Body Fluids and Circulation Class 11 Biology Notes

Key Definitions

Blood

A specialized connective tissue consisting of a fluid matrix (plasma) and formed elements (RBCs, WBCs, platelets).

Lymph

A colourless fluid containing specialized lymphocytes, formed from tissue fluid, important for immunity and fat absorption.

Cardiac Cycle

The sequence of events (atrial and ventricular systole/diastole) that occur from the beginning of one heartbeat to the beginning of the next.

ECG (Electrocardiograph)

A graphical representation of the electrical activity of the heart during a cardiac cycle, used to diagnose heart conditions.

Double Circulation

A circulatory system where blood passes through the heart twice for each complete cycle (pulmonary and systemic circulation).

Hypertension

High blood pressure, defined as blood pressure equal to or higher than 140/90 mmHg, a major risk factor for heart diseases and other ailments.

Systole

The phase of the cardiac cycle when the heart muscle contracts, pumping blood out of the chambers.

Diastole

The phase of the cardiac cycle when the heart muscle relaxes and the chambers fill with blood.

Composition and Functions of Blood

Blood, a vital fluid connective tissue, plays a central role in transport and homeostasis. It constitutes about 7% of the total body weight. Blood is broadly divided into two main components: plasma and formed elements.

Plasma makes up about 55% of the blood volume and is a straw-coloured, viscous fluid. It consists of approximately 90-92% water, 6-8% proteins (like fibrinogen for clotting, globulins for defense, and albumins for osmotic balance), and small amounts of minerals (Na+, Ca2+, Mg2+, HCO3-, Cl-), glucose, amino acids, lipids, and hormones. Plasma is crucial for transporting nutrients, waste products, hormones, and heat throughout the body.

The formed elements constitute about 45% of the blood and include erythrocytes, leukocytes, and platelets.

• Erythrocytes (Red Blood Cells - RBCs) are the most abundant cells in the blood. They are biconcave, anucleated in mammals, and contain haemoglobin, an iron-containing pigment responsible for oxygen transport. RBCs are produced in the bone marrow and have a lifespan of about 120 days.
• Leukocytes (White Blood Cells - WBCs) are nucleated and generally short-lived. They are the body's defense cells, divided into two main categories: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Neutrophils are phagocytic, eosinophils are involved in allergic reactions, basophils release histamine, lymphocytes are responsible for immune responses, and monocytes are phagocytic macrophages.
• Platelets (Thrombocytes) are cell fragments produced from megakaryocytes in the bone marrow. They are crucial for haemostasis (blood clotting) by releasing various factors that initiate the coagulation cascade. A reduction in platelet count can lead to clotting disorders.

Key Points to Remember

• The human heart is a four-chambered muscular organ located in the mediastinum, protected by a double-walled pericardium.
• The SA node (sino-atrial node) acts as the natural pacemaker of the heart, initiating the cardiac impulse.
• Cardiac output is the volume of blood pumped by each ventricle per minute (Stroke Volume x Heart Rate), typically around 5 litres.
• Blood groups (ABO and Rh) are determined by the presence or absence of specific antigens on the surface of RBCs.
• Type O blood is a universal donor (lacks A and B antigens), while Type AB is a universal recipient (lacks anti-A and anti-B antibodies).
• Double circulation ensures complete separation of oxygenated and deoxygenated blood, enhancing efficiency of oxygen delivery.
• ECG waves (P, QRS, T) represent atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively.
• Blood clotting (coagulation) is a complex process involving a cascade of enzymatic reactions leading to fibrin formation.
• Lymphatic system functions include draining excess tissue fluid, immune surveillance, and absorbing fats from the intestine.

Open vs. Closed Circulatory Systems

Steps in the Cardiac Cycle

1. Atrial Systole (0.1 sec) — Both atria contract due to SA node impulse. This pushes blood into the ventricles (already ~70% full from passive flow). Tricuspid and bicuspid valves are open.
2. Ventricular Systole (0.3 sec) — As ventricular pressure increases, tricuspid and bicuspid valves close (producing 'lub' sound). Semilunar valves open, and blood is ejected into the aorta and pulmonary artery.
3. Ventricular Diastole (0.4 sec) — Ventricles relax. As ventricular pressure falls, semilunar valves close (producing 'dub' sound) to prevent backflow. Tricuspid and bicuspid valves open, allowing ventricles to fill passively.
4. Joint Diastole — All four chambers are in a relaxed state. Blood flows from the great veins into the atria and passively into the ventricles.

Exam Tip: Blood Grouping & ECG

Pay close attention to blood grouping compatibility! Remember the antigens on RBCs and antibodies in plasma for ABO and Rh systems. Practice simple transfusion problems: a recipient with blood group A can receive blood from A and O donors, but not B or AB, due to anti-B antibodies in their plasma. For ECG interpretation, know what each wave (P, QRS, T) represents physiologically and how abnormalities (e.g., prolonged QRS, inverted T wave) indicate specific heart conditions. Diagrams of the heart and the cardiac cycle are frequently asked; practice drawing and labeling them accurately.

Practice Questions with Solutions

• Q: What is the primary function of haemoglobin? A: Haemoglobin primarily functions in the transport of oxygen from the lungs to the body tissues and also plays a minor role in carbon dioxide transport.
• Q: Name the pacemaker of the human heart and its location. A: The natural pacemaker of the human heart is the Sino-Atrial (SA) node, located in the upper right corner of the right atrium.
• Q: Briefly explain why a person with blood group AB is considered a universal recipient. A: A person with blood group AB has both A and B antigens on their RBCs but no anti-A or anti-B antibodies in their plasma. Therefore, they can receive blood from all ABO blood groups without causing agglutination by recipient antibodies.
• Q: What causes the 'lubb' sound during a heartbeat? A: The 'lubb' sound is caused by the closure of the tricuspid and bicuspid (mitral) valves at the beginning of ventricular systole.

Frequently Asked Questions