Mechanism of breathing

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Questions and Answers

What primarily initiates the process of inspiration?

Increasing intra pulmonary pressure

Contraction of the diaphragm and intercostal muscles (correct)

Contracting abdominal muscles

Relaxing the diaphragm

What percentage of respiration is attributed to the contraction of the diaphragm?

50%

75% (correct)

100%

25%

What occurs during the expiration phase of breathing?

Decrease in lung volume and increase in intra pulmonary pressure (correct)

Active process involving muscle contraction

Contraction of external intercostal muscles

Air rushing in due to negative pressure

Which muscles are involved in forced expiration?

Abdominal muscles and internal intercostal muscles Signup and view all the answers

What is the normal duration of one complete cycle of inspiration?

1.3 seconds Signup and view all the answers

What happens to intra pulmonary pressure during inspiration?

Decreases to about -2 mmHg Signup and view all the answers

Which accessory muscles are involved during forced inspiration?

Sternocleidomastoid, serratus anterior, and scalenes Signup and view all the answers

What role does the external intercostal muscle play in respiration?

Increases the antero-posterior and transverse diameter of the chest Signup and view all the answers

What is the typical intra pulmonary pressure during expiration?

+2 mmHg Signup and view all the answers

What is the function of surfactant in the lungs?

Prevent alveolar collapse Signup and view all the answers

Which factor is associated with bronchoconstriction?

Parasympathetic nervous system Signup and view all the answers

What is the normal amount of intra pleural fluid in each pleural space?

20-25 ml Signup and view all the answers

What happens if the formation of intra pleural fluid exceeds reabsorption?

Pleural effusion occurs Signup and view all the answers

Which layer of pleura adheres to the thoracic cavity?

Parietal pleura Signup and view all the answers

During normal expiration, what is the typical intra pleural pressure?

-3 mmHg Signup and view all the answers

Which of the following is NOT a chemical factor that causes bronchoconstriction?

Epinephrine Signup and view all the answers

What is the primary function of surfactant in the lungs?

To decrease the surface tension of alveolar fluid Signup and view all the answers

Which condition is characterized by the presence of air in the pleural cavity?

Pneumothorax Signup and view all the answers

How does negative intrapleural pressure (I.P.P) support venous return to the heart?

By creating a suction effect that facilitates blood flow Signup and view all the answers

What triggers the condition of dyspnea in pneumothorax?

Tension pneumothorax leading to lung collapse Signup and view all the answers

Which factor does NOT affect lung compliance?

The elasticity of tendons in the thorax Signup and view all the answers

Which statement correctly describes the role of surfactant concerning alveolar macrophages?

Surfactant enhances the bacteriolytic activity of macrophages Signup and view all the answers

What is one potential consequence of reduced surfactant levels?

Decreased ease of lung inflation Signup and view all the answers

What is the main characteristic of a closed pneumothorax?

Air accumulates without an external wound Signup and view all the answers

Study Notes

Mechanics of Breathing

The mechanism of breathing involves the transport of oxygen from the atmosphere to tissues, oxidation of food, and the removal of carbon dioxide.
It has three main stages:
- External respiration: This includes pulmonary ventilation (breathing), gas exchange between the alveoli and the blood, and the carriage of oxygen and carbon dioxide by the blood.
- Internal respiration: This includes gas exchange between the blood and the tissues.
Each respiratory cycle consists of inspiration (inhalation), expiration (exhalation), and an expiratory pause.

Muscles of Respiration

The diaphragm is responsible for about 75% of respiration.
Contraction of the diaphragm causes it to descend from 1.5 to 7 cm.
The phrenic nerve (cervical 3-5) supplies the diaphragm.
External intercostal muscles, when contracted, increase both the antero-posterior and transverse diameters of the chest.
External intercostal muscles are supplied by the 1-10 thoracic nerve segments.

Mechanics of Inspiration

Inspiration is an active process requiring the contraction of the diaphragm and intercostal muscles.
Contraction of these muscles increases the volume of the thoracic cavity.
This increase in volume causes a decrease in intra-pulmonary pressure to about -2 mmHg.
The intra-pleural pressure also decreases to -6 mmHg.
The decrease in pressure within the lungs relative to the atmosphere causes air to rush in (inspiration).

Mechanics of Expiration

Expiration is a passive process that occurs upon relaxation of the respiratory muscles.
Relaxation of the muscles causes the chest cavity to decrease in volume, increasing intra-pulmonary pressure to about +2 mmHg.
This increase in pressure causes air to rush out (expiration).

Forced Respiration

Forced inspiration is an active process requiring the contraction of accessory muscles of respiration such as the sternocleidomastoid, serratus anterior, and scalene muscles.
Forced expiration is an active process involving the contraction of abdominal muscles and internal intercostal muscles.

Pulmonary Ventilation

Pulmonary ventilation is the flow of air to the lungs and is influenced by several factors including:
- The resistance of the air passage
- Pressures in the thoracic cavity (intra-pulmonary pressure and intra-pleural pressure)
- Surfactant
- Lung and chest compliance

Factors Affecting Pulmonary Ventilation

Resistance of the respiratory passage: Bronchial muscle tone is influenced by both nervous and chemical factors. The sympathetic nervous system causes bronchodilation while the parasympathetic nervous system causes bronchoconstriction. Chemical factors like histamine and serotonin constrict the airways, while platelet-activating factor does the same.
Pressure relationships in the thoracic cavity: Intra-pulmonary pressure is the pressure inside the lung alveoli, connected to the atmosphere. During inspiration, it's -2 mmHg, +2 mmHg during expiration, and equal to atmospheric pressure during the expiratory pause.
Surfactant: This lipoprotein substance is synthesized and released by type II alveolar cells. It reduces the surface tension of the fluid lining the alveoli, aiding lung expansion. It also activates macrophages, possesses bacteriolytic properties, and assists with ciliary movement in the upper respiratory tract.
Chest and lung compliance: This refers to the ability of the lungs and chest to expand. It's influenced by factors like the elastic force of lung tissues (decreased compliance in cases of fibrosis) and surface tension of the fluid lining the alveoli. The chest wall's elasticity is determined by the natural elasticity of the muscles, tendons, and connective tissues.

Intra-pleural Space

The intra-pleural space is located between the parietal and visceral layers of pleura, which are serous membranes that enclose each lung.
It is filled with intra-pleural fluid, a lubricant that allows the pleura to slide easily during ventilation.

Intra-pleural Pressure

Intra-pleural pressure is the pressure inside the pleural cavity, typically a negative pressure.
It's -3 mmHg at the end of normal expiration, -6 mmHg at the end of normal inspiration, and can reach -30 mmHg during forced inspiration.

Causes of Negative Intra-pleural Pressure

The lack of air in the pleural cavity
Surface tension of the fluid lining the alveolar wall
Elastic recoil of the lung, which continuously pulls the visceral pleura away from the parietal pleura

Significance of Negative Intra-pleural Pressure

Aids in lung expansion
Helps venous return to the heart
Facilitates blood flow through pulmonary vessels
Supports lymphatic flow in the thoracic duct

Pneumothorax

Pneumothorax is the presence of air in the pleural cavity.
Air can enter the pleura through a wound in the chest wall.
Types of pneumothorax include:
- Closed pneumothorax
- Open pneumothorax
- Valvular pneumothorax
- Artificial pneumothorax
Manifestations of pneumothorax include sudden pleural pain, dyspnea, and cyanosis.

Surfactant

Surfactant is a lipoprotein substance secreted by type II alveolar cells.
It decreases the surface tension of the fluid lining the alveoli, facilitating lung expansion.
Surfactant also activates macrophages, possesses bacteriolytic properties, and supports ciliary movement in the upper respiratory tract.
Surfactant levels are decreased in respiratory distress syndrome and cigarette smokers.

Compliance

Compliance refers to the ability of the lungs and chest to expand.
It is influenced by the elastic force of lung tissues, surface tension of the fluid lining the alveoli, and the elastic properties of the chest wall.

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