Respiratory System: Part 2

Pulmonary Ventilation

Also known as Breathing. The mechanical process of inhalation and exhalation of air between the atmosphere and the alveoli.

External (Pulmonary) Respiration

Exchange of gases between the alveoli and the blood pulmonary capillaries

Pulmonary capillary blood gains O₂ and loses CO₂

Internal (Tissue) Respiration

The exchange of gases between systemic capillary blood and tissue cells, during which blood loses $O_2$ and gains $CO_2$.

Inhalation

It is the breathing in or inspiration. An active process involving muscular contraction that expands lung volume to decrease intrapulmonic pressure below atmospheric pressure, allowing air to flow in.

Diaphragm

The dome-shaped skeletal muscle forming the thoracic floor that flattens upon contraction to increase the vertical dimension of the thoracic cavity, accounting for about $75\%$ of air intake during quiet breathing.

External Intercostals

Muscles whose contraction elevates the ribs to expand the anteroposterior and lateral diameters of the thoracic cavity, responsible for about $25\%$ of air entering the lungs during normal quiet breathing.

Intrapleural Pressure

The pressure maintained within the pleural cavity, measuring approximately $756\,\text{mmHg}$ before inhalation and dropping to $754\,\text{mmHg}$ during quiet inhalation.

Alveolar (Intrapulmonic) Pressure

Lung volume increases = pressure within decreases

Pressure inside the lungs during inhalation is $758\,\text{mmHg}$.

Accessory Muscles of Inhalation

Muscles recruited during forceful inhalation, comprising the sternocleidomastoid (elevates sternum), scalene muscles (elevate ribs 1 and 2), and pectoralis minor muscles (elevate ribs 3, 4, and 5).

Exhalation

It is the passive process of breathing out or expiration wherein the pressure in the lungs is greater than the atmosphere. This process starts when inspiratory muscles relax.

Only becomes an active process during forceful breathing (exercise).

Elastic Recoil

The natural tendency of the stretched chest wall and lung tissues to spring back to their resting size.

Muscles of Forceful Exhalation

Abdominal muscles (compress abdominal viscera to force the diaphragm superiorly) and internal intercostals (depress the ribs), which actively contract during exercise or heavy breathing.

Surface Tension of Alveolar Fluid

A thin liquid film lining the alveoli that causes them to assume the smallest possible diameter, accounting for 2/3 of lung elastic recoil.

Surfactant

A substance present in alveolar fluid that reduces surface tension, preventing alveolar collapse at end-expiration and lowering the pressure needed for subsequent inflation.

Respiratory Distress Syndrome (RDS)

A breathing disorder in premature newborns caused by a deficiency of pulmonary surfactant, resulting in alveolar collapse, tachypnea, grunting, and intercostal retractions.

Compliance of the Lungs

The measure of effort required to stretch the lungs and chest wall, Related to two principal factors: elasticity and surface tension.

Airway Resistance

The opposition to airflow offered by the walls of the respiratory tract, primarily regulated by the degree of contraction & relaxation of smooth muscle in the airways walls

Eupnea

A normal, quiet, unlabored breathing pattern consisting of shallow, deep, or combined breathing.

Costal Breathing

Shallow chest breathing produced by the upward and outward movement of the chest via contraction of the external intercostal muscles.

Diaphragmatic Breathing

Deep abdominal breathing marked by the outward movement of the abdomen resulting from the contraction and downward displacement of the diaphragm.

Spirometer

An apparatus used to measure the lung volume and capacities, and the results is shown as a spirogram (see image).

Tidal Volume (TV)

The volume of air inspired or expired in a single quiet breath, equal to approximately $500\,\text{mL}$ ($350\,\text{mL}$ reaching the respiratory zone and $150\,\text{mL}$ in the anatomic dead space).

Anatomic Dead Space

The portion of inhaled air (about $150\,\text{mL}$) that remains in the conducting airways and does not participate in gas exchange.

Inspiratory Reserve Volume (IRV)

The maximal volume of air that can be inhaled after the tidal volume, averaging $3100\,\text{mL}$ in adult males and $1900\,\text{mL}$ in adult females.

Expiratory Reserve Volume (ERV)

The maximal volume of air that can be exhaled after the tidal expiration, averaging $1200\,\text{mL}$ in adult males and $700\,\text{mL}$ in adult females.

Residual Volume (RV)

The volume of air remaining in the lungs after a maximal exhalation ($1200\,\text{mL}$ in adult males, $1100\,\text{mL}$ in adult females), which continues to function in gas exchange.

Inspiratory Capacity (IC)

Calculated as $\text{Tidal Volume} + \text{Inspiratory Reserve Volume}$.

Functional Residual Capacity (FRC)

Calculated as $\text{Expiratory Reserve Volume} + \text{Residual Volume}$ .

Vital Capacity (VC)

Calculated as$\text{Inspiratory Reserve Volume} + \text{Tidal Volume} + \text{Expiratory Reserve Volume}$.

Total Lung Capacity (TLC)

Calculated as $\text{Vital Capacity} + \text{Residual Volume}$ .

Minute Ventilation

The total volume of air moved inspired and expired of the lungs every minute, calculated as $\text{Tidal Volume} \times \text{Respiratory Rate}$ (with a normal rate of $12\,\text{breaths/min}$).

Alveolar Ventilation

Total amount of air reaching the respiratory zone every minute

External Respiration

Exchange of gas between the alveoli and pulmonary capillaries. It converts deoxygenated blood → oxygenated blood

Internal Respiration

Exchange of gas between the systemic capillaries and tissue cells.

Oxygen-Hemoglobin Dissociation Curve

An sigmoidal curve that illustrates the relationship between the partial pressure of oxygen ($P_{O2}$ ) and the percentage saturation of hemoglobin with oxygen.

Fully Saturated Hemoglobin

All hemoglobin is converted to oxyhemoglobin, hemoglobin bound to oxygen.

Partially Saturated Hemoglobin

Mixture of oxyhemoglobin (Hb-O₂) and reduced hemoglobin (Hb)

Acidity

Acidity in blood comes from CO₂ due to carbonic anhydrase. ↑ Acidity = ↓ affinity of Hb for O₂

Bohr Effect

The physiological phenomenon in which an increase in blood acidity ($H^+$ ions) reduces hemoglobin's affinity for oxygen, promoting oxygen unloading into metabolically active tissues.

2,3-Bisphosphoglycerate (BPG)

A byproduct of metabolism that can be found in RBCs which helps in unloading O₂ in target tissues. ↑ BPG  = ↓ affinity of Hb for O₂

Fetal Hemoglobin (Hb-F)

Has higher affinity for O₂ versus adult-type hemoglobin.

Dissolved CO₂

About 7% are dissolved which diffuses out of the blood in the alveoli.

Carbamino compounds

CO₂ + Hgb → forms Carbaminohemoglobin (reversible). Around 23% of CO₂ are bound to Hemoglobin.

Carbon monoxide has higher affinity to hemoglobin compared to O₂ → displaces O₂ → Carbon monoxide poisoning

Bicarbonate ions

70% of the CO₂  is transported in blood plasma as bicarbonate ions

Haldane Effect

The relationship wherein a lower saturation of oxyhemoglobin ($\text{Hb-O}_2$) increases the carbon dioxide-carrying capacity of the blood.

2,3-Bisphosphoglycerate (BPG)

A metabolic byproduct within red blood cells that decreases hemoglobin's affinity for oxygen, facilitating oxygen unloading in peripheral target tissues.

Carbaminohemoglobin

A reversible chemical complex formed by the direct binding of carbon dioxide to hemoglobin, accounting for about $23\%$ of blood $CO_2$ transport.

Dorsal Respiratory Group (DRG)

Neurons in the medullary respiratory center controlling normal quiet breathing that send rhythmic motor impulses to the diaphragm via phrenic nerves and external intercostals via intercostal nerves for $2\,\text{seconds}$, followed by $2\,\text{seconds}$ of inactivity.

Ventral Respiratory Group (VRG)

Medullary neurons activated during forceful breathing that innervate the accessory muscles of inhalation and exhalation.

Pontine Respiratory Group (PRG)

A collection of pontine neurons (aka pneumotaxic area) that modulates the basic rhythm generated by the medullary centers during activities such as speaking, exercising, or sleeping.

Cerebral Cortex

Enables voluntary alteration of pattern of breathing and activities like swimming, or avoiding irritating gases.

Hypothalamus and Limbic System

Stimulates the respiratory center which allows emotional stimuli to alter breathing, as in laughing and crying.

Central Chemoreceptors

Sensory receptors located in the medulla oblongata that detect changes in hydrogen ion ($H^+$) concentration and carbon dioxide partial pressure ($PCO_2$) in cerebrospinal fluid.

Peripheral Chemoreceptors

Sensory receptors located within the aortic bodies and carotid arteries sensitive to systemic changes in $PO_2$, $H^+$, and $PCO_2$.

Hypercapnia

An elevation in arterial blood $PCO_2$ that stimulates central and peripheral chemoreceptors.

Hypocaonia

No stimulation from chemoreceptors → no impulse to DRG → CO₂ accumulates.

Hypoxic Hypoxia

A deficiency of tissue oxygenation caused by a low partial pressure of oxygen ($PO_2$) in arterial blood, often due to high altitude, airway obstruction, or fluid accumulation in the lungs.

Anemic Hypoxia

Tissue oxygen deficiency resulting from an inadequate amount of functional hemoglobin to transport oxygen, caused by conditions such as hemorrhage, anemia, or carbon monoxide poisoning.

Ischemic Hypoxia

Inadequate tissue oxygen delivery resulting from decreased local blood perfusion, occurring even when arterial $PO_2$ and hemoglobin saturation remain normal.

Histotoxic Hypoxia

A condition in which blood oxygen delivery is completely normal, but tissue cells are unable to utilize oxygen due to cellular toxins, such as cyanide blocking ATP synthesis.

Pulmonary Tuberculosis (PTB)

Scarring of lung tissue → histology is no longer normal, no elastic fibers

Pulmonary Edema

Fluid deposition

Surfactant Deficiency

High surface tension leads to alveolar collapse and reduced compliance

Paralysis of Intercostal Muscles

Impedes lung expansion

Emphysema

Destruction of elastic fibers in alveolar walls