Systemic veins carry CO2-rich blood from different parts of the body to the ductus cuveae. Blood then enters the sinus venosus from the ductus cavae. The different types of systemic veins are as follows.
(i) Anterior cardinal vein: It brings blood from different parts of the brain via ductus cuveae.
(ii) Inferior jugular vein: It supplies blood from different parts of the brain through the ductus cuveae.
(iii) Posterior Cardinal Vein: The right and left posterior cardinal veins are joined by the anastomosis vein. It carries blood from the back of the body through the ductus cuveae.

The vessels through which blood returns to the heart from different parts of the body are called veins. The vascular system of roe fish can be divided into two main parts. Systemic veins and portal veins
1. Systemic veins
Systemic veins carry CO2-rich blood from different parts of the body to the ductus cuveae. Blood then enters the sinus venosus from the ductus cavae. The different types of systemic veins are as follows.
(i) Anterior cardinal vein: It brings blood from different parts of the brain via ductus cuveae.
(ii) Inferior jugular vein: It supplies blood from different parts of the brain through the ductus cuveae.
(iii) Posterior Cardinal Vein: The right and left posterior cardinal veins are joined by the anastomosis vein. It carries blood from the back of the body through the ductus cuveae.
2. Portal vein
The veins that collect blood from different parts of the body and enter another organ and re-form the capillary network are called portal veins. It consists of hepatic portal and renal portal. Portal veins are:
(i) Caudal vein: The caudal vein enters the trunk from the tail and divides into two branches to form the renal portal vein. The renal portal vein brings blood from the digestive system to the liver.
(ii) Segmental vein: It collects blood from different parts of the body.
(iii) Transverse anastomosis veins: Right and left cardinal sinuses join with some transverse veins to form transverse anastomosis veins.
(iv) Sub clavian vein: It supplies blood directly from the thoracic fins to the sinus venosus.
(v) Hepatic veins: Two hepatic veins arise from the liver and bring blood to the sinus venosus.

The two lateral arteries join posteriorly to form the dorsal aorta. The branches of the dorsal aorta are:
(i) Celiaco-mesenteric artery: It supplies blood to the bladder, stomach, intestines, liver, pancreas, spleen and spleen.
(ii) Sub clavian artery: It supplies blood to pectoral fins and pectoral fins.
(iii) Parietal artery: It supplies blood to the body wall.
(iv) Pelvic/iliac artery: It supplies blood to the pelvic fins.
(v) Renal artery: It transports blood to the kidney.
(vi) Caudal artery: It supplies blood to the tail.

Arteries that carry blood from the gills to various parts of the body are called extrinsic arteries. Four pairs of gills give rise to 4 pairs of external arteries. The first extrinsic artery transports blood to the pseudobranch. From the pseudobranch, blood reaches the eye through the ophthalmic artery. The first and second external bronchial arteries join to form the lateral artery. The carotid artery arises from the tip of the lateral artery. The carotid artery supplies blood to the brain. The external branchial artery arising from the third and fourth gills first joins and then joins the lateral artery. The two lateral arteries join to form the oval arterial ring. It is called Circulus Cephalicus. Dr. Siddiq Publications

Arteries that carry CO2 rich blood from the heart to the gills are called internal bronchial arteries. The ventral aorta arises from the bulbous aorta and extends to the hyoid artery. Four pairs of internal bronchial arteries arise from the ventral aorta.
(i) First afferent branchial artery: The first pair of afferent bronchial arteries arises from the tip of the ventral aorta. They enter the first gill.
(ii) Second afferent bronchial artery: The second pair of afferent bronchial arteries arises from the medial part of the ventral aorta. They enter the second gill.
(iii) Third afferent branchial artery: The third pair of afferent bronchial arteries arises from the medial part of the ventral aorta. They enter the third gill.
(iv) Fourth afferent branchial artery: From the root of the ventral aorta arises the fourth pair of afferent bronchial arteries. They enter the fourth gill.
Branchial arteries arise from each internal bronchial artery. Branch arteries supply blood to each gill lamella.

The chambers and subchambers of roe fish open unidirectionally. Never opens backwards. Flows unidirectionally from the back of the body through the heart. For this reason it is called venous heart. It is called venous heart because it only carries blood containing CO2.Dr. Siddiq Publications
Blood from different parts of the body enters the sinus venosus through veins, ductus cuveae and hepatic sinuses. The sinus venosus contracts and blood enters the atrium. When the atria are full of blood, they contract and blood enters the ventricles. When the blood-filled ventricles contract, blood moves into the bulbous aorta. Blood enters the gills from the bulbous aota.

The heart through which venous CO2 rich blood is carried is called venous heart or venous heart. Blood with CO2 flows unidirectionally from the back to the front of the roe fish body. Sinus venosus enters from different parts of the body. Blood enters the atrium from the sinus venosus. Blood flows from the atrium. Then CO2 rich blood from the ventricles enters the bulbus arteriosus. In this way, the blood flows through the heart in a unidirectional manner through the veins. So roe fish heart is called venous heart or vein heart.

There are three types of valves in the heart of roe fish.
(i) Sino-atrial cavity: It lies between the sinus venosus and the atrium.
(ii) Atrio-ventricular chamber: It lies between the atria and the ventricles.
(iii) Ventriculo-bulbus capatica: It lies between the ventricles and the bulbous aorta.

Conus arteriosus is absent in the heart of roe fish. The root of the artery swells to form the bulbus arteriosus. The bulbus arteriosus is not part of the heart as it is not covered by myocardium or pericardium. It delivers blood from the heart to the ventral aorta. Dr. Siddiq Publications

The heart of ruhi fish is located in the pericardial cavity. It is covered by the pericardium. It is called a myogenic heart because the heart muscle in the heart wall produces the heartbeat. The parts of the heart are:
1. Subchamber or sinus venosus: Sinus venosus is a triangular shaped and elastic subchamber. It is thin walled. Blood enters the sinus venosus from the veins, ductus cuveae, and hepatic sinuses. Blood enters the atrium from the sinus venosus. Siddique Publications
2. Atrium: The atrium is the largest chamber of the heart. It is triangular, muscular and thin walled. One end is connected to the ventricles and the other end to the sinus venosus. It collects blood containing CO2 from the sinus venosus and delivers it to the lungs. Siddique Publications
3. Ventricle : Ventricle is a muscular, fleshy and thick walled cavity. One end of the ventricle is connected to the atrium and the other end to the bulbus arteriosus. It is filled with blood containing CO2. Dr. Siddiq Publications
4. Bulbus arteriosus: Conus arteriosus is absent in the heart of roe fish. The root of the artery swells to form the bulbus arteriosus. The bulbus arteriosus is not part of the heart as it is not covered by myocardium or pericardium. It delivers blood from the heart to the ventral aorta. Dr. Siddiq Publications
5. Cardiac valves: There are three types of valves in the heart of roe fish.
(i) Sino-atrial cavity: It lies between the sinus venosus and the atrium.
(ii) Atrio-ventricular chamber: It lies between the atria and the ventricles.
(iii) Ventriculo-bulbus capatica: It lies between the ventricles and the bulbous aorta.