Human Physiology: Gas Exchange
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6.4.1 Distinguish between ventilation, gas exchange, and cell respiration
- Cell Respiration
- • Occurs in the cytoplasm and mitochondria of cells and releases energy in the form of ATP for use inside the cell
- • In aerobic respiration oxygen is used and carbon dioxide is produced
- Gas Exchange
- • Process of exchanging one gas for another
- • Occurs in the alveoli of the lungs
- • Oxygen diffuses from the air in the alveoli to the blood in capillaries and carbon dioxide diffuses in the opposite direction
- • Diffusion occurs because of the concentration gradients of oxygen and carbon dioxide between the air and the blood
- • Air in the alveoli must be refreshed frequently in order to maintain the concentration gradients
- • The process of moving fresh air into the alveoli of the lungs and removing stale air
- • Air flows along air passages to the lungs
- • An active process requiring the contraction and relaxation of muscles
6.4.2 Explain the neccesity for a ventilation system
Air in the alveoli must be refreshed frequently in order to maintain the concetnration gradients
6.4.3 Describe the features of alveoli that adapt them to gas exchange
- 1. The lungs contain hundreds of millions of alveoli meaning that despite their small size they create a huge amount of surface area for gas exchange.
- 2. The wall of the alveolus is a single layer of very thin cell so that gases only have to diffuse over a very small distance.
- 3. The walls of the alveoli secrete a fluid which helps keep the inner surface moist allowing gases to dissolve. The fluid also contains a detergent to stop the sticking of walls.
- 4. They are covered by capillaries with high CO2 and low O2 concentration promoting the diffusion of O2 into the blood.
6.4.4 Draw and label diagram of the ventilation system, including trachea, lungs, bronchi, bronchioles and alveoli
6.4.5 Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by the internal and external intercostals muscles, diaphragm and abdominal muscles.
Inspiration: Achieved by increasing the space inside the thoracic cavity and therefore decreasing the air pressure inside the lungs. Air then flows toward the lower pressure. Always an active process involving muscle contractions.
- 1. External intercostals muscles contract causing the ribcage to expand and move upDiaphragm contracts and drops downwards.
- 2. Thoracic volume increases, lungs expand and the pressure inside the lungs decreases.
- 3. Air flows into the lungs in response to the pressure gradient.
Expiration: Achieved by decreasing the space inside the thoracic cavity and therefore increasing the air pressure inside the lungs, air then flows toward the lower pressure outside the lungs. During quiet breathing this is a passive process but during forced (active) breathing this is an active process involving muscle contractions.
- 1. Quiet breathing: External intercostals muscles and diaphragm relax – elasticity of the lung tissue causes recoilForced breathing: Internal intercostals muscles and abdominal muscles also contract to increase the force of the expiration.
- 2. Thoracic volume decreases and the pressure inside the lungs increases
- 3. Air flows out of the lungs in response to the pressure gradient.
H.6.1 Define partial pressure
The pressure exerted by each of the gases in a mixture of gases
H.6.2 Explain the oxygen dissociation curves of adult haemoglobin, fetal haemoglobin and myoglobin.
- • The shape of the dissociation curve is explained by the cooperation among the subunits of the Hb molecule.
- • Binding O2 to 1 subunit induces the remaining subunit to change their shape slightly so that their affinity for O2 increases.
- • Conversely when 1 subunit unloads its O¬2, the other 3 quickly follow suit as a conformational change lowers the affinity for O2.
- • Hence where the dissociation curve has a steep slope even a slight change in the partial pressure of O2 causes Hb to load or unload a substantial amount of O2.
- • The fetal hemoglobin has a higher affinity for O2 than adult hemoglobin which explains why O2 can be unloaded in the placenta from the mother’s blood to the fetus blood.
- • Myoglobin, a protein molecule similar to Hb but with a grater affinity for O2, is only a single heme group attached to a single globular protein.
- • Myoglobin acts as an oxygen store within muscles releasing the oxygen during periods of extreme muscular activity.
- • The myoglobin dissociation curve isn’t S-shaped because there is only 1 heme site so there is no co-operative binding.
H.6.3 Describe how carbon dioxide is carried by the blood, including the action of carbonic anhydrase, the chloride shift and buffering by plasma proteins.
- 1. In blood plasma
- • About 7% is dissolved in blood plasma
- 2. Bound to the hemoglobin molecule
- • About 23% is bound to amino groups of Hb forming carbaminohemoglobin
- 3. As bicarbonate (HCO3-) ions in RBCS and plasma
- • About 70% is transported this way as CO2 reacts with water using enzyme carbonic anhydrase inside the RBC
- Carbon dioxide produced by body tissues diffuses into the interstitial fluid and into the plasma. 7% remains in the plasma as dissolved CO2. The rest (70%) diffuses into red blood cells, where some (23%) is picked up and transported by hemoglobin.
- Most of the CO2 reacts with H20 in the red blood cells to form carbonic acid. Red blood cells contain the enzyme carbonic anhydrase, which catalyzes this reaction. Carbonic acid dissociates into a bicarbonate ion and hydrogen ion (H+). Hemoglobin (a plasma protein) binds most of the H+, preventing them from acidifying the blood.
- The reversibility of the carbonic acid- bicarbonate conversion also helps buffer the blood, releasing or removing H+ depending on the pH.
- Chlorine goes into the red blood cells when bicarbonate comes out. This is referred to as the chloride shift and prevents the imbalance of charge.
- Also some plasma proteins also bind to the H+ acting as a buffer.
H.6.4 Explain the role of the Bohr shift in the supply of oxygen to respiring tissues.
- At a given PO2, as pH increases (lower CO2) more O2 combines with Hb and as pH decreases (higher CO2) less O2 combines with Hb (the O2 is given off) this allows O2 to bind to Hb in the lungs (high pH)
- The oxygen dissociation curve for hemoglobin shift to the right at a lower pH --> happens because H+ binds to Hb changing its conformation so O2 binds less easily
- The difference between Hb saturation with O2 at high and low pH represents the amount of O2 released to the respiring tissues from the blood
H.6.5 Explain how and why ventilation rate varies with exercise
Basic rhythm of breathing is controlled by the respiratory centre located in the medulla oblongata
- Phrenic nerve: medulla -> diaphragm to stimulate contraction
- Intercostal nerves: medulla -> intercostal muscles
- Vagus nerve: stretch receptors in the bronchiloes and bronchi -> respiratory centre to inhibit inspiration
- 1.Chemoreceptors in medulla monitor pH of blood and slight drop in pH (increase in CO2 concentration ) stimulate the respiratory centre causing increase in the frequency of impulses down the phrenic and intercostal nerves which increases the rate of respiration and depth of breathing respiration and depth of breathing (exercise causes an increase in cellular respiration increasing CO2concentration)CO2 levels are the main factor that affects breathing. .
- 2. Chemoreceptor’s in the aorta and carotid arteries also monitor the pH of the blood and produce the same effect.
- 3. Vagus nerve triggered by stretch receptors in the bronchioles and bronchi.
- - exercise increases the rate of cellular respiration so that more CO2 is produced
- - this causes the pH of the blood to decrease slightly which is detected by chemoreceptors in the medulla, aorta and carotid arteries
- - the chemoreceptors send signals to the respiratory centre in the medulla oblongata which then increases the frequency of impulses down the phrenic and intercostal nerves causing the depth and frequency of breathing to increase
O2 concentration usually has little effect on the breathing control centre.
H.6.6 Outline the possible causes of asthma and its effects on the gas exchange system
- • Often an allergic reaction to allergens such as house dust mite feces, pollen, animal dander, and some fungi causes the release of histamines in the bronchioles which constricts the airways and causes accumulation of fluid and mucus- hygiene hypothesis (people living in very clean homes are more at risk).
- • Environmental factors such as cold air, exercise, air pollutants, viral infections and the presence of bacterium
- Effects on gas exchange
- • Narrowing of the airways and accumulations of fluid and mucus make ventilation difficult and gas exchange is reduced.
- • Sufferers show labored breathing with overexpansion of the chest cavity.
- • May have excessive coughing due to irritation of the airways.
- • Labored breathing can produce muscle ache or pain in the diaphragm and intercostals.
H.6.7 Explain the problem of gas exchange at high altitudes and the way the body acclimatizes.
- • At higher altitudes the partial pressure of oxygen decreases which can make individuals feel sick.
- • Altitude sickness is usually a mild illness resulting from high altitudes. People will get headaches, insomnia, poor appetite, nausea, vomiting, dizziness, tiredness, coughing, and breathlessness because not enough oxygen entering the body as a result of the decreased amount of oxygen.
- • Altitude sickness can lead to the accumulation of fluid in the brain (cerebral edma) and accumulation of fluid in the lungs (pulmonary edma).
- • The body will make adjustments to deal with the different altitude.
- • In order of when they take effect
- - Increased heart rate
- - Increased breathing
- - Concentration of blood
- - Increased red blood cell production
- - Increased capillary density
- - Muscles produce more myoglobin
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