Respiratory system- Gas exchange in the body

Important Facts

• The body consumes about 250 mL of oxygen per minute and produces 200 mL of carbon dioxide.

• An average adult male inhales about 6000 mL of air per minute, but only 4200 mL reaches the alveoli.

• Out of the air that reaches the alveoli, only 882 mL is oxygen, and only 250 mL of this oxygen diffuses into the blood. 21%

• Body consumes 360,000ml of oxygen per day

What is Internal Respiration?

Internal respiration refers to the exchange of gases between the blood in the systemic capillaries and the cells or tissues of the body. This process allows oxygen to be delivered to the cells and carbon dioxide to be removed from them.

What is External Respiration?

External respiration is the exchange of gases between the alveoli in the lungs and the blood. This process involves oxygen diffusing from the alveoli into the blood and carbon dioxide diffusing from the blood into the alveoli to be exhaled.

What is Partial Pressure and Oxygen Saturation?

Partial pressure is the concentration of a specific gas in a mixture of gases, expressed as a value. Oxygen saturation refers to the percentage of hemoglobin molecules in the blood that are bound to oxygen.

General Mechanism of Gas Exchanges in the Body (Internal and External)

• Gases move in and out of the lungs by breathing

• Oxygen and carbon dioxide are exchanged in the lungs via diffusion

• Gases dissolved in the blood are transported by the circulatory system

• Oxygen and carbon dioxide are exchanged in the tissues by diffusion

• Oxygen moves in carbon dioxide moves out

• External Respiration: Oxygen from inhaled air diffuses through the alveolar walls into the blood, while carbon dioxide diffuses from the blood into the alveoli to be exhaled.

• the air in the alveoli has a high PO2 and a low PCO2

• the blood in the pulmonary capillaries, which have come from the body has a low PO2 and a high PCO2

• the blood that returns to the heart now has a high PO2 and a low PCO2 and is pumped by the left ventricle into systemic circulation.

• Internal Respiration: Oxygen diffuses from the blood in the systemic capillaries into the body cells, while carbon dioxide diffuses from the cells into the blood.

• the arterial blood that reaches systemic capillaries has a high PO2 and a low PCO2.

• the body cells and tissue fluid have a low PO2 and a high PCO2 because cells continuously use oxygen in cell respiration(producing energy) and produce carbon dioxide in this process.

• therefore oxygen diffuses from the blood to tissue fluid and carbon dioxide diffuses from tissue fluid to blood.

• the blood that enters the systemic veins to return to the heart now has a low PO2 and a high PCO2 and is pumped by the right ventricle to the lungs to participate in external respiration

How is Oxygen Transported in the Blood and Hemoglobin?

Oxygen is transported in the blood primarily by binding to hemoglobin molecules in red blood cells. Each hemoglobin molecule can bind up to four oxygen molecules, forming oxyhemoglobin.

• Every liter of arterial blood contains about 200ml of oxygen, approx 3ml is dissolved in the plasma or in the cytosol of the erythrocytes. only the dissolved oxygen contributes to PO2 in blood

• The remaining 197ml is transported bound to a haemoglobin molecule.

• The bound oxygen does to contribute to the PO2, it is in equilibrium with the dissolved oxygen - the amount of oxygen bound to haemogolbin is a function of the PO2.

• Haemoglobin

• 4 subunits- each containg a globular polypeptide chain and a heme group containg iron

• each heme group can bind to one oxygen- each haemoglobin molecule can carry four oxygen molecule

• oxygen binds reversibly

What is the Oxygen Dissociation Curve and the Different Effects?

The oxygen dissociation curve is a graph that shows the relationship between the partial pressure of oxygen and the oxygen saturation of hemoglobin. It is typically S-shaped, indicating that hemoglobin's affinity for oxygen increases as more oxygen molecules bind to it. Factors such as pH, temperature, and carbon dioxide levels can shift the curve, affecting oxygen binding and release.

Oxygen Saturation and Partial Pressure

- Oxygen Saturation: The saturation of oxygen in the blood is a function of the partial pressure of oxygen.

- Higher Partial Pressure: Leads to more oxygen saturation in the blood.

- Arterial Blood: Oxygenated blood with a higher partial pressure of oxygen (~100 mm Hg), resulting in hemoglobin being about 98.5% saturated.

- Venous Blood: Deoxygenated blood with a lower partial pressure of oxygen (~40 mm Hg), resulting in hemoglobin being about 75% saturated.

Hemoglobin and Oxygen Binding

- Hemoglobin: Each hemoglobin molecule can carry four oxygen molecules.

- Higher Partial Pressure: Results in higher saturation of hemoglobin with oxygen.

- Lower Partial Pressure: Results in lower saturation of hemoglobin with oxygen.

Hemoglobin-Oxygen Dissociation Curve

- Curve Description: The relationship between the partial pressure of oxygen and the percentage of oxygen saturation of hemoglobin is depicted in the hemoglobin-oxygen dissociation curve.

- Affinity Increase: As one molecule of oxygen binds to hemoglobin, it increases the affinity of hemoglobin to bind more oxygen molecules.

- Curve Shift:

- Leftward Shift: Indicates increased affinity and higher oxygen saturation, occurring in systemic veins.

- Rightward Shift: Indicates decreased affinity, necessary for oxygen unloading into tissues, occurring in systemic arteries.

- Curve Shape: The curve is sigmoid (S-shaped), not linear. It shows a steep increase in saturation at lower partial pressures and flattens at higher partial pressures.

Temperature Effect on Dissociation Curve

- Increased Temperature:

- Causes the curve to shift to the right.

- Decreases the affinity of hemoglobin for oxygen.

- Facilitates the unloading of oxygen into the tissues.

- This is beneficial during exercise or fever when tissues require more oxygen.

- Decreased Temperature:

- Causes the curve to shift to the left.

- Increases the affinity of hemoglobin for oxygen.

- Makes it harder for oxygen to be released into the tissues.

- This can occur in cold environments or during hypothermia.

The rightward shift at higher temperatures ensures that more oxygen is released to meet the increased metabolic demands of tissues, while the leftward shift at lower temperatures helps conserve oxygen when metabolic demands are lower.

Effects of pH on the Hemoglobin-Oxygen Dissociation Curve

- Decreased pH (Acidosis):

- Causes the curve to shift to the right.

- Decreases the affinity of hemoglobin for oxygen.

- Facilitates the unloading of oxygen into the tissues.

- This is beneficial in conditions where tissues are producing more carbon dioxide and lactic acid, such as during intense exercise.

- Increased pH (Alkalosis):

- Causes the curve to shift to the left.

- Increases the affinity of hemoglobin for oxygen.

- Makes it harder for oxygen to be released into the tissues.

- This can occur in conditions where there is less carbon dioxide, such as in hyperventilation.

The rightward shift at lower pH levels ensures that more oxygen is released to meet the increased metabolic demands of tissues, while the leftward shift at higher pH levels helps conserve oxygen when metabolic demands are lower.

Carbon Dioxide Transport in Blood

- Forms of Carbon Dioxide Transport:

- CO2 in blood: 5-6% is dissolved 5-8% is bound to haemoglobin as carbaminohemoglbin

- Bicarbonate Ions (HCO3-): The majority, around 86-90%, of carbon dioxide is transported in the form of bicarbonate ions.

Formation of Bicarbonate Ions

- Carbon Dioxide Diffusion: Carbon dioxide diffuses from tissues into red blood cells.

- Carbonic Anhydrase Reaction: Inside red blood cells, carbon dioxide reacts with water to form carbonic acid (H2CO3), catalyzed by the enzyme carbonic anhydrase.

- Dissociation of Carbonic Acid: Carbonic acid quickly dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+).

- Chloride Shift: Bicarbonate ions diffuse out of red blood cells into the plasma, and chloride ions (Cl-) move into red blood cells to maintain electrical neutrality. This is known as the chloride shift.

Release of Carbon Dioxide in the Lungs

- Reverse Chloride Shift: In the lungs, bicarbonate ions re-enter red blood cells, and chloride ions move out.

- Formation of Carbonic Acid: Bicarbonate ions combine with hydrogen ions to form carbonic acid.

- Dissociation of Carbonic Acid: Carbonic acid dissociates into carbon dioxide and water.

- Carbon Dioxide Diffusion: Carbon dioxide diffuses out of red blood cells into the alveoli and is exhaled.

CO2 Summary

- Carbon dioxide is transported in three main forms: dissolved in plasma, bound to hemoglobin, and as bicarbonate ions.

- The majority of carbon dioxide is transported as bicarbonate ions, formed through the carbonic anhydrase reaction.

- The chloride shift helps maintain electrical neutrality during the transport of bicarbonate ions.

- In the lungs, the reverse chloride shift allows carbon dioxide to be released and exhaled.

Carbon Dioxide Transport and the Haldane Effect

- Haldane Effect: The Haldane effect describes how oxygenation of blood in the lungs displaces carbon dioxide from hemoglobin, increasing the removal of carbon dioxide.

- Oxygen Binding: When oxygen binds to hemoglobin, it reduces the affinity of hemoglobin for carbon dioxide and hydrogen ions.

- Deoxygenated Blood: In tissues, where oxygen levels are lower, hemoglobin has a higher affinity for carbon dioxide and hydrogen ions, facilitating the uptake of carbon dioxide.

- Oxygenated Blood: In the lungs, where oxygen levels are higher, hemoglobin releases carbon dioxide and hydrogen ions, facilitating the release of carbon dioxide into the alveoli for exhalation.

Carbon Dioxide Graph

- Graph Description: The graph illustrates the relationship between the partial pressure of carbon dioxide (PCO2) and the content of carbon dioxide in the blood.

- Deoxygenated Blood Curve: The curve for deoxygenated blood is higher, indicating that deoxygenated blood can carry more carbon dioxide.

- Oxygenated Blood Curve: The curve for oxygenated blood is lower, indicating that oxygenated blood carries less carbon dioxide.

- Effect of Oxygenation: As blood becomes more oxygenated, the ability of hemoglobin to carry carbon dioxide decreases, shifting the curve downward.

Summary

- The Haldane effect explains how oxygenation of blood affects carbon dioxide transport.

- Deoxygenated blood has a higher capacity to carry carbon dioxide, while oxygenated blood has a lower capacity.

- The carbon dioxide graph shows the relationship between PCO2 and carbon dioxide content in deoxygenated and oxygenated blood.

Control of Respiration

- Respiratory Centers: The control of respiration is primarily managed by the respiratory centers located in the brainstem, specifically the medulla oblongata and the pons.

- Medulla Oblongata: Contains the dorsal respiratory group (DRG) and the ventral respiratory group (VRG).

- Dorsal Respiratory Group (DRG): Responsible for the basic rhythm of breathing. It sends signals to the diaphragm and external intercostal muscles to initiate inspiration.

- Ventral Respiratory Group (VRG): Involved in forced breathing (both inspiration and expiration). It activates additional muscles during intense respiratory demands.

- Pons: Contains the pneumotaxic center and the apneustic center.

- Pneumotaxic Center: Regulates the rate and pattern of breathing by inhibiting the DRG, thus controlling the duration of inspiration.

- Apneustic Center: Promotes prolonged inspiration by stimulating the DRG.

Chemoreceptors

- Central Chemoreceptors: Located in the medulla oblongata, they respond to changes in the pH of cerebrospinal fluid (CSF), which is influenced by the partial pressure of carbon dioxide (PCO2) in the blood.

- Increased PCO2: Leads to a decrease in pH (more acidic), stimulating the central chemoreceptors to increase the rate and depth of breathing to expel more CO2.

- Decreased PCO2: Leads to an increase in pH (more alkaline), reducing the stimulation of central chemoreceptors and decreasing the rate and depth of breathing.

- Peripheral Chemoreceptors: Located in the carotid bodies and aortic bodies, they respond to changes in the partial pressure of oxygen (PO2), PCO2, and pH in the blood.

- Low PO2: Stimulates peripheral chemoreceptors to increase the rate and depth of breathing.

- High PCO2 and Low pH: Also stimulate peripheral chemoreceptors to enhance respiratory activity.

Mechanoreceptors

- Stretch Receptors: Located in the walls of the bronchi and bronchioles, they respond to the stretch of the lungs during inspiration.

- Hering-Breuer Reflex: Prevents over-inflation of the lungs by inhibiting the DRG when the lungs are stretched, thus ending inspiration.

Higher Brain Centers

- Cerebral Cortex: Allows voluntary control of breathing, such as holding one's breath or altering the breathing pattern.

- Hypothalamus: Can influence breathing in response to emotions, pain, and temperature changes.

Summary

- The control of respiration involves a complex interaction between the respiratory centers in the brainstem, chemoreceptors, mechanoreceptors, and higher brain centers.

- The medulla oblongata and pons play key roles in regulating the rhythm and pattern of breathing.

- Chemoreceptors monitor changes in blood gases and pH to adjust respiratory activity.

- Mechanoreceptors prevent over-inflation of the lungs, and higher brain centers allow voluntary and emotional influences on breathing.