I. The Basic Embryonic Vertebrate Plan
The fundamental blueprint for the vertebrate arterial system involves a series of paired arterial channels.
- Major Channels: Blood exits the heart via the ventral aorta, which extends anteriorly. This vessel gives rise to typically six paired aortic arches (I to VI) that ascend through the visceral (pharyngeal) arches.
- Aortic Arches and Dorsal Aorta: Each aortic arch connects the ventral aorta to the lateral dorsal aortae. Dorsally, the efferent branchial arteries from these arches join the lateral dorsal aorta (or the internal carotid artery, depending on the arch).
- Systemic Circulation: Posterior to the pharynx, the paired lateral dorsal aortae fuse to form a single, median dorsal aorta, which serves as the principal systemic artery, extending caudally as the caudal artery.
II. Comparative Anatomy: Primitive Conditions
Despite vast differences in adult circulation, the underlying architecture across all vertebrates adheres to this basic embryonic plan.
- Agnathans (Jawless Fish):
- Branchiostoma (Lancelets): Possess a high number, nearly 60 pairs, of aortic arches.
- Petromyzon (Lampreys): Exhibit 7 pairs of functional aortic arches.
- Myxine and Eptatretus (Hagfishes): The number varies, ranging from 6 (Myxine) to 15 pairs (Eptatretus).
III. Aortic Arches in Fishes: Gill Respiration
Fishes generally retain the arch system as essential components of the branchial circulation, specialized for gaseous exchange in the gills.
| Group | Functional Arches Retained (Typically) | Key Modification |
|---|---|---|
| Primitive Elasmobranchs (e.g., Heptanchus) | 7 pairs of arches | Retention of a high, primitive number. |
| Most Elasmobranchs (Sharks and Rays) | 5 pairs (II, III, IV, V, VI) | Arch I is lost or non-functional; the first gill slit often forms the spiracle. |
| Teleosts (Bony Fishes) | 3 pairs (III, IV, VI) | Arches I and II typically disappear in the adult. |
- Vascular Differentiation: In elasmobranchs, lungfishes, and teleosts, each aortic arch is divided by the gill capillary bed into an afferent branchial artery (carrying blood to the gill) and an efferent branchial artery (carrying blood away).
- Pulmonary Origin (Dipnoi and Polypterus): In lungfishes and the lobe-finned fish Polypterus, the pulmonary artery arises from the efferent segment of the VI arch, supplying blood to the air bladder or lungs. For example, in Protopterus, arches III and IV are interrupted by gill capillaries.
IV. Amphibians: Transition to Pulmocutaneous Respiration
The shift to a semi-terrestrial lifestyle and the introduction of lungs necessitates significant modification of the aortic system.
- General Adaptation: The role of gills is reduced or eliminated, reflected in the loss or reduction of several arches. The adult circulation typically features four pairs of arches ($III$ to $VI$).
- Arch Fate:
- Arch III (Carotid Arch):
- Forms the carotid arteries, supplying blood to the head.
- Arch IV (Systemic Arch):
- Forms the main systemic arteries (aortic arches).
- Arch V:
- Typically incomplete, reduced, or absent in the adult.
- Arch VI (Pulmocutaneous Arch):
- Gives rise to the pulmocutaneous artery, which supplies blood to the skin and lungs (the dual respiratory surfaces).
Anurans (Frogs and Toads)
- Metamorphosis: During the transition from the gilled tadpole stage, arches I, II, and V are lost.
- Duct Closure: Two embryonic shunts disappear:
- Ductus Caroticus: Its loss means the carotid arch (III) now supplies only oxygenated blood to the head.
- Ductus Arteriosus (Ductus Botalli): Its disappearance from the VI arch ensures pulmonary circulation (VI arch derivatives) carries venous blood exclusively to the lungs and skin for oxygenation.
- Functional Arches: Adult anurans retain:
- Functional systemic arches (IV) connecting to the dorsal aorta for body distribution.
- Pulmonary arches (VI).
V. Reptiles: Full Terrestrial Adaptation
- Lungs and Circulation: Reptiles are fully terrestrial, relying entirely on lungs, with a sophisticated aortic system and a partially divided ventricle.
- Ventral Aorta Division: The embryonic ventral aorta and conus arteriosus divide into three distinct trunks:
- Right Systemic Aorta (Arch IV): Originates from the left ventricle (oxygenated blood) and joins the carotid arch supply to the head.
- Left Systemic Aorta (Arch IV): Originates from the right ventricle (deoxygenated or mixed blood). Both systemic aortas join posteriorly to form the dorsal aorta.
- Pulmonary Trunk (Arch VI): Originates from the right ventricle, carrying deoxygenated blood to the lungs.
- Cold-Blooded Nature: Incomplete ventricular separation allows mixing of oxygenated and deoxygenated blood, supporting ectothermy.
- Embryonic Shunts: Ductus caroticus and ductus arteriosus are generally absent, except in some species like Sphenodon and certain turtles.
VI. Birds and Mammals: Endothermy
- Ventricular Separation: Complete division prevents mixing of oxygenated and deoxygenated blood, supporting high metabolic rates.
- Persistent Arches: Only arches III, IV, and VI persist in adults.
- Arch I and II: Lost entirely.
- Arch V: Disappears.
- Fate of Aortic Arches:
| Arch | Birds (Avian) | Mammals (Mammalian) |
|---|---|---|
| III | Carotid Arteries | Carotid Arteries |
| IV | Right Systemic Aorta (sole systemic arch) | Left Systemic Aorta (sole systemic arch) |
| VI | Pulmonary Trunk | Pulmonary Trunk |
- Systemic Trunk Origin:
- Birds: Single systemic aorta (from right IV arch) arises from the left ventricle.
- Mammals: Single systemic aorta (from left IV arch) arises from the left ventricle.
- Pulmonary Trunk: VI arch forms the pulmonary trunk, carrying deoxygenated blood from the right ventricle to the lungs.
- Post-Natal Changes: Embryonic ductus arteriosus (connection between pulmonary trunk and systemic arch) closes after birth/hatching, forming the non-functional ligamentum arteriosum.
