Chapter 14: The Respiratory System
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.
14.1 THE NEED FOR RESPIRATION release energy
Respiration is the biochemical process of releasing energy by breaking down glucose for carrying out life processes.
This chemical breakdown occurs by utilizing oxygen and is represented by the following overall reaction:
C6H12O6+ 60 - 6CO2 + 6H2O + energy(38 ATP)
There are five important points to remember about this chemical reaction in respiration.
1. This part of respiration, yielding energy, occurs inside the living cells and hence, it is better known as cellular or tissue respiration.
2. The breakdown of glucose (C6H12O6) to carbon dioxide and water does not occur in a step but in a series of chemical steps. Some of these steps occur in the cytoplasm of the cell and some inside the mitochondria.
3. Each breakdown step is due to a particular enzyme.
4. 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.
[As you have read in the case of plants when the energy in the form of ATP is used, it is converted to ADP (adenosine diphosphate) and again when more energy is available by further breakdown of glucose, the ADP is reconverted to ATP and so it goes on and on. ATP is a sort of "energy currency" inside the cell. One mole of glucose on complete oxidation yields 38 molecules of ATP.]
5. The essential steps of cellular respiration are same in plants and animals.
WHY WE NEED ENERGY
Body cells need energy for a vast variety of activities in them. Some of these are:
1. Synthesis of proteins from amino acids 2. Production of enzymes
3. Contraction of muscles for movement
4. Conduction of electrical impulse in a nerve cell 5. Production of new cells by cell division
6. In keeping the body warm (in warm-blooded animals, i.e. birds and mammals)
14.2 ANIMALS NEED MORE ENERGY
The need for 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 look after their eggs or young ones, and so on.
The birds and mammals need still more energy
The birds and mammals, including ourselves, have to produce a lot of heat for keeping the body rm. This heat comes through respiration in the Is. The amount of heat required 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.
Shivering and clattering of teeth (when one feels too cold) is an emergency activity of the muscle cells to produce extra heat to keep the body warm.
The energy used in all the cellular activities is obtained from the oxidation of glucose (CHO), carbohydrate.
4.3 GLUCOSE HAS NO ALTERNATIVE FOR RESPIRATION
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 re totally flesh-eaters. The main constituent of their diet is protein with very little 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.
14.4 TWO KINDS OF RESPIRATION AEROBIC AND ANAEROBIC
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 such as fast running, walking over long distances, swimming, wrestling, weight-lifting, etc., our muscles work too fast but not get 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 which may be called oxygen-debt. When you rest after such exercise, the lactic acid gets slowly oxidised by the oxygen later available and then the "debt is cleared" producing carbon dioxide in the process. CHEMICAL STEPS IN RESPIRATION Aerobic respiration in animals
The chemical changes taking place in aerobic respiration in animals are the same as in the aerobic respiration in plants. The overall chemical change can be represented by the equation:
CH1206+ 60,→ 600, 6CO2+ 6H2O + 686 kcal/mole Glucose Oxygen
Carbon Water dioxide
Energy
The above equation depicts the chemical substances in mole. 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 × 4.2) kJ.
In the above equation we can represent energy in the form of ATP as follows:
C6H12O6+602 → 6CO, + 6H,O+ 38ATP + 420 kcal
[One mole of ATP requires 7 kcal, so 38 ATP are produced by consuming 38 x 7 = 266 kcal, the rest of the energy i.e. 686-266 = 420 kcal is released as heat). 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. The overall chemical reaction in anaerobic respiration is summarised as follows
C6H12O6 -> lactic acid + 2ATP + heat energy
Glucose
Special points to note in the above chemical reaction in anaerobic respiration in animals, are as follows:
1. It is a slow process.
2. The reaction cannot continue for long time. The product lactic acid has a toxic effect on cells, which causes muscle fatigue and aches.
3. No CO2 is produced.
4. 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, the anaerobic respiration is different in the two in some respects.
Table 14.1 Differences in anaerobic respiration in plants and animals.
Anaerobic respiration
IN PLANTS
1. Products of glucose breakdown are ethanol and CO2-
2. Released heat energy is more.
Anaerobic respiration in ANIMALS
1. Product of glucose breakdown is lactic acid only (and no CO2).
2. Released heat energy is less.
14.5 PARTS OF RESPIRATION
Four major parts of respiration:
In humans (as in most other animals) there are
Out
1. Breathing: This is a physical process in which the atmospheric air is taken in and forced c of the oxygen-absorbing organs, the lungs.
2. Gaseous transport: The oxygen absorbed by the blood in the lungs is carried by the RBCs as oxyhaemoglobin 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:
(i) as bicarbonates dissolved in plasma, and
partly,
(ii) in combination with the haemoglobin of RBCs as carbamino-haemoglobin.
3. 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
4. Cellular respiration: The complex chemical changes which occur inside the cell to release energy from glucose.
A common misconception : Many people wrongly say "We inhale oxygen and exhale carbon dioxide". Instead we should say "We inhale air containing much oxygen and very little carbon dioxide and exhale air containing less of oxygen and more of carbon dioxide than before.
Where does respiration occur in a cell?
The cellular respiration occurs in two main phases at two different places inside the cell:
1 Glycolysis (breakdown of glucose):
CYTOPLASM (Glycolysis)
- occurs in cytoplasm outside the mitochondria breakdown into pyruvic acid which further breaks down into ethanol in plants and lactic acid in animals
- anaerobic (not requiring oxygen)
- very little energy released.
2 Krebs cycle:
- occurs inside mitochondria
MITOCHONDRION (Krebs cycle)
- step by step breakdown of pyruvic acid/lactic acid to produce ATP and CO, aerobic (needs oxygen) much energy produced
- H' ions released in the cycle are removed through the oxygen supplied by forming H,O.
So now you know why our body needs oxygen. Yes, to remove the H+ ions.
14.6 RESPIRATORY ORGANS (BREATHING) 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:
(1) It warms the air as it passes over
(2) It adds moisture to the air
(3) 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 (Fig. 14.2) When you smell something special, you give a little sniff which carries the odour 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 (oesophagus) 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
The Larynx: The larynx or the voice-box (popularly called "Adam's apple") is a hollow cartilaginous structure located at the start of the windpipe (Fig. 14.2 & 14.3). You can feel it with your fingers in the front part of your neck. When you swallow 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.
VOICE AND SPEECH
Voice is the sound produced by the vocal cords of the larynx.
Speech is a character given to the voice by the complex movements of lips, cheeks, tongue and jaws. Speech consists of words or syllables, and it is a speciality of only the human species.
The Trachea : The trachea or the windpipe emerges from the larynx (Fig. 14.3) down below in 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 back side. 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 which 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. A network of blood capillar of the alveoli are extremely thin (one-cell thick) a moist, thus allowing gaseous diffusion them (Fig. 14.4). Oxygen from air first dissolves a thin layer of water/fluid that covers the surface of alveoli.
The lungs provide an enormous surface area!
The number of alveoli in the two lungs in an adult human - about 700 million.
Total surface area of the alveoli - about 70 square metres (nearly equal to the area of a tennis court, or nearly hundred times the surface of the skin).
Protective inner Lining of Respiratory Passages
The entire inner lining of the larynx , trachea , bronchi , and bronchioles is formed of ciliated epithelium . During 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 towards the mouth.
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 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 (Fig. 14.3). 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 the 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 Fig. 14.6 represents the branching of respiratory passages and the blood circulation in the lungs. 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.
14.7 BREATHING - RESPIRATORY CYCLE
Respiration vs. Breathing
Respiration (Gk. re: again, spirare: to blow) is a broader term which includes intake of air/ oxygen and its utilization in the body cells to produce energy, But, breathing is simply a mechanical process of inhaling and exhaling the air, in other words it is a muscular process. So, "Respiration includes breathing but breathing doesn't include respiration."
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 new born breathes 60 times per minute. Slight increase in the CO2 content in blood increases the breathing rate.
1. Inspiration (or inhalation) is the result of increase in the size of thoracic cavity and this increase is due to the combined action of the ribs and the diaphragm (Fig. 14.7).
The ribs move upwards and outwards 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, a sheet of muscular tissue situated towards the base of the lungs, which normally remains arched upward like a dome, contracts and flattens from the dome-shaped outline to an almost horizontal plane and thus contributes to at lengthwise. As the diaphragm flattens, it presses the organs inside the abdomen and with the abdominal muscles relaxed, abdominal wall moves outwards leading to increase in volume of chest cavity and decrease of pressure.
Decreased pressure inside the lungs draws the air inward. The outside air being at a greater pressure, rushes equalize the pressure.
When the thoracic (chest) cavity increases in size, its internal pressure decreased. The lungs expand and as a result, the pressure inside the lungs is lowered below the atmospheric pressure].
2 . 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 move upwards to its dome-like outline. As a consequence of the above-mentioned movements of ribs and diaphragm, the volume of the thoracic cavity is decreased and the lungs are compressed, forcing the air out into the atmosphere.
When we breathe out forcibly or naturally as it happens during intense physical exercise, the internal intercostal muscles also contract causing further contraction of the rib cage to expel out more air for larger intake of oxygen.
Tissue or Internal Respiration: The process by which the cells of the body use the oxygen to oxidise the food and release energy. The carbon dioxide formed as a waste product of breakdown of glucose molecule is sent out in expiration.
Have you ever observed the w fall of people's belly alternately expiration? Now you knew them for
CONTROL OF BREATHING MOVEMENTS
The breathing movements are largely controlled by a respiratory centre located in the medulla oblongata of the brain. This centre is stimulated by the carbon dioxide content of the blood. More the carbon dioxide content in the blood, faster is 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 approximately as follows:
1. 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 trachea and bronchi where no diffusion of gases can occur = 150 mL
Alveolar air. The tidal air contained in air sacs = 350 mL 2. Inspiratory reserve volume. Air that can be drawn in forcibly over and above the tidal air (also called complemental air) = 3000 mL 3. Inspiratory capacity. Total volume of air a person can breathe in after a normal expiration.
= 3500 mL 4. Expiratory reserve volume. Air that can be forcibly expelled out after normal expiration (also called supplemental air) = 1000 mL
5. Vital capacity. The volume of air that can be taken in and expelled out by maximum inspiration and expiration = 4500 mL
6. Residual volume. Some air is always left in the lungs even after forcibly breathing out. This is the leftover (residual) air
= 1500 mL
7. Total lung capacity. Maximum air which can at any time be held in the two lungs = 6000 mL
14.9 INSPIRED AIR vs. EXPIRED AIR The air inside the lungs is never replaced completely. It is always a mixture of the air left inside and the air inspired. In other words, the air in the lungs is only becoming better and worse with each inspiration and expiration.
Qualitywise, the expired air differs from inspired air in the following respects: 1. It contains less oxygen.
2. It contains more carbon dioxide. 3. It contains more water vapour.
4. It is warmer (or at the same temperature as that of the body).
5. It may contain some bacteria.
Table 14.3 gives the average composition of the expired and inspired air of a person at rest and the basis of difference.
EFFECT OF ALTITUDE ON BREATHING
As we go higher up, the air we breathe in decreases in pressure accompanied by a gradual decrease in oxygen content. At about 4,500 metres above sea level, one may suffer from air sickness, in which lack of oxygen leads to dizziness, unsteady vision, loss of hearing, lack of muscular coordination and even complete blackouts.
14.10 HYPOXIA AND ASPHYXIATION
HYPOXIA is the deficiency of oxygen reaching the tissues. It may result due to sitting for long hours in a crowded room with poor ventilation. It may also be experienced at high altitudes where the oxygen content of the air is low.
ASPHYXIATION is a condition in which the blood becomes more venous by accumulation of more diminished. This may result due to several causes, such as, strangulation, drowning, or any obstruction in the respiratory tract. Death follows if the cause is not taken care of quickly. Artificial respiration is helpful in certain cases.
14.11 SOME EXPERIMENTS ON BREATHING AND RESPIRATION
1. To demonstrate that water is lost during breathing:
Gently breathe upon a cold surface such as a piece of glass or slate; the water droplets appearing on the surface prove the presence of moisture in expired air.
2. To demonstrate that CO, is given out in breathing :
Set up an apparatus as shown in Fig. 14.9. Clip (C) is opened and clip (D) is closed. Air is sucked in by the mouth, through the tube at the centre. Atmospheric air rushes in flask (A) bubbling through the lime water. Next, clip (C) is closed and clip (D) is opened and air is blown through same central tube. This time the air is forced into flask (B) bubbling through its lime water. The process is repeated about ten times. The lime water in flask (B) turns milky much faster than in flask (A). This proves that the expired air contains more carbon dioxide than the inspired air.
3. To demonstrate the action of the diaphragm during breathing: Set up an experiment as shown in Fig. 14.10. The rubber sheet tied around the bottom edge of the bell jar represents the diaphragm. When the sheet is pulled downward, volume is increased, pressure inside the bell jar lowered and the rubber balloons are expanded by the air rushing in through the tube at the top. When the sheet is pushed upward, volume is decreased, pressure inside the jar increased and the balloons collapse due to the air rushing out. The balloons represent the two lungs.
4. To measure the volume of expired air: Set up an apparatus as shown in Fig. 14.11. Fill your chest with air to the maximum, and then blow out through the short tube expelling as much air as you can. The water expelled from the other tube when measured gives the volume of the air exhaled.
5. To show that oxygen is taken in by animals during respiration : Use a small animal such as a cockroach or rat in this experiment. Take two conical flasks A and B. Place a live cockroach in one flask (A) and a dead cockroach that has be as a control. Fit a rubber cork in the mouth soaked in formalin to prevent decay in the othe flask (B). This flask with the dead cockroach ac each flask and make sure that the apparatus air-tight. Leave the flasks for a few hours, afte which introduce a small burning candle into eac flask as shown in the Fig. 14.12. Immediately afte flask tightly. Note the time taken for the cand introducing the candle, close the mouth of the flame to go out. You will find that the flame goes out faster in the flask (A) containing the flame of the candle goes out faster because the living animal. Since oxygen supports burning the living cockroach also consumes oxygen for respiration.