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Gills and lungs are richly vascularized organs that serve for gas exchange between the environment and the circulatory system.

The lungs differentiate from gills in that they are saclike structures always internal to the organism and specialized in gas exchange in terrestrial environment. Branchiae, in their turn, are internal or external laminar structures in direct contact with water and specialized in gas exchange in aquatic environment.

The pulmonary respiration is controlled by the neural respiratory center located within the medulla (the lower part of the brain continuous to the spinal cord). 

The circulatory system has an important role in cutaneous respiration, branchial respiration and pulmonary respiration. The respiratory function of the blood is tailored for transportation of gases for exchange between tissues and respiratory surfaces in contact with the exterior (skin, gills, lungs).

A. False The stretch receptors in internal carotid arteries are carotid sinus baroreceptors; the carotid bodies are separate structures nearby. 

B. False They have the greatest flow rate/unit volume yet described in the body. 

C. True They are not excited in anaemia where P_CO2 is normal but O₂ content is low. 

D. True Acidosis stimulates ventilation. 

E. True When carotid and aortic bodies are denervated, hypoxia depresses respiration.

Terrestrial molluscs and the arachnid arthropods are the invertebrates that present pulmonary-like respiration. Some terrestrial molluscs have a mantle cavity filled with air that makes contact with richly vascularized tissues that work as rudimentary lungs. Besides their tracheal respiration some arachnids have book lungs (thin folds resembling leaves of a book) that make gas exchange.

The increase in product formation in the chemical equilibrium of the formation of bicarbonate from carbon dioxide and water heightens the concentration of hydrogen ions and thus lowers the pH of the solution.

The chemical equation of the chemical equilibrium of the formation of bicarbonate having as reagents carbon dioxide and water is as follows:

CO₂ + H₂0 --> H₂CO₃ --> H⁺ + HCO₃^-

The reaction is catalyzed by the enzyme carbonic anhydrase present in red blood cells.

The shifting of the chemical equilibrium of the formation of bicarbonate from carbon dioxide and water into the reverse reaction (production of water and carbon dioxide) means spending of hydrogen ions and thus it increases the solution pH..

In mammals the muscles that participate in the breathing process are the diaphragm and the intercostal muscles. In respiratory insufficiency other muscles can help the respiration, the muscles of the shoulders, neck, thorax and abdomen.

A. False At rest, O₂ consumption (about 250 ml/min) is about 25 per cent of the total available. 

B. True All but 3 of the 200 ml/l is combined with hemoglobin, the rest is dissolved. 

C. False In anaemic hypoxia, cardiac output rises to compensate for the reduced oxygen content per litre of blood. 

D. False If P_O2 falls by 25 per cent from normal, there is relatively little change in the blood oxygen content. 

E. False The extraction ratio rises as oxygen consumption rises, e.g. during exercise.

Insects and arachnids are the arthropod animals that make tracheal respiration. In the body surface of these animals there are many orifices called spiracles that communicate with small tubules, the tracheae, through which air penetrates and carbon dioxide is expelled. The tracheae ramify into tracheoles that reach all tissues of the animal.

In the circulatory system of insects the blood only transports nutrients; gases are independently transported by the tracheal system.

A. False Values around pH 2–3 are normal. 

B. True Pepsin is the active proteolyic form of pepsinogen. 

C. False HCl reduces trivalent ferric iron to the divalent ferrous form in which it can be absorbed in the small intestine. 

D. False Pentagastrin is a powerful pharmacological stimulant of mucosal cells to produce HCl. 

E. False But it rises markedly after H₂ blockade which blocks gastric acid secretion.

A. False It is scanty and rich in enzymes; it is secreted reflexly in the ‘cephalic’ phase of pancreatic secretion when food is thought about or chewed. 

B. False It is copious, bicarbonate rich and poor in enzymes; it buffers the acid secretions entering the duodenum from the stomach. 

C. False This is the hormone released when acid enters the duodenum that stimulates the pancreas to produce the juice described in (B) above. 

D. True Pancreatic lipase. 

E. False Pancreatic amylase breaks down carbohydrates to dextrins and polysaccharides. 

The diaphragm (exclusive of mammals) and the intercostal muscles can contract or relax varying the volume of the thorax (the compartment where the lungs are located). The changing of the thorax volume forces inhalation or expiration.

When the thorax volume is increased an internal pressure lower than the atmospheric pressure (external) is created and gases naturally enter the lungs. When the thorax volume is lowered the internal pressure rises above the external pressure and the air is expelled from the lungs.

A. True The normal value is about 0.1 litre/cm H₂O (1 litre/kPa). 

B. False It is maximal over this range of chest movement. 

C. False The surface tension of alveolar fluid decreases compliance. 

D. True This decreases the surface tension. 

E. False The slope of this line indicates compliance and is unchanged by a parallel shift.

The arch reflex depends only on the integrity of the fibers at a single spinal level. In the arch reflex the motor response to the stimulus is automatic and involuntary and does not depend upon the passage of information to the brain. So it happens even if the spinal cord is damaged at other levels.

A. Option 4 Arterial blood oxygen saturation. At high altitudes the alveolar oxygen pressure falls to levels at which blood passing through the lungs is inadequately saturated with oxygen. Therefore arterial blood oxygen saturation is seriously reduced and this is the fundamental cause of impaired delivery of oxygen at high altitudes. 

B. Option 1 Total available oxygen. This equals the oxygen in five litres of normally saturated blood with a normal haemoglobin content. 

C. Option 3 Blood haemoglobin concentration. Here the reduction in total available oxygen is reduced in proportion to the reduction in the blood haemoglobin concentration. 

D. Option 4 Arterial blood oxygen saturation. As with (A) the hypoxic hypoxia is due to inadequate saturation of blood in the lungs. However this time the cause is impaired lung function rather than reduced inspired oxygen. 

E. Option 1 Total available oxygen. This depends on cardiac output, blood oxygen capacity and arterial blood saturation percentage. Impaired cardiac output causes stagnant hypoxia, impaired blood oxygen capacity causes anaemic hypoxia and reduced arterial blood saturation causes hypoxic hypoxia.

A. True For example, when the ‘satiety’ centres are stimulated. 

B. True For example, when the ‘hunger’ centres are damaged. 

C. True Appetite is relieved after a meal before the food products of digestion are absorbed into the blood. 

D. False The drive to eat does not depend on an intact stomach.

E. False It increases as hypothalamic ‘glucostats’ detect the low glucose and excite the ‘hunger’ centres to generate the emotional drive to eat.

A. False The concentration is higher, due partly to potassium released from cast off cells. 

B. False Intestinal secretion is not under vagal control; its enzymes are thought to be constituents of the mucosal cells and released when these are cast off into the lumen. 

C. True The mucosal cell brush border contains these enzymes, e.g. maltase and lactase. 

D. False Monosaccharides are end-products of digestion and absorbed as such. 

E. True For example, enterokinase which converts trypsinogen to trypsin. 

A. False The reverse is true, since fat tissue contains little water 

B. True Fat is the main energy store of the body. 

C. True It favours survival by increasing skin insulation. 

D. False Fat has a lower specific gravity than the lean body mass. 

E. True Actuarial tables show this to be true. 

A. False For adequate mixing, pulmonary artery blood is needed. 

B. False It falls due to increased oxygen extraction by the active muscles. 

C. True Due to large volumes of well oxygenated blood returning from skin. 

D. True Cyanide blocks uptake of oxygen by tissue enzyme systems. 

E. True Oxygen extraction is increased in stagnant hypoxia.

A. True Food, foreign bodies and the dentist are effective stimuli. 

B. True E.g. lemon juice; saliva is a useful buffer to protect teeth from acid. 

C. False Thinking of appetizing food results in salivary secretion by a conditioned reflex; conversely, thinking of unappetizing food inhibits secretion. 

D. True Saliva helps to buffer the acid vomitus when it reaches the mouth. 

E. True Sympathetic stimulation produces a scanty viscid juice; parasympathetic stimulation produces a copious watery juice.

A. False Serous glands such as the parotids produce a watery juice; mucous glands such as the sublinguals produce a thick viscid juice. 

B. False The functions of salivary amylase (ptyalin) can be affected by enzymes from other digestive glands. 

C. False It is saturated with calcium ions; calcium salts are laid down as plaque on the teeth. 

D. True Saliva is an important route of iodide excretion; its concentration in saliva is 20–100 times that in plasma.

E. False Saliva has a neutral pH; acidity in the mouth tends to dissolve tooth enamel. 

A. False The curves start and finish differently. The CO₂ curve has a steep initial slope which gradually decreases, but there is no plateau. It is the O₂ curve which is sigmoid in shape. 

B. False Content continues to rise as P_CO2 rises above normal alveolar levels. 

C. False CO₂ content is zero when P_CO2 is zero. 

D. True Oxyhaemoglobin is a stronger acid than reduced haemoglobin; the liberated H⁺. ions drive the reaction H⁺ + HCO₃^- → H₂CO₃ → CO₂H₂O. 

E. False This is shown by the oxygen dissociation curve, not the carbon dioxide dissociation curve.

A. True It constitutes a physiological shunt. 

B. True This would be an anatomical shunt. 

C. False Physiological dead space is relatively over ventilated. 

D. False It would tend to raise it. 

E. False Increased ventilation cannot increase oxygen saturation; compensation for carbon dioxide retention can occur.

A. False The reflex centres lie in the medulla oblongata.

B. True This plus closure of the glottis prevents food being aspirated into the airways. 

C. True The voluntary act is propulsion of a bolus of food onto the posterior pharyngeal wall. 

D. True These are essential for the peristaltic phase. 

E. True Gravity can assist the reflex; tablets are more difficult to swallow when lying down. 

A. False Bile contains no digestive enzymes; its bile salts assist in the emulsification and absorption of fat. 

B. False The bilirubin is conjugated by the hepatocytes before excretion. 

C. True By forming cholesterol micelles. 

D. False Iron is removed from haem in the formation of bilirubin. 

E. False It becomes more acid, which improves the solubility of bile solids.

A. False It occurs in the systemic capillaries. 

B. True This occurs when tissue metabolic activity increases. 

C. True More oxygen is released at any given P_O2. 

D. False Less oxygen is taken up at any given P_O2. 

E. True For example, a rise in temperature raises the P₅₀.

A. True This is its definition. 

B. True Men, on average, have bigger thoracic cages than women. 

C. False It is around 1–1.5 litres 

D. False Residual air cannot be exhaled; it is measured indirectly by a dilution technique. 

E. False It increases with age since the elastic recoil of the lungs decreases with age.

A. False Carboxy hemoglobin is pink and gives the skin a pinkish colour. 

B. True This blue pigment is a rare cause of central cyanosis. 

C. False The fingers are red; cold inhibits oxygen dissociation and reduces metabolism. 

D. False Cyanosis occurs when arterial blood contains more than 5 g/dl reduced hemoglobin; low hemoglobin values in anaemia make it difficult to reach this level. 

E. False Cyanide poisons the enzymes involved in O₂ uptake by the tissues; in cyanide poisoning the blood remains fully oxygenated and the skin is pink.

A. False It reduces it by at least half. 

B. False A single lung can maintain normal oxygenation at rest. 

C. True Maximum ventilation and maximum oxygen uptake are reduced. 

D. True It leads to a restrictive lung disease pattern. 

E. False The ratio is little affected.

Cystic fibrosis is caused by a change, or mutation, in a gene called CFTR (cystic fibrosis transmembrane conductance regulator). This gene controls the flow of salt and fluids in and out of your cells. If the CFTR gene doesn't work the way it should, a sticky mucus builds up in your body.

A. False The concentration of oxygen in the air breathed is not a determinant of oxygen consumption.

B. False It rises; metabolic rate is the prime determinant of oxygen consumption. 

C. True This increases metabolic rate by increasing the rate of cellular metabolism. 

D. True More thermogenesis is required to maintain body temperature. 

E. True Due to the specific dynamic action of the food, particularly protein.

A. False Residual volume increases and vital capacity decreases. 

B. True This gradually falls with normal ageing. 

C. False The ‘closing volume’ increases with age. 

D. True This increases residual and closing volumes. 

E. True There is, however, little change in oxygen saturation.

A. False It increases content, especially at tissue P_O2. 

B. True The blood does not release its oxygen adequately. 

C. True Cold shifts the curve to the left and may reduce O₂ delivery to cold tissues. 

D. True Such blood when transfused may not release its O₂ content adequately. 

E. True Fetal blood can thus take up O₂ at the low P_O2 levels seen in the placenta but fetal tissue P_O2 has to be low to permit its release.

A. False Other digestive tract enzymes can take over if salivary enzymes are absent. 

B. False But swallowing solids is difficult without saliva’s moisturizing and lubricant effects. 

C. True Nervous orators with dry mouths continually sip water. 

D. True In the absence of saliva, the mouth becomes infected and ulceration occurs. 

E. True Substances must go into solution before they can stimulate taste receptors.

The left bronchus is more elevated than the right bronchus because of the position of the heart in the left side of the chest, anterior and inferior to the left bronchus.

Accidentally aspired objects are frequently found in the right bronchus because the inferior angle between the trachea and this bronchus is lower than the inferior angle between the trachea and the left bronchus since the left bronchus is more horizontalized. Therefore aspired objects tend to fall in the right side (bronchus) and not in the left. 

A. False It increases as the walls between alveoli break down to form large sacs. 

B. True Destruction of elastic fibres holding airways open allows them to narrow. 

C. False It is increased as airways close more readily than usual during expiration. 

D. True It decreases as the residual volume increases. 

E. True Thus it is a typical obstructive airways disease.

A. False CO₂ retention causes respiratory acidosis. 

B. False In respiratory acidosis there is increased secretion of H ions in urine. 

C. False Carbon dioxide dilates peripheral blood vessels. 

D. True The vasodilator effect of high P_CO2 on cerebral vessels may lead to cerebral oedema and headaches. 

E. True The kidney manufactures bicarbonate to compensate the respiratory acidosis. 

If the body experiences acidosis the respiratory center located in the medulla gets the information and induces the increase of the respiratory frequency. The increment of the respiratory frequency makes the body eliminate more carbon dioxide and to shift the equilibrium of the formation of bicarbonate towards the spending of more hydrogen ions and thus the blood pH raises.

Acidosis is the condition in which the blood pH is abnormally low. Alkalosis is the condition in which the blood pH is abnormally high. Normal pH levels for the human blood are between 7.35 and 7.45 - slightly alkaline.

If the body undergoes alkalosis the respiratory center located in the medulla gets the information and induces the lowering of the respiratory frequency.

The reduction of the respiratory frequency makes the body retain more carbon dioxide and to shift the equilibrium of the formation of bicarbonate towards the production of more hydrogen ions and thus the blood pH lowers.

Ammonia is a highly toxic molecule if not diluted and quickly excreted out of the body. For this reason the ammoniotelic excretion was abandoned in terrestrial habitats because the availability of water for dilution is reduced in this medium and wastes cannot be excreted so promptly to the exterior.

A. True This ‘atelectasis’ is due to absorption of trapped air. 

B. False Local collapse of the lung lowers local intrapleural pressure. 

C. False Since the affected lung is collapsed it does not count as dead space. 

D. False Local hypoxia in the unventilated segment causes local vasoconstriction. 

E. False Vasoconstriction in the collapsed segment prevents deoxygenated blood passing through to the systemic circulation.