Body Fluids and Circulation - Comprehensive NEET Biology Notes

1. Introduction to Body Fluids and Circulation

All living cells require nutrients, oxygen (O2O_2O2​), and the removal of waste products. In simple organisms, such as sponges and coelenterates, water circulates through body cavities to facilitate the exchange of these substances. In complex organisms, special fluids like blood and lymph transport materials efficiently within the body. Blood is the most commonly used body fluid in higher organisms, including humans, while lymph assists in the transport of certain substances.

Did You Know?

The human heart pumps about 5 liters of blood per minute, ensuring the circulation of essential nutrients, gases, and waste materials throughout the body.

2. Blood

2.1 Composition of Blood

Blood is a specialized connective tissue composed of:

  • Plasma: A straw-colored, viscous fluid that makes up 55% of blood. Plasma is 90-92% water and contains proteins such as fibrinogen, globulins, and albumins. Plasma also contains electrolytes, nutrients, gases, and waste products.
  • Formed Elements: These include erythrocytes (RBCs), leucocytes (WBCs), and platelets (thrombocytes). These cells and cell fragments make up 45% of the blood.

Visual Aid Suggestion:

A labeled diagram of the components of blood, showing RBCs, WBCs, and platelets, along with a breakdown of plasma contents, can help illustrate the structure of blood.

2.2 Functions of Blood Components

  • Erythrocytes (RBCs): Responsible for oxygen transport via the protein hemoglobin. RBCs are biconcave, lack a nucleus, and have a lifespan of 120 days. They are formed in the red bone marrow.
  • Leucocytes (WBCs): Play a critical role in immunity. There are two main types of WBCs:
    1. Granulocytes: Includes neutrophils, eosinophils, and basophils.
    2. Agranulocytes: Includes lymphocytes and monocytes.
  • Platelets: Aid in blood clotting. These are small, cell-fragment-like structures involved in the coagulation process.

NEET Problem-Solving Strategy:

Remember the classification of WBCs and their functions, particularly the immune responses of lymphocytes, as these topics frequently appear in NEET exams.

Quick Recap

  • Blood consists of plasma and formed elements (RBCs, WBCs, and platelets).
  • RBCs transport oxygen, WBCs are involved in immunity, and platelets assist in clotting.

3. Lymph (Tissue Fluid)

3.1 Composition and Function of Lymph

Lymph is a colorless fluid that circulates in the lymphatic system. It is formed when interstitial fluid, derived from plasma, enters lymphatic vessels. Lymph is involved in:

  • Transporting nutrients, hormones, and waste products.
  • Absorbing fats from the intestine.
  • Playing a role in the immune response through specialized lymphocytes.

Real-life Application:

Lymphatic circulation is critical in maintaining fluid balance in tissues and facilitating the absorption of dietary fats in the intestines.

Quick Recap

  • Lymph is a colorless fluid involved in nutrient transport and immune responses.
  • It plays an essential role in the absorption of fats and the removal of excess tissue fluid.

4. Circulatory Pathways

4.1 Open and Closed Circulatory Systems

  • Open Circulatory System: Blood flows freely through body cavities, seen in arthropods and mollusks.
  • Closed Circulatory System: Blood circulates in a closed system of blood vessels, as seen in annelids and vertebrates. This system is more efficient in regulating the flow of blood to tissues.

4.2 Double Circulation in Humans

Humans have a double circulatory system, comprising two distinct pathways:

  1. Pulmonary Circulation: Blood is pumped from the right ventricle to the lungs for oxygenation and then returns to the left atrium.
  2. Systemic Circulation: Oxygenated blood from the left ventricle is distributed to the rest of the body, and deoxygenated blood returns to the right atrium.

Visual Aid Suggestion:

A diagram showing the flow of blood in both pulmonary and systemic circulation can help clarify the concept of double circulation.

Quick Recap

  • In a closed circulatory system, blood flows through a network of vessels.
  • Humans have double circulation, with separate pulmonary and systemic pathways for oxygenated and deoxygenated blood.

5. Regulation of Cardiac Activity

5.1 Cardiac Cycle

The cardiac cycle consists of systole (contraction) and diastole (relaxation) of the atria and ventricles. The cycle repeats approximately 72 times per minute, resulting in the heart pumping out about 70 mL of blood per beat (stroke volume).

Key Events:

  • Atrial Systole: Atria contract to push blood into the ventricles.
  • Ventricular Systole: Ventricles contract, forcing blood into the pulmonary artery and aorta.
  • Diastole: Relaxation of the heart chambers, allowing blood to refill.

5.2 Regulation by Nodal Tissue

The heart is myogenic, meaning it generates its own electrical impulses. The sinoatrial node (SAN) acts as the pacemaker, initiating action potentials that regulate the heartbeat.

NEET Tip:

Be familiar with the role of the sinoatrial node and atrioventricular node in regulating the heartbeat, as these are common topics in NEET questions.

Quick Recap

  • The cardiac cycle involves the rhythmic contraction and relaxation of the heart.
  • The sinoatrial node (SAN) sets the heart's pace and ensures coordinated contraction.

6. Disorders of the Circulatory System

6.1 Hypertension

Hypertension (high blood pressure) occurs when blood pressure consistently exceeds 140/90 mmHg. It is a major risk factor for heart disease, stroke, and kidney failure.

6.2 Coronary Artery Disease (CAD)

CAD occurs when cholesterol and other materials build up in the coronary arteries, restricting blood flow to the heart muscle. This condition can lead to heart attacks.

NEET Problem-Solving Strategy:

Be familiar with common disorders like hypertension and coronary artery disease, their causes, and effects, as questions about circulatory system disorders often appear in NEET.

Quick Recap

  • Hypertension refers to high blood pressure, a major risk factor for cardiovascular disease.
  • Coronary artery disease restricts blood flow to the heart, potentially leading to heart attacks.

NEET Exam Strategy

  • Focus on understanding the composition of blood, types of blood cells, and the functions of plasma.
  • Practice questions on the mechanism of double circulation, the cardiac cycle, and common circulatory disorders.
  • Be prepared for diagram-based questions, particularly on the human heart, blood vessels, and circulatory pathways.

Practice Questions

  1. Which of the following is responsible for blood clotting?
    Solution: c) Platelets
    • a) Erythrocytes
    • b) Leucocytes
    • c) Platelets
    • d) Plasma
  3. What is the primary function of erythrocytes (RBCs)?
    Solution: b) Oxygen transport
    • a) Immunity
    • b) Oxygen transport
    • c) Blood clotting
    • d) Absorption of fats
  5. Which blood vessel carries deoxygenated blood from the heart to the lungs?
    Solution: a) Pulmonary artery
    • a) Pulmonary artery
    • b) Aorta
    • c) Vena cava
    • d) Pulmonary vein
  7. What is the function of the sinoatrial node (SAN)?
    Solution: a) It acts as the pacemaker of the heart.
    • a) It acts as the pacemaker of the heart.
    • b) It prevents the backflow of blood.
    • c) It transports oxygenated blood to the body.
    • d) It filters blood in the kidneys.
  9. Which component of blood is primarily responsible for immunity?
    Solution: d) WBCs
    • a) RBCs
    • b) Platelets
    • c) Plasma
    • d) WBCs

Glossary

  • Plasma: The liquid part of blood, consisting mostly of water and proteins.
  • Lymph: A clear fluid involved in immune responses and nutrient transport.
  • Hypertension: A condition characterized by persistently high blood pressure.
  • Sinoatrial Node (SAN): The pacemaker of the heart, responsible for initiating the heartbeat.