Human Respiratory System — Short Questions

Respiratory surface is the area where actual gas exchange between environment and the blood takes place. Following are its properties.

(i) Moist thin and permeable epithelium
(ii) Located internally
(iii) Has blood supply
(iv) Well ventilated

Function of Nasal Cavity

1. As inhaled air flows through the nasal cavity it is warmed and humified by blood vessels present very close to its surface.
2. Hairs in the nose and mucous produced by mucous membranes trap larger foreign particles in air before they go deeper into respiratory tract.

Air → nostrils → nasal cavity → pharynx → larynx → trachea → bronchi→ bronchioles → alveolar duct → Alveoli.

Alveoli are made of moist epithelial tissues only 0.1 micro meter thick. A network of tiny blood capillaries surround the alveolus. Oxygen in the air inside the alveoli is absorbed by the blood via diffusion.

During inhalation the diaphragm contracts and take a more flattened shape. This causes expansion of lungs, lowering air pressure in lungs and the air moves in. During exhalation diaphragm relaxes it takes dome like shape. This causes contraction of lungs, increasing air pressure in lungs and the air moves out.

1. 72% of CO2 is transported in blood as bicarbonate.
2. 20% of CO2 is carried as Carboxyhaemoglobin.
3. 8% of CO2 is carried dissolved in water of blood plasma.

Millions of alveoli provide a large total surface area for gas exchange. They are also located internally and does not lose water to the atmosphere.

(a) Internal and external respiration
Inter respiration (also called as cellular respiration) is metabolism of glucose in the cell to got energy.
External respiration involves breathing and exchange of oxygen and carbon dioxide in capillaries

(b) Upper and lower respiratory tract
Upper respiratory tract includes nasal cavity, pharynx and larynx while lower respiratory tract includes trachea, bronchi, bronchioles and alveoli.

(c) Bronchi and bronchioles
Trachea branches at the bottom into two branches and called bronchi. These bronchi enter the lungs and form secondary bronchi which divides into tertiary bronchi which branch further to form small bronchioles.

(d) Haemoglobin and myoglobin
Haemoglobin is oxygen carrying protein found in RBCs in blood. Myoglobin is oxygen storing protein found in the muscles.

Lungs are the largest organs of respiratory system which are covered by two membranes. Each lung is divided into lobes. Right lung is larger having three lobes. The left lung is smaller and contains two lobes.

The following properties enable respiratory surfaces for effective diffusion of gases across them.

• It is moist and permeable, so that gases may pass through it.
• It has thin walls, so that gases have to travel minimum distance.
• It has blood supply, so that gases can diffuse in and out of blood.
• It has structural support, to remain open and does not collapse.
• It is located internally, that is why it has moist surface which does not lose water to the atmosphere.
• Air ventilates over it i.e., moves towards and away from it.
• Air reaches to it after passing a branched tubular way, so that air becomes saturated with water vapour before reaching it.

The organs of the respiratory system form a continuous system of passages, called the respiratory tract, through which air flows into and out of the body. The respiratory tract has two major divisions: the upper respiratory tract and the lower respiratory tract.

Upper Respiratory Track It consists of nasal cavity, pharynx and larynx.

Lower Respiratory Track The lower respiratory tract includes trachea, bronchi, bronchioles and lungs.

Vocal cards are two bands of smooth muscles. The vocal cords vibrate when air flows over them to produce sound.

Larynx is called voice box it contains two bands of smooth muscles called vocal cords. These vocal cords vibrate when air flows over them and produce sound.

If swallowed material enter the larynx it irritates the larynx and stimulates a strong cough reflex. This helps to expel the material out of the Larynx and into the throat.

Muscles in the larynx move the vocal cords apart to allow breathing. Other muscles in the larynx move the vocal cords together to allow the production of vocal sounds. The latter muscles also control the pitch of sounds and help control their volume.

The lungs are the largest organs of the respiratory tract. The outside of each lung is covered by two membranes. First membrane, visceral pleura, lines the lungs while the second membrane, parietal pleura, lines the inner wall of thoracic cavity.

The small space between visceral and parietal pleural membranes, called pleural cavity is filled with fluid. This fluid allows the lungs to expand and contract freely during breathing.

The right lung is larger and contains three lobes. The left lung is smaller and contains two lobes. The smaller left-lung allows space for the heart.

Some epithelial cells of alveoli secrete a liquid called surfactant, which lines the inside of alveoli. It prevents the alveoli from collapsing and sticking together when air moves out of them. In healthy lungs, surfactant is constantly secreted and reabsorbed.

Pulmonary arteries carry deoxygenated blood to the lungs and pulmonary veins carry oxygenated blood back to the heart.

The movement of the air in and out of the body is called breathing or ventilation. It is done by creating negative and positive pressures in the lungs. This role is played by two sets of muscles i.e., (i) diaphragm (dome like large skeletal muscle that separates thoracic cavity and abdomen) and (ii) the intercostal muscles.

Atmospheric pressure is lower at high altitude. It requires greater increase in the space of thorax for inspiration. That is why it is difficult to breathe at high altitudes. Due to this reason athlete often undertake high altitude training before competitions

Intercostal muscles are present between each pair of ribs. During inspiration the intercostal muscles contract and raise the ribs to expand rib cage while during expiration the intercostal muscles relax. Ribs are lowered and ribcage moves inward.

Inspiration

• Taking in air into the lungs is called inspiration or inhalation.
• Diaphragm contracts and take more flattened shape.
• Intercostal muscle contract.
• Ribs moves upwards and outwards, expanding the ribs.
• Thoracic cavity volume increases.
• Lungs expand due to pleural membrane adherence

Expiration

• Moving the air out of lungs is called expiration or exhalation.
• Diaphragm relaxes and becomes dome-shaped.
• Intercostal muscles relax.
• Ribs moves downwards and inwards, reducing rib cage volume.
• Thoracic cavity volume decreases.
• Lungs recoil due to elastic tension.

The process of breathing is controlled by respiratory center located in the medulla oblongata i.e. part of brain stem. Neuron from this center send impulse to diaphragm and intercostal muscles, stimulating them to contract causing inspiration and vice versa.

Blood plasma can dissolve a maximum of only about 3ml O2 per litre. Yet whole blood carries almost 200ml O2 per litre! The reason is that most of the oxygen is not dissolved in blood plasms but is bound to molecules of haemoglobin inside the RBCs.

Oxyhaemoglobin is bright red while deoxyhaemoglobin is dark red, this imparts blueish tinge to tissues. Due to these color changes, arteries carrying oxygenated blood are indicated with red color while veins carrying deoxygenated blood with blue color.

The oxygen reserve ensures that the blood contains enough oxygen to maintain life for four to five minutes if breathing is interrupted or if heart stops pumping.

The CO2 produced by tissues lowers the pH of blood. This lowered pH reduces haemoglobin's affinity for oxygen and thus causes it to release oxygen more readily. The effect of pH on haemoglobin's affinity for oxygen is known as the Bohr effect.

Approximately 72% of carbon dioxide is carried in the blood as bicarbonate ions. CO2 enters the RBCs and combines with water to form carbonic acid (H2CO3) in the presence of enzyme carbonic anhydrase.
CO2 + H2O —Carbonic anhydrase—> H2CO3

Formation of carbonic acid is important in maintaining the acid base balance of the blood because bicarbonate serves as the major buffer of the blood plasma.

Carbonic acid (H2CO3) disassociates to form hydrogen ions (H+) and bicarbonate ions (HCO3-). The hydrogen ion readily associates with oxyhaemoglobin and oxygen of oxyhaemoglobin is released to the tissue. While the bicarbonate ions (HCO3-) is facilitated by a transporter that exchanges one chloride ion (Cl-) for a bicarbonate ion (this is called the "chloride shift" or "Hamburger phenomenon".).

CO2 binds to the protein portion of haemoglobin while O2 binds to the haem irons. So, both do not compete for attachment to haemoglobin.

• Incomplete combustion of fuels like wood, gasoline, propane or natural gas produce. CO (carbon monoxide) gas.
• If gas heaters are left burning overnight in closed rooms, CO accumulate in them.
• Carbon monoxide enters the body through inhalation and binds to haemoglobin with much higher affinity than oxygen.
• This binding reduces the amount of haemoglobin to transport oxygen to the body tissues, leading to tissue oxygen efficiency hypoxia.
• This leads to condition called CO poisoning.
• Its symptoms include headache, dizziness, weakness, nausea, confusion shortness of breath, chest pain and loss of consciousness.
• In severe cases it can cause permanent brain damage, and even death.

Respiratory pigments are special proteins in blood or tissues and are involved in transporting oxygen throughout body. They also serve for other purposes e.g., O2 storage, CO2 transport, and transport of substances other than respiratory gases. The two well-known respiratory pigments are haemoglobin and myoglobin.

Haemoglobin is a protein present in RBCs. A haemoglobin molecule is composed of four globin (globular) polypeptide chains (two α chains and two β) and four haem groups. There are 141 and 146 amino acids in the α and β chains, respectively. Each polypeptide chain is folded in such a way that it contains a pocket where the haem group binds. So, each chain is associated with a haem group. A haem group consists of an iron held in a porphyrin ring. The iron ion is attached with four nitrogen atoms of the polypeptide chain. Under higher partial pressure of oxygen, iron ion attaches a molecule of O2. In this way, one haemoglobin molecule can carry up to four O2 molecules.

The four polypeptide chains of haemoglobin are bound to each other by salt bridges, hydrogen bonds, and hydrophobic effect.

Myoglobin is the oxygen-binding protein in skeletal and cardiac muscle cells of vertebrates. It gives a distinct red or dark gray colour to muscles.

It is a monomer composing of a singly polynucleotide chain (made of 153 amino acids) and contains a single haem group. Therefore, it is capable of binding with a single O2 molecule. The binding affinity of myoglobin is high as compared to that of haemoglobin. As a result, myoglobin serves as the oxygen-strong protein in muscles. It releases oxygen when the partial pressure of oxygen is below 20mm Hg. In this way, myoglobin provides oxygen to the muscles when they need.

Respiratory disorders can range from common cold to more severe diseases.

Upper respiratory tract infections

• Sinusitis
• Otitis media

Lower respiratory tract infections

• Pneumonia
• Pulmonary tuberculosis

Disorders of lungs

• COPD
• Emphysema

It is the inflammation of the lining off the sinuses (four paired air-filled spaces that surround the nasal cavity i.e., under the eyes; above the eyes; between the eyes and behind the eyes). It may be acute (lasts for 7 to 10 days) or chronic (lasts longer than 12 week).

Symptoms of sinusitis include fever, plugged nose, pus-like nasal discharge, loss of sense of smell, facial pain, a feeling that phlegm is falling from the back of nose into throat, and headache that is sometimes aggravated by bending over

It is the inflammation of the middle ear. Otitis may be acute (rapid onset) or chronic (lasts more than six weeks).

Cause The common cause of otitis media accumulation of fluid in Eustachian tube, which cannot be drained from the middle ear. When this fluid is not drained, it allows the growth of bacteria and viruses in the middle ear that lead to otitis media.

Symptoms Symptoms of otitis media include severe ear pain, pulling at one or both ear, fever draining from ear(s), loss of balance, and hearing difficulties.

Treatment Treatment include oral and topical pain killers and antibiotics (if caused by bacterial infection).

Pneumonia is a form of acute respiratory infection.

Cause It can lead to life-treating illness. In pneumonia, the alveoli of one or both lungs are inflamed and are filled with pus and fluid. It makes breathing painful and limits oxygen intake.

Pneumonia is most commonly caused by viruses or bacteria. It is the single largest infectious cause of death in children worldwide. A variety of organisms, primarily bacteria (particularly Streptococcus pneumonia or viruses (e.g., human rhinovirus) and less commonly fungi, can cause pneumonia.

Pneumonia killed more than 808000 children under the age of 5 in 2017, according for 15% of all deaths of children under 5 years.

Tuberculosis (TB) is a chronic infection caused by bacteria Mycobacterium tuberculosis. The tuberculosis of the lungs is called pulmonary tuberculosis. It is highly contagious and spreads through cough or sneezes. The bacteria enter the lungs, multiply and cause inflammation and damage to the lung tissue, including the alveoli. The damage to the alveoli can lead to the formation of small cavities or holes in the lung tissue, which can make it difficult for the lungs to function properly

Treatment Treatment includes the use of multiple antibiotics over a long period of time (for 9 months) regularly.

Chronic obstructive pulmonary disease (COPD) is a chronic inflammatory disease of lungs.

Causes The common causes of COPD are tobacco smoking, long terms exposure to harmful pollutants and chemical fumes etc. A small percentage of genetic predisposition (protein alpha-1 antitrypsin deficiency) can also develop COPD, even without smoking or significant exposure to pollutants.

Symptoms The symptoms of COPD are persistent cough with mucus (sputum), shortness of breath wheezing chest, fatigue and frequent respiratory tract infections.

Treatment COPD is incurable but by minimizing exposure to smoke pollutants, and chemicals this disease can slow its progression. Other therapies include bronchiodilators, inhaled corticosteroids, pulmonary rehabilitation and oxygen therapy. In some severe cases, surgery such as lung transplantation may be considered.

Chronic bronchitis Chronic bronchitis is a type of COPD. It involves inflammation and narrowing of the bronchial tubes in the lungs. It leads to increased mucus production, which can further block the airways and make breathing difficult. This disease lasts for three months to two years. It is caused by long-term exposure to irritants such as cigarette smoke, air pollution or industrial dusts.

Symptoms Symptoms of chronic bronchitis are almost same as of COPD such as wheezing, shortness of breath, chest tightness and frequent respiratory infections.

Management Chronic bronchitis can be managed by quitting smoking. Other treatments are bronchodilators, pulmonary rehabilitation, and in some cases oxygen therapy.

Emphysema is a type of COPD. In emphysema, the inner walls of alveoli are damaged, causing them to eventually rupture. This creates one larger air space instead of many small ones and reduces the surface area available for gas exchange

Cause The primary cause of emphysema is smoking. It can also be caused by long-term exposure to air pollution, dust, or chemical fumes. Emphysema disease can also be caused by a genetic deficiency of a protein called alpha-1 antitrypsin.

See Q5. of theory.

See Q7 of theory.

See Q9. of theory.

See Q10. of theory

Alveoli wall is only 0.1 micrometre thick and also moist. Due to these two factors the gaseous exchange is optimized.

In Chronic obstructive pulmonary disease cause inflammation resulting in narrowing of bronchial tubes in the lungs. It leads to increase mucus production which block the airways. This ultimately affects the gaseous exchange efficiency.

During external gaseous exchange i.e. intake of O2 and out take of CO2 takes place at the alveoli level. While during internal respiration the oxygen is taken in and CO2 is given out at cellular level during cellular respiration.

About 97% of the haemoglobin within RBCs combines with oxygen and becomes oxyhaemoglobin. As the blood travels through the blood capillaries oxyhaemoglobin releases oxygen. Here 75% of haemoglobin is left saturated It means that 22% of the oxyhaemoglobin has released its oxygen to the tissues for use in cellular respiration.

Gaseous exchange can be influenced by environmental factors like temperature, humidity, altitude, and air pollution.