Problem 171 - Objective Biology for NEET

Question

Assertion: Circulation in amphibians and reptiles is called incomplete double circulation. Reason: Deoxygenated and oxygenated blood get mixed up in the single ventricle pumps out mixed blood.

Step-by-Step Solution

Answer

Both the assertion and reason are correct, and the reason is the correct explanation for the assertion.

Step 1: Identifying the Circulation Type

Incomplete double circulation is a type of circulatory system where the heart pumps blood to the lungs and the body in two separate pathways, but there is some mixing of oxygenated and deoxygenated blood.

Step 2: Understanding the Heart Structure

Amphibians and reptiles, except crocodiles, possess three-chambered hearts, meaning they have two atria and one ventricle. The pulmonary and systemic circulations are linked by this ventricle.

Step 3: Understanding Blood Mixing

Because both deoxygenated blood (coming from the body to the heart) and oxygenated blood (coming from the lungs to the heart) are received by the heart and must pass through a common ventricle, there is some mixing of the blood prior to being sent out to the body again. This leads to the blood being 'mixed' or incompletely separated.

Step 4: Verifying the Assertion and Reason

Based on the heart structure in amphibians and reptiles and the fact that there is some mixing of oxygenated and deoxygenated blood, the assertion is correct that circulation in amphibians and reptiles is called incomplete double circulation. Additionally, the reason that this occurs is also correct because deoxygenated and oxygenated blood gets mixed up in the single ventricle that pumps out mixed blood.

Key Concepts

Circulatory System Overview

The circulatory system is the body's delivery system, responsible for transporting nutrients, oxygen, and hormones to cells throughout the body, and removing metabolic wastes such as carbon dioxide and nitrogenous waste. It typically consists of the heart, a series of blood vessels, and the blood itself.

There are two primary types of circulatory systems found in animals: open and closed. In an open circulatory system, the blood is not always contained within blood vessels and can bathe organs directly. In contrast, a closed circulatory system means that blood circulates entirely within blood vessels, providing a more efficient system of delivery to specific areas of the body.

In terms of complexity, circulation can be categorized as single circulation, which is found in fish where the blood passes through the heart once per circuit of the body. On the other hand, double circulation, found in amphibians, reptiles, and mammals, involves blood passing through the heart twice - once to be oxygenated in the lungs or gills and once to deliver oxygen to the rest of the body. However, in some creatures like amphibians and most reptiles, this double circulation is 'incomplete' due to the mixing of blood.

Amphibian Heart Structure

Amphibians, such as frogs and salamanders, possess a unique heart structure that is pivotal to their circulatory system. Their hearts have three chambers: two atria and one ventricle. The right atrium receives deoxygenated blood from the body, whereas the left atrium receives oxygenated blood from the lungs or skin.

Path of Blood Flow in the Amphibian Heart

Once blood enters the atria, it moves into the single ventricle. Due to the presence of only one ventricle, there is an inevitable mixing of oxygen-rich and oxygen-poor blood. This inefficiency is somewhat mitigated by the timing of heart contractions and physical structures within the heart that help to direct blood flow, reducing but not eliminating the mixture.

The mixed blood is then pumped to both the lungs and body. The skin also plays a significant role in gas exchange in amphibians, thus complementing the function of their hearts and compensating for the imperfect separation of blood.

Blood Mixing in Reptiles

Reptilian circulatory systems are a step up in evolution from amphibians but still possess characteristics of incomplete double circulation. Most reptiles have a heart with three chambers similar to amphibians - two atria and one ventricle.

Function and Adaptation

However, within the reptilian ventricle, there are partial septa that somewhat divide the ventricle into distinct regions, which help to reduce the mixing of oxygenated and deoxygenated blood. This anatomical feature allows reptiles to be more efficient than amphibians in their circulatory process, but it is not as effective as the complete separation seen in mammals and birds.

As cold-blooded creatures, reptiles can tolerate lower levels of oxygen in the blood, which is an adaptation that helps them survive with blood mixing. Nevertheless, this incompletely separated circulation is one reason why reptiles are less active and have a lower metabolic rate compared to animals with complete double circulation, such as mammals and birds, whose four-chambered hearts ensure that oxygen-rich and oxygen-poor blood do not mix at all.