Respiration and Gas Exchange

Most fish exchange gases using gills on either side of the pharynx (throat). Gills are tissues which consist of cloth and fabric structures called as filaments. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective bony cover called an operculum. Fish do not have lungs and do not absorb oxygen through mouth or tail. 

Frog are amphibians and they can survive on land and water both. In water they breathe through gills while on land they have fully developed lungs to breathe. During hibernation, when they are not active they take oxygen from the moist skin by the process of diffusion. 

Frog are amphibians and they can survive on land and water both. In water they breathe through gills while on land they have fully developed lungs to breathe. It undergoes hibernation as it is cold blooded and the body temperature fluctuates with the surrounding. During hibernation the diffused oxygen is taken from moist skin.  

While both the tadpole and the frog like other organisms need to take in oxygen from the environment, the respiratory mechanisms differ. The tadpoles make use of gills and the mature frogs respire need to take in oxygen from their environment, and the way they respire also changes as they develop. Tadpoles use gills, while mature frogs have three types of respiration- skin, buccal pumping and lungs

Animal P always breathes through gills - fish; animal Q breathes both through moist skin and lungs - frog; animal R breathes only through its moist skin - earthworm; animal S breathes through its cell membrane - amoeba. Oxygen diffuses in and carbon dioxide diffuses out of its cell membrane.

In frogs the process of respiration takes place through three respiratory surfaces - skin, lungs and lining of the mouth. However, when submerged in water, the frog respires through its cutaneous membranes on the skin as it is quite permeable to water and contains a large network of blood vessels. The cutaneous membrane allows the respiratory gases to readily diffuse directly. Even when out of water, mucus glands present in the skin help in the cutaneous respiration.

Respiration of aquatic animals is a process wherein they obtain the oxygen from the water. The large aquatic animals have well developed gills which helps them in their respiratory function. 

Respiration in frogs is both through lungs and skin. However, during hibernation, frogs make use of the skin to breathe. However, there are exceptions such as the wood frog which stops to breathe completely during its hibernating period. During hibernation most of these amphibians receive their necessary oxygen through their skin referred to as cutaneous gas exchange. Their skins soaks in the oxygen required by their bodies require.

The mucus glands present in the skin of the frog helps to absorb dissolved oxygen from the air. However, the frogs have three respiratory surfaces and exchange of gases with the surroundings through the skin is a continuous process.
Therefore, the correct answer is option D.

Gills are the respiratory organs of large aquatic animals. These organs are specifically adapted to their function. In Osteichthyes, the gills have four gill arches, Chondrichthyes have two, fish has 7 gill baskets with a long bony cover for the gills which is used to push water. 

Respiratory quotient or R.Q. is the ratio of volume of CO$ _2$ produced to volume of O$ _2$ consumed during cellular respiration. It is measured by Ganong's Respirometer. R.Q. for carbohydrates is 1, organic acid is >1 and fats and proteins is <1.

The respiratory quotient is used in calculations of BMR when estimated from carbon dioxide production. It is calculated from the ratio of carbon dioxide produced by the body to oxygen consumed by the body. A  value of 0.7 indicates that lipids are being metabolized, 0.8 for proteins, and 1.0 for carbohydrates. RQ can be used as an indicator of over or underfeeding. Underfeeding, which forces the body to utilize fat stores, will lower the RQ and is marked by an RQ below 0.85, while overfeeding, which causes lipogenesis, will increase it and is indicated by RQ greater than 1.0.

If RQ value is more than 1, the respiration would be aerobic and anaerobic respiration.

The urge to breathe comes from the respiratory center, located at the base of brain. It sends signals via the spinal cord to diaphragm and the muscles between ribs telling them when to contract or relax. During inspiration, the diaphragm -- the large muscle that divides chest and abdomen -- contracts and moves downward. Additionally, ribs move outward. This enlarges your chest and lungs expand. Lung expansion creates a vacuum. Air enters the nose and mouth and is pulled into windpipe -- the trachea. The trachea divides into smaller airways called bronchi. These continue to divide as they get farther from the trachea, like the branches of a tree. Finally, the tiny airways deliver the air to the smallest structures in your lung -- the alveoli -- where gas exchange takes place.