Respiratory Physiology Overview

Respiratory Physiology Overview

• Specific processes include ventilation (mechanical), gas exchange (external and internal respiration), gas transport in the blood (circulatory), and regulation of respiratory function (autonomic and somatic).

Physics of Ventilation

• Air behaves like a fluid, moving from high to low pressure.
• Normal atmospheric pressure is 760 mmHg.
• Ventilation occurs in response to differences in intra-alveolar pressure compared to atmospheric pressure.
• Alveolar pressure less than 760 mmHg = inspiration.
• Alveolar pressure greater than 760 mmHg = expiration.

Ventilation Mechanics (Inspiration)

• Diaphragm and external intercostals contract, enlarging the thoracic cavity.
• This decreases intra-alveolar pressure, drawing air into the lungs.
• Air enters lungs until intra-alveolar pressure equals atmospheric pressure.
• Elastic recoil of lungs and thorax resists expansion.
• Additional muscles are involved in forced inspiration (sternocleidomastoid, pectorals, serratus anterior).

Ventilation Mechanics (Expiration)

• Relaxation of inspiratory muscles decreases thoracic cavity volume.
• This increases intra-alveolar pressure, forcing air out of the lungs.
• The pleural membranes resist collapse, and there is a positive pressure gradient.
• Additional muscles are involved in forced expiration (abdominals and internal intercostals).

Lung Volumes

• Tidal volume (TV): Volume of air exhaled after normal inspiration (~500 mL).
• Inspiratory reserve volume (IRV): Volume of air that can be forcibly inspired following normal inspiration (~3300 mL).
• Expiratory reserve volume (ERV): Volume of air that can be forcibly expired following a normal expiration (~1200 mL).
• Residual volume (RV): Volume of air remaining in the respiratory tract following maximum expiration (~1200 mL).
• Pulmonary volumes are measured using a spirometer.

Pulmonary Capacities

• Vital Capacity (VC): Largest volume of air moved in and out of the lungs (TV + IRV + ERV ~4500-5000 mL).
• Inspiratory Capacity (IC): Maximum volume of inspiration following normal expiration (TV + IRV ~3500-3800 mL).
• Functional Residual Capacity (FRC): Volume of air remaining in the lungs after a normal expiration (ERV + RV ~2200-2400 mL).
• Total Lung Capacity (TLC): Total volume held by the lungs (TV + IRV + ERV + RV ~5700-6200 mL).

Dead Space

• Only air entering the respiratory zone participates in gas exchange with the blood.
• Anatomical dead space is the volume in the conducting airways not available for gas exchange.
• Physiological dead space also includes alveolar dead space—alveoli that are not perfused (not receiving blood).

Partial Pressures

• Dalton's law states that the partial pressure of a gas in a mixture is proportional to its concentration.
• Partial pressures of gases in air and liquid determine flow direction.
• Atmospheric Po2 = 21% X 760 = 159.6 mm Hg.
• Alveolar Po2 = 100 mm Hg.
• Arterial Po2 = 100 mm Hg.
• Venous Po2 = 37 mmHg.

Pulmonary Gas Exchange

• Gases (O2 and CO2) move across the respiratory membrane down respective pressure gradients.
• Factors affecting O₂ diffusion rate include: Po₂ gradient between alveolar air and blood, functional surface area of respiratory membrane, respiratory minute volume, and alveolar ventilation.

Oxygen Diffusion

• Alveolar Po₂ changes in relation to atmospheric pressure changes.
• Functional surface area of the respiratory membrane reduces due to pulmonary pathologies (like emphysema).
• Minute volume can be reduced by pharmaceuticals.

Structure Determines Function

• Gas exchange mechanisms have thin diffusion distances (0.3–0.4 µm) between alveoli and capillaries and large surface areas maximizing gas exchange efficiency.
• Large blood volume within pulmonary capillaries efficiently facilitates gas exchange.
• Narrowness of pulmonary capillaries affects speed and efficiency.

Gas Transport

• Gasses dissolve in blood plasma, but only a limited amount.
• Hemoglobin carries most O₂ (forming HbO₂) and some CO₂ (forming carbaminohemoglobin).

Hemoglobin (Hb)

• Hb is a protein molecule in red blood cells.
• It can bind four O2 molecules.
• It also binds CO2 in a different form.

Oxygen Transport

• O2 is carried in blood dissolved in plasma (a small amount) or bound to hemoglobin (HbO2; majority).
• Oxygen carrying capacity depends on Hb concentration. ( ~1.34 mL O₂ / 1 g Hb, 15 g Hb / 100 mL blood)

Oxyhemoglobin Curve

• The sigmoid shape of the curve indicates how oxygen affinity of hemoglobin changes with PO2 changes.
• Small changes in PO2 have a larger effect on O2 content at lower PO2 levels.

Carbon Dioxide Transport

• The majority of CO2 is carried in blood as bicarbonate ions (HCO3-).
• Small amounts are carried dissolved in plasma and bound to hemoglobin (carbaminohemoglobin).

Carbon Dioxide Transport

• The majority of CO2 is carried in blood as bicarbonate ion (HCO3-), which is formed after CO2 reacts with water.
• This reaction is catalyzed by carbonic anhydrase.
• A portion of the H⁺ dissociates, and it is exchanged in the RBC for chloride (chloride shift).
• CO2 carrying capacity is affected by the amount of CO2 present.

CO2 and pH

• Production of carbaminohemoglobin or bicarbonate ions generates protons (H+).
• Lower pH (increased acidity) is a characteristic of blood carrying higher amounts of CO2.

Systemic Gas Exchange

• As tissues metabolize O2, intracellular/interstitial PO2 decreases.
• Dissolved arterial O2 diffuses into the tissues down its gradient.
• O2 dissociates from HBO2 to be released into the tissues.
• CO2 increases in tissues, diffuses into capillaries, initiating the formation of carbaminohemoglobin and bicarbonate.

Regulation of Breathing

• Blood gas homeostasis is primarily controlled by alterations in ventilation (regulation of the rate and volume of air exchange in the lungs).
• Integrators for respiratory control are in the brainstem.
• Inspiratory/expiratory control centers in the medulla.

Chemical Control

• Chemoreceptors detect chemical changes in blood.
• Central chemoreceptors are in the medulla.
• Peripheral chemoreceptors are in carotid bodies and aortic bodies.

Peripheral Receptors -Blood pH

• Peripheral chemoreceptors are sensitive to changes in blood pH (acid-base balance) and are stimulated by acid or base imbalances.

Hypoxic Drive

• Hypoxic drive is the reduced sensitivity to oxygen (PaO2) as the primary drive for breathing.
• It is commonly increased and important in COPD.

Vascular Resistance and Flow

• Decreased alveolar oxygen tension directly influences blood vessels to the alveoli, causing vasoconstriction.
• This hypoxic pulmonary vasoconstriction is important to maintain efficient ventilation-perfusion matching.

Blood Pressure

• Aortic and carotid baroreceptors are sensitive to blood pressure and regulate breathing rate based on arterial pressure.
• Sudden pressure increases slow ventilation rate, whereas a sudden decrease in pressure will increase ventilatory rate and depth.

Hering-Breuer Reflex

• Lung expansion to tidal volume stimulates stretch receptors, inhibiting the inspiratory center.
• Lung relaxation inhibits stretch receptors.

Other Factors

• The cerebral cortex voluntarily modifies and can override automatic breathing rhythms (to a point).
• Reflexive apneas can occur in response to sudden painful stimulation, cold exposure, or irritation of the larynx or pharynx.

Explore the fundamental concepts of respiratory physiology, including mechanical ventilation, gas exchange, and the regulation of respiratory functions. Understand the physics behind how air moves in and out of the lungs, and the mechanics involved in inspiration and expiration.