Breathing and Exchange of Gases Test 10

The frog has three respiratory surfaces on its body that it uses to exchange gas with the surroundings as the skin, the lungs and the lining of the mouth. The most important is the skin. The skin is composed of the thin membranous tissue that is quite permeable to water and contains a large network of blood vessels.
Therefore, the correct answer is option D.

• Lungs are poorly developed in frogs. They are oval, thin-walled, and highly elastic. They are freely suspended inside the peritoneal cavity on either side of the heart.

Besides pulmonary respiration, frogs respire through cutaneous respiration and buccal respiration as well.

Cellular respiration is defined as a set of metabolic reactions and processes that take place in the cells of organisms. These reactions and processes convert chemical energy from oxygen molecules or nutrients into adenosine triphosphate (ATP) and then release waste products.

The glucose molecule is the primary fuel for cellular respiration. Without it, the entire process would not be able to start as there would be no pyruvate to initiate Kreb's cycle (second step in cellular respiration).

Hence, the correct answer is option C.

"Respiratory distress syndrome" also known as "Hyaline membrane disease" or "Surfactant deficiency disorder" is a syndrome caused due to structural immaturity of the lungs and lack of a slippery substance in the lungs called surfactant. It can also result from a genetic problem linked with the production of surfactant associated proteins. Most cases of RDS occur in babies born before 37 weeks. The less the lungs are developed, the higher the chance of RDS after birth. The problem is uncommon in babies born full-term (at 40 weeks).
Surfactant is a surface-active lipoprotein complex secreted by the cells of the alveoli that serves to reduce the surface tension of the lungs. Surfactant increase pulmonary compliance ( the ability of lungs and thorax to expand). It prevents atelectasis (collapse of the lung) at the end of expiration. It facilitates recruitment of collapsed airways. It reduces fluid accumulation and keeps the airways dry by reducing these forces.
"Dipalmitoyl lecithin" act as a surfactant in the lungs. It is found in the extracellular lipid layer, lining the pulmonary alveoli, allowing it to ventilate. It develops in the lungs after the 30th week of gestation. Premature infants do not have an adequate amount of this surfactant, which may cause death due to respiratory distress syndrome.

Gas exchange through skin is normal and  important for amphibians which have moist and well vascularised skin. Frogs also have a respiratory surface on the lining of their mouth on which gas exchange takes place readily. While at rest, this process is their predominate form of breathing, and frog only fills the lungs occasionally. This is because the lungs, which only adults have, are poorly developed. In frogs the relative roles of skin and lungs change through the year. When oxygen uptake is low the skin takes up more oxygen than lungs. When oxygen consumption is high the uptake through lungs increases several folds. In vertebrates filling of lungs can take place with the use of pressure pump as in amphibians or a suction pump as in most reptiles, birds and mammals. A frog fills its lungs by taking air into its mouth cavity, closing its mouth and nostrils and pressing air into its lungs by elevating the floor of its mouth.  As a result of this filling mechanism, a frog can continue to take in repeated volumes of  air several times in sequence without  letting air out. If a frog is swimming in water it cannot move the floor of its mouth to push air into lungs. Hence, during swimming frog will breathe through skin or buccopharyngeal cavity only.

In buccal respiration, the mouth remains closed while nostrils are open. The floor of buccal cavity is alternately raised and lowered so that no air is drawn into or expelled out of buccal cavity. During buccal respiration glottis remain closed so that no air enters or leave lungs. The mucous layer of buccal cavity is richly supplied with blood capillaries, which absorb the oxygen of the air and releases carbon-dioxide.

The structure of hemoglobin is sensitive to partial pressure of carbon dioxide in addition to partial pressure of oxygen. When circulating hemoglobin encounters an environment where the carbon dioxide levels are elevated, the carbon dioxide decreases hemoglobins affinity for oxygen and oxygen is released to the tissue. Carbon dioxide reduces hemoglobins ability to bind oxygen in two different ways, directly and indirectly. $$CO_2$$ can bind directly to the amino-terminal ends of the alpha and beta chains that make up the globin. The binding of $$CO_2$$ to hemoglobin causes a conformational change, reducing hemoglobins hold on oxygen and, as a consequence, oxygen is released. The sensitivity of hemoglobin to carbon dioxide levels can be illustrated on a saturation curve. The saturation curve for hemoglobin shifts to the right as higher carbon dioxide levels are encountered, a phenomenon called the Bohr effect. A shift to the right of the oxygen saturation curve represents a decrease in the affinity of hemoglobin for oxygen.

 In vertebrates filling of lungs can take place with the use of pressure pump as in amphibians or a suction pump as in most reptiles, birds and mammals. A frog fills its lungs by taking air into its mouth cavity, closing its mouth and nostrils and pressing air into its lungs by elevating the floor of its mouth.  As a result of this filling mechanism, a frog can continue to take in repeated volumes of  air several times in sequence without  letting air out. Amphibians lack ribs and hence use floor of the buccal cavity to force air in and out of the lungs. Frogs and toads modify 2nd, 3rd and 4th visceral arches to produce a plate-like hyobranchial apparatus that lies in the floor of oral cavity and is connected to squamosal bone of skull by petrohyal muscle and to sternum by sternohyal muscle. One breathing cycle is completed in four steps in anurans that is affected by contraction of these two muscles.

Gas exchange through skin is normal and  important for amphibians, which have moist and well vascularised skin. In fact, some small salamanders have no lungs; and all gas exchange takes place through the skin surface except for a small contribution by the oral mucosa. Frogs also have a respiratory surface on the lining of their mouth on which gas exchange takes place readily. While at rest, this process is their predominate form of breathing, and frog only fills the lungs occasionally. This is because the lungs, which only adults have, are poorly developed. In frogs the relative roles of skin and lungs change through the year. In winters when oxygen uptake is low the skin takes up more oxygen than lungs. In summers, when oxygen consumption is high, the uptake through lungs increases by several folds.