NCERT Solutions for Class 11 Biology Chapter 18 – Body Fluids and Circulation
If you have been wondering why your heart beats continuously without taking a break, Chapter 18 of Class 11 Biology is where you find the answer. Body Fluids and Circulation is one of the most important chapters in the NCERT Biology textbook, and students preparing for CBSE board exams as well as NEET will find it unavoidable. Must check the CBSE resources and NCERT Solutions.
This chapter explains the composition of blood and lymph, the structure of the human heart, the electrical conduction system, and the different types of circulatory systems found in animals. The NCERT Solutions for Class 11 Biology Chapter 18, available on Myclass24, are written by subject experts and cover each question with detailed, step-by-step explanations so that you do not just memorise answers but actually understand the concepts. Whether you are revising blood groups before an exam or trying to understand cardiac output, these solutions will make your preparation more confident and organised.
Download PDF – NCERT Solutions for Class 11 Biology Chapter 18 Body Fluids and Circulation
Students can download the chapter-wise PDF of NCERT Solutions for Class 11 Biology Chapter 18 from Myclass24. The PDF covers all textbook questions including short-answer, long-answer, and diagram-based questions with properly labelled illustrations. It is formatted to be mobile-friendly so you can read it easily on your phone during revision. The PDF is completely free and does not require any registration.
Chapter 18 Body Fluids and Circulation – Detailed Notes and Key Facts
Body Fluids and Circulation is a chapter that deals with how substances are transported throughout the body. Humans have a closed circulatory system, meaning blood is always enclosed within blood vessels. This is different from open circulatory systems, such as in insects, where blood flows freely in body cavities called haemocoel. Let us look at the most important concepts from this chapter.
Blood is a connective tissue and is the primary body fluid responsible for transport. It is composed of plasma and formed elements. Plasma makes up about 55% of the blood and is largely water (around 90-92%), with proteins like albumin, globulin, and fibrinogen dissolved in it. The remaining 45% consists of formed elements, which include Red Blood Cells (RBCs), White Blood Cells (WBCs), and platelets. A healthy adult male has about 5 million RBCs per cubic millimetre of blood, while a healthy adult female has around 4.5 million. Check out NCERT Solutions for Class 11 Biology and NCERT Solutions for Class 11 for the rest of the chapters.
Haemoglobin is the pigment found in RBCs that carries oxygen. It is a conjugated protein with four iron-containing haem groups. Each haemoglobin molecule can carry four molecules of oxygen. Carbon dioxide is also carried by blood, with about 70% transported as bicarbonate ions in plasma, around 23% bound to haemoglobin as carbaminohaemoglobin, and the rest dissolved in plasma.
WBCs, or leucocytes, are part of the immune system. They are of two main types: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes and monocytes). Neutrophils are the most abundant WBCs. Platelets, also called thrombocytes, are essential for blood clotting.
The ABO blood grouping system, discovered by Karl Landsteiner, classifies human blood into four types: A, B, AB, and O. This system is based on the presence or absence of antigens A and B on the surface of RBCs and their corresponding antibodies in plasma. Blood group O is considered the universal donor, while AB is the universal recipient. The Rh factor is another important antigen. Individuals with the Rh antigen are Rh-positive, and those without are Rh-negative. Rh incompatibility between mother and foetus can cause erythroblastosis foetalis.
The lymphatic system is the second circulatory system in the body. Lymph is a pale yellow fluid similar in composition to plasma but with fewer proteins and no RBCs. Lymph capillaries absorb excess tissue fluid and fats from the intestine (as chyle) and return them to the blood. Lymph nodes filter the lymph and are important sites for immune responses.
Quick Comparison: Open vs Closed Circulatory System
| Feature | Open Circulatory System | Closed Circulatory System |
|---|---|---|
| Blood Vessels | Absent or poorly developed | Well-developed arteries, veins, capillaries |
| Blood Flow | Flows freely in body cavity (haemocoel) | Always enclosed in blood vessels |
| Pressure | Low | High |
| Speed of Transport | Slow | Fast |
| Examples | Insects, Molluscs (except cephalopods) | Annelids, Vertebrates including humans |
Blood Components at a Glance
| Component | Normal Count / Value | Main Function |
|---|---|---|
| RBCs (Erythrocytes) | 4.5–5.5 million/mm³ | Oxygen and CO₂ transport |
| WBCs (Leucocytes) | 6,000–8,000/mm³ | Immunity and defence |
| Platelets (Thrombocytes) | 1.5–3.5 lakh/mm³ | Blood clotting |
| Plasma | ~55% of blood volume | Transport of nutrients, hormones, waste |
| Haemoglobin | 12–18 g/dL | Carries O₂ via haem groups |
The human heart is a four-chambered muscular organ located in the thoracic cavity between the two lungs in a region called the mediastinum. It is protected by a double-layered membrane called the pericardium. The heart has two atria (upper chambers) and two ventricles (lower chambers). The right side of the heart deals with deoxygenated blood, and the left side handles oxygenated blood.
Valves prevent backflow of blood within the heart. The tricuspid valve lies between the right atrium and right ventricle; the bicuspid (mitral) valve lies between the left atrium and left ventricle. The semilunar valves guard the exit of blood from the ventricles into the pulmonary artery and aorta. The lub-dub sound of the heartbeat is produced by the closure of these valves.
The heart has its own electrical conduction system. The Sino-Atrial (SA) node, located in the right atrium, generates electrical impulses and is called the pacemaker of the heart. The impulse travels to the Atrio-Ventricular (AV) node, then through the Bundle of His and Purkinje fibres to cause coordinated contraction of the ventricles. A normal heartbeat rate is 70–75 beats per minute.
The cardiac cycle refers to one complete sequence of events in the heart, lasting about 0.8 seconds. It includes atrial systole (0.1 sec), ventricular systole (0.3 sec), and joint diastole (0.4 sec). Cardiac output is the volume of blood pumped by each ventricle per minute. With an average stroke volume of 70 mL and 72 beats per minute, cardiac output is approximately 5040 mL (about 5 litres per minute).
Cardiac Cycle – Time Chart
| Phase | Duration | Event |
|---|---|---|
| Atrial Systole | 0.1 seconds | Atria contract, blood pushed into ventricles |
| Ventricular Systole | 0.3 seconds | Ventricles contract, blood pumped to lungs and body |
| Joint Diastole | 0.4 seconds | Both atria and ventricles relax, heart fills with blood |
| Total Cardiac Cycle | 0.8 seconds | ~75 cycles per minute |
Electrocardiography (ECG) is a diagnostic tool that records the electrical activity of the heart. A normal ECG shows the P wave (atrial depolarisation), QRS complex (ventricular depolarisation), and T wave (ventricular repolarisation). Any deviation from the normal ECG pattern can indicate cardiac disorders.
Blood pressure is the pressure exerted by blood on the walls of blood vessels. Normal blood pressure in a healthy adult is 120/80 mm Hg (systolic/diastolic). Hypertension (high blood pressure) and hypotension (low blood pressure) are common circulatory disorders. Angina pectoris is chest pain caused by reduced blood supply to the heart. Myocardial infarction (heart attack) occurs when blood supply to part of the heart muscle is blocked.
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