RS Physiology Quiz

Pleural Cavity and Inspiration/Expiration

• The pleural cavity is a crucial anatomical space located between the lungs and the chest wall. It contains a thin layer of pleural fluid, which facilitates smooth movement of the lungs during breathing.

• During inspiration, the chest cavity expands, increasing intrathoracic volume

  • This is caused by contraction of the diaphragm and inspiratory chest wall muscles
  • The pressure in the thorax and pleural cavity decreases
  • The lungs expand, drawing air in

• This is an active process

• During quiet expiration, the chest cavity recoils, decreasing its volume

  • This is caused by relaxation of the diaphragm and inspiratory chest wall muscles
  • The pressure in the thorax and pleural cavity increases
  • The lungs recoil, pushing air out

• This is a passive process

Forced Ventilation

• When ventilation is stimulated, such as during exercise, additional muscles are recruited (intercostals & abdominals)
• This enhances the movement of the chest wall:
  • Stronger inspiratory efforts increase lung volumes
  • Stronger expiratory efforts decrease lung volumes
• Both of these are active processes

Alveolar Ventilation

• Not all inspired air undergoes gas exchange because it must reach the alveoli
• The volume of air that does not exchange with blood is called physiologic dead space
  • This includes the anatomic dead space: airways up to the respiratory bronchioles just short of the alveoli
• The volume of air that reaches the alveoli per minute is called alveolar ventilation
• The formula for alveolar ventilation is: (Tidal volume - Anatomic dead space) x Breaths per min

Key Concepts in Ventilation

• Ventilation is effected by changes in thoracic volumes and pressures
• It requires the integrity of the lungs and pleura, muscles, and innervation
• Alveolar ventilation is crucial for gas exchange
  • Inspired air reaches the alveoli to deliver oxygen to the blood
  • Expired air with carbon dioxide is expelled from the alveoli

Gas Exchange at Alveoli

• Gas exchange occurs by diffusion across the alveolar-capillary membranes
• Diffusion is the movement of gases down their partial pressure (concentration) gradients
• Oxygen diffuses from the alveoli into the capillaries
• Carbon dioxide diffuses from the capillaries into the alveoli

Overview of Respiratory Process & Partial Pressures

• The partial pressure of oxygen in the alveoli (PAO2) is typically around 100 mmHg
• The partial pressure of carbon dioxide in the alveoli (PACO2) is typically around 40 mmHg

Oxygen Dissociation Curve

• The oxygen dissociation curve shows the relationship between the partial pressure of oxygen (PO2) and the saturation of hemoglobin with oxygen
• At a PO2 of 100 mmHg, hemoglobin is approximately 98% saturated with oxygen
• This means that most of the oxygen carried in the blood is bound to hemoglobin

Systemic vs Pulmonary Circulation

• The pulmonary circulation is a low pressure, high volume system
• It receives 100% of the cardiac output
• Blood flow through the pulmonary capillaries is regulated by arteriolar tone

Ventilation and Perfusion Ratios

• Ventilation (V) refers to the amount of air that reaches the alveoli
• Perfusion (Q) refers to the amount of blood flow through the pulmonary capillaries
• The ventilation/perfusion (V/Q) ratio describes the matching of ventilation and perfusion
  • A high V/Q ratio indicates that there is more ventilation than perfusion
  • A low V/Q ratio indicates that there is more perfusion than ventilation

Local Control Mechanisms

• Local control mechanisms help to match ventilation and perfusion in the lungs
• If there's a large airflow and a small blood flow (apex of the lungs), the blood vessels constrict to redirect blood to areas with more ventilation
• If there's a large blood flow and a small airflow (base of the lungs), the alveoli dilate to increase ventilation

Limitations Respiratory in Response to Disease

• Reduced alveolar ventilation can occur due to:
  • Decreased tidal volume (volume of air inhaled/exhaled per breath)
  • Increased dead space (air that doesn't participate in gas exchange)
• Decreased tidal volume can be caused by conditions that restrict lung or chest wall movement like pain
• Increased dead space can be caused by conditions that affect the alveoli like fibrosis

Summary of Gas Exchange & Perfusion

• For efficient gas exchange, fresh air must reach the alveoli, and the alveoli must be perfused by blood
• Diffusion at the alveoli must be efficient, which depends on:
  • Partial gas pressure (difference in pressure between the alveoli and blood)
  • Rate of blood flow
  • Diffusion barrier (alveolar-capillary thickness and surface area)

Summary of Parts I & II: Gas Exchange & Perfusion

• The pulmonary circulation has special properties:
  • It is a low-pressure, high-volume system
  • It is gravity-dependent
  • Blood flow can be regulated by arteriolar tone

Ventilation Terminology

• The concept of "dead space" refers to the air that does not undergo gas exchange. This is usually mainly air in the conducting airways.
• More forceful inspiration and expiration (vs. quiet breathing) requires more effort from the respiratory muscles

Alveolar Area and Membrane Thickness

• The surface area of the alveoli is critical for efficient gas exchange
• A thinner alveolar-capillary membrane also facilitates quicker gas exchange.
• Disease processes that decrease alveolar surface area or thicken the membrane impair gas exchange

Lung Elastic Recoil and Compliance

• The ability of the lung to expand and recoil is called elastic recoil
• Lung compliance is the ease with which the lungs can be stretched
• Surfactant, a substance that lines the alveoli, helps to maintain alveolar stability by reducing surface tension

Surfactant

• Surfactant is a mixture of phospholipids, lipids, and proteins
• Surfactant lines the alveoli and reduces surface tension
• If surfactant is absent (like in premature babies), the alveoli can collapse

Airway Resistance

• The resistance to airflow through the trachea, bronchi, and bronchioles
• Narrowing of the airways increases resistance due to:
  • Smooth muscle contraction (bronchoconstriction)
  • Decreased lung volume
  • Mucus accumulation
• Increased airway resistance increases the work of breathing

Factors Affecting Airway Resistance

• Bronchoconstriction is stimulated by irritants like smoke, cold air, and parasympathetic nerves
• Bronchodilation is stimulated by drugs like b2 adrenergic agonists (e.g., epinephrine) and sympathomimetic mediators

Oxygen and Carbon Dioxide Transport

• Oxygen is transported in the blood bound to hemoglobin or dissolved in the plasma
• Carbon dioxide is transported in the blood as bicarbonate (HCO3-), dissolved in the plasma, or bound to hemoglobin

Respiratory Changes During Exercise

• During exercise, ventilation increases to meet the increased oxygen demand
• This is achieved by increasing both tidal volume and respiratory rate
• The partial pressure of carbon dioxide in the blood (PaCO2) decreases during exercise due to increased ventilation

Important Key Points:

• Alveolar Ventilation is an important concept in lung physiology, and is crucial for gas exchange.
• Understanding how ventilation and perfusion are controlled and how they can be affected by disease is essential for understanding lung disease.
• The effectiveness of respiration is dependent upon the structure and function of the components of the respiratory system.

Alveolus and Surrounding Environment

• The alveolar-capillary membrane is the thin barrier responsible for gas exchange between air and blood, measuring approximately 0.5µm.
• Type I alveolar cells are responsible for gas exchange within the alveoli.
• Type II alveolar cells are responsible for producing surfactant, a phospholipid mixture which reduces surface tension and promotes alveolar stability.
• Alveolar macrophages reside within the alveoli and fight off foreign particles.
• Pulmonary capillaries surround the alveoli, allowing for blood to flow close to the air for gas exchange.
• Interstitial fluid, containing monocytes, fills the space between the alveolar epithelium and the capillary endothelium.
• Elasin fibers are present in the surrounding tissue, aiding in the expansion and contraction of the lungs.

Gas Exchange

• Gas exchange occurs primarily through diffusion, moving from a higher concentration to a lower concentration.
• Oxygen diffuses from the alveoli into the blood within the capillaries.
• Carbon dioxide diffuses from the blood in the capillaries into the alveoli.

Perfusion of Lungs

• Blood flow within the pulmonary capillaries is crucial for gas exchange.
• Perfusion refers to the rate of blood flow through the alveoli.
• The rate of blood flow must be sufficient for adequate gas exchange.

Innervation and Musculature of Airways

• Airways are comprised of various structures:
  • Trachea and bronchi: primarily composed of cartilage with smaller amounts of smooth muscle.
  • Bronchioles and terminal bronchioles: predominantly composed of smooth muscle.
• Smooth muscle innervation:
  • Sympathetic nervous system: triggers bronchodilation, relaxing smooth muscle.
  • Parasympathetic nervous system: causes bronchoconstriction, contracting smooth muscle.
• Asthma is characterized by excessive bronchoconstriction.
• Bronchodilators, such as b2-adrenergic receptor agonists relax smooth muscle in the airways, aiding in easier airflow.

Ventilation Terminology

• Minute ventilation: represents the volume of air inhaled or exhaled per minute.
• Tidal volume: the volume of air inhaled or exhaled during a normal breath.
• Respiratory rate: the number of breaths per minute.
• Dead space: the volume of air that does not participate in gas exchange, usually due to air remaining within airways.
• Alveolar ventilation: represents the volume of fresh air that reaches the alveoli per minute.

Control of Respiration

• Chemical control:
  • Carbon dioxide (CO2) is the primary respiratory regulator, acting on central chemoreceptors, stimulating increased breathing to expel CO2.
  • Hydrogen ion (H+) concentration is monitored by peripheral chemoreceptors located in the carotid and aortic bodies.
  • Oxygen (O2) levels are also monitored by peripheral chemoreceptors, becoming active when arterial PO2 falls below 60 mmHg.
• Physical control:
  • Stretch receptors in the lungs send signals to the brain stem to adjust the rate and depth of breathing.
• Nervous control:
  • The respiratory center in the brainstem dictates involuntary breathing.
  • Higher brain areas (cerebral cortex) can voluntarily adjust breathing patterns (holding your breath, singing, etc.).

Factors Influencing Gas Exchange

• Partial pressures of gases: The driving force for gas diffusion across the alveolar-capillary membrane depends on differences in partial pressure.
• Thickness of barrier: A thinner barrier allows for faster diffusion.
• Surface area: Larger surface area allows for faster diffusion.
• Blood flow: The rate of blood flow through the alveoli contributes to the efficiency of gas exchange.
• Perfusion of alveoli: Efficient gas exchange requires an adequate blood supply to every alveolus.

Work of Breathing: Compliance of Lungs and Chest Wall

• Compliance refers to the stretchability of the lungs and chest wall.
• It is influenced by surface tension within the alveoli.
• Compliance is measured as the change in lung volume per unit change in airway pressure (ΔV/ΔP).
• Lower compliance means less stretchability and requires more effort to breathe.
• Surfactant reduces surface tension, promoting compliance and reducing the work of breathing.
• Infant respiratory distress syndrome (IRDS) occurs when premature babies lack sufficient surfactant, leading to collapsed alveoli.

Work of Breathing: Airway Resistance

• Airway resistance is the opposition to airflow caused by the narrowing of airways.
• Bronchoconstriction (narrowing of airways) increases resistance due to smooth muscle contraction, leading to increased work of breathing.
• Bronchodilators help relax smooth muscle in the airways, decreasing resistance and improving breathing.
• Stimulants such as smoke and cold air can trigger bronchoconstriction.
• Sympathetic nervous system can induce bronchodilation.
• Parasympathetic nervous system can induce bronchoconstriction.

Alveolus and Surrounding Environment

• Alveolus is a small air sac in the lungs where gas exchange occurs.
• The alveolar-capillary membrane is a 0.5µm barrier separating air and blood.
• Alveolar macrophages reside in the alveolus, responsible for engulfing foreign particles.
• Type I alveolar cells provide a thin barrier for gas exchange.
• Type II alveolar cells produce surfactant, a substance that reduces surface tension in the alveoli.
• Pulmonary capillaries surround the alveoli, facilitating gas exchange.

Gas Exchange

• Gas exchange occurs by diffusion across the 0.5µm alveolar-capillary membrane.
• Diffusion occurs from areas of high partial pressure to low partial pressure.
• Blood in the pulmonary capillaries spends about 0.75 seconds in contact with the alveoli, allowing for complete oxygenation at rest.

Innervation & Musculature of Airways

• The trachea and bronchi primarily consist of cartilage with minimal smooth muscle.
• Bronchioles, up to the terminal bronchioles, are primarily composed of smooth muscle.
• The autonomic nervous system innervates smooth muscle in the bronchi and bronchioles.
• Sympathetic stimulation causes bronchodilation, while parasympathetic stimulation causes bronchoconstriction.
• Asthma is characterized by excessive bronchoconstriction.

Protective Mechanisms of Airways

• The respiratory epithelium (mucosa) is protected by humidifying the air in the upper passages and secreting mucus.
• The lungs are protected by the mucociliary trapping of foreign matter, the ciliary escalator, alveolar macrophages, and airway reflexes.

The Ciliary Escalator

• The epithelium of the upper airways, trachea, bronchi, and bronchioles contain cilia.
• The epithelium is covered by mucus.
• Cilia beat to move particles away from the lungs, a process known as the ciliary escalator.
• Macrophages ingest small particles that reach the lungs.

Functions of the Respiratory System

• Gas Exchange: Pulmonary circulation transports blood from the body to the lungs for gas exchange and back to the heart for distribution. Gas exchange occurs between blood in alveolar capillaries and air in alveoli.
• Immunological & Protective Functions: The respiratory system plays a role in immunity and protection against foreign particles.
• Endocrine Functions: The respiratory system participates in endocrine function, for instance, producing angiotensin II.
• Voice Production: The respiratory system is involved in voice production.
• Water and Heat Loss: The respiratory system contributes to water and heat loss from the body.

Ventilation

• Ventilation refers to the movement of air into and out of the respiratory tract.
• Minute ventilation is calculated by multiplying tidal volume (TV) by the respiratory rate.
• Normal minute ventilation at rest is around 6 liters/minute, while it increases significantly during exercise.
• Hyperventilation: Increased ventilation.
• Hypoventilation: Decreased ventilation.
• Tachypnoea: Increased rate of respiration.
• Dyspnoea: Distressful sensation of breathing.

Alveolar Ventilation

• Alveolar ventilation (VA) is the volume of air that reaches the alveoli per minute.
• Not all air breathed in reaches the alveoli, as some remains in the dead space.
• VA is calculated by subtracting the dead space volume (VD) from tidal volume (TV) and multiplying the result by the respiratory rate.

Control of Respiration

• CO2 Transport in Blood:
• CO2 is transported in the blood in three forms:
  • Dissolved CO2 (7%)
  • Carbaminohemoglobin (23%)
  • Bicarbonate ions (HCO3-) (~70%)
• Chemoreceptor Control:
  • Central chemoreceptors in the medulla are sensitive to changes in PCO2.
  • Peripheral chemoreceptors (carotid and aortic bodies) are sensitive to changes in both PO2 and H+.

Chemical Control of Breathing

• PCO2: The primary respiratory regulator, acting primarily on central chemoreceptors. CO2 can pass the blood-brain barrier, while H+ cannot.
• [H+]: Monitored by the carotid and aortic bodies.
• PO2: Monitored by the carotid and aortic bodies, stimulating peripheral chemoreceptors when arterial PO2 drops below 60 mmHg.

Factors Affecting Gas Diffusion

• Difference in Partial Pressure: Gases diffuse from higher to lower partial pressure; a larger difference in partial pressure leads to faster diffusion.
• Diffusion Barrier: A thicker alveolar-capillary barrier slows down diffusion.
• Area Available for Diffusion: A smaller area for diffusion results in slower diffusion.
• Diffusion Properties of Gas: More soluble gases in blood diffuse faster (e.g., CO2 > O2).

Abnormal Diffusion

• Decreased Pressure Difference: Decreased partial pressure difference across the membrane can lead to reduced diffusion.
• Thickened Barrier: Abnormal thickening of the alveolar-capillary barrier (due to fibrosis, edema) slows down diffusion.
• Reduced Alveolar Volume: Decreased alveolar volume or fewer functional alveoli decrease the area for diffusion.

Gas Exchange at Alveoli: Diffusion vs. Blood Flow

• Decreased Blood Flow: Reduced blood flow (e.g., pulmonary embolism) leads to decreased gas exchange.
• Increased Blood Flow: Increased blood flow (e.g., during exercise) can also lead to decreased gas exchange due to less time for equilibration, especially in diseased alveoli with thickened membranes.

Work of Breathing

• Compliance: Compliance refers to the ease with which the chest wall and lungs can stretch.
• Airway Resistance: Resistance to airflow in the respiratory passages can influence the work of breathing.

Summary

• Ventilation is essential for gas exchange in the lungs.
• Alveolar ventilation specifically refers to the volume of air reaching the alveoli per minute.
• The control of respiration is primarily regulated by CO2 through central chemoreceptors and peripheral chemoreceptors sensitive to both PO2 and H+.
• Gas diffusion is influenced by differences in partial pressure, barrier thickness, available area, and gas solubility.
• Abnormal diffusion can occur due to factors like reduced pressure difference, thickened barriers, and reduced alveolar volume.
• Blood flow also affects gas exchange, with decreased flow reducing gas exchange and excessive flow potentially leading to incomplete equilibration in diseased alveoli.
• The work of breathing is affected by compliance (ease of stretching) and airway resistance.

Test your knowledge on the pleural cavity, inspiration, and expiration processes. This quiz covers the mechanics of breathing, including active and passive respiration, as well as forced ventilation during exercise. Challenge yourself to understand these essential respiratory concepts.