CIRCULATORY AND RESPIRATORY SYSTEMS

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DOGFISH RESPIRATORY SYSTEM

In the shark, the circulatory and respiratory systems is one as the heart pumps unoxygenated blood to the gills for oxygenation and from their oxygenated blood is distributed to the body. Gas exchange also takes place in the skin, but primarily in the gills. Look at the diagrams and follow in your shark the passage of water in the mouth and spiracles (which have a one way valve) and through the five gill slits. The gill slits close and the pharyngeal chamber expands to suck in water. When the pharynx is filled, the mouth closes and the gill chambers expand and fill with water. Then the gill slits open and the chambers constrict to flush out the water. If you look down the gill slits, on each side you will see one half of the gill, a demibranch. The internal septum, blood vessels, nerves, muscles and two demibranchs make up each holobranch. At their base are gill rakers, which project into the mouth and protect the gills from mechanical injury. The demibranchs have primary lamellae, which can be easily seen, and secondary lamellae, which can be seen if you remove a portion of the gill and examine it closely. The oxygen rich water flows in a countercurrent pattern to the blood and allows efficient oxygen and carbon dioxide exchange to take place.

DOGFISH HEART AND AORTIC ARCHES

To expose the blood vessels, first skin the roof of the mouth where the efferent (e=from) branchial arteries are located. They are pink and usually well injected. Make a shallow cut just inside the teeth and peel the skin off by flaying it. You will have to go well down the pharynx to the place where the arteries meet.

The floor of the mouth, under the attached tongue, has the afferent (a=to) branchial arteries and posteriorly the heart. The yellow or blue latex does not make it through the heart so the vessels are not injected and are delicate. They are colourless or brown with blood. Remove only the skin with a scalpel. Expose the heart by removing its protective cartilage and identify the sinus venosus, auricle and ventricle. Trace the ventricle anteriorly into the conus arteriosus, and into the short ventral aorta. Pry off the cartilage on the afferent vessels with forceps and tease it loose with dissecting needles but do not use the scalpel any more in this area. The vessels are almost stuck to the cartilage. Expose all 5 afferent branchial arteries as far as the right gills. Follow one into the gill as far as you can. Expose the efferent arteries on each side of this afferent one and find how the major efferent duct loops up to the proceeding gill. In the region of the gills, skin a pair of gill arches and trace one loop of the efferent branchial arteries around a gill slit.

Using your diagrams, trace the pathway of the blood through the afferent branchial arteries, the gills and the efferent branchial arteries. Learn how the gills work by answering the following questions.

QUESTIONS:

* How many afferent arteries are there and what direction does the blood flow in them?

* How many efferent arteries are there and what direction does the blood flow in them?

* Where does the blood become oxygenated?

* Where does the unoxygenated blood enter the gills?

* How does it become oxygenated?

* What is the path of water flow over the gills?

* How does oxygenated blood leave the gills?

* How are the afferent and efferent blood vessels of the gills connected to each other within the gills? Describe in your own words.

* Describe the efferent loop around a gill slit.

* Describe the path of the blood through the heart.

NECTURUS RESPIRATORY SYSTEM

Much of the respiration in amphibians is through the skin. Up to 75% of the oxygen and 90% of the carbon dioxide is exchanged this way in aquatic forms. In addition, Necturus has three pairs of gills and one pair of lungs. External gills develop before the two gill slits open and can be waved about by branchial muscles. Lungs developed from swim bladders, which sometimes function as lungs in fish that gulp air. Look for the glottis, a longitudinal slit in the pharynx floor, which is the opening to the lungs. The larynx is a pair of cartilages surrounding the glottis and will not be noticed. The trachea is short. The lungs are long, slender and saclike. They run along the dorsal sides of the pleuroperitoneal cavity and are attached to the body wall by the pulmonary ligament on the left side and on the right, the hepatocavopulmonary ligament that also supports the liver. The lungs are used as hydrostatic organs and only contribute about 2% of the gas exchange. Air is gulped via the mouth as it is in lungfish.

NECTURUS HEART AND AORTIC ARCHES

Carefully skin the insides of the pharynx, top and bottom, concentrating on the area between the gills to the transverse septa. Expose the heart on the ventral floor of the mouth and the afferent arteries leading to the gills. The heart consists of a left and right atrium and a single, large, muscular ventricle. Lift the ventricle to see the thin- walled, poorly injected, sinus venosus. Blood enters the sinus venosus via the lateral common cardinal veins and the posterior, postcaval vein. Unoxygenated blood enters the heart from the body via the right atrium and from the lungs via the left atrium. It enters the ventricle where it is pumped into the bulbus arteriosus. Follow the branches of the ventral aorta and its branches to the gills (afferent arteries) and head (external carotid artery). On the roof of the mouth, find the efferent arteries from the gills, internal carotid going to the head and subclavians going to the arms. Note the thin pulmonary artery, which joins at the junction of the two posteriormost efferent arteries. These vessels join the dorsal aorta, which carries oxygenated blood to the body.

QUESTIONS

* How many afferent arteries are there and what direction does the blood flow in them?

* How many efferent arteries are there and what direction does the blood flow in them?

* Where does the blood become oxygenated?

* Describe the path of the blood through the heart.

MAMMALIAN RESPIRATORY SYSTEM

Open the thoracic cavity by cutting through the diaphragm, which separates the abdominal from the thoracic cavity. Cut through the ribs at both sides of the sternum, and deflect the flaps, breaking the ribs, but leaving the sternum in position. This exposes the thoracic cavity. Note the shiny membranes covering the lungs and the inside of the thoracic cavity. These are the pleura. Note also the mediastinum which is the shiny membrane dividing the right and left pleural cavities. The heart lies in the center of the thoracic cavity. Remove the sternum by cutting through the mediastinum. Be careful not to damage the heart or the accompanying blood vessels.

The lungs are the brownish lobes to the right and left of the heart. Entering the lungs on either side from the trachea are the bronchi. They are supported, as is the trachea, by incomplete cartilage rings. Cut through a lung and trace the divisions and subdivisions of the bronchi. They divide into: secondary bronchi, one to each lobe and bronchioles, within each lobe. Too small to see are the small end branches, the infundibula, which are lined with alveolar sacs. Different animals vary in the number of lobes to each lung. The cat and rat have 4 right lobes, humans have 3. The cat has 3 left lobes, rats have one and humans have 2.

Trace the trachea anteriorly. The rings supporting it are incomplete. Note that the esophagus lies dorsal to the trachea and the gaps in the rings allow the esophagus to attach on the dorsal surface of the trachea. Trace the trachea anteriorly to the larynx, or voice box. Note that it is supported by cartilages. These are the hyoid bone and thyroid and cricoid cartilages.

What is the evolutionary origin of these cartilages?

The air enters the nasal passageway at the nostrils and is separated from the oral passageway by the hard and soft palate. A nasal septum divides the air from the two nostrils. You should be able to look down the pharynx and see a triangular flap, the epiglottis which moves up to open the air passageway (glottis) or down to close the air passageway and allow food down the esophagus. Inside the larynx at the base of the epiglottis are two lateral flaps, the vocal cords, which open and close to produce sound.

In the upper part of the pharynx, slit the soft palate to see the nasal passage. Into the upper back of this passage way opens the eustachian tubes which lead to the middle ear. This opening allows you to "pop" your ears, thus equalizing the pressure on both sides of the tympanic membrane to prevent it from rupturing.

MAMMAL HEART AND AORTIC ARCHES

Remove the thymus gland on the anterior of the heart. The heart lies in the pericardial cavity and is enclosed by the pericardial sac. In the cat, these sacs have to be removed. It consists of two atria, and two ventricles. The sinus venosus is now incorporated in the wall of the right atrium. The auricles, or atria are small dark structures on the antero-lateral border of the heart. They are divided from the ventricles by a groove, and have much thinner walls than the ventricles. The ventricles appear as one externally, but if you feel them, the larger left ventricle is hard due to its thick walls, the smaller right ventricle is soft as its walls are thinner. When you have finished finding the arteries and veins below, slit open the ventricles from the posterior tip towards the atria and lift up the top flap to see the division of the ventricles and their valves.

Deoxygenated blood enters the heart from the body via the branched superior vena cava, which drains the head and arms, and the inferior vena cava, which drains the abdominal cavity. These both enter the right atrium and are injected blue.

From the right atrium, deoxygenated blood flows through a tricuspid valve into the right ventricle which pumps the blood through a semilunar valve to the blue injected pulmonary aorta which divides into two pulmonary arteries, one to each lung. In the lung the blood vessels subdivide into capillary beds around the alveoli and gas exchange takes place.

The oxygenated blood flows back from the lungs via the colourless pulmonary veins, which run a parallel, course to the pulmonary arteries but enter the left atrium. From the left atrium the oxygenated blood flows through the bicuspid (mitral) valve to the left ventricle.

The muscular left ventricle must pump oxygenated blood to the entire body including the heart itself. Blood is pumped through the semilunar valve to the systemic aorta (IV) where a branch, the paired coronary arteries go to the surface of the heart. The systemic aorta then bends to the left as the aortic arch. At the top of the arch, vessels go to the arms and head. In the rat and human there are three main vessels, first the branchiocephalic (=innominate) then the left common carotid (III) to the head then the left subclavian (IV) to the left arm. In the cat the left common carotid branches off the branchiocephalic rather than having a separate origin. The branchiocephalic sends a branch, the right subclavian (IV) to the right arm and continues as the right common carotid to the head. The common carotids travel up the neck on either side of the larynx and supply the thyroid, larynx, throat, and face muscles, tongue and part of the brain. The internal carotids supply the deep parts of the head; the external carotids supply the more superficial areas.

After the head and arm vessels leave the aortic arch the systemic aorta continues towards the diaphragm as the thoracic aorta and posterior to the diaphragm as the abdominal aorta. These are equivalent to the dorsal aorta in the dogfish.

Comparative Aspects

The purpose of this laboratory session is to give you an understanding of:

A) The loss in number of aortic arches in the evolution from Fish to Mammal, and the change in function of those arches that remain.

B) The increase in complexity that has occurred from the two-chambered fish heart to the four-chambered mammal heart. To study these features examine the models of hearts and aortic arches displayed in the lab, the illustrations in your lab and textbook and what you have learned about the dogfish, mudpuppy, cat and rat.

Assignment:

Colour your drawings to show the pathway of the blood from the heart through the afferent branchial arteries, the gills, and the efferent branchial arteries to the dorsal aorta. In mammals trace the path of blood from the heart to and from the lungs and out to the upper body.

Blue: Unoxygenated blood.

Red: Oxygenated blood.

Fill in the Tables below.

Table 1. Comparison of Aortic Arches. Check if present.
Aortic Arch	Fish	Amphibia	Reptile	Bird	Mammal	Name in Mammal
I
II
III
IV
V
VI

Table 2. Comparison of the Heart in Vertebrates.
Vertebrate	# atria/ventricles	Innovations (Changes)
Fish
Salamander
Frog
Turtle
Crocodile
Mammal

Updated by Sandra Millen, December, 2003

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