Respiration in scoliodon or shark

Respiratory System of Scoliodon

🦈Scoliodon is an aquatic animal. 

🦈It depends completely upon oxygen dissolved in sea water for respiration. 

🦈Thus, respiration is aquatic and carried on entirely by vascular gills.

Respiratory Organs:

🦈There are five pairs of gill- pouches bearing gills, arranged in a series behind the hyoid arch in the lateral walls of the pharynx. 

🦈 Each gill-pouch is compressed antero-posteriorly and communicates with the cavity of the pharynx through a large internal branchial aperture and with the exterior through a narrow external branchial 

aperture also  called gill- slit.

🦈Two adjacent gill-pouches are completely separated from each other by a vertical fibro-muscular partition, the interbranchial septum or gill septum.

🦈The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays. 

🦈The septum is covered by an epithelium and contains blood vessels, nerves, etc.

🦈The mucous membranes of a septum are raised into numerous horizontal leaf-like folds, called gill lamellae or gill filaments. 

🦈These constitute the gill proper and are richly supplied with bloodcapillaries.

🦈 Each septum bears two sets of gill-lamellae, one on its anterior face and the other on its posterior face. 

🦈Each set makes a half gill called demibranch or hemibranch, while both the sets attached to a gill arch and its gill septum constitute a complete gill called holobranch. 

🦈 In Scoliodon  the interbranchial septa extend well beyond their gill lamellae to form flaps which protect the gills as well as external gill slits. Such gills are called lamelliform. 

🦈In  front of hyoid arch or the first internal gill slit, on either lateral side of pharynx, there is an oval pit of spiracle. 

🦈It has no gill lamellae and 'no external opening and is regarded a vestigial gill pouch.

Mechanism of respiration

🦈During respiration, water taken into the mouth, passes through internal gill slits bathing gill lamellae and passes out of the external gill slits.

1. Inspiration:

🦈 The floor of the buccopharyngeal cavity is depressed by the contraction of hypobranchial (hypoglossal) muscles due to which the visceral arches expand the wall of the pharynx, so that sea-water containing dissolved oxygen rushes in through the opened mouth. 

🦈Entry of water into external branchial apertures is prevented by an anterior fold of skin on each gill-pouch.

🦈 As the external gill slits are tightly closed, water enters the enlarged buccopharyngeal cavity through the open mouth

2. Expiration:

🦈During expiration the mouth becomes closed by the action of adductor muscle.

🦈 At the same time the constrictor and inter-branchial muscles contract raising the floor of pharynx and reducing its volume. 

🦈As a result, water is forced into gill pouches, over the gill lamellae, and out through the open external gill slits.

🦈 The spiracles are occasionally used as accessory pathways for entry of  water for respiration, instead of the mouth.

Physiology of respiration: 

In the branchial lamellae the blood flows from the tip towards the base, that is in a direction opposite to that of the water current, so that the blood just before leaving the lamellae meets the highest concentration of oxygen and the lowest of carbon dioxide, thus, an efficient exchange of oxygen and carbon dioxide takes place between the blood and sea-water.Sea-water entering the gill- pouches with the respiratory current contains oxygen dissolved in it. This water is separated from the blood 

contained within the capillaries of the gill-lamellae merely by the thin and permeable membranous walls of the capillaries. The oxygen of the water passes by endosmosis through the thin capillary walls into the blood, and at  the same time the carbon dioxide of the blood passes into the water by a process of exosmosis.The oxygen is conveyed by the blood to all the parts of the body, while carbon dioxide brought to the gills in the venous blood is eliminated by the water of the outgoing respiratory current. As the blood makes a complete circuit in the capillaries of the gills in a very short time, it is evident that exchange of gases also takes place very quickly