Circulatory System | Zoology Optional Notes for UPSC PDF Download

Heart in Vertebrates

1. Layers:

   • The heart consists of three layers:
     • Endocardium: Inner lining comprising endothelium and elastic tissue.
     • Myocardium: Muscular layer located between endocardium and epicardium.
     • Epicardium: Outer fibrous tunica, covered by visceral pericardium.
2. Pulsation and Evolutionary Changes:
   • The heart pulsates due to the response of muscle cells to electrolytes.
   • Evolutionary changes in the heart can be traced from a primitive, simple tube in protochordates to the more complex structures in fishes, tetrapods, birds, and mammals.

1. Protochordate Heart:

   • Protochordates (tunicates and Amphioxus) possess a simple, tubular heart that beats through peristaltic waves.
   • Lack of respiratory function in pharyngeal clefts leads to a less specialized heart.

2. Early Vertebrate Heart (Agnathans):

   • Agnathans (jawless fishes) have a main systemic heart and three accessory hearts.
   • Circulatory system includes a portal heart, cardinal heart, and caudal heart.
   • Sinus venosus in hagfish is attached to the left side of the atrium.
   • Blood pressure is adapted to the low arterial pressure required for bottom-dwelling.

1. Fish Heart:

   • Fishes have a true heart with specialized chambers (atrium and ventricle).
   • Blood flows through sinus venosus, atrium, ventricle, and conus arteriosus.
   • Semilunar valves prevent backflow, and the bulbous arteriosus regulates pressure.
   • Opercular movement is synchronized with heart pulsation for steady blood flow.

2. Early Tetrapod Heart:

   • Evolutionary changes involve the development of the air bladder in pre-tetrapod ancestors.
   • Lungfishes exhibit a dual circulation system, with a partially divided heart.
   • Tetrapods show further subdivisions in the heart, including a complete separation of the atrium.

3. Amphibian Heart:

   • Amphibians have a three-chambered heart with a partially divided ventricle.
   • Presence of a spiral valve directs blood to systemic and pulmocutaneous arches.
   • Modifications in septum and valve are observed in lungless salamanders and species with reduced lung function.
   • Changes in the sinus venosus reflect adaptations to terrestrial life.

Evolutionary changes in the vertebrate heart have been closely linked to the development of respiratory organs and the transition from aquatic to terrestrial life.

Late Ectotherm Heart in Reptiles

1. Chelonian/Squamate Heart:

   • The sinus venosus is reduced in size or absent.
   • The myogenic center, known as the sinoatrial node, initiates each heartbeat in the right atrium.
   • The conus arteriosus disappears; semilunar valves persist at the bases of the pulmonary and systemic trunks.
   • Atrium is divided into right and left atria.
   • The ventricle is partially divided into three compartments: cavum venosum, cavum pulmonale, and cavum arteriosum.

2. Blood Flow Patterns:

   • The pattern of blood flow varies depending on whether reptiles breathe air or hold their breath (apnea).
   • In air-breathing conditions, blood from cavum venosum goes to the lungs and oxygenated blood from the lungs is directed to systemic tissues.
   • During apnea or diving, a right-to-left cardiac shunt occurs, redirecting blood from the cavum venosum to the systemic circuit instead of the pulmonary circuit.

3. Crocodile Heart:

   • The ventricle is completely divided into right and left chambers by a complete interventricular septum.
   • The pulmonary trunk and the left aortic arch open off the right ventricle, and the right aortic arch opens off the left ventricle.
   • Blood flow is regulated through the foramen of Panizza, connecting the right and left aortic arches.
   • Semilunar valves prevent direct flow from the right ventricle to the aorta, ensuring a specific path for blood flow.

4. Comparison:

   • The heart in crocodiles shows variations, especially in the complete division of the ventricle and the presence of the foramen of Panizza.

Endothermic Heart in Birds and Mammals

Birds:

• Sinus Venosus: Reduced but still exists as a small discrete area.
• Conus Arteriosus: Transiently present in embryonic conditions, giving rise to the pulmonary trunk and a single aortic trunk in the adult.
• Pacemaker: The sinus venosus reduction results in a patch of Purkinje fibers (sinoatrial node) in the right atrium, acting as a pacemaker.
• Valves: Birds possess semilunar and atrioventricular valves, similar to mammals.

Mammals:

• Sinus Venosus: Reduced to a patch of Purkinje fibers (sinoatrial node) in the right atrium, functioning as a pacemaker.
• Conus Arteriosus: During embryonic development, it splits to form the pulmonary trunk and a single aortic trunk.
• Valves: Mammals have semilunar valves and two sets of atrioventricular valves: tricuspid and bicuspid (mitral) valves.
• Double Circulation: The heart has evolved into a four-chambered structure with two atria and two ventricles, facilitating double circulation without cardiac shunting.

Comparisons:

• Structural Similarities: Although structurally similar, avian and mammalian hearts evolved independently from different reptilian groups, reflected in their embryonic development.
• Functional Similarities: Both avian and mammalian hearts function similarly, with parallel pumps supporting double circulation circuits. The right side gathers deoxygenated blood, pumping it into the pulmonary circuit, while the left side pumps oxygenated blood from the lungs into the systemic circuit.

Circulatory Patterns:

• Single Circulation (Fish): Blood passes only once through the heart during each complete circuit.
• Double Circulation (Birds and Mammals): Blood passes twice through the heart in every circuit, traveling from the heart to the lungs, then to systemic tissues, and back to the heart.

Evolutionary Significance:

• Double Circulation Evolution: The evolution of double circulation, where blood passes through the heart twice in a circuit, is considered an adaptive advantage for transitional forms that moved onto land.

The adaptations seen in the endothermic hearts of birds and mammals highlight their efficiency in supporting the high metabolic demands associated with endotherm.

Arterial System in Vertebrates: Aortic Arches Overview

• The arterial system in vertebrates, despite variations in arrangement, is built upon a fundamental plan.
• Evolutionary changes in heart complexity from lower forms to crocodilians, birds, and mammals are associated with modifications in the blood vascular system.
• Developmental modifications adapt aortic arches for respiration, either through gills or lungs.

Aortic Arches Overview:

• Embryonic Development: In all vertebrates, the anterior end of the ventral aorta divides into two arches, known as aortic arches, coursing dorsally into the mandibular region. Paired dorsal aortae continue posteriorly.
• Number of Aortic Arches: The basic embryonic pattern includes six pairs of aortic arches. Exceptions exist, such as eight in lamprey, fifteen in hagfishes, and ten or twelve in some shark species.
• Naming Convention: The arches are named mandibular, hyoid, and from III to VI using Roman numerals.
• Blood Flow: Blood pumped anteriorly by the heart passes through the ventral aorta to aortic arches, which carry it to paired dorsal aortae and ultimately to the single dorsal aorta for distribution to the body.

Fishes:

• Gill-Bearing Vertebrates: Aortic arches in gill-bearing vertebrates primarily facilitate blood flow through gills for oxygenation.
• Reduction in Number: Adult fishes show a reduction in the number of aortic arches. Elasmobranchs retain the IInd arch, while lungfishes adapt arches III and IV for lung respiration.
• Pulmonary Veins: Pulmonary veins bring oxygenated blood from the lungs to the heart.

Amphibians:

• Transition to Lung Respiration: Amphibians transition from gill to lung respiration during metamorphosis.
• Reduction of Arch Pairs: Larval frogs retain aortic arches III to VI, but arches I and II disappear. During metamorphosis, arch V is lost in anurans but retained in some urodeles.
• Carotid Arteries: The anterior continuation of the ventral aorta becomes external and internal carotid arteries.
• Pulmocutaneous Artery: Arch VI becomes the pulmocutaneous artery, supplying the developing lung and skin.

Caudate Amphibians:

• Fifth Arch Persistence: In some salamanders, the fifth arch may persist.
• Neotenic Salamanders: Neotenic salamanders like Necturus retain gills throughout life. In Necturus, the fifth arch persists, and the pulmonary artery arises from it.

These adaptations in the arterial system highlight the evolutionary transitions from aquatic to terrestrial life and the changes in respiratory organs across vertebrate groups.

Evolution of Aortic Arches in Vertebrates: Summary

• The evolution of aortic arches in vertebrates involves adaptations for different respiratory mechanisms and changes in the circulatory system.

1. Primitive Fishes:

   • Deliver deoxygenated blood to gill respiratory surfaces.
   • Distribute oxygenated blood to the head region via carotid arteries and to the body through the dorsal aorta.

2. Lung Fishes and Tetrapods:

   • Form two types of circuits:
     • Arterial circuit to lungs through pulmonary arch.
     • Arterial circuit to the rest of the body through systemic arches.
   • Carotid arteries supply blood to the head in tetrapods.

3. Reptiles:

   • Retain left and right arches.
   • Reduced to a single systemic arch in birds (right) and mammals (left).

4. Evolutionary Progression (See Fig. 5.24):

   • Hypothetical ancestor with six aortic arches corresponding to six pairs of gill pouches.
   • Primitive fishes (sharks) also have six paired gill arches.
   • Teleost fishes show a reduction in arches I and II, with only four pairs in caudal branchial arches.
   • Lungfishes have gills and a pulmonary circulation.
   • Amphibians lose gill arches during metamorphosis.
   • Reptiles like turtles retain bilateral fourth aortic arch.
   • Mammals and birds show further modifications, with asymmetrical aortic arch arteries.

• The evolution of aortic arches reflects adaptations to different respiratory organs and the transition from aquatic to terrestrial life in vertebrates. The changes highlight the diversity and specialization of the circulatory system across evolutionary stages.

Types of Arteries:

• Supply body regions derived from the embryonic epimere.
• Distributed to dorsal musculature and vertebral column.
• Named based on body regions, e.g., intercostal, dorsolumbar, sacral.

(2) Visceral Arteries:

• Paired Arteries: Segmentally arranged, supplying urinogenital organs. Terms include renal, genital, ovarian, spermatic, and urinogenital arteries.
• Unpaired Arteries: Supplying spleen and digestive tract derivatives.
  • Celiac Artery: Supplies anterior viscera (stomach, spleen, pancreas, liver, duodenum).
  • Superior Mesenteric Artery: Supplies the entire small intestine (except pyloric end of duodenum).
  • Inferior Mesenteric Artery: Supplies posterior part of large intestine and rectum.

Lymphatic System in Vertebrates: Overview

• Resembles the blood vascular system in structure.
• Comprises vessels, fluids (lymph), and associated organs.
• Lymphatics are thin-walled vessels with one-way valves similar to veins.

Key Differences:

• Lymph flows only towards the heart.
• Lymphatic system includes lymphatic tissue containing leukocytes, plasma cells, and macrophages.

Lymphatics and Lymph:

• Lymphatics penetrate soft tissues, starting as blind-end lymph capillaries collecting interstitial fluids.
• Fluid inside lymph capillaries is called lymph, a colorless or pale-yellow fluid containing metabolites and secretions.

Lymphatic Tissue:

• Consists of connective tissue and free cells (leukocytes, plasma cells, macrophages).
• Found diffusely, in patches, or encapsulated in lymph nodes.
• Lymph nodes act as checkpoints in the immune system.

Lymphatic System in Different Vertebrates:

1. Fishes:

   • Extensively developed lymphatic vessels.
   • Peripheral and deep channels following veins.
   • Lymph hearts may be absent or present (e.g., in eels and European catfish).
   • Lymph nodes are lacking.

2. Amphibians:

   • Two main sets of lymphatic vessels.
   • Superficial vessels beneath the skin and deeper channels following the dorsal aorta.
   • Lymph hearts (up to 20) in various forms.
   • More numerous in larval stages.

3. Reptiles:

   • Well-developed lymphatic system.
   • Large subvertebral trunk entering precaval veins.
   • Lymph hearts, especially in snakes, pumping into iliac veins.

4. Birds:

   • Lymphatic vessels enter two thoracic lymph ducts joining precaval veins.
   • Transitory lymph hearts during embryonic development.
   • Mainly lack lymph hearts in adulthood.

5. Mammals:

   • Lack lymph hearts.
   • Thoracic duct drains lymph from the posterior body, left side of the head, neck, and thoracic regions.
   • Numerous lymph nodes in superficial and body cavity regions, serving immune functions.

1. Muscular Activity: Various body parts' muscular activity.
2. Pressure: Built up in smaller vessels by osmosis and tissue fluid absorption.
3. Lymph Hearts: Pulsating lymph hearts with contractile walls, helping drive lymph.

• The lymphatic system, specialized for immune function, varies across vertebrates, showing adaptations to different environments and physiological needs.