Pulmonary Function, Respiratory Cycle, Respiratory Rates, and Regulation of Ventilation Notes :

Pulmonary Function, Respiratory Cycle, Respiratory Rates, Regulation of Ventilation

Introduction

• The respiratory system has three primary functions:

  • Providing oxygen for the body's energy needs.

  • Eliminating carbon dioxide (CO2).

  • Helping maintain the pH of the blood plasma.

• The respiratory cycle works with the circulatory system to achieve these functions.

Gas Exchange

• There are two main sites of gas exchange (oxygen and carbon dioxide) in the body:

  • Lungs

  • Tissues of the body

• External respiration: Gas exchange between the air in the alveoli and the blood in the pulmonary capillaries.

  • "External" refers to the involvement of air from the external environment, even though the exchange occurs within the lungs.

• Internal respiration: Gas exchange between the blood in the systemic capillaries and the tissue fluid (cells) of the body.

• Inhaled air:

  • Approximately 21% oxygen

  • Approximately 0.04% carbon dioxide

  • Approximately 78% nitrogen (not physiologically available and is exhaled)

• Exhaled air:

  • Approximately 16% oxygen (some oxygen is retained within the body)

  • Approximately 4.5% carbon dioxide (produced by cells and exhaled)

Anatomy of the Respiratory System

The respiratory system is divided into:

1. Upper respiratory tract

2. Lower respiratory tract

Upper Respiratory Tract

• Consists of parts outside the chest cavity:

  • Air passages of the nose

  • Nasal cavities

  • Pharynx

  • Larynx

  • Upper trachea

• Mainly concerned with conduction of gases from the atmosphere to and from the lungs.

Lower Respiratory Tract

• Partly concerned with conduction and mainly with gaseous exchange.

• Consists of parts within the chest cavity:

  • Lower trachea

  • Lungs (including bronchial tubes and alveoli)

• Also includes:

  • Pleural membranes

  • Respiratory muscles (diaphragm and intercostal muscles) that form the chest cavity

Anatomy of the Upper Airways

• Air is drawn into the lungs via the nasal cavity and passes highly vascular nasal mucous membranes, which are covered by ciliated columnar epithelium (except at the nasal entrance).

• Functions of nasal membranes:

  • Moistening the entering air

  • Removing large particles of dust

  • Warming the air (raising the temperature from 6°C to 30°C)

• Inhaled air is fully humidified and warmed to body temperature (37°C) as it travels through the trachea.

• Expired air is slightly below body temperature because it loses heat when it leaves the nasal passages.

• Counter-current exchange system prevents excessive heat loss from the body core.

• Pharynx:

  • Divided by the soft palate into an upper nasopharyngeal and a lower oropharyngeal region.

  • Contains lymphoid structures such as the adenoids and tonsils.

• Airway opening:

  • Can be opened during anesthesia or emergency resuscitation by tilting the head backwards at the atlanto-occipital joint (between C1 and skull).

  • Can also be opened by protruding the jaw to lift the tongue forward.

• Partial blockage:

  • Partial blockage of the airways by the tongue, uvula, or soft palate during sleep leads to turbulence in airflow, which is heard as snoring.

• Larynx:

  • Consists of articulated cartilages, vocal cords, muscles, and ligaments.

  • Keeps the airway open during breathing and closed during swallowing.

  • Can remain closed and withstand high pressures generated by the thorax (90 cmH2O) prior to sudden release during coughing.

  • Innervated by the laryngeal nerve.

• Trachea:

  • Begins at the lower border of the cricoid cartilage of the larynx at the level of the sixth cervical vertebra.

  • Mean diameter of 1.8 cm and a length of 11 cm.

  • Extends from the larynx to the primary bronchi.

  • Supported by C-shaped cartilaginous rings to prevent kinking during head and neck movement.

  • Can be compressed by moderate external pressure of between 50 cmH2O and 70 cm H2O or by internal pressure from a hematoma (blood collection) following surgery or accident.

The Tracheobronchial Tree

• The trachea divides into two bronchi (right and left primary bronchi) that enter the lungs.

• Right bronchus:

  • Wider than the left bronchus.

  • Makes a smaller angle with the trachea.

  • More likely to receive inhaled foreign bodies.

• Structure is similar to that of the trachea, with C-shaped cartilages and ciliated epithelium.

• Subdivisions:

  • The trachea divides into two main bronchi, then into four lobar bronchi, 16 segmental bronchi, and thereafter into small bronchi, terminal bronchioles, respiratory bronchioles, and alveolar ducts.

• Bronchial tree: The further branching of the bronchial tubes.

• No cartilage is present in the walls of the bronchioles.

• The tree has Approximately 23 generations of division result in approximately 8 million alveolar sacs.

• Alveolar sacs

  • Form the last generation of blind air passages.

  • Approximately 17 alveoli arise from each sac.

  • Account for approximately half of the 250–300 million alveoli, the others arising directly from the alveolar ducts.

  • Total surface area of approximately 75 m2 (adult male).

• Trachea, bronchi, and bronchioles:

  • Tubular structures for conducting air.

  • Walls consist of an outer fibrous layer with supporting pieces of cartilage and bronchial smooth muscle.

  • Bronchial smooth muscle is arranged in clockwise and anticlockwise helical bands with a matrix of elastic tissue.

  • The lumen of the airways decreases in size with progressive numbers of divisions in the tracheobronchial tree.

Breathing – Process of Respiration

• Processes of respiration:

  1. External respiration

  2. Gas transport

  3. Internal respiration

• External respiration: Mechanisms by which a person obtains oxygen from the external environment and eliminates carbon dioxide into the external environment.

• Gas transport: Mechanisms used to distribute oxygen to and remove carbon dioxide from cells.

• Internal respiration: Chemical reactions of cellular metabolism in which oxygen is consumed and carbon dioxide is produced.

Overview of Ventilatory Control

• The ventilatory control system consists of three basic elements:

  1. Sensors: Peripheral and central chemoreceptors and pulmonary mechanoreceptors that gather information and feed it to the central controller.

  2. Central controller:

     • Respiratory control center in the brain.

     • Integrates and coordinates the information and sends signals to the effectors.

  3. Effectors: Respiratory muscles (including the diaphragm) that produce changes in the ventilatory pattern.

Mechanism of Breathing

• Ventilation: The movement of air to and from the alveoli.

• Two aspects of ventilation:

  • Inhalation

  • Exhalation, which are brought about by the nervous system and the respiratory muscles.

• Respiratory centers are located in the medulla and pons.

Muscles Involved in the Process of Breathing

• Diaphragm and the external and internal intercostal muscles.

• Diaphragm:

  • A dome-shaped muscle below the lungs.

  • When it contracts, the diaphragm flattens and moves downward.

• Intercostal muscles: Found between the ribs.

  • External intercostal muscles pull the ribs upward and outward.

  • Internal intercostal muscles pull the ribs downward and inward.

• Accessory Muscles of Inspiration:

  • Sternocleidomastoid (elevates sternum)

  • Scalenes Group (elevate upper ribs)

  • Pectoralis minor

• Principal Muscles of Inspiration:

  • External intercostals (Interchondral part of-internal intercostals also elevates ribs)

  • Diaphragm (dome descends, thus increasing vertical dimension of thoracic cavity; also elevates lower ribs)

• Muscles of Expiration:

  • Quiet breathing: Expiration results from passive, elastic recoil of the lungs, rib cage and diaphragm

  • Active breathing: Internal intercostals, except interchondral part (pull ribs down), Abdominals (pull ribs down, compress abdominal contents thus pushing diaphragm up)

  • Quadratus lumborum (pulls ribs down)

Ventilation

• Ventilation is the process by which air moves in and out of the lungs.

• The incoming air is composed of a volume that fills the conducting airways (dead space ventilation) and a portion that fills the alveoli (alveolar ventilation).

• Minute (or total) ventilation (\"), VE: The volume of air that enters or leaves the lung per minute:

  V̇E = f × VT$$

  • f is the frequency or number of breaths per minute.

  • VT (also known as TV) is the tidal volume, or volume of air inspired (or exhaled) per breath.

• Tidal volume varies with age, sex, body position, and metabolic activity.

  • In an average-sized adult at rest, tidal volume is 500 mL.

  • In children, it is 3 to 5 mL/kg.

Pressures Involved in Breathing

• Three types of pressure are important:

  • Atmospheric pressure: The pressure of the air around us.

  • Intrapleural pressure: The pressure within the potential pleural space between the parietal pleura and visceral pleura.

  • Intrapulmonic pressure: The pressure within the bronchial tree and alveoli. This pressure fluctuates below and above atmospheric pressure during each cycle of breathing.

Inhalation and Exhalation

• Inspiration (breathing in):

  • The diaphragm contracts and moves downwards.

  • The intercostal muscles contract and move the ribs upwards and outwards.

  • This increases the size of the chest and decreases the air pressure inside it, which sucks air into the lungs.

• Expiration (breathing out):

  • The diaphragm relaxes and moves back to its domed shape.

  • The intercostal muscles relax so the ribs move inwards and downwards under their own weight.

  • This decreases the size of the chest and increases the air pressure in the chest, so air is forced out of the lungs.

• During inspiration, skeletal muscles (such as the diaphragm and external intercostals) contract, thereby increasing volume within the thoracic cavity and lungs.

  • The increased volume creates less pressure within the lungs than the atmosphere, so air rushes into the lungs.

• During resting expiration, the inspiratory muscles relax, causing the volume of the thoracic cavity and the lungs to be reduced.

  • This reduction forces gas back into the atmosphere.

  • Normally, unlabored expiration at rest is a passive event determined by relaxation of inspiratory muscles.

• During exercise or during forced exhalation (e.g., coughing), expiration becomes an active event dependent upon contraction of expiratory muscles that pull down the rib cage and compress the lungs.

• During inspiration, oxygen drawn into the lungs diffuses to the pulmonary capillaries and is transported to cells via erythrocytes (red blood cells).

• The cells use oxygen to supply energy for metabolic processes.

• When producing energy, these cells then release carbon dioxide as a waste product.

• Some of the carbon dioxide reacts with water in the body to form carbonic acid, which then dissociates to H+ and bicarbonate.

• The erythrocytes transport CO2 and H+ back to the lungs.

• Once in the lungs, the H+ and HCO3- recombine to form water and CO2.

Factors Involved in the Regulation of Ventilation

• The basic breath pattern is affected by:

  • Higher centers in the brain.

  • Feedback from peripheral and central chemoreceptors in the arterial system and medulla, respectively.

  • Stretch receptors in the lungs.

  • Other sensory receptors in the body.

• Cerebral control is evident during speech, which requires expiratory air to pass over the vocal cords.

Hyperventilation

• The separate chemoreceptors sense O2, CO2, and H+ levels in the blood and in the cerebrospinal fluid of the medulla.

• In hyperventilation (excess ventilation), the breathing rate and depth are increased, so that the lungs rid the body of carbon dioxide faster than it is being produced.

• Hydrogen ions are removed from body fluids, and the pH becomes elevated.

• This tends to depress ventilation until normal carbon dioxide and hydrogen ion levels are restored.

• The temporary cessation of breathing after voluntary hyperventilation is known as apnea vera.

Hypoventilation

• In hypoventilation (insufficient ventilation - shallow and/or slow breathing), the lungs gain carbon dioxide in body fluids (hypercapnia) since the lungs fail to remove carbon dioxide as rapidly as it is being formed.

• The increased formation of carbonic acid results in a net gain of hydrogen ions, lowering pH in body fluids.

• The chemoreceptor feedback causes ventilation to increase until carbon dioxide levels and extracellular fluid pH return to normal.