Transport of respiratory gases Option D.6 (AHL)

U: Oxygen dissociation curves show the affinity of hemoglobin for oxygen Partial pressure is pressure exerted by a single type of gas when it is found within mixture of gases The partial pressure of a given gas will depend on: The concentration of the gas in the mixture (e.g. O2 levels may differ in certain environments) The total pressure of the mixture (air pressure decreases at higher altitudes)

Haemoglobin is composed of four polypeptide chains, each with an iron-containing heme group capable of reversibly binding oxygen As each oxygen molecule binds, it alters the conformation of haemoglobin, making it easier for others to be loaded (cooperative binding) Conversely, as each oxygen molecule is released, the change in haemoglobin makes it easier for other molecules to be unloaded

Oxygen dissociation curves show the relationship between the partial pressure of oxygen and the percentage saturation of oxygen carrying molecules At low O2 levels (i.e. hypoxic tissues) percentage saturation will be low, while at high O2 levels (e.g. in alveoli) molecules will be fully saturated Because binding potential increases with each additional oxygen molecule, haemoglobin displays a sigmoidal (S-shaped) dissociation curve 

Dissociation curves displays a typical sigmoidal shape (due to cooperative binding) There is low saturation of oxygen when partial pressure is low (corresponds to environment of the tissue, when oxygen is released) There is high saturation of oxygen when partial pressure is high (corresponds to environment of the alveoli, when oxygen is taken up)

U: Carbon dioxide is carried in solution and bound to hemoglobin in the blood CO2 is carried in 3 forms in blood: Dissolved as CO2 - (5%) Bound to hemoglobin - (10%) Bicarbonate ions (HCO-3) - (85%)

Example data based question a) Calculate the percentage of CO2 found as bicarbonate ions in the plasma of venous blood at rest. [1] (b) (i) Compare the changes in total CO2 content in the venous plasma due to exercise. [1] (ii) Identify which form of transport shows the greatest increase due to exercise. [1] (c) Explain the pH differences shown in the data. [3]

Answers a) Calculate the percentage of CO2 found as bicarbonate ions in the plasma of venous blood at rest. [1] 93 % ± 1 % (b) (i) Compare the changes in total CO2 content in the venous plasma due to exercise. [1] increases by 0.63 mmol/ l of blood / rises from 15.59 to 16.22 mmol/ l blood (ii) Identify which form of transport shows the greatest increase due to exercise. [1] dissolved CO2 (c) Explain the pH differences shown in the data. [3] CO2 makes the blood more acidic and the pH drops; pH of venous blood at rest has decreased compared to arterial blood; because the blood is carrying waste CO2 (from cellular respiration) back to lungs for removal; pH of venous blood after exercise has decreased compared to arterial blood; and dropped even further than venous blood at rest; because the blood is carrying more waste CO2 than normal due to exercise;

U:CO2 is transformed in RBCs into hydrogencarbonate (bicarbonate) ions In tissues, where CO2 is generated, rxn proceeds to right, lowering pH of blood In lungs, where CO2 leaves blood, rxn proceeds to left

U: The Bohr shift explains the increased release of oxygen by hemoglobin in respiring tissues Exercise increases CO2 CO2 lowers pH of blood which causes hemoglobin to release its O2 This is known as the Bohr effect – when pH is lower than normal, hemoglobin does not bind O2 as well Shifts the O2 dissociation curve to right Hemoglobin's oxygen binding affinity is inversely related both to acidity and to the concentration of CO2 Hence more O2 is released at the same partial pressure of oxygen, ensuring respiring tissues have enough O2 when their need is greatest

U:During exercise the rate of ventilation changes in response to the amount of CO2 in blood During exercise metabolism increases, O2 becomes limited & there is build up of CO2 and lactic acid in blood This lowers blood pH, which is detected by chemoreceptors in the carotid artery and the aorta These chemoreceptors send impulses to the breathing center in the brain stem (medulla) to increase rate of respiration Impulses are sent to diaphragm and intercostal muscles to change the rate of muscular contraction, hence changing the rate of breathing This process is under involuntary control (reflex response) – as breathing rate increases, CO2 levels in the blood will drop, restoring blood pH Long term effects of continual exercise include an improved vital capacity

U: The rate of ventilation is controlled by the respiratory control centre in medulla oblongata 2 sets of nerves travel to lungs from medulla: Intercostal nerves – stimulate intercostal muscles (between ribs) Phrenic nerves stimulate diaphragm When lungs expand, stretch receptors in chest wall & lungs send signal to medulla which stops signals to inhale until animal exhales, then new signal is sent.

U: Chemoreceptors are sensitive to changes in blood pH If increase in blood CO2 or drop in blood pH is detected, chemoreceptors in carotid artery & aorta send message to medulla oblongata Nerve impulses sent from medulla to diaphragm and intercostal muscles, increasing ventilation rate Leads to increased rate of gas exchange

U: Fetal hemoglobin is different from adult hemoglobin allowing transfer of O2 in placenta onto the fetal hemoglobin Fetal hemoglobin has higher affinity for O2 at all partial pressures than adult hemoglobin Hb-F can carry up to 30% more O2 Maternal blood’s O2 readily transferred to fetal blood

App: pH of blood is regulated to stay within the narrow range of 7 App: pH of blood is regulated to stay within the narrow range of 7.35 to 7.45 If too acidic, chemoreceptors signal to hypothalamus to increase breathing rate (“hyperventilation”) Takes CO2 from blood, raising pH In kidney, H+ ions can be secreted into urine to raise pH If hyperventilating makes blood to alkaline, can breath in a bag to re- breath your own CO2 to lower pH

Skill: Analysis of dissociation curves for hemoglobin & myoglobin Myoglobin = special O2 transport protein in muscles Only has one subunit (hemoglobin has 4) Much higher affinity for O2 than hemoglobin Only releases O2 when partial pressure is low (like during heavy exercise) When graph plateaus, myoglobin is saturated with O2, hemoglobin is giving up O2

App: consequences of high altitude for gas exchange High altitude = low pO2 in air Hemoglobin may not become fully saturated, so cells may not be adequately supplied with O2 Human body can adapt: RBCs increase Breathing rate increases Myoglobin increases Lung surface area increases Vital capacity increases Oxygen dissociation curve shifts to right, encouraging release of O2 into cells

NOS: Scientists have a role in informing the public: scientific research has led to a change in public perception of smoking Early 20th century, believed smoking improved ventilation; docs prescribed smoking of medicine for asthma 1930’s & 1940’s, smoking very popular Also rising concerns about smoking Tobacco companies advertised docs & scientists smoking 1964 US Surgeon General warned about link of smoking to bronchitis & cancer Smoking is on a decline; half of smokers have quit Public health depts. Have pushed for policy measures due to scientific evidence

App: Causes & treatments of emphysema Emphysema = walls between alveoli break down, increase their size, reduction in SA for gas exchange, less O2 uptake in blood Cause: long-term exposure to airborne irritants, especially tobacco smoke, air pollution, coal & silica dust; or lack of enzyme alpha-1-antitrypsin (A1AT) Barrel chest = lungs trapped in inspiration position Treatment = O2 therapy, breathing techniques, quitting smoking, meds, surgery, lung transplat

Skill: Identification of pneumocytes, capillary endothelium cells and blood cells in light micrographs & electron micrographs of lung tissue Type 1 pneumocytes = thin lung cells 90% of alveolus surface Function: gas exchange Type 2 pneumocytes = thicker lung cells Covered in microvilli Function: secrete surfactant, which reduces surface tension, preventing alveoli from sticking and lung collapsing