The Respiratory System

What is Respiration?

• Respiration is a vital process in all living organisms. It goes on non-stop throughout life.
• This chapter explains the various aspects related to respiration — the raw material used, the end products formed and the amount of energy liberated, etc.
• Some experiments to demonstrate the mechanism of breathing are very interesting.

Need for Respiration:

• There are five important points to remember about this chemical reaction in respiration.
  • This part of respiration, yielding energy, occurs inside the living cells and hence, it is better known as cellular or tissue respiration.
  • The breakdown of glucose to carbon dioxide and water does not occur in a single step but in a series of chemical steps.
  • Some of these steps occur in the cytoplasm of the cell and some inside the mitochondria.
  • Each breakdown step is due to a particular enzyme.
  • The energy liberated in the breakdown of the glucose molecule is not all in the form of heat, but a large part of it is converted into chemical energy in the form of ATP - a chemical substance called adenosine triphosphate.
  • The essential steps of cellular respiration are the same in plants and animals.

Animals need more Energy:

• The need for the production of energy is greater in animals than in plants.
  • This is because animals consume more energy in doing physical work.
  • They have to move about for obtaining food or run away to escape enemies.
  • They have to chew their food and have to take after their eggs or young ones, and so on.
    • Birds and mammals including we have also to produce a lot of heat for keeping their body warm. This heat comes through respiration in the cells.
    • The amount of heat to keep the body warm is quite large.
  • Think about the cold winter days when the outside temperature is far below our body temperature.
    • We are constantly losing heat to the outside air, and more of it has to be continuously produced to make up for the loss.
    • Liver cells in particular produce much heat, and the muscle cells also contribute to it.
    • The energy used in all cellular activities is obtained from the oxidation of glucose (C6H 20 6), a carbohydrate.

Glucose:

• If the simple carbohydrate (glucose) is not available directly, the cells may break down the proteins or fats to produce glucose for respiratory needs.
  • Think for a while about the wild animals which are totally flesh-eaters.
  • The main constituent of their diet is a protein with very few carbohydrates.
    • The excess amino acids absorbed through protein- digestion are broken down in the liver to produce sugar (glucose) and the nitrogenous part is converted into urea which gets excreted out.
  • The glucose thus produced may be used immediately or may get stored in the liver cells as glycogen for future needs.
• A similar process takes place in humans if they take excessively protein-rich food.

Aerobic and Anaerobic Respiration:

• In animals, there is normally aerobic respiration using oxygen.
  • Anaerobic respiration (in the absence of oxygen) is only exceptional in some cases as in the tapeworms living inside the human intestines.
  • Anaerobic respiration may occur even in our own body in the fast-working skeletal muscles temporarily.
    • During continuous physical exercise as fast running, walking over long distances, swimming, wrestling, weight-lifting, etc., our muscles work too fast but not getting enough oxygen.
    • In this situation, the muscles are working in the absence of oxygen (anaerobic respiration) to provide extra energy.
    • The product of anaerobic respiration in such muscles is lactic acid.
    • Accumulation of lactic acid gives the feeling of fatigue.
    • This is a condition that may be called oxygen debt.
  • When you rest after such exercise, the lactic acid gets slowly oxidized by the oxygen later available and then the “debt is cleared” producing carbon dioxide in the process.

Aerobic respiration in animals:

• The chemical changes taking place in aerobic respiration in animals are the same as in aerobic respiration in plants.
  • Thus, by taking 180 g of glucose the energy released is 686 kilocalories, or if expressed in kJ (kilojoules) the energy released is about 2890 (686 x 4 2) kJ.

Anaerobic respiration in animals:

• In animal cells, particularly in the skeletal muscle cells, anaerobic respiration may occur when they have to work very fast with insufficient oxygen.
  • It is a slow process.
  • The reaction cannot continue for a long time.
    • The product lactic acid has a toxic effect on cells, which causes muscle fatigue and aches.
    • No CO2 is produced.
    • The total energy released per mole of glucose is much less compared to aerobic respiration.
    • The basic steps in cellular respiration are the same in plants and animals.
    • However, anaerobic respiration is different between the two in some respects.

Parts of Respiration:

• In humans (as in most other animals) there are four major parts of respiration:
  • Breathing: This is a physical process in which the atmospheric air is taken in and forced out of the oxygen-absorbing organs, the lungs.
  • Gaseous transport: The oxygen absorbed by the blood in the lungs is carried by the RBCs as oxyhemoglobin, throughout the body by means of arteries.
  • The carbon dioxide from the tissues is transported to the lungs by the blood by means of veins in two ways:
    • as bicarbonates dissolved in plasma,
    • in combination with the hemoglobin of RBCs as carbamino-hemoglobin.
  • Tissue respiration: The terminal blood vessels, i.e., the capillaries deliver the oxygen to the body cells or tissues where oxygen diffuses through their thin walls, and in a similar way, the capillaries pick up the carbon dioxide released by them.
  • Cellular respiration: The complex chemical changes which occur inside the cell to release energy from glucose.

Respiratory Organs:

• The respiratory system in humans consists of air passages (nose, pharynx, larynx, trachea, bronchi) and the lungs.
  • The nose: The external part of the nose bears two nostrils separated by a cartilaginous septum.
  • The hairs present in the nostrils prevent large particles from entering the system.
  • The two nostrils open into a pair of nasal chambers.
    • The inner lining of  the nasal chambers performs three functions:
    • It warms the air as it passes over.
    • It adds moisture to the air.
    • Its mucous secretion entraps harmful particles.
      • So, always breathe through the nose and not through the mouth.
    • An additional function of the nose is to smell.
      • The sensory cells of smell are located in a special pocket situated high up in the nasal chambers.
      • When you smell something special, you give a little sniff which carries the odor up into this pocket.

The Pharynx:

• The nasal chambers open at the back into a wide cavity, the pharynx, situated at the back of the mouth.
  • It is a common passage for air and food. It leads into an air tube, the trachea (windpipe), and a food tube (esophagus) located dorsally behind the trachea.
  • When not in use, the food tube is partially collapsed as it has soft walls.
    • The entrance to the trachea is guarded by a flap called epiglottis which closes it at the time of swallowing food.
  • Incomplete closure of epiglottis during swallowing causes cough.

Larynx:

• The larynx or the voice box (popularly called “Adam’s apple”) is a hollow cartilaginous structure located at the start of the windpipe.
  • You can feel it with your fingers in the front part of your neck.
  • When you so allow something, this part rises and falls.
  • The larynx contains two ligamentous folds called vocal cords not shown in the figure.
  • Air expelled forcibly through the vocal cords vibrates them producing sound.
  • By adjusting the distance between the two cords and their tension by means of attached muscles, a range of sounds can be produced.

Trachea:

• The trachea or the windpipe emerges from the larynx down below the neck where it is partly covered by the thyroid gland.
  • Its walls are strengthened by C-shaped rings of cartilage, the incomplete parts of the rings being on the backside.
  • The rings provide flexibility and keep the trachea distended permanently.

The Bronchi:

• Close to the lungs, the trachea divides into two tubes, called the bronchi {sing. bronchus), which enter the respective lungs.

  • On entering the lungs, each bronchus divides into fine secondary bronchi, which further divide into still finer tertiary bronchi.
  • The cartilaginous rings, as those present on the trachea, are also present on the smaller bronchi to keep them distended.

• Bronchioles are the subsequent still finer tubes of tertiary bronchi that acquire a diameter of about 1 mm and are without cartilage rings.

  • By repeated branching, the bronchioles ultimately end in a cluster of tiny air chambers called the air sacs or alveoli {sing. alveolus).
  • A network of blood capillaries surrounds the wall of each alveolus.
  • The walls of the alveoli are extremely thin (one-cell thick) and moist, thus allowing gaseous diffusion through them.
  • Oxygen from the air first dissolves in a thin layer of water/fluid that covers the surface of the alveoli.
  • The protective inner lining of respiratory passage:

• The entire inner lining of the larynx, trachea, bronchi, and bronchioles is formed of ciliated epithelium.

  • During their lifetime the cilia are constantly in motion driving any fluid (mucus) that is on them and also any particles that may have come in with the air toward the mouth.

The Lungs:

• The Lungs are a pair of spongy and elastic organs formed by the air sacs, their connecting bronchioles, blood vessels, etc.
  • The two lungs are roughly cone-shaped, tapering at the top and broad at the bottom.
  • The left lung has two lobes, and the right lung has three.
  • The left lung is slightly smaller to accommodate the heart in between.

Membranous Covering of the Lungs:

• Membranous coverings of the lungs. 
  • Each lung is covered by two membranes — the inner (visceral) pleura and outer (parietal) pleura with a watery fluid (pleural fluid) in the pleural cavity found between the two membranes.
  • This arrangement provides lubrication for free movement of the expanding and contracting lungs.
  • The lungs occupy the greater part of the thoracic cavity.
    • They are located close to the inner surface of the thoracic wall and their lower bases closely rest on the diaphragm.

Blood Supply to Lungs:

• The right auricle pumps all the deoxygenated blood received in it from the body into the right ventricle, which in turn, pumps it into the lungs through the main pulmonary artery.
  • The pulmonary artery, soon after its emergence, divides into two branches entering their respective lungs.
  • Inside the lungs, they divide and redivide several times to ultimately form capillaries around the air sacs.
  • Veins arising from these capillaries join and rejoin to form two main pulmonary veins from each lung which pour the oxygenated blood into the left auricle of the heart.
    • The bright red parts represent oxygenated blood, and the dull brownish parts represent deoxygenated blood.
  • The interconnecting capillaries between arteries and veins have not been shown in the upper figure to avoid complexity in the diagram.
    • The lower figure shows a small part of the lung highly magnified depicting air sacs (alveoli), the capillaries surrounding them, and the connected pulmonary artery and pulmonary vein.

Breathing - The Respiratory Cycle:

• The respiratory cycle consists of inspiration (breathing in), expiration (breathing out), and a very short respiratory pause. In normal adults, the breathing rate is 12-18 breaths per minute.
  • A newborn breath 60 times per minute.
  • A slight increase in CO, content in the blood increases breathing rate.
    • Inspiration (or inhalation) is the result of an increase in the size of the thoracic cavity and this increase is due to the combined action of the ribs and the diaphragm.
    • The ribs are moved upward and outward due to the contraction of the external intercostal muscles stretched between them, thus enlarging the chest cavity all around. (The internal intercostal muscles are relaxed).
    • The diaphragm is a sheet of muscular tissue, which normally remains arched upward like a dome, towards the base of the lungs, contracts and flattens it presses the organs inside the abdomen, and from the dome-shaped outline to an almost horizontal the abdominal muscles relaxed, the abdominal wall plane and thus contributes to the enlargement of the moves outwards leading to increase in the volume of chest cavity lengthwise.
    • As the diaphragm flattens, the chest cavity decreases pressure.
      • Decreased pressure inside the lungs draws the air inward.
      • The outside air being at a greater pressure rushes in to equalize the pressure.
    • When the thoracic (chest) cavity increases in size, its internal pressure is decreased.
    • The lungs expand and as a result, the pressure inside the lungs is lowered below the atmospheric pressure.
      • Expiration (or exhalation) is the result of reverse movements of the ribs and diaphragm.
      • The external intercostal muscles relax and the ribs move in automatically.
        • The diaphragm is relaxed and moves upwards to its dome-like outline.
      • As a consequence of the above-mentioned movements of the ribs and diaphragm, the volume of the thorax cavity is decreased and the lungs are compressed, forcing the air out into an atmosphere.
      • When we breathe out forcibly or naturally as it happens during intense physical exercise, the internal intercostal muscles also contract to cause further contraction of the rib cage to expel out more air for a larger intake of oxygen.

Control of Breathing Movement:

• The breathing movements are largely controlled by a respiratory center located in the medulla oblongata of the brain.
  • This center is stimulated by the carbon dioxide content of the blood.
  • More carbon dioxide content in the blood faster the breathing.
  • The breathing movements are normally not under the control of the will, i.e., they are involuntary, but to some extent, one can consciously increase or decrease the rate and extent of breathing.
    • If you forcibly hold your breath, a stage would come when you cannot hold it any longer.

Capacities of the Lungs:

• Capacities of the lungs or the Respiratory volumes in a normal human adult are approximate as follows:
  • Tidal volume. Air breathed in and out in a normal quiet (unforced) breathing = 500 mL.
  • Dead air space. Some tidal air is left in respiratory passages such as the trachea and bronchi where no diffusion of gases can occur = 150 mL
  • Alveolar air. The tidal air contained in air sacs = 350 mL.
  • Inspiratory reserve volume. Air that can be down in forcibly over and above the tidal air (also called complemental air) = 3000 mL.
  • Inspiratory capacity. The total volume of air a person can breathe in after a normal expiration. = 3500 mL.
  • Expiratory reserve volume. Air that can be forcibly expelled out after normal expiration (also called supplemental air) = 1000 mL
  • Vital capacity. The volume of air that can be taken in and expelled out by maximum inspiration and expiration = 4500 mL
  • Residual volume. Some air is always left in the lungs even after forcibly breathing out.
  • This is the leftover (residual) air = 1500 mL
  • Total lung capacity. Maximum air which can at any time be held in the two lungs = 6000 mL

\