Phylogeny of the Chordate Circulatory System. Amphioxus

General characteristics of the circulatory system

During the embryonic development of chordates, the circulatory system begins to form on the ventral side of the embryo. The lateral plates of the mesoderm serve as its source. All representatives of the phylum Chordata are characterized by having a closed circulatory system. At the same time, a true muscular heart appears only in representatives of the subphylum Vertebrata, whereas in the subphylum Acrania (Cephalochordata), which includes the class Amphioxus, this organ is absent.

Amphioxi are cold-blooded and sedentary animals. Due to their low metabolic rate, hemodynamics is maintained by a primitive mechanism: the pulsating ventral aorta, as well as the bases of the afferent branchial arteries, take on the function of a pump.

Arterial system

Despite its name, only venous blood flows through the ventral aorta of amphioxus. It is directed toward the anterior end of the animal's body. The further pathway of the blood is as follows:

1. Numerous vessels branch off from the ventral aorta — from 100 to 150 pairs of afferent branchial arteries.
2. These arteries deliver venous blood directly to the interbranchial septa.
3. Gas exchange takes place in the region of the gills, as a result of which the blood becomes saturated with oxygen and turns into arterial blood.
4. Oxygen-enriched arterial blood collects in the efferent branchial arteries.
5. Next, it enters the roots of the dorsal aorta, from where it passes into the dorsal aorta itself for distribution throughout the body.

The anterior end of the body is supplied with nutrients via the carotid arteries, through which arterial blood flows.

Venous system

The collection of venous blood that has released its oxygen to the tissues occurs along two main pathways: from the body walls (somatic outflow) and from the internal organs (visceral outflow).

Somatic drainage:

• From the posterior part of the body, blood collects into the posterior cardinal veins.
• From the anterior part of the body, venous blood is drained by the anterior cardinal veins.
• On each side of the body (left and right), the anterior and posterior cardinal veins merge together, forming the so-called Cuvierian ducts (modern term: common cardinal veins).
• The right and left Cuvierian ducts empty into the venous sinus located at the origin of the ventral aorta.

Visceral drainage:

• From the intestine, the spent blood collects into the subintestinal vein.
• This vein heads toward the hepatic caecum, where it breaks up into capillaries, forming the hepatic portal system.
• After passing through the hepatic caecum, the blood collects into the hepatic vein, which also empties into the venous sinus.

Topography of the venous sinus vessels

The venous sinus is a vital collecting vessel that gathers all the venous blood before sending it to the gills. The vascular bed in this zone is strictly structured.

Vessels bringing blood (emptying into the sinus): Exactly three vessels empty into the venous sinus:

1. Right Cuvierian duct.
2. Left Cuvierian duct.
3. Hepatic vein.

Vessels draining blood (exiting the sinus): Only one large vessel exits the venous sinus — the ventral aorta. It takes the collected venous blood and propels it forward, toward the afferent branchial arteries, thereby completing the single circle of blood circulation.

Briefly: answers to topical questions

What else to study on this topic

• Topography of the circulatory system primordia within the lateral plates of the mesoderm.
• The mechanism of pulsation of the ventral aorta and the bases of the afferent branchial arteries.
• Structure and functions of interbranchial septa during gas exchange.
• Features of the formation of the hepatic caecum portal system.
• Differences between the venous bed of amphioxi (Cuvierian ducts) and higher vertebrates.

Other topics in the section «Phylogeny of the vertebrate circulatory system»

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