Respiratory Physiology: Anatomy, Mechanics, and Function

• Respiratory physiology includes the anatomy, mechanics of breathing, and compliance of the respiratory system.

Respiratory System: Primary Functions

• Exchange of gases between the atmosphere and the blood occurs.
• Regulation of pH is achieved by releasing or retaining CO2.
• Protection from inhaled pathogens
• Vocalization

Cellular vs. External Respiration

• Cellular respiration is the intracellular reaction of O₂ with organic molecules to produce CO2 and ATP.
• External respiration involves the movement of gases between cells and the environment.
• The four processes of external respiration are:
  • Ventilation between the atmosphere and air sacs (alveoli) in the lungs.
  • Exchange of O₂ and CO2 between air in the alveoli and blood in the pulmonary capillaries.
  • Transport of O₂ and CO2 by the blood between the lungs and tissues.
  • Exchange of O₂ and CO2 between the blood in systemic capillaries and tissue cells.

General Anatomy

• The respiratory system is divided into the upper and lower respiratory tracts.
• The upper respiratory tract consists of the mouth, nasal cavity, pharynx, and larynx.
• The lower respiratory tract includes the trachea, bronchioles, and branches.

Respiratory Functions

• Passageway for air
• Warms the air
• Adds water vapor to dry gas exchange surface
• Filters out foreign materials like dust and viruses

Lower Respiratory Tract functions

• Pull air in from upper respiratory tract
• Absorb oxygen
• Release carbon dioxide
• The three main components of the respiratory system are:
  • Airways: system of tubes/airways
  • Alveoli: structures for gas exchange
  • Bones and Muscles of thorax and abdomen that assist in ventilation

Lungs and Thoracic Cage

• Lungs are spongy tissue with volume that is mostly air-filled spaces.
• Airways connect to the lungs via the trachea and bronchi.
• The thoracic cage consists of:
  • Bones: ribs, sternum, and vertebrae
  • Muscles: intercostals, sternocleidomastoid, and scalenes

Pleural Membranes

• The pleural membranes are a double-walled membranous sac that covers each lung.
• One side is attached to the lung, and the other is attached to the ribs and muscles.
• The inside of the pleural sac contains pleural fluid, which provides a moist, slippery surface for breathing.
• It ensures that the two layers stick together so that they slide over each other easily yet still stick to each other.

Lower Respiratory Tract: Trachea

• Structurally, the trachea is 12cm long and 2.5cm in diameter.
• Located anterior to the esophagus and has a unique structure.
• Responsible for air humidification, filtration, and warming.
• Contains Hyaline cartilage (C-shaped) and smooth muscle (m. trachealis).
• Contains Pseudostratified ciliated columnar epithelium (PSCC)
• PSCC components ensure mucus is produced, trapping pathogens.
• Smoking irritates and destroys cilia, impairing mucociliary transport.

Lower Respiratory Tract: Bronchial Tree

• The bronchial tree is a series of tubes that carry air.
• Mainstem bronchi are left and right and separate distal to the trachea.
• Bronchi branch repeatedly about 22 times, becoming smaller, and the amount of cartilage decreases.
• Eventually replaced with more smooth muscle.
• Bronchi have some cartilage, while bronchioles have no cartilage.
• Divisions increase which makes smaller tubes.
• Nerves increase surface area maximizing the amount of gas exchange.

Control of Airflow

• Bronchoconstriction is the constriction of smooth muscle in bronchioles, which leads to a smaller radius and more resistance to airflow and is influenced by parasympathetic input through muscarinic receptors.
• Bronchodilation is the relaxation of smooth muscle in bronchioles, which leads to an increased radius and less resistance to airflow due to sympathetic activation of B₂ receptors by epinephrine/norepinephrine.

Lower Respiratory Tract: Respiratory Bronchiole

• Cartilage rings gradually decrease in size and number as branches divide, while the mucous membrane becomes thinner.
• The terminal bronchioles turn into respiratory bronchioles
• Respiratory bronchioles consist of smooth muscle, elastic tissue, and thin mucous membranes
• Further divide into alveolar ducts.

Lung Lobule

• A lung lobule contains alveoli and is the final destination of air.
• Primary function is gas exchange, done through a Single layer of cells line each alveolus
• Close association with capillary network, almost always continuous with sheet of blood in contact with air filled alveoli
• Very little room between alveoli for rapid diffusion

Internal Structure of Alveoli

• There are two types of cells:
  • Type 1 Alveolar Cells: 95% of the surface and is very thin: facilitates diffusion of gases.
  • Type II Alveolar Cells: Thicker cells that create surfactant, which removes excess fluid.

Pulmonary Circulation

• Pulmonary circulation has high flow and low pressure due to
  • Shorter length of blood vessels in the pulmonary circuit
  • Large total cross-sectional area of blood vessels
• Cardiac output is 5L/min, meaning that this value of blood is going through the lungs/minute.

Mechanics of Breathing

• Gas Laws

Gas Laws

• Air is similar to blood because both are fluids.
• Air is compressible
• These laws govern the properties of gases. - Ideal Gas Law and Boyle's Law: P₁V₁ = P₂V₂
• Both equations express the inverse relationship between volume and pressure.

Dalton's Law: Air is a Mixture of Gases

• The total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each individual gas
• Partial pressure is the pressure exerted by an individual gas.
• Partial pressure of a gas = Patm x % of gas in atmosphere.
• Oxygen makes up 21% of air. Normal atmospheric pressure is 760 mmHg.
• Poxygen = 760 mmHg x 21% oxygen, Poxygen = 760 mmHg x 0.21 = 160 mmHg

Mechanics of Breathing

• Air flow occurs when a pressure gradient is present
• Flows from an area of high pressure to an area of low pressure.
• Diffusion of a single gas moves from areas of high partial pressure to areas of low partial pressure.
• The movement of air into the lungs is influenced by pressure gradients.

Pressure Relationships in the Thoracic Cavity

• Patm is the pressure of gases in the atmosphere, equal to 760mmHg or 1atm.
• Ppl is the pressure in the pleural cavity, approximately 756mmHg or -4mmHg.
• Ppul pressure is exerted by the alveoli in the lungs.
• Ppul must be greater than Ppl or lungs will collapse.

Basic Principles

• Air always travels from an area of high pressure to an area of low pressure.
• As volume increases, pressure decreases (Boyle's Law).
• Atmospheric pressure
• Intrapulmonary pressure and volume
• Intrathoracic pressure and volume
• Intrapleural pressure

Negative Pleural Pressure: Opposing Forces

• Influenced by the elastic recoil of lungs
• Thoracic wall is always trying to expand, while the pleura is pulled in opposite directions causing intraplerual space = negative pressure
• The Pleura is pushed to to thoracic wall
• Intrapleural space = negative pressure
• Pleural fluid contains water, creating a hydrostatic force btween the layers that makes the lungs and visceral pleura move with the thoracic wall

Before Inspiration

• No air moving and thoracic wall is stationary
• Patm= Pintrapulmonary
• Ppl = negative

Inspiration

• The diaphragm contracts and moves inferiorly.
• External intercostals contract and move the ribs up and out.
• Muscles cause extra force pulling the thoracic wall out which overcomes the elastic recoil force of the lungs
• Causes thoracic wall to expand

Inspiration: Greater Negative Pressure

• Increases the volume in the thoracic cavity
• The extra muscular force pulling on the pleura creates greater negative pleural pressure.
• Ensures the lungs stick to the moving thoracic wall and increases the volume of the lungs.
• Because the lungs have a lower pressure than the atmosphere, air rushes into the lungs (Patm > Pintrapulmonary).

Inspiration Sequence of Events

• Inspiratory muscles contract (diaphragm descends; rib cage rises).
• Thoracic cavity volume increases.
• Lungs are stretched, and intrapulmonary volume increases.
• Intrapulmonary pressure drops (to -1 mm Hg).
• Air (gases) flows into lungs down its pressure gradient until intrapulmonary pressure is 0 (equal to atmospheric pressure).

Intrapulmonary and Intraplural Pressures

• At rest, no net difference between Ppul and Patm
• During inspiration there is negative pressure
• During expiration there is positive pressure

Expiration Sequence of Events

• Inspiratory muscles relax (diaphragm rises; rib cage descends due to recoil of costal cartilages).
• Thoracic cavity volume decreases.
• Elastic lungs recoil passively; intrapulmonary volume decreases.
• Intrapulmonary pressure rises (to +1 mm Hg).
• Air (gases) flows out of lungs down its pressure gradient until intrapulmonary pressure is 0.

Work of Breathing

• Ventilation is the bulk flow of air from the atmosphere to the alveoli
  • Respiratory cycle consists of inspiration followed by and expiration
• Work of breathing encompasses the required work to:
  • Expand the lungs against elastic forces (compliance work).
  • Overcome the viscosity of the lung and chest wall structures (tissue resistance work).
  • Overcome airway resistance during the movement of air into the lung (airway resistance work).

Factors that affect ventilation

• Compliance: How easily the lungs can inflate
• Elastance: ability to resist being deformed
• Lungs must have a natural elastic fiber

Elastic Properties of the Lung

• Compliance = △Volume/△Pressure
• Forces:Elastic forces of the lung tissue
• Elastic forces caused by
• Fluid that lines the walls

Compliance

• Lowest at maximal inspiratory volumes Lowest at maximal inspiratory volumes
• Lower in children
• Highest in large lungs

Compliance Diagram of the Lung

• The differences in compliance between the air-filled and water-filled lung is a result of different surfaces

Surface tension

• LePlace's Law: States that the pressure inside of a bubble formed by thin fluid is a function of:
  • Surface tension/ Radius of the bubble

Surface Tension

• Surface tension is the ratio of pressure:P = 2T/r (where P = pressure, T = surface tension, r = radius).
• The smaller bubble will have higher pressure due to surface tension

Surface tension in Alveoli

• Alveoli are lined with a thin layer of water, and water has a high surface tension. Which means small alveoli have high pressure and wont inflate

Surface Tension in Alveoli and Surfactant

• Type 2 Alveolar Cells release surfactant to disrupt cohesive properties of water
• Surfactants reduce surface tension.
• Reduces the pressure in the alveolus

Surfactant

• Surfactant is more concentrated in smaller alveoli
• Reduces the surface tension of smaller alveoli more than large ones
• Results in an equalization of pressure across all alveoli
• Alveoli inflate uniformly

Compliance Work

• Balanced during the end of normal expiration (FRC)
• Increased with exercise
• Increased by diseases that cause fibrous of the lungs (CF)

Resistive Work

• Resistance = Pressure/Flow
• Increased by exercise
• Increased caused by diseases that restrict airflow

Airway Resistance

• Velocity of flow increases turbulence
• Increases resistance
• Velocity = Flow/Cross Sectional Area

Obstructive vs Restrictive Disease

• Obstructive disease: Resistive work is increased because the airways are obstructed, such as in asthma, emphysema, chronic bronchitis, and COPD.
• Restrictive disease: Elastic work is increased because the lung is stiff and non-compliant, as seen in interstitial lung disease, obesity hypoventilation syndrome, scoliosis, and cystic fibrosis.