GAS exchange
Gas Exchange
Learning Objectives
Define gas exchange:
The uptake of O2 from the environment and the excretion of CO2 to the environment.
Understand the partial pressure of respiratory gases:
Partial pressure affects the diffusion of gases from the lungs to the blood and from the blood to the tissues.
Describe the transport mechanisms for O2 and CO2 in blood:
Different forms in which these gases are carried in the blood.
Understand the oxygen dissociation curve:
Analyze how changes in partial pressure (of O2 or CO2), pH, and activity affect hemoglobin saturation and its affinity for oxygen.
Understand regulation of breathing:
The physiological control of respiration in response to internal and external factors.
Gas Exchange Overview
Definition:
Gas exchange refers to the uptake of O2 from the environment and the excretion of CO2 back into the environment.
Diffusion Process:
Movement of respiratory gases occurs down a partial pressure gradient:
At the Lungs:
O2 moves from the lungs into arterial blood.
CO2 moves from venous blood into alveoli.
At the Tissues:
O2 moves from arterial blood into cells.
CO2 moves from cells into venous blood.
Partial Pressure
Definition:
Partial pressure (Pp) is the pressure exerted by a particular gas in a mixture of gases.
Applies to gases dissolved in liquids such as water.
Diffusion Mechanism:
A gas diffuses from a region of higher partial pressure to a region of lower partial pressure, driven by a partial pressure gradient.
Gas exchange occurs in the alveoli of the lungs through this mechanism.
Anatomy Summary of the Respiratory System
Structure:
The trachea branches into two primary bronchi.
Components:
Larynx
Trachea with cartilage rings.
Primary bronchi which further divide into additional bronchi and terminate in alveoli.
Oxygen concentration ( extit{PO2}):
Highest in the environment, decreases as it progresses through the respiratory system and blood vessels.
Path of Airflow:
Inhaled air → airways → alveolar air → arterial blood in pulmonary capillaries → blood in tissue capillaries.
In tissues, cells perform cellular respiration, consuming O2 and producing CO2 which diffuses into the blood.
Transport of Oxygen
Solubility in Blood:
The solubility of O2 in blood is very low.
A minimal amount of O2 is transported dissolved in blood.
Role of Respiratory Pigments:
Respiratory pigments (e.g. hemoglobin) are proteins that significantly enhance the oxygen-carrying capacity of blood.
Hemoglobin Composition:
Consists of four protein chains, containing a heme group with iron.
Each hemoglobin can bind up to 4 O2 molecules through the iron in the heme groups.
Transport of Carbon Dioxide (CO2)
Routes of CO2 Transport:
Dissolved in blood (7%).
Bound to hemoglobin (23%).
Dissolved in plasma as bicarbonate ion (HCO3⁻, 70%).
Chemical Reactions Involved:
In red blood cells:
CO2 + H2O
ightleftharpoons H2CO3
ightleftharpoons H^+ + HCO3^-Carbonic acid (H2CO3) dissociates to release hydrogen ions ($H^+$) and bicarbonate ($HCO3^-$).
Hemoglobin Oxygen Saturation Curve
Axes Description:
X-axis: Partial pressure of O2 in the blood.
Y-axis: Percent of Hb molecules that are bound to oxygen.
Key Observations:
At the lungs, hemoglobin is fully saturated with O2.
Saturation decreases at the tissues where O2 is delivered.
Effects of pH on the Hemoglobin Oxygen Dissociation Curve
Bohr Effect:
The change in hemoglobin oxygen affinity due to pH variations.
Low pH (acidosis) → right shift in the curve → decreased hemoglobin oxygen affinity.
High pH (alkalosis) → left shift in the curve → increased hemoglobin oxygen affinity.
Effects of Temperature on Hemoglobin Oxygen Affinity
Temperature Influence:
An increase in temperature results in a decrease in hemoglobin's oxygen affinity (right shift).
Cells produce heat during metabolism; higher temperatures favor the unloading of oxygen to tissues.
Oxygen Transfer to Tissues
Unbinding Process:
For oxygen to enter cells, it must first unbind from hemoglobin due to a partial pressure gradient favoring diffusion into tissues.
Hemoglobin oxygen saturation is lower at tissue level compared to the lung level.
Regulation of Breathing
Chemoreceptor Function:
Central chemoreceptors monitor CO2 levels in cerebrospinal fluid to help regulate respiration.
Peripheral chemoreceptors (located in carotid and aortic bodies) monitor levels of CO2, O2, and hydrogen ions ($H^+$).
Hypoventilation Consequences:
Decreasing breathing increases CO2 levels leading to increased acidity (acidosis).
Increasing breathing decreases CO2 levels leading to decreased acidity (alkalosis).
The lung’s role in regulating pH disturbances is significant, managing about 75% of such issues rapidly.
Gas exchange is the vital physiological process by which your body takes in oxygen (O2) from the environment and releases carbon dioxide (CO2) back into it. This process is driven by differences in partial pressure, which is the pressure exerted by a particular gas in a mixture. Gases always diffuse from an area of higher partial pressure to an area of lower partial pressure.
In your lungs, specifically in the alveoli, O2 from the inhaled air (high PO2) diffuses into your arterial blood (lower PO2). Simultaneously, CO2 from your venous blood (high PCO2) diffuses into the alveoli (lower PCO2) to be exhaled.
Once in the arterial blood, O2 is transported throughout the body. Because oxygen has very low solubility in blood, most of it binds to a special protein called hemoglobin (Hb) found in red blood cells. Each hemoglobin molecule can bind up to four O2 molecules, thanks to its iron-containing heme groups. At the tissues, where cells are consuming O2 for cellular respiration, the PO2 is lower than in the arterial blood, causing O2 to unbind from hemoglobin and diffuse into the cells.
Carbon dioxide, a waste product of cellular respiration, is transported in the blood in three main ways:
About 7% is dissolved directly in the blood plasma.
Roughly 23% binds to hemoglobin.
The majority, about 70%, is transported as bicarbonate ions (HCO3^-) in the plasma. This involves a crucial reaction in red blood cells:
CO2 + H2O \rightleftharpoons H2CO3 \rightleftharpoons H^+ + HCO3^-
This reaction shows that CO2 combines with water to form carbonic acid (H2CO3), which then dissociates into a hydrogen ion (H^+) and a bicarbonate ion (HCO3^-).
Hemoglobin's affinity for oxygen isn't constant; it's influenced by several factors, as illustrated by the oxygen dissociation curve. This curve plots the percentage of hemoglobin saturated with oxygen against the partial pressure of O2. Key factors affecting it include:
pH (Bohr Effect): A lower pH (more acidic conditions, like those found in active tissues due to CO2 production) causes the curve to shift to the right, decreasing hemoglobin's affinity for O2 and promoting its release to tissues. Conversely, a higher pH increases affinity.
Temperature: Increased temperature (also common in active, metabolizing tissues) similarly shifts the curve to the right, reducing affinity and facilitating oxygen unloading.
Your breathing is tightly regulated to maintain appropriate O2 and CO2 levels and, consequently, pH balance. Chemoreceptors play a crucial role:
Central chemoreceptors in your brain monitor CO2 levels in the cerebrospinal fluid.
Peripheral chemoreceptors in your carotid and aortic arteries monitor levels of O2, CO2, and H^+ ions.
For example, if you hypoventilate (decrease breathing), CO2 levels in your blood rise, leading to increased acidity (acidosis). Conversely, hyperventilation (increased breathing) lowers CO2 and reduces acidity (alkalosis). Your lungs are highly effective at managing these pH disturbances, handling about 75% of them rapidly.