Circulatory System

The circulatory system is a vital transport network found in most animals, responsible for the movement of essential substances like nutrients, gases, hormones, and waste products throughout the body. While its complexity varies across species, the fundamental function remains the same: to ensure every cell receives what it needs to survive and function.

1.0Types of Circulatory Systems

Animal circulatory systems are broadly classified into two main types:

• Open Circulatory System: In this system, blood (or a fluid called hemolymph) is not always contained within vessels. It is pumped by a heart into a body cavity called a hemocoel, where it directly bathes the organs and tissues. The fluid then re-enters the heart through openings called ostia. This system is less efficient and is typical in invertebrates like insects, spiders, and most molluscs.
• Closed Circulatory System: Here, blood is continuously confined within a network of blood vessels. A heart pumps blood through arteries, which branch into capillaries, and then flows back to the heart through veins. This allows for more efficient and rapid transport. This system is found in vertebrates (including humans), some invertebrates like earthworms, and cephalopods (e.g., squids, octopuses).

2.0Components of the Circulatory System

Regardless of the type, the core components of a circulatory system are:

1. Heart: The muscular organ that acts as the pump, generating pressure to move the circulatory fluid. The structure of the heart varies. For instance, fish have a two-chambered heart, amphibians and most reptiles have a three-chambered heart, and birds and mammals have a four-chambered heart.

2. Circulatory Fluid: This is the medium that transports substances.

• Blood: A specialised fluid connective tissue in closed systems, consisting of plasma and blood cells(like red blood cells, white blood cells, and platelets).
• Hemolymph: The fluid in open systems, which combines the functions of blood and interstitial fluid.

3. Blood Vessels: The tubes that carry blood in a closed system.

• Arteries: Carry blood away from the heart.
• Veins: Carry blood back to the heart.
• Capillaries: Tiny vessels where the exchange of gases, nutrients, and waste occurs between the blood and tissues.

Key Functions

• Nutrient and Oxygen Delivery: Transports oxygen and nutrients to cells, muscles, and organs. 
• Waste Removal: Carries away waste products, such as carbon dioxide, from cells for elimination from the body. 
• Hormone Transport: Delivers hormones to target organs. 
• Temperature Regulation: Helps to distribute heat throughout the body. 
• Immune Defence: Transports white blood cells and other immune components to fight pathogens. 

3.0Circulatory System Process

The circulatory process involves the following steps:

1. Circulatory fluid receives oxygen and nutrients from the respiratory and digestive systems.
2. The pumping organ pushes this fluid through a network of vessels.
3. The fluid reaches tissues, where exchange of gases, nutrients, and wastes takes place.
4. Deoxygenated fluid returns to the pumping organ or reservoir for reoxygenation or purification.

This continuous loop sustains metabolic activities and homeostasis.

4.0Types of Circulation in Vertebrates

Vertebrates exhibit different patterns of circulation that correspond to their heart structure:

• Single Circulation: Found in fish, blood passes through the heart once during a complete circuit. The two-chambered heart pumps deoxygenated blood to the gills, where it is oxygenated, and then it flows directly to the rest of the body before returning to the heart.
• Double Circulation: Found in amphibians, reptiles, birds, and mammals, blood passes through the heart twice in one complete circuit. This system is more efficient because it separates the circulation to the lungs from that to the body tissues. It consists of two loops:
• • Pulmonary Circulation: The loop that transports blood between the heart and the lungs for gas exchange.
  • Systemic Circulation: The loop that transports blood between the heart and the rest of the body.

5.0Heart and Circulatory Organs

The heart is a muscular pumping organ found in most animals with a closed system.

• In fish, the heart has two chambers (one atrium, one ventricle).
• In amphibians, it has three chambers.
• In reptiles, it is three or partially four-chambered.
• In birds and mammals, it is four-chambered, allowing complete separation of oxygenated and deoxygenated blood.

In simpler animals like insects, the “heart” is a tubular structure that pumps hemolymph through the body cavity.

6.0Circulatory Adaptations in Different Organisms

Various organisms have evolved unique circulatory adaptations:

• Sponges and cnidarians lack a true circulatory system; exchange occurs via diffusion.
• Annelids like earthworms have a closed system with multiple hearts.
• Insects have an open system with dorsal vessels and hemolymph.
• Fish exhibit single circulation with gill-based oxygenation.
• Amphibians and reptiles show partial separation of blood.
• Birds and mammals have a double circulation system for efficient oxygen supply.

These adaptations reflect evolutionary specialisation based on complexity and metabolic demand.

7.0Functions of the Circulatory System

The circulatory system performs several critical functions to maintain homeostasis:

• Transport of Gases: Delivers oxygen from the lungs (or gills) to the body's cells and carries carbon dioxide back to be expelled.
• Transport of Nutrients and Hormones: Distributes nutrients absorbed from the digestive system and hormones from endocrine glands to target cells.
• Waste Removal: Collects metabolic waste products, such as urea, and transports them to organs like the kidneys for removal.
• Immune Response: The white blood cells circulating in the blood are crucial for fighting infections and diseases.
• Thermoregulation: Helps regulate body temperature by distributing heat.