Gaseous Exchange in Mammals

Gaseous Exchange in Mammals

Overview

• Mammalian breathing systems involve pulmonary ventilation through inhalation.

• Key processes include gaseous exchange in the lungs and mechanisms controlling this exchange.

Mammalian Breathing System

• Inhalation Process: Air enters through the nasal cavity, where it is warmed and humidified.

• The respiratory tract is lined with mucus to protect tissues from direct air contact.

• Protective Mechanisms: Warming, humidifying, and particle removal prevent damage to the trachea and lungs.

• Mucus and cilia in the nasal passages trap and remove particulate matter.

• Inhalation serves multiple roles, including delivering oxygen and protecting the respiratory system.

Passage of Air in the Pulmonary System

• Air Pathway: Nasal cavity → Pharynx (throat) → Larynx (voice box) → Trachea.

• Trachea Function: Channels inhaled air to the lungs and exhaled air out of the body.

• Trachea Structure:

  • Cylinder approximately 10-12 cm long and 2 cm in diameter.

  • Located in front of the esophagus, extending from the larynx into the chest cavity.

  • Divides into two primary bronchi at the midthorax.

  • Made of incomplete rings of hyaline cartilage and smooth muscle.

  • Lined with mucus-producing goblet cells and ciliated pseudo-stratified epithelium.

• Cilia Function: Propel foreign particles trapped in mucus toward the pharynx.

• Cartilage Function: Provides strength and support to keep the trachea open.

• Smooth Muscle Function: Contraction decreases the trachea's diameter, causing forceful exhalation to expel mucus during coughing.

• Smooth muscle contraction and relaxation respond to external stimuli and the body's nervous system.

Lungs: Bronchi and Alveoli

• Lung Division: The trachea bifurcates into the right and left lungs.

• Lung Size: The right lung is larger with three lobes, while the left lung has two lobes.

• Diaphragm: The muscular diaphragm facilitates breathing and is inferior to the lungs, marking the end of the thoracic cavity.

Flow of Air into the Lungs

• Air is directed into smaller passages or bronchi within the lungs.

• Two primary (main) bronchi channel air into the lungs.

• Bronchial Divisions: Primary bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles.

• Bronchi Structure: Made of cartilage and smooth muscle, similar to the trachea.

  • In bronchioles, cartilage is replaced by elastic fibers.

• Bronchi Innervation: Nerves from the parasympathetic and sympathetic nervous systems control muscle contraction and relaxation.

• Respiratory bronchioles, smaller than 0.5 mm in diameter, lack cartilage and depend on inhaled air for support.

• The relative amount of smooth muscle increases as passageways decrease in diameter.

Alveoli Structure and Function

• Terminal bronchioles subdivide into respiratory bronchioles, which further subdivide into alveolar ducts.

• Alveolar ducts are surrounded by numerous alveoli and alveolar sacs.

• Alveolar sacs resemble bunches of grapes at the end of bronchioles.

• Alveoli Characteristics:

  • Approximately 100 alveolar sacs per duct, each containing 20-30 alveoli.

  • Alveoli are 200 to 300 microns in diameter.

  • Gas exchange occurs exclusively in alveoli.

  • Made of thin-walled parenchymal cells (typically one-cell thick).

  • In direct contact with capillaries (one-cell thick) of the circulatory system.

• Gas Exchange Process: Oxygen diffuses from alveoli into the blood, and carbon dioxide diffuses from the blood into alveoli to be exhaled.

• Alveoli Quantity: Approximately 300 million alveoli per lung, providing a sponge-like consistency.

• Surface Area: The surface area of alveoli in the lungs is approximately 75 m2.

• This large surface area and thin walls facilitate efficient gas diffusion.

Alveoli Structure and Function

• Alveoli Structure: Each alveolus is a hollow, cup-shaped cavity surrounded by capillaries.

• Distance Between Alveoli and Capillaries: Approximately 0.2 μm due to the thin walls.

• Alveolar Cells: Walls are lined with simple squamous epithelium cells.

• Connective Tissue: A thin layer (0.0001 mm thick) underlies and supports the alveolar cells.

• Other Cells: Septal cells and macrophages are also found inside alveoli.

Alveolar Fluid and Surfactant

• Septal Cells: Produce alveolar fluid that coats the inside surface of the alveoli, acting as a surfactant.

• Functions of Surfactant:

  1. Moistens the alveoli.

  2. Maintains the elasticity of the lungs.

  3. Prevents alveolar walls from collapsing.

  4. Lowers surface tension, reducing the effort needed to breathe in and inflate the lungs.

  5. Speeds up the transport of $O<em>2$ and $CO</em>2$ between air and the alveolar lining.

  6. Helps kill bacteria that reach the alveoli.

• Surfactant is continuously secreted and reabsorbed in healthy lungs.

• Production begins in the fetus at 23 weeks of gestation.

• Premature babies are at risk of respiratory distress syndrome due to surfactant deficiency, leading to breathing difficulties and potentially premature death.

The Pleura

• Pleura Structure: Double-layered serous membranes surrounding each lung.

• Layers:

  • Parietal pleura: Outer layer attached to the walls of the thoracic cavity.

  • Visceral pleura: Inner layer covering the outer surface of the lungs.

• Pleural Cavity: Hollow space between the parietal and visceral pleura, allowing lung expansion during inhalation.

• Serous fluid, secreted by the pleural membranes, lubricates the cavity to prevent irritation of the lungs during breathing.

Gaseous Exchange in the Alveoli

• Gaseous exchange primarily occurs in the alveoli.

• Blood serves as the transport medium for oxygen and carbon dioxide between the lungs and body cells.

• Gases are exchanged between the air and blood in the alveoli.

• Alveoli are closely connected to a vast network of blood capillaries, with each alveolus having its own blood supply.

Gas Exchange Process

• Gases dissolve in the fluid on the cell surface membrane.

• Gases diffuse through the thin walls of the alveolus and neighboring capillaries into the blood.

• Oxygen enters the blood, and carbon dioxide leaves the blood and diffuses into the alveolus.

• Oxygen passes into the blood plasma and combines with hemoglobin in red blood cells to form oxyhemoglobin.

Key Aspects of Gas Exchange

• The site of gas exchange in mammals is the epithelium of the alveoli.

• A diffusion gradient must be maintained at the alveolar surface to ensure a constant oxygen supply.

• Alveoli Adaptations for Efficient Diffusion:

  • Large surface area due to a large number of alveoli.

  • Short diffusion distance due to thin alveolar and capillary walls.

  • Presence of surfactant to prevent alveolar collapse during exhalation.

  • Steep diffusion gradient maintained by ventilation.

Exchange and Transport of Gases

• Diffusion primarily accounts for the exchange of gases between the air in the alveoli and the blood in the pulmonary capillaries.

Transport of Oxygen

• Most oxygen entering the blood combines with hemoglobin (Hb) to form oxyhemoglobin ($HbO_2$).

• $Hb + O<em>2 \rightarrow HbO</em>2$

• Oxygen is transported in blood:

  1. Bound to hemoglobin (98.5% of all $O_2$).

  2. Dissolved in Plasma (1.5%).

• The relationship between the concentration (partial pressure) of $O_2$ and Hb saturation (the percentage of Hb molecules carrying oxygen) is critical.

Hemoglobin (Hb)

• Hemoglobin is an intracellular protein and the primary vehicle for transporting oxygen in the blood.

• Oxygen is also carried in plasma but to a much lesser extent.

• Hemoglobin is contained in erythrocytes (red blood cells).

• Oxygen bound to hemoglobin is released into or absorbed from body tissue based on certain conditions.

Hemoglobin and Oxygen Association

• The iron in Hb forms a loose association with $O_2$.

• The combination of $O<em>2$ and Hb to form $HbO</em>2$ occurs when the concentration of $O_2$ is high, such as in the lung alveolar capillaries.

• When the concentration of $O<em>2$ is low, the bond holding $HbO</em>2$ becomes unstable, and $O_2$ is released, diffusing into surrounding cells.

• Dissociation: Release of $O_2$ from Hb.

• $HbO<em>2 \rightarrow Hb + O</em>2$

Hemoglobin Molecule Structure

• Each hemoglobin molecule has a limited capacity for holding oxygen molecules.

• Oxygen saturation is the amount of capacity filled by oxygen bound to hemoglobin at any time.

• Expressed as a percentage, it is the ratio of the amount of oxygen bound to the hemoglobin to the oxygen-carrying capacity of the hemoglobin.

• The oxygen-carrying capacity is determined by the amount of hemoglobin present in the blood.

Oxygen Binding and Partial Pressure

• The amount of oxygen bound to hemoglobin is related to the partial pressure of oxygen to which the hemoglobin is exposed.

• In the lungs, at the alveolar-capillary interface, the partial pressure of oxygen is typically high; therefore, oxygen readily binds to hemoglobin.

• As blood circulates to other body tissues where the partial pressure of oxygen is less, the hemoglobin releases oxygen because it cannot maintain its full bound capacity in the presence of lower oxygen partial pressures.

Oxyhemoglobin Dissociation Curve

• Shows the extent to which the Hb in blood is combined with $O<em>2$, depending on the partial pressures of $O</em>2$ of the blood.

• The amount of $O<em>2$ that can combine with Hb is determined by the $O</em>2$ concentration or partial pressure.

• $O_2$-binding characteristics of Hb can be studied by examining the oxyhemoglobin dissociation (saturation) curve.

• This curve indicates the relationship between $O_2$ levels and Hb saturation.

Factors Affecting the Dissociation Curve

• The curves show the percentage of $O<em>2$-binding sites of Hb that are carrying $O</em>2$ at various $O<em>2$ partial pressures ($PO</em>2$).

• As the $PO<em>2$ decreases, Hb gives up its $O</em>2$ more easily.

• Higher temperature and higher acidity both promote this effect.

• At normal partial pressures of $O<em>2$ in the lungs, Hb becomes practically saturated with $O</em>2$, but at the $PO<em>2$ in the tissues, oxyhemoglobin quickly gives up much of its $O</em>2$. $HbO \rightleftharpoons Hb + O_2$

Bohr Effect

• In tissues with increased partial pressure of $CO<em>2$, the $O</em>2$-dissociation curve shifts to the right.

• This shift has a physiological advantage, facilitating the release of $O_2$ from Hb.

• Where $CO<em>2$ concentration is higher (i.e., in respiring tissues), $O</em>2$ is released readily; where $CO<em>2$ concentration is low (i.e., at the respiratory surfaces), $O</em>2$ is taken up readily by Hb.

• The Bohr Effect allows for enhanced unloading of oxygen in metabolically active peripheral tissues, such as exercising skeletal muscle.

Impact of Muscle Activity

• Increased skeletal muscle activity results in localized increases in the partial pressure of carbon dioxide, which in turn reduces the local blood pH.

• This results in enhanced unloading of bound oxygen by hemoglobin passing through the metabolically active tissue due to the Bohr Effect, thus improving oxygen delivery.

• The Bohr Effect enhances oxygen delivery proportionally to the metabolic activity of the tissue.

Metabolism and Oxygen Unloading

• As more metabolism takes place, the carbon dioxide partial pressure increases, causing larger reductions in local pH, allowing for greater oxygen unloading.

• This is especially true in exercising skeletal muscles, which may also release lactic acid, further reducing local blood pH and enhancing the Bohr Effect.

Bohr and Haldane Effects

Factors affecting Oxygen-Hemoglobin Dissociation Curve

• Factors favoring rightward shift include:

  • Increased temperature

  • Increased $P<em>{CO</em>2}$ (Bohr effect)

  • Increased $H^+$ ion concentration (pH)

• Reduces Hb affinity for $O<em>2$, thus improves $O</em>2$ unloading.

• Factors favouring leftward shift ie. Increases High affinity of Hb for $O_2$ include:

  • Decreased temperature

  • Decreased $P<em>{CO</em>2}$

  • Decreased $H^+$ ion concentration ( pH)

  • 2, 3 diphosphoglycerate (2,3-DPG)

Fetal Hemoglobin (Hb F)

• Fetal hemoglobin (Hb F) more avidly binds oxygen than adult hemoglobin (Hb A).

• This binding of oxygen shifts the oxygen-hemoglobin disassociation curve to the left, and less oxygen is unloaded.

• HbF has a greater affinity for $O<em>2$ than HbA, enabling it to obtain $O</em>2$ from the mother's Hb in the placenta.

• At birth, the production of HbF gives way to that of the adult type (HbA).

• The larger affinity for $O<em>2$ of HbF compared with HbA facilitates $O</em>2$ transfer from the mother to the fetus.

Factors Affecting the Oxygen-Hemoglobin Dissociation Curve

Factors Favoring Leftward Shift (Increased Affinity of Hb for O2)

• Decreased temperature

• Decreased $P<em>{CO</em>2}$

• Increased pH

Factors Favoring Rightward Shift (Decreased Affinity of Hb for O2)

• Increased temperature

• Increased $P<em>{CO</em>2}$ (Bohr effect)

• Decreased pH

• Increased 2,3-diphosphoglycerate (2,3-DPG)

Transport of Carbon Dioxide

• Gaseous $CO_2$ is produced in large quantities by metabolically active tissues and diffuses into the bloodstream, where it is transported to the lungs for elimination.

• It must not be allowed to accumulate in the body because it forms an acid in solution and could lead to fatal changes in blood pH.

• $CO_2$ is transported in three main ways:

  1. Dissolved in the plasma forming aqueous solution (5%).

  2. In combination with Hb as carbaminoHb (10%).

     • $Hb-NH<em>2 + CO</em>2 \rightleftharpoons Hb-NHCOO + H^+$

  3. As bicarbonate ($HCO_3^-$) (85%).

     • $H<em>2O + CO</em>2 \rightleftharpoons H<em>2CO</em>3 \rightleftharpoons H^+ + HCO_3^-$

• The $O_2$ so released diffuses out of the RBC, through the capillary wall and tissue fluid into a respiring tissue cell.

• The $HCO<em>3^-$ ion diffuses out of the RBC into the plasma, where it combines with $Na^+$ ions from the dissociation of $NaCl$ to form $NaHCO</em>3$.

• Most of the $CO<em>2$ is transported in the form of $NaHCO</em>3$ to the lungs.

• In the lungs, the process is reversed, $CO_2$ is released and diffuses out of the body.

• Reversal of these reactions in the lungs is enhanced by the presence of high oxygen partial pressures.

Chloride Shift

• The loss of negatively charged $HCO_3^-$ ions from RBC is balanced by the inward diffusion of negative $Cl^-$ ions from the dissociation of $NaCl$.

• In this way, the electrochemical neutrality of the RBC is restored.

• This is known as the Chloride shift.

• Question: Summarize how $CO<em>2$ in the blood is expelled as gaseous $CO</em>2$ by the lungs.

Adaptations of O2 Uptake by High Altitude Dwellers

• High altitude dwellers include mountaineers or mountain climbers.

• The amount of $O_2$ in the atmosphere is the same at high altitude as it is at sea level, namely 21%.

• The respiratory problems associated with living at high altitude are a result of reduced atmospheric pressure.

• The reduced pressure means that it is more difficult to load Hb with $O_2$.

• Above about 6000M, the pressure is inadequate to load Hb effectively.

Acclimatization/Adaptation Factors Include:

• Adjustment of blood pH:

  • The reduced loading of the Hb leads to deep breathing (Hyperventilation) in order to compensate for lack of $O<em>2$ in the blood. This leads to excessive removal of $CO</em>2$ and a raised blood pH.

• Increased rate and depth of Breathing

  • In order to take in more $O<em>2$ and to have broader arteries and capillaries thereby allowing much higher rates of blood flow and subsequently greater amounts of $O</em>2$ delivered to their muscle.

• Increased $O_2$ uptake

  • More $O_2$ is absorbed by the lungs as a result of an improved capillary network in the lungs, and a deeper breathing.

• Improved transport of $O_2$ to the tissues

  • This is the result of:

    • Increased RBCs conc'n and capillaries to carry more $O_2$From 45% to 60% of the total blood volume

    • Increased Hb conc'n in the RBCs- by 20%

• Increased myoglobin levels in muscles.

  • With its high affinity for $O<em>2$, this facilitates the exchange of $O</em>2$ from the blood to the tissues.

• Increased lung size to facilitate diffusion of $O<em>2$ and $CO</em>2$

• Increased vascular network of muscle to enhance the transfer of gases.

• Even though $PO_2$ differs by 20 mmHg there is almost no difference in hemoglobin saturation.