What is Respiratory System, Human Respiratory System

Human 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.

THE NEED FOR RESPIRATION

This chemical breakdown occurs by utilizing oxygen and is represented by the following overall reaction:

C₆H₁₂O₆ + 60₂ → 6CO₂ + 6H₂O + energy

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 (C₆H₁₂0₆) 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 same in plants and animals.

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.

Birds and mammals need still more energy

The birds and mammals including ourselves have also to produce a lot of heat for keeping the 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 the loss. Liver cells in particular produce much heat, and the muscle cells also contribute to it. The energy used in all the cellular activities is obtained from the oxidation of glucose (C₆H₁₂0₆) a carbohydrate.

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 are 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.

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 as in 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 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-

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 x 4.2) kJ.

In the above equation we can represent energy in the form of ATP as follows-

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

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 same in plants and animals. However, the anaerobic

respiration is different in the two in some respects.

Differences in anaerobic respiration in plants and animals.

PARTS OF RESPIRATION

In humans (as in most other animals) there are four major parts of respiration-

1. Breathing-

This is a physical process in which the atmospheric air is taken in and forced out 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.

Tissue respiration-

The terminal blood vessels, 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 (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 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. 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.

The Trachea-

The trachea or the windpipe emerges from the larynx 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 (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 air first dissolves in a thin layer of water/fluid that covers the surface of alveoli.

Protective inner lining of respiratory passage. 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

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. 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.) 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. 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 — 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 new born breathes 60 times per minute. Slight increase in CO₂ content in blood increases 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.

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 a sheet of muscular tissue, which normally remains arched upward like a dome,
towards the base of the lungs, contracts and flattens from the dome-shaped outline to an almost horizontal plane and thus contributes to the enlargement of the chest cavity lengthwise. As the diaphragm flattens,

it presses the organs inside the abdomen and with the abdominal muscles relaxed, the 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 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).

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 thorax 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.

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 the 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 = 500ml

Dead air space- Some tidal air is left in respiratory passages such as trachea and bronchi where no diffusion of gases can occur = 150ml
Alveolar air- The tidal air contained in air sacs = 350ml

2. Inspiratory reserve volume- Air that can be drawn in forcibly over and above the tidal air (also    called complemental air) = 3000ml

3. Inspiratory capacity- Total volume of air a person can breathe in after a normal expiration. = 3500ml

4. Expiratory reserve volume- Air that can be forcibly expelled out after normal expiration (also called supplemental air) = 1000ml

5. Vital capacity- The volume of air that can be taken in and expelled out by maximum inspiration and expiration = 4500ml

6. Residual volume- Some air is always left in the lungs even after forcibly breathing out. This is the leftover (residual) air = 1500ml

7. Total lung capacity- Maximum air which can at any time be held in the two lungs = 6000ml

All the above respiratory volumes (in ml) are available in a normal adult human.

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,

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.

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 carbon dioxide and the oxygen supply is 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 removed quickly. Artificial respiration is helpful in certain cases.

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 CO2 is given out in breathing-

Set up an apparatus as shown in (Fig.) 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 the exhaling air\ is blown through the 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.) 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, 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, 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.) 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 snail in this experiment. Take two conical flasks A and B. Place a live cockroach in one flask (A) and a dead cockroach that has been soaked in formalin to prevent decay in the other flask (B). This flask with the dead cockroach acts as a control. Fit a rubber cork in the mouth of each flask and make sure that the apparatus is air-tight. Leave the flasks for a few hours, after which introduce a small burning candle into each flask as.