Chapter 18: Body Fluids and Circulation – Part 1

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Chapter 18: Body Fluids and Circulation – Part 1

📘 Class 11 Biology | NEET PYQs + Concept Series
🔗 Visit: Biologyatease | NEET PYQ All

🧠 Introduction to Body Fluids and Circulation

The circulatory system is a critical component of human physiology. It helps in the transport of essential substances like oxygen, carbon dioxide, nutrients, hormones, and waste materials throughout the body. In this chapter, we begin with the body fluids — especially blood and lymph, and then dive into the types of circulatory systems and the structural and functional details of the human heart.

🩸 Types of Body Fluids

1. Blood

Blood is the most vital transport fluid in the human body. It is a connective tissue composed of plasma and formed elements.

  • Plasma: The liquid matrix of blood (~55%). It contains:

    • 90-92% water

    • 7-8% proteins (albumin, globulin, fibrinogen)

    • Salts, hormones, enzymes, nutrients, and waste materials.

  • Formed Elements (~45% of blood):

    • Red Blood Cells (RBCs) – Transport oxygen via haemoglobin.

    • White Blood Cells (WBCs) – Immune defense; types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

    • Platelets – Help in clotting of blood.

🔬 NEET Tip:

  • RBCs are enucleated in mammals.

  • Lifespan of RBC = 120 days

  • Normal RBC count = 5 million/mm³

2. Lymph (Tissue Fluid)

  • Lymph is a colourless fluid similar to plasma but with fewer proteins.

  • Formed by filtration of blood in capillaries.

  • Functions:

    • Carries digested fats from the intestine (via lacteals).

    • Transports WBCs, especially lymphocytes.

❤️ Circulatory System Types

Organisms have developed different types of circulatory systems.

1. Open Circulatory System

  • Found in arthropods and molluscs.

  • Blood (hemolymph) is pumped into open spaces (sinuses).

  • Less efficient; slower circulation.

2. Closed Circulatory System

  • Found in annelids and vertebrates.

  • Blood flows within vessels — arteries, veins, and capillaries.

  • Efficient for nutrient and gas exchange.

✅ Humans have a closed circulatory system.

💓 Structure of the Human Heart

The heart is a muscular pumping organ, located in the thoracic cavity, slightly tilted to the left.

External Features:

  • Conical in shape, weighs ~300g in adults.

  • Protected by a double-layered pericardium filled with pericardial fluid.

  • Made of cardiac muscles (involuntary, striated, and autorhythmic).

Internal Structure:

The heart has four chambers:

  • Right Atrium (RA)

  • Right Ventricle (RV)

  • Left Atrium (LA)

  • Left Ventricle (LV)

Valves in the Heart:

  • Tricuspid Valve – Between RA and RV

  • Bicuspid (Mitral) Valve – Between LA and LV

  • Semilunar Valves – At exits of pulmonary artery and aorta

These valves prevent backflow of blood, ensuring unidirectional flow.

Important Points:

  • Right side of the heart handles deoxygenated blood

  • Left side handles oxygenated blood

💉 Major Blood Vessels Associated with the Heart

  • Superior & Inferior Vena Cava – Bring deoxygenated blood to the right atrium.

  • Pulmonary Artery – Carries deoxygenated blood from right ventricle to lungs (exceptional artery).

  • Pulmonary Veins – Carry oxygenated blood from lungs to left atrium (exceptional vein).

  • Aorta – Carries oxygenated blood from left ventricle to the entire body.

🔄 Double Circulation in Humans

Humans exhibit double circulation — meaning blood passes through the heart twice during one complete cycle.

1. Pulmonary Circulation:

  • Right ventricle → Lungs → Left atrium

  • Purpose: Oxygenation of blood.

2. Systemic Circulation:

  • Left ventricle → Body → Right atrium

  • Purpose: Distribution of oxygen and nutrients to tissues.

This ensures efficient separation of oxygenated and deoxygenated blood — a key adaptive advantage in mammals.

📝 Key NEET PYQ Concepts Covered (Part 1)

Here are some high-yield concepts often asked in NEET:

  • What is the function of lymph?

  • Why is the pulmonary artery an exception?

  • Which valves prevent backflow in heart chambers?

  • Which blood vessel carries oxygenated blood from lungs to the heart?

📌 Summary

In this first part of Chapter 18, we've covered:

  • Body fluids: Blood and Lymph

  • Circulatory system types

  • Human heart structure and chambers

  • Major blood vessels

  • Concept of double circulation

These foundational topics form the base for understanding circulation-related mechanisms and disorders, which we’ll explore in Part 2 of this chapter.

🔗 Keep Learning

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