STRUCTURE AND FUNCTIONLecture—1

Dr.Zahoor Ali Shaikh

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Nasal passages (Nose)

Pharynx (Throat )

Larynx (Voice box)

Trachea –Divides into Right main bronchus and Left main bronchus

Bronchi

Bronchioles—large and small

Terminal Bronchioles

Respiratory Bronchioles

Alveolar Duct

Alveoli

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Trachea divides into Right and Left Bronchi which enter Right and Left Lungs

Within each lung, bronchi continue to branch into narrow (small diameter),shorter and more numerous airways like branching of a tree.

Small braches are known as Bronchioles-lastly Terminal bronchioles

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At the end of Terminal Bronchioles, are Respiratory Bronchioles , Alveolar duct, Alveoli.

Tiny sacs(Alveoli) where gas exchange takes place between alveolar air and blood capillaries

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Trachea and large bronchi have cartilaginous rings that prevent these from compressing

Very small bronchioles have no cartilage to hold them open. Their wall has smooth muscle

This smooth muscle is innervated by Autonomic Nervous System, and is also sensitive to some hormones and local chemicals, which affect the air flow in small bronchioles

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1- Breathing Oxygen in, and breathing out Carbon dioxide

Helps in regulation of pH of blood (Acid –base balance) , by adjusting the rate of removal of CO2.

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-- Cellular Respiration ( Internal Respiration)

--External Respiration

Cellular Respiration

It refers to intracellular metabolic process in the Mitochondria, which uses O2 and produces CO2 and energy ATP from food.

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CELLULAR RESPIRATION

On a mixed diet ( Carbohydrate, Fat, Protein ) O2used is 250 ml/min and CO2 produced is 200 ml/min.

We use the Term Respiratory Quotient (RQ) CO2 produced = 200

RQ= O2 used = 250

- On a mixed diet RQ = O.8 -- RQ depends on the type of food used -- when Carbohydrate is used RQ= 1-- when Fat is used RQ= 0.7-- when Protein used RQ=0.8

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Exchange of O2and CO2 between External environment and cells of body. It has 4 steps.

1 – Gas exchange between the atmosphere and alveoli.

2- Exchange of O2 and CO2 between air in the alveoli and blood in pulmonary capillaries.

Transport of O2 and CO2 by the blood to the tissues.

4 – Exchange of O2 and CO2 between system capillaries and tissue cells

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It is route for water loss and heat elimination. Inspired air is humidified and warmed by the respiratory airways before it is expired .

Respiratory pump – helps in venous return.

Helps in regulation of PH of blood.

It enables speech, singing .

It defends against inhaled foreign material.

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Prostaglandins are inactivated in the lungs.

Conversion of angiotensinI to angiotensinII hormone, by ACE ( angiotensin converting enzyme ).

Nose– part of respiratory system , organ of smell.

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O2 and CO2 diffuse through alveoli.

Rate of diffusion depends on thickness of alveolar membrane, surface area and partial pressure of O2 and CO2.

Alveolar wall consists of single layer of alveolar cells [type 1].

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Each alveolus is surrounded by a network of pulmonary capillaries, which is also single layer .

The interstitial space between an alveolus and capillary is very thin 0.5 µm which facilitates gas exchange.

Respiratory Membrane [Alveolar wall and Capillary wall].

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Lungs contain about 500 million alveoli, each about 300 µm in diameter [surface area exposed between alveolar air and pulmonary capillary blood is about 75 m2, size of tennis court].

In alveoli, there are Type II alveolar cells . They secrete Pulmonary Surfactant.

Pulmonary Surfactant is a phospholipoprotein complex that helps in lung expansion.

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Also in alveolar lumen, present are alveolar macrophages which help in defense [Phagocytosis].

Pore of Kohn – are present between adjacent alveoli. Their presence permits air flow between adjacent alveoli. This process is called Collateral Ventilation.

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Two lungs- Right lung is divided into 3 lobes [upper, middle, lower] by oblique and transverse fissure.- Left lung is divided into 2 lobes [upper, lower and has lingula] by oblique fissure.

Lung – has alveoli, blood vessels and large quantities of elastic connective tissues.

Changes in lung volume and alveolar volume are brought about through changes in dimensions of thoracic cavity.

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The outer chest [Thorax] is formed by 12 pairs of curved ribs, which join the sternum anteriorly and thoracic vertebrae posteriorly.

Diaphragm – forms floor of thoracic cavity. Diaphragm is sheet of skeletal muscle that separates thoracic cavity from abdominal cavity. It is penetrated by esophagus and blood vessels.

In the lung and chest wall, there is considerable amount of elastic connective tissue.

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Pleura – separates each lung from the thoracic wall.

Pleura which surround each lung has two layers –Visceral Pleura [inner layer] which surrounds the lung and Parietal Pleura [outer layer] which is under thorax.

Interior of pleural sac(space between parietal and visceral pleura) is known as Pleural Cavity.

Surfaces of pleura secrete intrapleural fluid which lubricates surfaces as they slide on each other during respiratory movements.

Clinical application – pleurisy – [inflammation of pleura]. It causes pain during inspiration and expiration, and friction rub.

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Ventilation - air flow into and out of lungs.

We will consider

1. Atmospheric [barometric] pressure

2. Intra-alveolar pressure or Intra-pulmonary pressure

3. Intra-pleural pressure

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It is pressure exerted by weight of air in the atmosphere on objects on Earth, as Earth surface.

At sea level, atmospheric pressure is 760mmHg.

Atmospheric [Barometric] pressure decreases at high altitude as layers of air decrease in thickness.

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It is pressure within alveoli. It is 760mmHg. It decreases slightly during inspiration and increases during expiration.

Intra-Pleural Pressure or Intra-thoracic Pressure

It is pressure within pleural sac.

It is pressure exerted from outside the lungs within thoracic cavity.

Intra-pleural pressure is -4 mmHg [756mmHg which is 4mmHg less than atmospheric pressure of 760mmHg]

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Flow of air into and out off lung occurs due to cyclic changes in intra-alveolar pressure.

Intra-alveolar pressure is less than atmospheric pressure during inspiration.

Intra-alveolar pressure is greater than atmospheric pressure during expiration.

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When thorax expands, the lungs also expand that is lungs follow the movements of chest wall.

Transmural Pressure Gradient It is the pressure difference between alveolar

pressure and intra-pleural pressure in the lungs. Intra-alveolar pressure equals to atmospheric

pressure of 760mmHg. Intra-pleural pressure is 756mmHg. So there is greater pressure in the lungs as

compared to pleura. This Transmural pressure of +4mmHg causes

stretching or opening of alveoli ,therefore lungs are always forced to expand.

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Pneumothorax – air in the pleural cavity.

If there is chest injury, air rushes into the pleural cavity from high atmospheric pressure when chest is punctured e.g. broken rib or stab wound.

In Pneumothorax, pressure in the pleural cavity increases and causes the collapses of the lung.

Pleural Effusion – Abnormal collection fluid in the pleural cavity e.g. Tuberculosis

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Functional Anatomy of Respiratory System Functions of Respiratory System External and Cellular Respiration Non-respiratory functions of respiratory system Gas Exchange between alveoli and pulmonary

capillaries Respiratory membrane (Alveolar wall & capillary wall) Lungs and Thoracic Cavity Visceral, Parietal Pleura and Pleura Cavity Atmospheric pressure, Intra-alveolar pressure and

Intra-pleural pressure Transmural Pressure and its importance Applied Aspects – Pleurisy, Pneumothorax, pleural-

effusion

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