Biology Class 11 NCERT Solutions: Chapter 18 Body Fluids and Circulation Part 3

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Q: 8. Describe the evolutionary change in the pattern of heart among the vertebrates.

Answer:

All vertebrates possess a heart - a hollow muscular organ composed of cardiac muscle fibres. The function of the heart is to pump oxygen to all parts of the body. The evolution of the heart is based on the separation of oxygenated blood from deoxygenated blood for efficient oxygen transport.

In fishes, the heart was like a hollow tube. This evolved into the four-chambered heart in mammals.

Piscean heart:

Fish has only two chambers in its heart - one auricle and one ventricle. Since both the auricle and the ventricle remain undivided, only deoxygenated blood passes through it. The deoxygenated blood enters the gills for oxygenation from the ventricle. It has additional chambers such as sinus venosus and conus arteriosus.

Q 8 Image of Piscean Heart

Q 8 Image of Piscean Heart

Q 8 Image of Piscean Heart

Amphibian Heart:

Amphibians, such as frogs, have three-chambered hearts, with two auricles and one ventricle. The auricle is divided into a right and a left chamber by an inter-auricular septum, while the ventricle remains undivided.

Additional chambers such as sinus venosus and conus arteriosus are also present. The oxygenated blood from the lungs enters the left auricle and simultaneously, the deoxygenated blood from the body enters the right auricle. Both these auricles empty into the ventricle, wherein the oxygenated and deoxygenated blood gets mixed to some extent.

Q 8 1 Image of Amphibian Heart

Q 8 1 Image of Amphibian Heart

Q 8 1 Image of Amphibian Heart

Reptilian Heart:

Reptiles have incomplete four-chambered hearts, except for crocodiles, alligators, and gharials. They have only one accessory chamber called sinus venosus. The reptilian heart also shows mixed blood circulation.

Q 8 2 Image of Reptilian Heart

Q 8 2 Image of Reptilian Heart

Q 8 2 Image of Reptilian Heart

Avian and mammalian hearts:

They have two pairs of chambers for separating oxygenated and deoxygenated bloods. The heart is divided into four chambers. The upper two chambers are called atria and the lower two chambers are called ventricles. The chambers are separated by a muscular wall that prevents the mixing of the blood rich in oxygen with the blood rich in carbon dioxide.

Q 8 3 Image of Avian and Mammalian Heart

Q 8 3 Image of Avian and Mammalian Heart

Q 8 3 Image of Avian and Mammalian Heart

Q: 19. Why do we call our heart myogenic?

Answer:

In the human heart, contraction is initiated by a special modified heart muscle known as sinoatrial node. It is located in the right atrium. The SA node has the inherent power of generating a wave of contraction and controlling the heartbeat. Hence, it is known as the pacemaker. Since the heart beat is initiated by the SA node and the impulse of contraction originates in the heart itself, the human heart is termed myogenic. The hearts of vertebrates and molluscs are also myogenic.

Q: 10. Sino-atrial node is called the pacemaker of our heart. Why?

Answer:

The sino-atrial (SA) node is a specialised bundle of neurons located in the upper part of the right atrium of the heart. The cardiac impulse originating from the SA node triggers a sequence of electrical events in the heart, thereby controlling the sequence of muscle contraction that pumps blood out of the heart. Since the SA node initiates and maintains the rhythmicity of the heart, it is known as the natural pacemaker of the human body.

Image shows the Sino-atrial node

Image Shows the Sino-Atrial Node

Image shows the Sino-atrial node