Circulatory System: Evolution, Structure and Function in Organisms and Humans

The circulatory system is essential for transporting nutrients, oxygen, and waste products throughout the body. Different organisms have developed unique circulatory patterns to meet their metabolic needs. Vertebrates, for instance, have evolved hearts with varying chambers, from the two-chambered heart in fish to the four-chambered heart in birds and mammals. The human circulatory system, comprising the heart, blood vessels, and blood, ensures efficient circulation and overall bodily function.

Understanding Circulatory Systems: From Organisms to Humans

A. Circulatory Patterns in Different Groups of Organisms

• Transoprt Mechanism in Organisms: Different groups of organisms have evolved diverse methods of transport.
  • These methods ensure the delivery of nutrients, oxygen, and other essential substances to their cells.
  • They also facilitate the removal of waste and harmful substances produced by various metabolic processes.
• Types of Circulatory Patterns: Two types of circulatory patterns exist:
  • Open Circulatory System: Found in arthropods and molluscs, where blood pumped by the heart flows through large vessels into open spaces or body cavities known as sinuses.
  • Closed Circulatory System: Present in annelids and chordates, where the heart pumps blood through a closed network of blood vessels.
• Circulatory Systems in Vertebrates: All vertebrates feature a muscular, chambered heart. A brief outline of circulatory systems in different groups of vertebrates:
  • Fishes have a 2-chambered heart with an atrium and a ventricle.  
    • In fish, the heart pumps out deoxygenated blood, which is oxygenated by the gills and supplied to the body parts, from where deoxygenated blood is returned to the heart (single circulation).
  • Amphibians and reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle. 
    • In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from other body parts. 
    • However, they get mixed up in the single ventricle, which pumps out mixed blood (incomplete double circulation).
  • Crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles. In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria, respectively, passes on to the ventricles of the same sides. 
    • The ventricles pump it out without any mixing up, i.e., two separate circulatory pathways are present in these organisms; hence, these animals have double circulation.

B. Human Circulatory System

• Components Human Circulatory System: It (also called the blood vascular system) consists of amuscular,r chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated.

• Heart: It is situated in the thoracic cavity, in between the two lungs, slightly tilted to the left and is the size of a clenched fist. 
  • Heart Chambers: Our heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called ventricles (separated from each other by walls called septum). 
  • Septal Openings: However, each of these septums is provided with an opening (guarded by valves– tricuspid or bicuspid) through which the two chambers on the same side are connected. 
  • Valves: The valves in the heart allow only unidirectional flow of blood, preventing any backward flow.
• Cardiac Cycle: The sequential pumping of the heart, which is cyclically repeated, is called the cardiac cycle, and it consists of the systole (contraction) and diastole (expansion) of both the atria and ventricles. 
  • Heart Rate and Stroke Volume: The heart beats 72 times per minute (beats per minute/bpm), and during a cardiac cycle, each ventricle pumps out approximately 70 ml of blood, which is called the stroke volume. 
  • Cardiac Output: The stroke volume multiplied by the heart rate (bpm) gives the cardiac output. 
    • The body has the ability to alter the stroke volume as well as the heart rate and, thereby, the cardiac output
    • Example: the cardiac output of an athlete will be much higher than that of an ordinary man.
• Blood Vessels: The blood flows strictly by a fixed route through the Blood Vessels-the arteries, veins and capillaries. 
  • Anatomy of Arteries and Veins: Each artery and vein consists of three layers: 
    • Tunica Intim: an inner lining of squamous endothelium, the; 
    • Tunica Media: a middle layer of smooth muscle and elastic fibres, the, 
    • Tunica Externa: external layer of fibrous connective tissue with collagen fibres, the tunica externa. 
    • The tunica media is comparatively thin in the veins. 
  • Oxygenation Status of Arteries and Veins: All arteries except the pulmonary artery (which carries blood from the heart to lungs) carry oxygen-rich blood, and all veins except the pulmonary vein  (which carries blood from the lungs to heart) carry deoxygenated blood. 
  • Capillaries: These are the smallest and most numerous of the blood vessels. These form the connection between arteries and veins. 
    • The primary function of capillaries is the exchange of materials between the blood and tissue cells.

Conclusion

• The circulatory system plays a crucial role in maintaining health by transporting essential substances and removing waste. 
• The evolution of different circulatory patterns in organisms highlights its importance. In humans, the heart, blood vessels, and blood work together seamlessly to sustain life. 
• Understanding this system underscores its significance in supporting metabolic processes and overall well-being.

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