CBSE Class 11 Biology Exemplar NCERT Solutions: Body Fluids and Circulation

Basophils are a type of granulocyte, making up a very small percentage (0.01% to 0.3%) of circulating white blood cells. Their primary role is in mediating specific inflammatory responses, particularly those associated with allergic reactions. Unlike other white blood cells such as monocytes, neutrophils, and macrophages, basophils do not exhibit significant phagocytotic activity. Monocytes can migrate into tissues and differentiate into macrophages, which are highly phagocytic. Neutrophils are also actively phagocytic, targeting bacteria and fungi. Macrophages are specialized cells that engulf and digest cellular debris, foreign substances, and microbes.

Dengue fever is a viral infection transmitted by mosquitoes. One of the most common and concerning symptoms is a significant drop in the platelet count (thrombocytopenia). Low platelet levels can impair blood clotting, leading to an increased risk of bleeding, which can be life-threatening. While other blood cell counts might be affected to some extent, a drastic reduction in platelets is a hallmark of severe dengue infections.

A cardiac cycle encompasses one complete heartbeat, involving the coordinated contraction (systole) and relaxation (diastole) of the heart chambers. The primary function of the cardiac cycle is to efficiently pump blood throughout the body. The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery, while the left ventricle pumps oxygenated blood to the rest of the body via the aorta. For the circulatory system to function effectively, the volume of blood ejected by both the right and left ventricles during each cycle (known as stroke volume) must be equal. Any significant imbalance can lead to serious health issues, including heart failure. Therefore, options (b), (c), and (d) are incorrect as they suggest differences in volumes that should ideally be matched for proper circulation.

The autonomous neural system plays a crucial role in regulating cardiac function. Neural signals transmitted through the sympathetic nerves can significantly increase the heart rate and the force of ventricular contraction (stroke volume). This leads to an overall increase in cardiac output, preparing the body for 'fight or flight' responses. Conversely, the parasympathetic nervous system, primarily via the vagus nerve, generally has an inhibitory effect, slowing down the heart rate. Therefore, the sympathetic system is responsible for stimulating both heart rate and stroke volume, making option (b) the correct answer. Options (a), (c), and (d) incorrectly describe the functions of these systems.

Blood coagulation, or clotting, is a vital process that prevents excessive bleeding following an injury. This complex process requires the presence of specific factors. At the site of injury, blood platelets disintegrate and release substances that initiate clotting. Key among these are platelet factors, such as platelet factor 3 (also known as platelet thromboplastin), and thromboplastin released from injured tissues. Calcium ions ($C{a}^{2+}$) are also essential cofactors that play a critical role in several steps of the coagulation cascade. Heparin, on the other hand, is an anticoagulant that inhibits clotting. Oxalates and citrates are also anticoagulants, often used in blood collection tubes to prevent clotting by binding with calcium ions. Therefore, calcium ions and platelet factors are the essential pair for initiating blood coagulation.