Types of Closed Circulatory System | Single and Double Circulation

Types of Closed Circulatory System

The circulatory system is responsible for transporting nutrients, oxygen, hormones, and waste products throughout the body. In animals, two major types of circulatory systems exist: open and closed. 

In a closed circulatory system, the blood remains confined within blood vessels as it circulates, ensuring efficient and rapid transport of substances. This system is found in vertebrates and some invertebrates such as annelids.

Closed circulatory systems can be further classified into two main types: single circulation and double circulation. Let’s explore each in detail.

Single Circulatory System

The single circulatory system is the simpler form of closed circulation and is primarily observed in fishes. In this system, the heart pumps blood in one continuous loop through the body. Unlike double circulation, the blood passes through the heart only once during a complete cycle.

Structure of the Heart in Single Circulation

In organisms with single circulation, such as fishes, the heart is relatively simple compared to that of mammals and birds. It is designed to handle only deoxygenated blood and consists of four parts arranged in sequence, although only two are considered main chambers:

Sinus Venosus

• A thin-walled sac that first receives deoxygenated blood from the body.
• It acts as a collection chamber, regulating the smooth entry of blood into the atrium.

Atrium (Auricle)

• A muscular chamber that receives blood from the sinus venosus.
• Its contraction pushes the blood into the ventricle.

Ventricle

• A thick-walled, muscular chamber that pumps blood with strong force.
• It is responsible for sending blood to the gills through the ventral aorta.

Conus Arteriosus (or Bulbus Arteriosus in some fishes)

• A muscular or elastic structure located just beyond the ventricle.
• It contains valves that prevent the backflow of blood and helps to maintain continuous flow toward the gills.

Thus, although the heart is often described as having "two chambers" (atrium and ventricle), it effectively works as a four-part organ in fish, specialized for single circulation.

Pathway of Blood Flow in Single Circulation

The journey of blood in single circulation is straightforward but highly efficient for aquatic life. The cycle can be broken down into clear steps:

1. Collection of Deoxygenated Blood

Deoxygenated blood from various body tissues collects in the sinus venosus and then moves into the atrium.

2. Transfer to the Ventricle

The atrium contracts, pushing blood into the ventricle. The ventricle, being muscular, generates enough pressure to drive the blood out forcefully.

3. Pumping to the Gills

Blood is pumped into the ventral aorta, which branches into afferent branchial arteries leading to the gill capillaries.

At the gills, gas exchange occurs: carbon dioxide diffuses out into the water, and oxygen diffuses into the blood.

4. Distribution of Oxygenated Blood

From the gills, oxygen-rich blood flows into the dorsal aorta, which distributes it throughout the body. 

Organs and tissues receive oxygen and nutrients, and in turn, release carbon dioxide and other wastes into the blood.

5. Return to the Heart

Now deoxygenated, the blood flows back through veins into the sinus venosus, restarting the cycle.

This flow can be represented as:

Body → Sinus Venosus → Atrium → Ventricle → Conus Arteriosus → Gills (oxygenation) → Body → Heart

Characteristics of Single Circulation

The single circulatory system has several defining features that distinguish it from double circulation:

1. Single Loop of Circulation

Blood passes through the heart only once during a complete cycle.

The flow pattern is: Heart → Gills → Body → Heart.

2. Two-Chambered Heart

Functionally, the heart has one atrium and one ventricle, with supporting structures (sinus venosus and conus arteriosus).

It deals only with deoxygenated blood, unlike in double circulation where oxygenated and deoxygenated blood are separated.

3. Gills as the Site of Gas Exchange

Oxygenation of blood occurs at the gills, where oxygen from water diffuses into the blood and carbon dioxide diffuses out.

4. Low Blood Pressure Beyond the Gills

As blood flows through the narrow capillaries of the gills, there is a significant drop in pressure.

Consequently, blood moves more slowly when it reaches the systemic circulation (rest of the body).

5. Adaptation to Aquatic Life

This type of circulation is well-suited to fish and other aquatic organisms, whose metabolic demands are relatively lower compared to terrestrial animals.

Advantages of Single Circulation

Although it is less complex than double circulation, the single circulatory system has several benefits:

1. Simplicity of Design

The system is relatively straightforward and easy to maintain, reducing the energy cost of operating the circulatory system.

2. Efficiency in Water-Dwelling Animals

Fish and aquatic organisms live in buoyant environments that reduce their energy needs, making this circulation adequate for survival.

3. Direct Oxygen Delivery

Blood picks up oxygen at the gills and is immediately transported to the body tissues without passing through another chamber of the heart.

4. Energy Conservation

Because the system requires fewer heart chambers and a single circuit, it is less energetically expensive compared to a double circulatory system.

Limitations of Single Circulation

Despite its simplicity, the single circulatory system has several drawbacks that limit the activity level of animals that possess it:

1. Low Blood Pressure in Systemic Circulation

Blood loses much of its pressure after passing through the gill capillaries, which slows down its flow to the rest of the body.

2. Slower Oxygen Supply

Because of reduced pressure, tissues receive oxygen and nutrients at a slower rate. This restricts the organism’s ability to sustain high activity levels.

3. Limited Metabolic Efficiency

Fish and other animals with single circulation generally have lower metabolic rates and cannot sustain high levels of aerobic activity for long periods.

4. Less Adaptable to Terrestrial Environments

On land, where animals require higher oxygen delivery for walking, running, and other activities, this system would not provide enough efficiency. This is one reason why terrestrial vertebrates evolved double circulation.

Examples of Single Circulation

The single circulatory system is primarily found in aquatic vertebrates, particularly fishes, as it suits their oxygen and energy requirements. Some key examples include:

1. Bony Fishes (Osteichthyes)

Examples: Salmon, Goldfish, Tuna, Carp. 

These fishes have a two-chambered heart (atrium and ventricle) and rely on gills for oxygenation.

2. Cartilaginous Fishes (Chondrichthyes)

Examples: Sharks, Rays, Skates.

Similar to bony fishes, they have a simple two-chambered heart and a single-loop circulation.

3. Jawless Fishes (Agnatha)

Examples: Lampreys, Hagfish. 

These primitive fishes also exhibit single circulation with a simple heart structure.

All these animals live in aquatic environments, where buoyancy reduces energy demands, making single circulation sufficient for transporting oxygen and nutrients.

Double Circulatory System

The double circulatory system is an advanced type of closed circulatory system found in amphibians, reptiles, birds, and mammals. It is called “double circulation” because blood passes through the heart twice during a single complete cycle through the body.

This system allows for efficient separation of oxygen-rich and oxygen-poor blood, which is crucial for animals with higher metabolic demands. 

In terrestrial environments, oxygen needs are greater due to increased activity, thermoregulation, and complex organ function. The double circulatory system evolved to meet these needs, enabling faster and more effective delivery of oxygen and nutrients to tissues.

Key Advantages in Evolution

Supports high activity levels, such as running, flying, or hunting.

Maintains higher blood pressure in systemic circulation, ensuring rapid oxygen delivery. 

Prevents mixing of oxygenated and deoxygenated blood (especially in birds and mammals), optimizing energy efficiency.

Key Features of Double Circulatory System

The double circulatory system is a more complex and efficient type of closed circulation, found in amphibians, reptiles, birds, and mammals. 

It is called “double” because the blood passes through the heart twice during each complete circuit of the body. 

This arrangement allows oxygen-rich and oxygen-poor blood to be separated, improving the efficiency of oxygen delivery to tissues. The key features are as follows:

1. Two Separate Circuits

The double circulatory system consists of two distinct loops:

Pulmonary Circulation

• Carries deoxygenated blood from the heart to the lungs (or lungs and skin in amphibians).
• Blood picks up oxygen and releases carbon dioxide.
• Oxygenated blood then returns to the heart.

Systemic Circulation

• Carries oxygenated blood from the heart to the rest of the body.
• Delivers oxygen and nutrients to tissues and organs and returns deoxygenated blood to the heart.

This separation ensures that oxygen-rich and oxygen-poor blood do not mix significantly (completely separated in mammals and birds).

2. Heart Chambers

The heart in double circulation is more complex than in single circulation:

Three-Chambered Heart (amphibians and most reptiles)

• Two atria and one ventricle.
• Some mixing of oxygenated and deoxygenated blood occurs.

Four-Chambered Heart (birds and mammals)

• Two atria and two ventricles.
• Complete separation of oxygen-rich and oxygen-poor blood ensures highly efficient circulation.

3. Higher Blood Pressure and Faster Circulation

Blood pumped to the systemic circuit maintains high pressure, allowing rapid delivery of oxygen and nutrients. 

Pulmonary circulation operates at a lower pressure, protecting delicate lung capillaries from damage.

4. Adaptation to High Metabolic Demands

Animals with double circulation, especially birds and mammals, have high metabolic rates.

The system supports endothermy (warm-bloodedness) by efficiently supplying oxygen to sustain constant body temperature and energy-intensive activities like flying, running, or hunting.

5. Efficient Oxygen Supply

Complete separation in four-chambered hearts ensures that tissues receive fully oxygenated blood, which maximizes energy production in cells.

This efficiency allows animals to be highly active and survive in terrestrial environments where oxygen demands are higher than in aquatic habitats.

6. Flexibility Across Species

The double circulatory system has evolved in different forms across vertebrates, providing adaptive advantages in both aquatic and terrestrial environments. For example:

• Amphibians: use pulmocutaneous circulation, sending blood to lungs and skin for oxygenation.
• Reptiles: partially divided ventricles reduce mixing of blood.
• Birds and mammals: fully divided four-chambered hearts for maximum efficiency.

Pathway of Blood Flow in Double Circulation

In double circulation, the blood follows two distinct circuits: pulmonary (or pulmocutaneous in amphibians) and systemic circulation.

1. Pulmonary (or Pulmocutaneous) Circulation

Step 1: Deoxygenated Blood to Heart

• Deoxygenated blood from body tissues returns to the right atrium of the heart.

Step 2: Right Ventricle to Lungs

• Blood is pumped from the right ventricle into the pulmonary artery.

Step 3: Gas Exchange in Lungs

• In the lungs (or lungs and skin in amphibians), blood releases carbon dioxide and picks up oxygen.

Step 4: Oxygenated Blood Returns to Heart

• Oxygen-rich blood flows through pulmonary veins into the left atrium.

2. Systemic Circulation

Step 1: Left Ventricle Pumps Blood

• Oxygenated blood from the left atrium moves into the left ventricle, which contracts to pump blood into the aorta.

Step 2: Delivery to Body Tissues

• Blood flows through arteries to deliver oxygen and nutrients to organs and tissues.

Step 3: Return of Deoxygenated Blood

• After exchanging gases and nutrients, deoxygenated blood returns to the right atrium, completing the cycle.

Simplified Flow Diagram:

Right Atrium → Right Ventricle → Lungs (Oxygenation) → Left Atrium → Left Ventricle → Body → Right Atrium

This double passage through the heart ensures that systemic tissues receive fully oxygenated blood at high pressure, while pulmonary circulation operates at lower pressure to protect delicate lung capillaries.

Advantages of Double Circulatory System

The double circulatory system offers significant benefits over single circulation, particularly for animals with higher metabolic demands. Its advantages can be grouped into physiological efficiency, adaptability, and evolutionary success.

1. Efficient Oxygen Transport

• The complete separation of oxygen-rich and oxygen-poor blood ensures that body tissues receive fully oxygenated blood.
• This is crucial for animals that require high levels of aerobic respiration, as it maximizes ATP production in cells.
• Efficient oxygen transport allows sustained activity, such as running, flying, swimming, or hunting.

2. High Systemic Blood Pressure

• Blood leaving the left ventricle is pumped at high pressure to systemic circulation.
• This ensures rapid delivery of oxygen and nutrients even to distant organs and tissues.
• Meanwhile, pulmonary circulation operates at lower pressure to avoid damage to delicate lung capillaries, maintaining a balance between efficiency and safety.

3. Supports Endothermy (Warm-Bloodedness)

• Birds and mammals maintain a constant body temperature regardless of the environment.
• Double circulation provides the oxygen and nutrients needed to sustain high metabolic rates, supporting energy-intensive thermoregulation.
• This allows survival in diverse climates, from freezing mountains to tropical forests.

4. Enhanced Metabolic Capacity

• By ensuring a continuous, high-pressure supply of oxygen-rich blood, the system supports high-energy lifestyles.
• Animals can perform prolonged or intense activities without fatigue, such as long-distance migration in birds or endurance hunting in mammals.

5. Efficient Removal of Metabolic Wastes

• Deoxygenated blood is returned to the heart and pumped to the lungs for carbon dioxide removal.
• This quick waste removal helps maintain homeostasis and prevents the buildup of toxic substances in tissues.

6. Adaptive Flexibility Across Habitats

• Double circulation allows animals to thrive in terrestrial, aerial, and aquatic environments.
• Amphibians, reptiles, birds, and mammals have modified versions of the system that suit their ecological niche:
• Amphibians use lungs and skin for oxygenation.
• Birds use high-pressure four-chambered hearts to support flight.
• Mammals support sustained activity and thermoregulation.

7. Supports Complex Organ Systems

• Efficient oxygen delivery enables the development of highly active brains, muscles, and organs.
• This has allowed mammals and birds to evolve complex behaviors, advanced learning abilities, and sophisticated locomotion.

The double circulatory system is a hallmark of highly efficient, adaptive, and evolutionarily advanced vertebrates. Its combination of high-pressure systemic circulation, complete separation of oxygenated and deoxygenated blood, and flexibility across species has been key to the success of terrestrial and aerial animals.

Limitations of Double Circulatory System

Despite its efficiency, double circulation has some limitations:

1. Complex Heart Structure

Requires a three or four-chambered heart, which is more complex to develop and maintain than a simple two-chambered heart in single circulation.

2. Energy Demands

Pumping blood twice through the heart requires more energy, increasing cardiac workload.

3. Partial Mixing in Some Reptiles

In three-chambered hearts (amphibians and most reptiles), some mixing of oxygenated and deoxygenated blood can occur, slightly reducing efficiency.

4. Dependence on Oxygen Supply

Highly active animals with double circulation are more vulnerable to oxygen deprivation, as tissues require a constant high level of oxygen.

Examples of Double Circulation

Double circulation is present in most terrestrial vertebrates and some highly active aquatic species. Key examples include:

1. Amphibians

• Examples: Frogs, Toads, Salamanders
• Have a three-chambered heart (two atria, one ventricle) with pulmocutaneous circulation, allowing gas exchange through both lungs and skin.

2. Reptiles

• Examples: Lizards, Snakes, Crocodiles
• Most reptiles have three-chambered hearts, but crocodiles have a four-chambered heart, minimizing mixing of blood.

3. Birds

• Examples: Eagles, Pigeons, Sparrows
• Possess a four-chambered heart, with complete separation of oxygenated and deoxygenated blood. This supports high metabolic rates for flight.

4. Mammals

• Examples: Humans, Tigers, Whales, Elephants
• All mammals have a four-chambered heart, ensuring efficient oxygen delivery to support warm-bloodedness and high activity levels.

Double circulation has evolved to meet the demands of active lifestyles and terrestrial environments, providing a significant survival advantage over single circulation in these groups.

Conclusion

The circulatory system is a vital component of all animals, ensuring the transport of oxygen, nutrients, hormones, and wastes throughout the body. Among vertebrates, the closed circulatory system provides an efficient and organized means of circulation.

There are two main types of closed circulation:

Single Circulation: Found in fishes, where blood passes through the heart once per cycle. While simple and energy-efficient, it has lower blood pressure and slower oxygen delivery.

Double Circulation: Found in amphibians, reptiles, birds, and mammals, where blood passes through the heart twice. This system separates oxygenated and deoxygenated blood, supports higher metabolic rates, and allows active lifestyles.

Overall, the evolution from single to double circulation reflects the increasing metabolic demands and activity levels of animals. The efficiency of double circulation has enabled vertebrates to occupy diverse habitats and develop complex behaviors, making it a key factor in their evolutionary success.

Short Questions and Answers

1. What is a single circulatory system?

A. A system in which blood passes through the heart only once during each complete circuit. Common in fishes.

2. What is a double circulatory system?

A. A system in which blood passes through the heart twice per complete circuit, separating oxygenated and deoxygenated blood. Found in amphibians, reptiles, birds, and mammals.

3. What are the main advantages of double circulation?

A. Efficient oxygen delivery, high systemic blood pressure, supports high metabolic rates, thermoregulation, and active lifestyles.

4. Give examples of animals with single circulation.

A. Bony fishes (goldfish, tuna), cartilaginous fishes (sharks, rays), and jawless fishes (lampreys).

5. Give examples of animals with double circulation.

A. Amphibians (frogs, salamanders), reptiles (lizards, crocodiles), birds (pigeons, eagles), and mammals (humans, whales, tigers).