�������Pulmonary Ventillation, Type of Breathing, Anoxia, Surfactant, Elasticity, Pleura, compliance�&�Mechanism of Gas Transport and Regulation�

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Dr. Amar Chaudhary, ^{DVM, MS}

Assistant Professor

Department of Physiology and Biochemistry

Agriculture and Forestry University,

Rampur, Chitwan, Nepal

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Pulmonary Ventillation

• Pulmonary ventilation refers to part of respiration, which is the process by which oxygen is transported to tissues and carbon dioxide is expelled
• Pulmonary ventilation, also known as breathing, involves the mechanical processes of inspiration (inhalation) and expiration (exhalation)
• Components of Pulmonary Ventilation
• Pulmonary ventilation relies on several physiological components:
• Lungs:
• The primary organs responsible for gas exchange.
• Thoracic cavity:
• The chest cavity that houses the lungs and is surrounded by the ribs, diaphragm, and intercostal muscles.
• Airways:
• The nose, pharynx, larynx, trachea, bronchi, and bronchioles form the conducting zone of the respiratory system.
• Muscles of respiration:
• Diaphragm, intercostal muscles, and accessory muscles are involved in the mechanical movement of air.
• Pleural membranes:
• These include the parietal pleura (lining the thoracic wall) and visceral pleura (covering the lungs), which create a pressure differential between the lungs and chest wall.

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�Mechanics of Pulmonary Ventilation�

• Inspiration (Inhalation):
• The active phase of breathing in which air is drawn into the lungs
• Diaphragm contraction:
• The diaphragm contracts and moves downward, increasing the vertical volume of the thoracic cavity
• External intercostal muscles:
• These muscles contract, elevating the ribs and expanding the chest wall, thus increasing the transverse and anteroposterior diameters of the thorax
• Increased lung volume:
• As the thoracic cavity expands, the lungs follow, causing the intrapulmonary (alveolar) volume to increase
• Pressure difference:
• The expansion of the lungs reduces the intrapulmonary pressure, causing it to fall below atmospheric pressure
• Airflow:
• Air flows from the higher atmospheric pressure into the lower pressure in the lungs, filling the alveoli with fresh air

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Mechanics of Pulmonary Ventilation

• Expiration (Exhalation):
• The passive or active phase depending on the intensity of breathing
• Diaphragm Relaxation:
• The diaphragm relaxes and moves upwards, and the internal intercostal muscles contract, pulling the ribs downward
• Decrease in Thoracic Volume:
• These movements reduce the volume of the thoracic cavity.
• Increase in Intrapulmonary Pressure:
• As the volume decreases, the intrapulmonary pressure rises above atmospheric pressure
• Air Expulsion:
• The higher pressure inside the lungs causes air to be expelled through the airways out into the atmosphere.

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Control of Pulmonary Ventilation

• Ventilation is tightly regulated by neural and chemical mechanisms to meet the body’s metabolic needs.
• Neural Control:
• Medullary Respiratory Centers:
• Dorsal respiratory group (DRG): Primarily controls inspiration by sending signals to the diaphragm and external intercostal muscles.
• Ventral respiratory group (VRG): Activates during forceful breathing (both inspiration and expiration) and involves abdominal muscles.
• Pontine Respiratory Centers: Modulate the rhythm of breathing
• Central Pattern Generator (CPG): In the medulla, this rhythmic control generates the basic rhythm of breathing.
• Chemical Control:
• Chemoreceptors: Located in the carotid and aortic bodies and the medulla, they detect changes in the levels of carbon dioxide (CO₂), oxygen (O₂), and pH in the blood.
• Mechanoreceptors:
• Stretch receptors in the lungs prevent over-inflation (Hering-Breuer reflex).
• Irritant receptors in the airways initiate coughing or sneezing responses.

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Types of Breathing

There are various types of breathing patterns, categorized by the nature of the airflow and breathing rhythms.

• Normal Breathing (Eupnoea)
• Definition:
• Eupnoea is the normal, quiet breathing pattern, typically at a rate of 12-20 breaths per minute in adults.
• Characteristics:
• It is a regular and rhythmic pattern that involves normal inhalation and exhalation, driven by normal respiratory muscle contractions.
• Deep Breathing (Hyperpnea)
• Definition:
• Hyperpnea is deep and forceful breathing, usually seen during physical activity or when the body requires more oxygen
• Characteristics:
• It is characterized by increased tidal volume (amount of air inhaled or exhaled per breath) and increased respiratory rate
• Shallow Breathing (Hypopnea)
• Definition:
• Hypopnea refers to breathing that is shallow and slow, where the tidal volume is reduced.
• Characteristics:
• This type of breathing is often seen in conditions like sleep apnoea or respiratory diseases where the depth of breathing is compromised.

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Types of Breathing

• Laboured Breathing
• Definition:
• Laboured breathing occurs when it is difficult to breathe, often due to an obstruction or disease in the airways (e.g., asthma, COPD)
• Characteristics:
• It may involve the use of accessory muscles (neck and abdominal muscles) to help expel air, indicating difficulty in breathing
• Cheyne-Stokes Breathing
• Definition:
• A pattern characterized by periods of deep, rapid breathing followed by periods of apnoea (no breathing)
• Characteristics:
• Often seen in conditions like heart failure, stroke, and brain injuries.
• Kussmaul Breathing
• Definition:
• A type of deep, rapid breathing usually associated with metabolic acidosis, particularly diabetic ketoacidosis.
• Characteristics:
• It is a compensatory mechanism to expel excess CO₂ from the body and balance the acid-base status.

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Regulation of Respiration

• Stretch Receptors
• Location:
• Located in the lungs and bronchi.
• Function:
• They protect the lungs from overinflation
• When the lungs stretch too much, they send inhibitory signals to stop further inhalation, a reflex known as the Hering-Breuer reflex
• Respiratory Centres
• Location:
• Found in the medulla oblongata and pons of the brainstem
• Medullary Respiratory Centres:
• Dorsal Respiratory Group (DRG): Primarily responsible for controlling inspiration by sending rhythmic impulses to the diaphragm and external intercostal muscles
• Ventral Respiratory Group (VRG): Involved in both inspiration and expiration, especially during forceful breathing
• Pontine Respiratory Centres:
• Pneumotaxic Center: Regulates the rate and pattern of breathing by inhibiting the DRG and preventing overinflation of the lungs
• Apneustic Centre: Promotes deep, prolonged inspiration by stimulating the DRG

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Anoxia

• Definition:
• Anoxia refers to a condition where the body or a specific tissue is deprived of oxygen
• This can lead to severe metabolic disturbances and cell death if not corrected.
• Causes of Anoxia:
• Hypoxic Anoxia: Low levels of oxygen in the blood (e.g., due to high altitude or pulmonary diseases)
• Anaemic Anoxia: Low oxygen-carrying capacity of the blood (e.g., in anaemia or carbon monoxide poisoning)
• Ischemic Anoxia: Reduced blood flow to tissues (e.g., due to blockage of blood vessels, such as in a stroke or heart attack)
• Histotoxic Anoxia: Tissues are unable to use oxygen due to toxins (e.g., cyanide poisoning)
• Effects of Anoxia:
• When oxygen levels fall significantly, cellular metabolism switches from aerobic to anaerobic pathways, leading to the accumulation of lactic acid and a drop in pH
• Severe anoxia can lead to irreversible damage to tissues, especially in the brain, which is highly sensitive to oxygen deprivation
• Compensation Mechanisms:
• In the early stages of anoxia, the body tries to compensate by increasing ventilation to bring in more oxygen and increasing heart rate to improve oxygen delivery.
• However, if anoxia persists, it can lead to organ failure, coma, and death.

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Elasticity

• Definition:
• Elasticity in the lungs refers to the ability of lung tissues to stretch and then return to their original shape and size once the stretching force is removed
• This is primarily due to the elastin and collagen fibres within the lung parenchyma
• Importance in Respiration:
• Elastic recoil:
• During inspiration, the lungs expand to accommodate the incoming air, and during expiration, the lungs recoil to their resting volume, aiding in the expulsion of air from the lungs.
• The elastic properties of lung tissue help maintain the normal ventilation process, and they work in conjunction with the diaphragm and intercostal muscles to facilitate breathing.
• Lung stiffness or loss of elasticity, seen in diseases like pulmonary fibrosis, can restrict lung expansion and lead to difficulty breathing.

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Surfactant

• Definition:
• Surfactant is a complex mixture of lipids and proteins produced by type II alveolar cells in the lungs
• Its primary function is to reduce the surface tension of the fluid that lines the alveoli.
• Role in Respiration:
• Reduces surface tension:
• In the lungs, the alveolar walls are coated with a thin film of fluid, creating surface tension that would otherwise cause the alveoli to collapse (due to the tendency of liquid molecules to attract one another)
• Prevents alveolar collapse:
• Surfactant lowers the surface tension, preventing the alveoli from collapsing during expiration and making it easier for them to expand during inspiration
• Improves lung compliance:
• By reducing surface tension, surfactant increases lung compliance, making the lungs easier to inflate and reducing the work of breathing

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Pleural Cavity

• Definition:
• The pleural cavity is the space between the two pleural membranes (visceral and parietal pleura) that surround the lungs
• Role in Respiration:
• The pleura is a double-layered serous membrane that surrounds each lung
• The visceral pleura is directly attached to the lung surface, while the parietal pleura lines the thoracic cavity.
• The pleural cavity is filled with a small amount of pleural fluid, which helps reduce friction between the lung and the chest wall as the lungs expand and contract during breathing
• The negative pressure within the pleural cavity (intrapleural pressure) is essential for keeping the lungs inflated and preventing lung collapse

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Pleural liquid( Pleural fluid)

• Definition:
• Pleural fluid is a thin layer of fluid present between the visceral and parietal pleura in the pleural cavity
• Role in Respiration:
• Lubrication:
• The pleural fluid acts as a lubricant, allowing the lungs to slide smoothly against the chest wall during breathing
• This reduces friction and prevents damage to the lung tissue
• Pressure Maintenance:
• The pleural fluid helps maintain the negative intrapleural pressure, which is critical for lung expansion during inspiration
• The pleural pressure is essential for the lungs to stay inflated, preventing atelectasis (collapse of the alveoli or lung)

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��Oxygen-Hemoglobin Dissociation Curve��

Definition:

• The Oxygen-Hemoglobin Dissociation Curve shows the relationship between the partial pressure of oxygen (pO₂) and the saturation of hemoglobin with oxygen (SaO₂)
• It represents how hemoglobin binds to oxygen and releases it under varying conditions
• Sigmoid Curve:
• The curve has a sigmoid (S-shaped) form, meaning that at lower oxygen levels, hemoglobin’s affinity for oxygen is lower, and as oxygen levels increase, the affinity increases steeply
• Clinical Relevance:
• Acidosis (low pH), hypercapnia (high CO₂), and fever can lead to a rightward shift in the curve, improving oxygen delivery to tissues
• Alkalosis (high pH) and hypothermia can cause a leftward shift, making it harder for tissues to get oxygen

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Mechanism of Gas Transport and Regulation

• The transport of oxygen (O₂) and carbon dioxide (CO₂) involves complex physiological processes that facilitate gas exchange and regulation between the lungs, blood, and tissues
• Transport and Exchange of Oxygen (O₂)
• Oxygen in the Lungs::
• Oxygen from the atmosphere diffuses across the alveolar-capillary membrane into the blood in the pulmonary capillaries
• This diffusion occurs due to the concentration gradient, where the partial pressure of oxygen (PO₂) in the alveoli is higher than that in the capillary blood
• Hemoglobin Binding:
• Once oxygen enters the blood, it binds to hemoglobin (Hb) in red blood cells to form oxyhemoglobin (HbO₂).
• Hemoglobin serves as the main transport molecule for oxygen.
• It can carry up to four oxygen molecules per hemoglobin molecule (one on each heme group).

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Mechanism of Gas Transport and Regulation

• Oxygen Transport in the Blood:
• Oxygen Carrying Capacity:
• Oxygen is transported in two forms in the blood:
• Bound to hemoglobin (98.5% of O₂).
• Dissolved in plasma (1.5% of O₂). Only the dissolved oxygen is free to diffuse across tissues and cells.

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• Oxygen Exchange in Tissues:
• Oxygen Delivery to Cells:
• As blood reaches tissues, the PO₂ in the capillaries is lower than in the arterial blood, which promotes the release of oxygen from hemoglobin.
• Oxygen diffuses from the blood into the cells of tissues, where it is used in aerobic metabolism (oxidative phosphorylation) to produce ATP.
• The remaining deoxygenated hemoglobin (reduced hemoglobin) returns to the lungs for re-oxygenation.

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Mechanism of Gas Transport and Regulation

• Transport and Exchange of Carbon Dioxide (CO₂)
• Carbon Dioxide in the Tissues:
• Carbon dioxide is produced as a waste product of cellular metabolism, particularly during the breakdown of glucose in the mitochondria (via glycolysis and the citric acid cycle)
• It diffuses from the tissues into the capillaries.
• Forms of CO₂ in the Blood:
• Dissolved CO₂:
• A small amount of CO₂ (about 7%) is dissolved in the plasma, which is the form that diffuses across membranes.
• Carbaminohemoglobin:
• Approximately 23% of CO₂ binds directly to hemoglobin, forming carbaminohemoglobin.
• CO₂ binds to the amino groups on the hemoglobin molecule, which facilitates its transport back to the lungs.
• Bicarbonate (HCO₃⁻):
• The majority of CO₂ (about 70%) is converted into bicarbonate ions in red blood cells.
• This is facilitated by the enzyme carbonic anhydrase

Mechanism of Gas Transport and Regulation

• CO₂ Transport in the Blood:
• HCO₃⁻ in Plasma:
• The primary transport form of CO₂ in the blood is bicarbonate (HCO₃⁻)
• The high solubility of bicarbonate in plasma allows it to be transported over long distances back to the lungs
• CO₂ Exchange in the Lungs:
• Bicarbonate Conversion:
• As blood enters the pulmonary capillaries, the process reverses
• The low CO₂ concentration in the alveoli promotes the diffusion of CO₂ from the blood into the alveoli.
• Inside red blood cells in the lungs, bicarbonate (HCO₃⁻) reacts with hydrogen ions (H⁺) to form carbonic acid (H₂CO₃), which is then broken down into CO₂ and H₂O by carbonic anhydrase.
• The CO₂ diffuses across the alveolar-capillary membrane into the alveoli and is then exhaled.

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