Respiration

• Respiration is defined as the biochemical process by which the digested foods are oxidized liberating the energy. In the process, oxygen is utilized and carbon-dioxide is released.
• Overall respiration process involves three process.
• I. External respiration: it is a simple process of exchange of gases (O2 and CO2) between the respiratory surface and the environment.
• II. Transport of gases between the respiratory surface and the body tissue.
• III. Internal or cellular respiration: cellular respiration occur in mitrochondria of every cell, where digested food is oxidized releasing energy in the storable form ie. ATP. And when the cell require energy for vital activities, ATP broke down into ADP+ iP + energy.

Types of respiration

There are two types of respiration depending upon availability of oxygen

1. Aerobic respiration:

• It occur in the presence of Oxygen.
• The food is oxidized in the presence of oxygen in cellular level liberating CO2 and water along with energy in the form of ATP.
• Complete oxidation of 1 molecule of glucose liberate 38 molecule of ATP during glycolysis.

2. Anaerobic respiration:

• In the absence of oxygen, food is oxidized anaerobically (without utilizing o2).
• Anaerobic respiration is also known as fermentation as organic compounds are also produced as byproducts.
• One glucose molecule on anaerobic oxidation releases 2 ATP molecule only.
• Anaerobic respiration occur in deep seated tissue, in germinating seeds, parasites and bacteria.

Anatomy of Human respiratory system

The organs of the respiratory system are:

• nose (external nares and nasal chamber)
• Internal nares and pharynx
• larynx
• trachea
• two bronchi (one bronchus to each lung)
• bronchioles and smaller air passages
• two lungs and their coverings, the pleura
• muscles of breathing – the intercostal muscles and the diaphragm.

1. Nose (external nare and nasal chamber)

• Structurally the nose can be divided into the external portion which is in fact termed as the nose and the internal portions being the nasal cavities.
• The nose is the only visible part of the respiratory system, protruding from the face, and lying in between the forehead and the upper lip.
• Opening is known as nostril. The two nostrils are separated by nasal septum. The two nostril openings leading to two nasal chambers or cavities.
• The nasal cavity is the main route of air entry, and consists of a large irregular cavity divided into two equal passages by a septum.
• The posterior bony part of the septum is formed by the perpendicular plate of the ethmoid bone and the vomer. Anteriorly, it consists of hyaline cartilage.
• The nasal cavity is lined with very vascular ciliated columnar epithelium which contains mucus-secreting goblet cells.

The functions of nasal passage are:

• Prevent entry of dust particle into lungs
• Warm the incoming air entering the nasal cavity
• Moisten the dry air
• Olfactory receptor present in the roof of nasal cavity detect the smell
• Hold and sweep the microorgnisms entering the nasal chamber

2. Internal nares and pharynx

• The internal nares are the openings from the nasal cavity into the pharynx.
• The pharynx (throat) is a passageway that  extends from the posterior nares, and runs behind the mouth and the larynx to the level of the 6th thoracic vertebra, where it becomes the oesophagus.
• Structurally the pharynx can be divided into three anatomical parts ie. nasopharynx (posterior to the nasal chambers), the oropharynx (posterior to the mouth), and the laryngopharynx (posterior to the pharynx).

3. Larynx

• The larynx or ‘voice box’ links the laryngopharynx and the trachea. It lies in front of the laryngopharynx and the 3rd, 4th, 5th and 6th cervical vertebrae.
• Until puberty there is little difference in the size of the larynx between the sexes. Thereafter, it grows larger in the male, which explains the prominence of the ‘Adam’s apple’ and the generally deeper voice.
• The larynx is composed of several irregularly shaped cartilages attached to each other by ligaments and membranes.
• The main cartilages are:1 thyroid cartilage, 1 cricoid cartilage, 2 arytenoid cartilages and 1 epiglottis
• The vocal cords are two pale folds of mucous membrane with cord-like free edges, stretched across the laryngeal opening. They extend from the inner wall of the thyroid prominence anteriorly to the arytenoid cartilages posteriorly.

Sound production:

• When the muscles controlling the vocal cords are relaxed, the vocal cords open and the passageway for air coming up through the larynx is clear; the vocal cords are said to be abducted.
• Vibrating the vocal cords in this position produces low-pitched sounds.
• When the muscles controlling the vocal cords contract, the vocal cords are stretched out tightly across the larynx, and are said to be adducted (closed).
• When the vocal cords are stretched to this extent, and are vibrated by air passing through from the lungs, the sound produced is high pitched.
• The pitch of the voice is therefore determined by the tension applied to the vocal cords by the appropriate sets of muscles.
• When not in use, the vocal cords are adducted. The space between the vocal cords is called the glottis.

4. Trachea

• The trachea or windpipe is a continuation of the larynx and extends downwards to about the level of the 5^th thoracic vertebra where it divides at the carina into the right and left primary bronchi, one bronchus going to each lung.
• It is approximately 10–11 cm long and lies mainly in the median plane in front of the oesophagus.
• The tracheal wall is composed of three layers of tissue, and is held open by between 16 and 20 incomplete (C-shaped) rings of hyaline cartilage lying one above the other.
• The rings are incomplete posteriorly where the trachea lies against the oesophagus.
• The tracheal mucosa consists of pseudo stratified, ciliated columnar epithelium, while its submucosa contains cartilage, smooth muscle, and seromucous glands.

5. Bronchus

• The trachea divides into the two main bronchi (primary bronchi) ie. the right bronchus wider, shorter and more vertical than the left bronchus.
• The right bronchus: This is wider, shorter and more vertical than the left bronchus and is therefore more likely to become obstructed by an inhaled foreign body. It is approximately 2.5 cm long. After entering the right lung at the hilum it divides into three branches, one to each lobe. Each branch then subdivides into numerous smaller branches.
• The left bronchus. This is about 5 cm long and is narrower than the right. After entering the lung at the hilum it divides into two branches, one to each lobe. Each branch then subdivides into progressively smaller airways within the lung substance.
• The bronchial walls contain the same three layers of tissue as the trachea, and are lined with ciliated columnar epithelium. The bronchi progressively subdivide into bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts and finally, alveoli.

6.  Lungs

• There are two lungs, one lying on each side of the midline in the thoracic cavity. They are cone-shaped and have an apex, a base, a tip, costal surface and medial surface.
• The apex: This is rounded and rises into the root of the neck, about 25 mm above the level of the middle third of the clavicle. It lies close to the first rib and the blood vessels and nerves in the root of the neck.
• The base: This is concave and semilunar in shape, and lies on the upper (thoracic) surface of the diaphragm.
• The costal surface: This is the broad outer surface of the lung that lies directly against the costal cartilages, the ribs and the intercostal muscle.
• The right lung is divided into three distinct lobes: superior, middle and inferior. The left lung is smaller because the heart occupies space left of the midline. It is divided into only two lobes: superior and inferior. The divisions between the lobes are called fissures.
• Lungs are enclosed in the pleural cavity lined by transparent pleural membrane. The pleura consists of a closed sac of serous membrane which contains a small amount of serous fluid.
• The lung is pushed into this sac so that it forms two layers: one adheres to the lung called visceral pleura and the other to the wall of the thoracic cavity called parietal pleura.

7. Alveoli

• Within each lobe, the lung tissue is further divided by fine sheets of connective tissue into lobules.
• Each lobule is supplied with air by a terminal bronchiole, which further subdivides into respiratory bronchioles, alveolar ducts and large numbers of alveoli (air sacs).
• There are about 150 million alveoli in the adult lung. It is in these structures that the process of gas exchange occurs.
• As airways progressively divide and become smaller and smaller, their walls gradually become thinner until muscle and connective tissue disappear, leaving a single layer of simple squamous epithelial cells in the alveolar ducts and alveoli.
• These distal respiratory passages are supported by a loose network of elastic connective tissue in which macrophages, fibroblasts, nerves and blood and lymph vessels are embedded.
• The alveoli are surrounded by a dense network of capillaries.
• Exchange of gases in the lung (external respiration) takes place across a membrane made up of the alveolar wall and the capillary wall fused firmly together. This is called the respiratory membrane.

Respiration, types of respiration and anatomy of Human respiratory system

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• In normal quiet breathing there are about 15 complete respiratory cycles per minute. The amount of air exchanged during breathing is measured by an instrument called Spirometer or Resprometer.
• The amount of air present in lung under different condition is known as pulmonary air volume and the capacities of lungs to hold air varies according to conditions.
• some of the pulmonary air volume are:

1. Tidal volume (TV):

•  This is the amount of air passing into and out of the lungs during each cycle of breathing.
• It is about 500 mL at rest.

2. Inspiratory reserve volume (IRV):

• This is the volume of air that can be inhaled into the lungs during normal inspiration.
• It is about 1500 ml.

3. Expiratory reserve volume (ERV):

• This is the total volume of air which can be expelled from the lungs forcefully during normal expiration.
• It is about 1100ml.

4. Inspiratory capacity (IC):

• This is the amount of air that can be inspired with maximum effort.
• It consists of the tidal volume (500 ml) plus the inspiratory reserve volume. IC = TV + IRV
• It is about 2000ml

5. Functional residual capacity (FRC):

• This is the amount of air remaining in the lungs after normal expiration.
• It is equal to ERV+ RV= 1100+1200= 2300ml.ue the pr ocess of respiration. this means that exchange of gases is not interrupted between breath.
• The functional residual volume also prevents collapse of the alveoli on expiration.

6. Residual volume (RV):

• This is the volume of air remaining in the lungs after forceful expiration.
• It is about 1200ml.

7. Vital capacity (VC):

• This is the maximum volume of air which can be expired after forceful inspiration in single breath.
• VC= TV+IRV+ERV= 500+1500+1100
• VC of athletes is more than normal person.

8. Total lung capacity (TLC):

•  This is the maximum amount of air the lungs can hold after forceful inspiration.
• It is normally about 5000-6000 ml in adult.
• TLC=VC + RV.

9. Dead space:

• The lungs and the air passages are never empty.
• Out of 500ml of air inspired during normal respiration, 350ml are exchanged across the walls of the alveolar ducts.
• About 150ml of air always remains in the respiratory passage called as dead space.

Pulmonary air volume and capacities

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• The effect of carbon dioxide and acidity favor the formation of Oxyhaemoglobin at low concentration of CO2 and H+ ion and causes the dissociation of Oxyhaemoglobin releasing O2 at high concentration of CO2 and H+ ion.
• This shift in curve of oxyhaemoglobin due to concentration of carbondioxide at given partial pressure of oxygen, is known as Bohr effect.
• The amount of Oxygen take up by  Haemoglobin at particular time to from Oxyhaemoglobin is called percentage saturation.
• The graph of percentage of O2 saturation of haemoglobin plotted against partial pressure of Oxygen (PO2) is called Oxygen dissociation curve.
• The Oxygen dissociation curve is S-shaped (sigmoidal shape).
• The curve indicates that haemoglobin has high affinity to Oxygen.
• In human arterial blood have PO2 of about 95-100 mmHg, at this level percentage of O2 saturation of Hb is about 97 %.  This indicates the formation of Oxyhaemoglobin is favored.
• Similarly, the  venous blood have PO2 of 40mmHg,at this level percentage of O2 saturation of Hb is about 70%.

Effect of Carbon-dioxide on Oxygen dissociation curve:

• The effect of CO2 on Oxygen dissociation curve is known as Bohr effect.
• It has been found that increase in concentration of CO2 decreases the amount of oxyhaemoglobin formation.
• according to Bohr effect, for any particular partial pressure of Oxygen, the affinity of Haemoglobin toward Oxygen decreases and favors dissociation of oxyhaemoglobin when the partial pressure of carbondioxide increases.
• It means, higher CO2 concentration causes the dissociation of HbO2 releasing free O2.
• Increase in PCO2 shifts the O2 dissociation curve downwards. Higher PCO2 lowers the affinity of haemoglobin for O2.

Fig. Oxygen dissociation curve of haemoglobin at different partial pressure of CO2

• Bohr effect is very important physiological phenomenon, because uptake of oxygen in lungs and its releases in the tissue is regulated by the concentration of CO2 and H+ ion as well as the partial pressure of O2. So, this phenomenon made possible the cellular transport and release of O2.
• PCO2 is lower in lungs than tissue, so Hb has higher affinity for O2, therefore it favors HbO2 formation and transport of O2 from lungs to tissue. similarly PCO2 is higher in tissue, so it favors dissociation of HbO2 releasing free O2 and transport of CO2 from tissue to lungs.

Bohr effect- Oxygen dissociation curve and effects of CO2

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• The greater proportion (70%) of carbon dioxide is transported in the form of bicarbonates.
• The CO2 reacted with the water of the cytoplasm in the presence of enzyme carbonic anhydrase to form carbonic acid.
• The carbonic acid (H2CO3) is a weak acid, which undergoes partial dissociation to yield hydrogen ion (H+) and bicarbonate ion (HCO3-).
• The given reaction mostly occur inside RBCs, because the enzyme carbonic anhydrase is abundant there.

Fig. Diagrammatic representation of chloride shift.

• In RBCs, CO2 combines with water to from carbonic acid which  dissociates to gives H+ ion and bicarbonate  (HCO3-) ion in the presence of enzyme carbonic anhydrase.
• The bicarbonate ion then diffuses outside the RBC in the plasma and combines with Sodium ions to from Sodium bicarbonate (NaHCO3).
• Loss of bicarbonate ions from RBC causes positive charge inside RBC which is balanced by diffusion of chloride (Cl-) ion from plasma into the RBC.
• This exchange of Cl- ion and HCO3- ion between plasma and RBC is known as chloride shift.
• This phenomenon of chloride shift maintain the electrical neutrality of cell.
• This phenomenon is also known as Hamburger phenomenon.
• Reverse of chloride shift occurs in tissues.

Chloride shift/Hamburger phenomenon

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1. Breathing or pulmonary ventilation
2. External respiration
3. Transport of O2 to tissue
4. Internal respiration
5. Transport of CO2 from tissue

1. Breathing or Pulmonary ventilation

• This is movement of air into and out of the lungs.
• Breathing supplies oxygen to the alveoli, and eliminates carbon dioxide.
• The main muscles involved in breathing are the intercostal muscles  and the diaphragm.
• There are 11 pairs of intercostal muscles occupying the spaces between the 12 pairs of ribs. They are arranged in two layers, the external and internal intercostal muscles.
• The diaphragm is a dome-shaped muscular structure separating the thoracic and abdominal cavities.
• Breathing depends upon changes in pressure and volume in the thoracic cavity. Since air flows from an area of high pressure to an area of low pressure, changing the pressure inside the lungs determines the direction of airflow.
• Breathing involves two process

i. Inspiration

• It takes place when the volume of thoracic cavity is increased and the air pressure is decreased.
• Simultaneous contraction of the external intercostal muscles and the diaphragm expands the thorax.
• As the diaphragm + external intercostals contracts (moves downward) lung volume increases.

It involves following events

• First of all, external intercoastal muscle contracts and internal intercoastal muscles relaxes.
• Due to contraction of external intercoastal muscles, ribs is pulled upward, resulting in increase in thoracic cavity size
• The thoracic cavity further enlarges due to contraction of diaphragm, lowering the diaphragm and increases the size of thoracic cavity.
• With increase in size of thorax, lungs expand simultaneously.
• As lungs expands, the air pressure is reduced inside, so equalize the pressure, atmospheric air rushes inside the lungs

ii. Expiration

It takes place when the size of thoracic cavity is reduced and air pressure is increased.

involves following events

• The internal intercoastal muscle contracts and external intercoastal muscles relaxes.
• Due to contraction of internal intercoastal muscle, ribs are pulled inward, resulting in decrease in size of thoracic cavity
• Furthermore the diaphragm is pushed upward due to its relaxation
• With the decrease in size of thoracic cavity, lungs is compressed
• As lungs is compressed, pressure increases, so the air is forced outside.

2. External respiration

• This is the exchange of gases by diffusion between alveoli and blood in the alveolar capillaries, across respiratory membrane.
• Diffusion of oxygen and carbon dioxide depends on pressure differences, e.g. between atmospheric air and the blood, or blood and the tissues.
• Gas exchange during the respiration process takes place in the alveolus at its surface that separates the alveolus with the capillary.
• The exchange of O2 and CO2 occurs through diffusion which is the net movement of gas molecules from a region that has a higher partial pressure to another region that has a lower partial pressure.
• The venous blood in alveolar capillaries contains high level of CO2 and low level of O2.
• Co2 then diffuses from higher level (venous blood) to lower level (alveoli) until equilibrium is maintained. By the same process O2 diffuses from alveoli to venous blood until equilibrium.

3. Transport of Oxygen to tissue

• Oxygen is carried in the blood to the tissue in two from:

i) Oxyhaemoglobin (98.5%): it is a chemical combination of O2 with haemoglobin

Hb4    +   4O2………………………………………… Hb4O8 (oxyhaemoglobin)

ii) Solution in plasma water (1.5%): O2 dissolve in plasma of blood and carried to tissues.

• when the level of O2 is high in blood, it combines with haemoglobin to from oxyhaemoglobin.
• Oxyhaemoglobin is unstable, and under certain conditions readily dissociates releasing oxygen. Factors that increase dissociation include low O2 levels, low pH and raised temperature.

4. Internal respiration

• Internal respiration is exchange of gases which  takes places in tissue, so also known as cellular respiration.
• In tissue, oxygen carried in the form of Oxyhaemoglobin get dissciated to liberating free O2.

Hb4O8———dissociates to give ——— Hb + O2

• The free O2 then oxidized the glucose in the presence of respiratory enzymes to liberate CO2, water and energy.

C6H12O6  + 6O2 ———- 6CO2 +  6H2O + Energy

• Energy is utilized by the tissue for its vital activities, while the CO2 is diffused from the tissue.

5. Transport of Carbondioxide from tissue to lungs

• Carbon dioxide is one of the waste products of metabolism.
• It is excreted by the lungs and is transported by three mechanisms:

i) as Carbonic acid (H2CO3) (7%): some CO2 dissolved in the plasma to form carbonic acid

• carbondioxide mixed with water of blood plasma to form carbonic acid.
• CO2+ H2O——————H2CO3

ii) bicarbonate ions (HCO3−) in the plasma (70%)

• carbonic acid formed in blood plasma quickly ionizes to from bicarbonates and hydrogen ions in the presence of enzyme carbonic anhydrase.
• CO2 + H2O—————H+    + HCO3-
• bicarbonate ions combined with sodium or potassium present in blood to form sodium bicarbonate (NaHCO3) or Potasssium bicarbonate (KHCO3) and transported in this form

iii) as carbaminohaemoglobin (23%): some CO2 combines with Haemoglobin to form carbaminohaemoglobin in RBCs.

• CO2 +  NHbNH2————–HbNH.COOH (carbaminohaemoglobin).

finally, CO2 are carried to lungs and expelled out by expiration process of breathing.

Mechanism of Respiration in Human

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