Circulatory System & Its Types MCQ - Practice Questions with Answers

The circulatory system and its types

• Simple organisms such as sponges and coelenterates have a mechanism to circulate water of their surroundings through their body cavities 
• The complex organisms including humans have special body fluids such as blood and lymph to circulate these substances
• The system of organs which is primarily concerned with the circulation of substances through the body fluids is called the circulatory system. 

Types of the circulatory system

• The two basic types of circulatory systems that have evolved in animals are open circulatory system and closed circulatory system 
• These circulatory systems can be differentiated by the presence or absence of capillaries at tissue level

Open circulatory system

• In this system, the blood comes in direct contact with tissue cells.
• Blood may or may not flow in larger blood vessels
• When vessels are completely absent, blood flows in blood sinuses or lacunae or open spaces e.g arthropods
• In most molluscs, blood flows in larger blood vessels but at tissue level vessels are absent

Closed circulatory system

• In this system, blood does not come in direct contact with tissue cells. 
• In this circulation, fine blood vessels called capillaries are present at the tissue level to prevent direct connectivity of tissue and blood, e.g. in annelids, echinoderms, some molluscs and all vertebrates
• The closed circulatory system increases the efficiency of circulation and regulates blood flow. 

Circulation circuits

• The manner in which body fluids circulate through their circulatory system is called circulation circuits. 
• There are two types of circulation circuits 

                1. Single circuit
                2. Double circuit
                       - Incomplete or mixed double circuit
                       - Complete double circuit
Single circuit 

• The heart of fish is two-chambered having an atrium and ventricle.
• Heart pumps deoxygenated blood to the gills where it is oxygenated by the gills.
• The oxygenated blood is then continues through the rest of the body before arriving back at the atrium to complete the circuit. This is called systemic circulation.
• Thus, the flow of blood is unidirectional as there is only a single circulation i.e. systemic circulation

 Incomplete or mixed double circuit

• The double circuit contains two distinct loops which operates through a 3 or 4-chambered heart. 

The Incomplete or mixed double circuit in amphibians

• Incomplete  double circuit is found in amphibians and reptiles. 
• Amphibians and reptiles possess three-chambered heart, with two atria and one ventricle. 
• The left atrium receives oxygenated blood from gills/skin/lungs whereas the right atrium receives deoxygenated blood from body parts
• Atria receive blood from two different routes- 

             - Through the lungs/gills/skin known as pulmonary circulation 
             - From the rest of the body and its organs, including the brain known as systemic circulation

• The oxygenated and deoxygenated blood gets mixed up in the single ventricle which then pumps out mixed blood. This reduces the efficiency of oxygenation
• Due to this mixing of blood in circulation, it is called incomplete or mixed double circuit.
• The mixing is mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit where gas exchange occurs in the lungs and through the skin. 
• In amphibians, there is a SA node present in the sinus venosus.
• SA node is responsible for the pulsatory activity of the heart

The incomplete or mixed double circuit in reptiles 

• The mixed or incomplete circuit of reptiles is much more complicated than that of amphibians as they have a three-chambered heart with incompletely divided ventricle (except in crocodiles as they have 4-chambered heart)
• Sinus venosus is present in reptiles but is less functional
• The conus arteriosus is divided into right systemic, left systemic and pulmonary arches
• The left and right systemic arches are connected through a connection tube called foramen of panizzae
• The SA node is present in the right auricle. 

Complete double circuit 

• This type of circuit is found in animals with four-chambered heart with two atria and two ventricles e.g. crocodiles, birds and mammals 
• In this circulation circuit, oxygenated and deoxygenated blood are completely separated.
• Sinus venosus and truncus arteriosus are absent
• It involves pacemaker i.e. SA node and pacesetter i.e.AV Node
• The complete double circuit involves two circuits 

I. Systemic circulation

• This circuit involves circulation of blood between heart and body organs
• The left atrium receives oxygenated blood from the lungs. This blood is pumped out by the left ventricle into the systemic aorta, which supplies it to all the body parts which take oxygen from it
• The deoxygenated blood from these body parts is then returned to the right auricle through two large veins called the superior and inferior vena cava
• This circuit supplies blood to most of the tissues
• Left Ventricle ------------- Body------------Right Auricle

II. Pulmonary circulation

• This circuit involves the circulation of deoxygenated blood received from different body parts (except the lungs)
• This blood is pumped out by the right ventricle to the pulmonary arch through which it reaches to lungs for oxygenation. 
• The oxygenated blood is then returned to the left auricle by pulmonary veins. 
• Right Ventricle ---------- Lungs ---------  Left Auricle