Respiratory system

CHAPTER-8

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INTRODUCTIONRespiration is the process by which oxygen is taken in and carbon dioxide is given out. The first breath takes place only after birth. Fetal lungs are non-functional. So, during intrauterine life the exchange of gases between fetal blood and mother’s blood occurs through placenta.After the first breath, the respiratory process continues throughout the life. Permanent stoppage of respiration occurs only at death.

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TYPES OF RESPIRATIONRespiration is classified into two types:1. External respiration that involves exchange of respiratory gases, i.e. oxygen and carbon dioxide between lungs and blood2. Internal respiration, which involves exchange of gases between blood and tissues.

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PHASES OF RESPIRATIONRespiration occurs in two phases:1. Inspiration during which air enters the lungs from atmosphere2. Expiration during which air leaves the lungs.During normal breathing, inspiration is an active process and expiration is a passive process.

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ANATOMY OF RESPIRATORY TRACTThe respiratory system consists of the NOSEPHARYNX (THROAT)LARYNX (VOICE BOX)TRACHEA (WIND PIPE) BRONCHUS LUNGS

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Structurally, the respiratory system consists of two parts:

The upper respiratory system includes the nose, pharynx, and associated structures.

The lower respiratory system includes the larynx, trachea, bronchi, and lungs.

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Functionally, the respiratory system also consists of two parts:

(1) The conducting zone consists of a series of interconnecting cavities and tubes both outside and within the lungs.

These include the nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles; their function is to filter, warm, and moisten air and conduct it into the lungs.

(2) The respiratory zone consists of tissues within the lungs where gas exchange occurs .

These include the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli; they are the main sites of gas exchange between air and blood.

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right

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FUNCTION OF RESPIRATORY SYSTEM1. Provides for gas exchange-intake of O2 for

delivery to body cells and elimination of CO2 produced by body cells.

2. Helps regulate blood pH.3. Contains receptors for the sense of smell, filters

inspired air, produces vocal sounds (phonation), and excretes small amounts of water and heat.

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NOSEThe nose can be divided into external and

internal noses.The external nose is the portion of the nose

visible on the face.The frontal bone, nasal bones, and maxillae

form the bony framework of the external noseOn the undersurface of the external nose are

two openings called the external nares or nostrils.

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External nose have three functions such as

(1) warming, moistening, and filtering incoming air;

(2) detecting olfactory stimuli; and (3) modifying speech vibrations as they

pass through the large, hollow resonating chambers.

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The internal nose is a large cavity beyond the nasal vestibule in the anterior aspect of the skull that lies inferior to the nasal bone and superior to the mouth;

Anteriorly, the internal nose merges with the external nose, and posteriorly it communicates with the pharynx through two openings called the internal nares or choancae.

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Ducts from the paranasal sinuses (which drain mucus) and the nasolacrimal ducts (which drain tears) also open into the internal nose.

Skull bones containing the paranasal sinuses are the frontal, sphenoid, ethmoid, and maxillae.

The space within the internal nose is called the nasal cavity

The anterior portion of the nasal cavity just inside the nostrils, called the nasal vestibule.

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LARYNXThe larynx (voice box), is a short passageway that connects the laryngopharynx with the trachea. It lies in the midline of the neck anterior to the oesophagus and the fourth through sixth cervical vertebrae (C4–C6).The wall of the larynx is composed of nine pieces of cartilage. Three occur singly (thyroid cartilage, epiglottis, and cricoid cartilage), and three occur in pairs (arytenoid, cuneiform, and corniculate cartilages). The extrinsic muscles of the larynx connect the cartilages to other structures in the throat; the intrinsic muscles connect the cartilages to one another.

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TRACHEAThe trachea or windpipe, is a tubular passageway for air that is about 12 cm (5 in.) long and 2.5 cm (1 in.) in diameter. It is located anterior to the oesophagus and extends from the larynx to the superior border of the fifth thoracic vertebra (T5), where it divides into right and left primary bronchi.The layers of the tracheal wall, from deep to superficial, are the (1) mucosa, (2) submucosa, (3) hyaline cartilage, and (4) adventitia (composed of areolar connective tissue).

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The mucosa of the trachea consists of an epithelial layer of pseudostratified ciliated columnar epithelium and an underlying layer of lamina propria that contains elastic and reticular fibers.

Pseudostratified ciliated columnar epithelium consists of ciliated columnar cells and goblet cells that reach the luminal surface, plus basal cells.

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BRONCHUSAt the superior border of the fifth thoracic vertebra, the trachea divides into a right primary bronchus windpipe), which goes into the right lung, and a left primary bronchus, which goes into the left lung. The right primary bronchus is more vertical, shorter, and wider than the left. As a result, an aspirated object is more likely to enter and lodge in the right primary bronchus than the left..

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BRANCHING OF BRONCHIAL TREE

Trachea

Primary bronchi

Secondary bronchi

Tertiary bronchi

Bronchioles

Terminal bronchioles

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Larynx

Trachea

Left primary bronchusLeft secondary bronchus

Left tertiary bronchus

Left bronchiole

Left terminal bronchiole

Diaphragm

Pleural cavity

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Larynx

Trachea

Right primary bronchus

Right secondary bronchus

Right tertiary bronchusRight bronchiole

Right terminal bronchiole

Diaphragm

Pleural cavity

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At the point where the trachea divides into right and left primary bronchi an internal ridge called the carina.

On entering the lungs, the primary bronchi divide to form smaller bronchi—the secondary (lobar) bronchi, one for each lobe of the lung. (The right lung has three lobes; the left lung has two.)

The secondary bronchi continue to branch, forming still smaller bronchi, called tertiary (segmental) bronchi, that divide into bronchioles.

Bronchioles in turn branch repeatedly, and the smallest ones branch into even smaller tubes called terminal bronchioles.

This extensive branching from the trachea resembles an inverted tree and is commonly referred to as the bronchial tree.

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LungsThe lungs ( lightweights, because they float) are paired cone-shaped organs in the thoracic cavity. They are separated from each other by the heart and other structures in the mediastinum, which divides the thoracic cavity into two anatomically distinct chambers. As a result, if trauma causes one lung to collapse, the other may remain expanded. Each lung is enclosed and protected by a double-layered serous membrane called the pleural membrane.The superficial layer, called the parietal pleura, lines the wall of the thoracic cavity; the deep layer, the visceral pleura, covers the lungs themselves

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Apex

Superior lobe

Inferior lobe

Base

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STRUCTURE OF RESPIRATORY UNIT Respiratory unit starts from the respiratory bronchioles. Each respiratory bronchiole divides into alveolar ducts. Each alveolar duct enters

an enlarged structure called the alveolar sac. Space inside the alveolar sac is called antrum. Alveolar sac consists of a cluster of

alveoli. Few alveoli are present in the wall of alveolar duct also. Thus, respiratory unit includes: 1. Respiratory bronchioles 2. Alveolar ducts 3. Alveolar sacs 4. Antrum

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TerminalbronchiolePulmonary

arteriole Lymphaticvessel

Respiratorybronchiole

Alveolarducts

Alveolarsac

Alveoli

Alveoli

Pulmonaryvenule

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Blood Supply to the LungsThe lungs receive blood via two sets of arteries: pulmonary arteries and bronchial arteries. Deoxygenated blood passes through the pulmonary trunk, which divides into a left pulmonary artery that enters the left lung and a right pulmonary artery that enters the right lung. (The pulmonary arteries are the only arteries in the body that carry deoxygenated blood.)Return of the oxygenated blood to the heart occurs by way of the four pulmonary veins, which drain into the left atrium

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Mechanism of respiration The process of gas exchange in the body, called respiration, has three basic steps:1. Pulmonary ventilation or breathing2. External (pulmonary) respiration 3. Internal (tissue) respiration

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1. Pulmonary ventilation or breathing, is the inhalation (inflow) and exhalation (outflow) of air and involves the exchange of air between the atmosphere and the alveoli of the lungs.

2. External (pulmonary) respiration is the exchange of gases between the alveoli of the lungs and the blood in pulmonary capillaries across the respiratory membrane. In this process, pulmonary capillary blood gains O2 and loses CO2.

3. Internal (tissue) respiration is the exchange of gases between blood in systemic capillaries and tissue cells. In this step the blood loses O2 and gains CO2. Within cells, the metabolic reactions that consume O2 and give off CO2 during the production of ATP are termed cellular respiration.

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LUNG VOLUMELung volumes are the volumes of air breathed by an individual. Each of these volumes represents the volume of air present in the lung under a specified static condition (specific position of thorax).Lung volumes are of four types:1. Tidal volume2. Inspiratory reserve volume3. Expiratory reserve volume4. Residual volume.

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TIDAL VOLUMETidal volume (TV) is the volume of air breathed in and out of lungs in a single normal quiet respiration. Tidal volume signifies the normal depth of breathing.Healthy adult averages 12 breaths a minute, with each inhalation and exhalation moving about 500 mL of air into and out of the lungs. The volume of one breath is called the tidal volume (TV). The minute ventilation (MV )—the total volume of air inhaled and exhaled each minute is respiratory rate multiplied by tidal volume:MV = 12 breaths/min X 500 mL/breath

= 6 liters/min

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INSPIRATORY RESERVE VOLUMEInspiratory reserve volume (IRV) is an additional volume of air that can be inspired forcefully after the end of normal inspiration.Normal Value is 3,300 mL (3.3 L).

EXPIRATORY RESERVE VOLUMEExpiratory reserve volume (EVR) is the additional volume of air that can be expired out forcefully, after normal expiration.Normal Value is 1,000 mL (1 L).

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RESIDUAL VOLUMEResidual volume (RV) is the volume of air remaining in lungs even after forced expiration. Normally, lungs cannot be emptied completely even by forceful expiration. Some quantity of air always remains in the lungs even after the forced expiration.Residual volume is significant because of two reasons:1. It helps to aerate the blood in between breathing and during expiration2. It maintains the contour of the lungs.

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The apparatus commonly used to measure the volume of air (lung volume) exchanged during breathing and the respiratory rate is a spirometer or respirometer.

The recording is called the spirogram. Inhalation is recorded as an upward deflection, and

exhalation is recorded as a downward deflection. Lung volumes measured by spirometry include tidal

volume, minute ventilation, alveolar ventilation rate, inspiratory reserve volume, expiratory reserve volume, and FEV1.0. Other lung volumes include anatomic dead space, residual volume, and minimal volume.

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LUNG CAPACITYLung capacities are the sum of two or more lung volumes such as include inspiratory, functional, residual, vital, and total lung capacities. lung capacities are of four types:1. Inspiratory capacity2. Vital capacity3. Functional residual capacity4. Total lung capacity.

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Total lung capacity (TLC) is the volume of air present in lungs after a deep (maximal) inspiration. It includes all the volumes.

TLC = IRV + TV + ERV + RV.INSPIRATORY CAPACITY

Inspiratory capacity (IC) is the maximum volume of air that is inspired after normal expiration (end expiratory position).

It includes tidal volume and inspiratory reserve volume. IC = TV + IRV = 500 + 3,300 = 3,800 mL

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VITAL CAPACITY (VC)Vital capacity (VC) is the maximum volume of air that can be expelled out forcefully after a deep (maximal) inspiration. VC includes inspiratory reserve volume, tidal volume and expiratory reserve volume.VC = IRV + TV + ERV= 3,300 + 500 + 1,000 = 4,800 mL

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TOTAL LUNG CAPACITYTotal lung capacity (TLC) is the volume of air present in lungs after a deep (maximal) inspiration. It includes all the volumes.TLC = IRV + TV + ERV + RV= 3,300 + 500 + 1,000 + 1,200 = 6,000 mL

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REGULATION OF RESPIRATION Spontaneous respiration is produced by rhythmic discharge of motor neurons that innervate the respiratory muscles.This discharge is totally dependent on nerve impulses from the brain; breathing stops if the spinal cord is transected above the origin of the phrenic nerves. The rhythmic discharges from the brain that produce spontaneous respiration are regulated by alterations in arterial Po2, PCO2, and H+ concentration.Two separate neural mechanisms regulate respiration. One is responsible for voluntary control and the other for automatic control.The voluntary system is located in the cerebral cortex and sends impulses to the respiratory motor neurons via the corticospinal tracts. 

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Contd… The automatic system is driven by a group of pacemaker

cells in the medulla. Impulses from these cells activate motor neurons in the

cervical and thoracic spinal cord that innervate inspiratory muscles.

The main components of the respiratory control pattern generator responsible for automatic respiration are located in the medulla. Notes taken from

http://accessmedicine.mhmedical.com/content.aspx?bookid=393&sectionid=39736784&jumpsectionID=39741437

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DISORDERS OF RESPIRATORY SYSTEM

Bronchial asthma or asthmaCyanosisCaisson’s diseaseMountain sickness

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Bronchial asthma or asthmaBronchial asthma is the respiratory disease characterized by difficult breathing with wheezing. Wheezing refers to whistling type of respiration. It is due to bronchiolar constriction, caused by spastic contraction of smooth muscles in bronchioles, leading to obstruction of air passage. Obstruction is further exaggerated by the oedema of mucus membrane and accumulation of mucus in the lumen of bronchioles.

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Because of difficulty during expiration, the lungs are not deflated completely, so that the residual volume and functional residual capacity are increased.

There is reduction in: i. Tidal volume ii. Vital capacity iii. Forced expiratory volume in 1 second (FEV1) iv. Alveolar ventilation v. Partial pressure of oxygen in blood. Carbon dioxide accumulates, resulting in acidosis, dyspnoea

and cyanosis.

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CYANOSISCyanosis is defined as the diffused bluish coloration of skin and mucus membrane. It is due to the presence of large amount of reduced haemoglobin in the blood.Quantity of reduced haemoglobin should be at least 5 to 7 g/dL in the blood to cause cyanosis.CONDITIONS WHEN CYANOSIS OCCURS1. Any condition which leads to arterial hypoxia and stagnant hypoxia. 2. Conditions when altered haemoglobin is formed. Due to poisoning, haemoglobin is altered into methemoglobin or sulfhemoglobin, which causes cyanosis.3. Conditions like polycythemia when blood flow is slow.

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MOUNTAIN SICKNESSMountain sickness is the condition characterized by adverse effects of hypoxia at high altitude. It is commonly developed in persons going to high altitude for the first time. It occurs within a day in these persons, before they get acclimatized to the altitude.The symptoms are loss of appetite, nausea and vomiting, increased heart rate, increased force of contraction of heart, pulmonary oedema (breathlessness), headache, depression, disorientation, irritability, lack of sleep, weakness and fatigue

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CAISSON’S DISEASECaisson’s disease is the disorder that occurs when a person returns rapidly to normal surroundings (atmospheric pressure) from the area of high atmospheric pressure like deep sea. It is also known as dysbarism, compressed air sickness, decompression sickness, bends or diver’s palsy.High barometric pressure at deep sea leads to compression of gases in the body. Compression reduces the volume of gases.Among the respiratory gases, oxygen is utilized by tissues. Carbon dioxide can be expired out. But nitrogen, which is present in high concentration, i.e. 80% is an inert gas. So, it is neither utilized nor expired

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When nitrogen is compressed by high atmospheric pressure in deep sea, it escapes from blood vessels and enters the organs.

As it is fat soluble, it gets dissolved in the fat of the tissues and tissue fluids. It is very common in the brain tissues.

As long as the person remains in deep sea, nitrogen remains in solution and does not cause any problem.

But, if the person ascends rapidly and returns to atmospheric pressure, decompression sickness occurs.

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Due to sudden return to atmospheric pressure, the nitrogen is decompressed and escapes from the tissues at a faster rate. Being a gas, it forms bubbles while escaping rapidly.

The bubbles travel through blood vessels and ducts. In many places, the bubbles obstruct the blood flow and produce air embolism, leading to decompression sickness.

Underground tunnel workers who use the caissons (pressurized chambers) also develop decompression (caisson disease) sickness.

Pressure in the chamber is increased to prevent the entry of water inside.

Decompression sickness also occurs in a person who ascends up rapidly from sea level in an airplane without any precaution.

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Symptoms are: 1. Severe pain in tissues, particularly the joints, 2. Sensation of numbness, tingling or pricking (paresthesia) and itching 3. Temporary paralysis due to nitrogen bubbles in the myelin sheath of

motor nerve fibers 4. Muscle cramps associated with severe pain 5. Occlusion of coronary arteries followed by coronary ischemia, caused

by bubbles in the blood 6. Occlusion of blood vessels in brain and spinal cord also 7. Damage of tissues of brain and spinal cord because of obstruction of

blood vessels by the bubbles 8. Dizziness, paralysis of muscle, shortness of breath and choking occur 9. Finally, fatigue, unconsciousness and death.

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COUGH REFLEX The function of both the cough reflex and the sneeze reflex

is to dislodge foreign matter or irritating material from respiratory passages.

The bronchi and the trachea contain sensory receptors that are sensitive to foreign particles and irritating substances.

The cough reflex is initiated with the sensory receptors detect these substances and initiate action potentials that pass along the vagus nerves to the medulla oblongata, where the cough reflex is triggered.

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The movements resulting in a cough occur as follows: about 2.5 litters (L) of air are inspired, the epiglottis closes, and the vestibular folds and vocal cords close tightly to trap the inspired air in the lung; the abdominal muscles contract to force the abdominal contents up against the diaphragm; and the muscles of expiration contract forcefully.

As a consequence, the pressure in the lungs increase to about 100 mm Hg. Then the vestibular folds, the vocal cords, and the epiglottis open suddenly, and the air rushes from the lungs at a high velocity, carrying foreign particles with it.

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SNEEZE REFLEX The sneeze reflex is similar to the cough reflex, but it differs in

several ways. The source of irritation that initiates the sneeze reflex is in the

nasal passages instead of in the trachea and bronchi, and the action potentials are conducted along the trigeminal nerves to the medulla, where the reflex is triggered.

During the sneeze reflex the uvula and the soft palate are depressed so the air is directed primarily through the nasal passages, although a considerable amount passes through the oral cavity.

The rapidly flowing air dislodges particulate matter from the nasal passages and propels it a considerable distance from the nose.

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