Exchange In the Lungs

The pulmonary artery carries deoxygenated, dark red blood to the lungs. There it branches into an extensive network of small capillaries. These capillaries completely surround each alveolus. The air in the alveoli and the blood in the capillaries contain gases in different concentrations. Therefore, diffusion oc­curs through the thin, moist membranes of both the alveoli and capillaries. Oxygen diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air.

The Transport of Oxygen

        Oxygen is not very soluble in the plasma of blood. It is even less soluble at our body tem­perature of 37°C. Oxygen would be more soluble at lower temperatures. Remember, the erythrocytes contain a substance called hemoglobin. Most oxygen is transported from the lungs to the body tissues by the hemoglobin. Hemoglobin is a unique iron-containing protein. Its most important charac­teristic is that it combines readily with oxygen. This reaction is reversible, depend­ing on the oxygen concentration. In the lungs, where the oxy­gen concentration is high, hemoglobin (Hb) combines with oxygen (O₂) to form oxyhemoglobin (HbO₂). When the blood reaches the capillaries of the body tissues, where the oxygen concentration of the surrounding tissues is low, the oxyhemoglobin breaks down into oxygen and hemoglobin. The oxygen diffuses from the blood into the body cells, where it is used in cellular respiration.

Blood low in oxygen is a dark red or dull purple color be­cause of the hemoglobin. Blood rich in oxygen is a bright red color because of the oxyhemoglobin.

Hemoglobin has another important characteristic. The attraction of hemoglobin for oxygen decreases with an increase in acidity. During exercise, lactic acid is produced by the active muscle cells. This causes the hemoglobin to release more of its oxygen than it would normally.

The Transport of Carbon Dioxide

     Cellular respiration produces carbon dioxide. Thus the concentration of carbon dioxide is greater in the body cells than in the capillary blood. There­fore, the carbon dioxide diffuses out of the cells and into the blood. Carbon dioxide is transported by the blood to the lungs in several ways.

When carbon dioxide diffuses into the blood, it combines with water, forming carbonic acid.

                          CO₂+ H₂O——> H₂CO₃

The H₂CO₃ quickly breaks down (ionizes), forming hydrogen ions and bicarbonate ions,

                          H₂CO₃     —>      H⁺ + HCO₃^-

These reactions are speeded up by the presence of an enzyme in the red blood cells. Most of the carbon dioxide (about 70 percent) is carried in the plasma in the form of bicarbonate ions.

Some of the carbon dioxide (about 20 percent) is carried in the red blood cells as carboxy-hemoglobin.

                          CO₂ + Hb——> HbCO₂

A small amount of carbon dioxide (about 10 percent) is car­ried in solution in the plasma,

All these reactions are reversible, and in the lungs carbon dioxide is released.

Oxygen Debt

During times of great muscular activity, the cells need more oxygen than the body can supply. The lungs cannot take in oxygen fast enough, nor can the blood deliver it fast enough. When this happens, the cells switch to anaerobic respiration. This means that oxygen is not used. Instead, pyruvic acid becomes the hydrogen acceptor in the process of energy exchange. For a short period, the cells have enough energy to function and survive. The anaerobic process produces lactic acid. This collects in the tissues, causing a feeling of fatigue. A buildup of lactic acid signals the brain's respiratory center to increase the breathing rate and supply the tissues with more oxygen.

If the heavy exercise continues, lactic acid keeps build­ing up. This is called a state of oxygen debt. It continues until the heavy exercise ends. Then during a half-hour rest, some lactic acid is oxidized. Some is converted to glycogen. Carbon dioxide and excess water are excreted. The oxygen debt is paid. The body is ready for more exercise.

Environmental Effects on Breathing and Respiration

The air's temperature, moisture, oxygen, and carbon diox­ide content all influence the rate of breathing and respira­tion. Certain of these factors involve ventilation. If the air in a room is stuffy, it is likely to be too warm and moist. Very rarely is it caused by a build-up of carbon dioxide and lack of oxygen.

Carbon Monoxide

    Far too often you read of people who have died in a closed garage where an automobile engine was running. The cause of death is given as carbon monoxide poisoning. Actually, the death is not caused by poison­ing but by tissue suffocation. Carbon monoxide will not support life. Yet it combines with the hemoglobin of the blood 250 times more readily than does oxygen. As a result, the blood becomes loaded with carbon monoxide. Its oxy­gen-combining power decreases. The tissues suffer from oxygen starvation. The victim becomes light-headed. Soon paralysis sets in. Death follows from tissue suffocation.

High Altitudes

    You live at the bottom of a large ocean of air. If you were to climb a high mountain, the air pressure would become less. At this height, the molecules of nitro­gen, oxygen, and carbon dioxide are spread farther apart. You may have experienced your ears "popping" during an altitude change. Your middle ear must equalize the pres­sure,

Air and Space Travel

     When an airplane approaches an altitude of 6,000 meters , the pressure becomes so low that a pilot has difficulties in seeing and hearing. This condition is called hypoxia. It is the result of oxygen starvation of the cells. Passengers in modern airliners fly at high altitudes in pressurized cabins.

Diving

SCUBA (underwater breathing device) divers are well aware of the problems of pressure and respiration. The weight of water causes an increase in pressure as a diver descends. The air that passes into the lungs must be under a greater pressure than that of the water. This pressure means that the molecules of nitro­gen, oxygen, and carbon dioxide are closer together. The blood and tissues of a diver, then, dissolve more molecules of these gases than you have in your body now. If a diver returns too quickly to the surface, gas bubbles (mostly nitrogen) form in the tissues. These can cause pain and even death. This condition is commonly called the bends.

Diseases of the Respiratory System

The following list includes some of the common disorders of the respiratory system.

1.        Asthma: is a severe allergic reaction in which contraction of the bronchioles makes breathing difficult.

2.        Bronchitis is an inflammation of the linings of the bron­chial tubes. The passageways to the alveoli become swollen and clogged with mucus. The condition is generally marked by severe coughing and by difficulty in breathing.

3.        Emphysema is a condition in which the lungs lose their elasticity. The walls of the air sacs break down, reducing the respiratory surface. Emphysema is marked by shortness of breath.

4.        Pneumonia is a condition in which the alveoli become filled with fluid, preventing the exchange of gases in the lungs.

5.        Lung cancer is a disease in which tumors (masses of tis­sue) form in the lungs as a result of irregular and uncontrolled cell growth. Numerous studies have demonstrated a definite relationship between lung cancer and smoking.

Smokers also run a greater risk of developing bronchitis and emphysema than nonsmokers.