NSEJS Biology Body Fluids and Circulation

Blood is a vital fluid connective tissue that continuously circulates throughout the human body. It transports oxygen, nutrients, hormones, and waste products while helping maintain temperature, immunity, and internal balance. 

Understanding blood and circulation is essential for learning how different organs function together efficiently. This topic explains the composition of blood, including plasma and formed elements, along with blood groups and clotting mechanisms. 

It also covers the structure and working of the human heart, circulatory pathways, cardiac cycle, blood pressure, and electrocardiogram. Additionally, students learn about common circulatory disorders and the importance of a healthy cardiovascular system.

Also Read: 

• NSEJS Syllabus
• NSEJS Previous Question Papers

Blood: A Fluid Connective Tissue

Blood is a fluid connective tissue, appearing red due to haemoglobin.

Components of Blood

Blood is separated into two main parts via centrifugation:

1. Plasma: The fluid part.

2. Formed Elements: The cellular part.

Plasma Composition

Plasma constitutes 55% of total blood volume. It is 90-92% water, with other components including proteins, clotting factors, minerals, ions, glucose, amino acids, and salts.

Formed Elements Composition

Formed elements make up 45% of total blood volume. They comprise three cell types: Red Blood Cells (RBCs), White Blood Cells (WBCs), and Platelets.

Plasma Proteins and Their Functions

Plasma contains specific proteins:

• Fibrinogen: Essential for blood clotting.

• Globulins: Provide immunity by forming immunoglobulins (antibodies) against pathogens.

• Albumin: Maintains colloidal osmotic pressure and osmotic balance.

Plasma vs. Serum

• Plasma: Contains clotting factors.

• Serum: Plasma without the clotting factors.

Formed Elements: Blood Cells

Blood contains three main cell types:

1. Red Blood Cells (RBCs) / Erythrocytes

• Structure: Biconcave and anucleated in mature mammals. They lose their nucleus upon maturation to accommodate more haemoglobin.

• Function: Haemoglobin transports oxygen and carbon dioxide, and gives blood its red colour.

• Synthesis (Erythropoiesis): Process of RBC formation.

• Count: Approximately 5-5.5 million per cubic millimetre of blood.

• Lifespan: 120 days.

• Graveyard of RBCs: Spleen, where old RBCs are broken down.

2. White Blood Cells (WBCs) / Leukocytes

• Function: Provide immunity and fight pathogens.

• Count: 6,000-8,000 per cubic millimetre of blood.

• Nucleus: WBCs are nucleated.

• Types:

• Granulocytes (contain cytoplasmic granules):

• Neutrophils: Most numerous (60-65%), multilobed nucleus, perform phagocytosis.

• Eosinophils: 2-3%, bilobed/headphone-shaped nucleus, linked to allergic reactions.

• Basophils: Least numerous (0.5-1%), S-shaped nucleus, release histamine (inflammation), serotonin, and heparin (anticoagulant).

• Agranulocytes (lack granules):

• Lymphocytes: 20-25%, large nucleus, include B-cells (produce antibodies) and T-cells (cell-mediated immunity).

• Monocytes: 6-8%, kidney/bean-shaped nucleus, become macrophages in tissues, perform phagocytosis.

• Diapedesis: WBCs can change shape and move out of capillaries into tissues to combat infection.

• Memory Tip: To remember the order of WBCs by percentage (most to least): Never Let Monkeys Eat Bananas (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils).

3. Platelets / Thrombocytes

• Function: Crucial for blood clotting.

• Count: 150,000-350,000 per cubic millimetre of blood.

• Formation: Formed from fragments of megakaryocytes in bone marrow; hence, they are anucleated.

• Lifespan: Approximately 7-8 days.

Disorders Related to Blood Cell Counts

• RBC Disorders:

• Polycythemia: Excess RBCs, leading to thick blood and circulation difficulty.

• Anaemia: Deficiency of RBCs or haemoglobin.

• WBC Disorders:

• Leukocytosis: Increased WBC count, often due to infection.

• Leukopenia: Decreased WBC count.

• Leukaemia: Uncontrolled increase of abnormal WBCs (blood cancer).

• Platelet Disorders:

• Thrombocytosis: Excess platelets.

• Thrombocytopenia: Deficiency of platelets, impairing clotting and causing excessive bleeding.

Blood Coagulation (Clotting)

Blood clotting is a complex process involving platelets, clotting factors, calcium ions, and Vitamin K.

1. Injury: Activates platelets, releasing clotting factors.

2. Thrombokinase Release: Activated platelets release thrombokinase.

3. Prothrombin to Thrombin: Thrombokinase, with calcium ions, converts inactive prothrombin to active thrombin.

4. Fibrinogen to Fibrin: Thrombin converts soluble fibrinogen into insoluble fibrin threads.

5. Clot Formation: Fibrin forms a mesh, trapping blood cells to form a clot (thrombus).

Blood Grouping

The ABO blood grouping system is based on two surface antigens (A and B) on RBCs and two plasma antibodies (anti-A and anti-B).

• Universal Donor: Blood Group O (no antigens).

• Universal Recipient: Blood Group AB (no antibodies).

Rh Factor

The Rh factor is another antigen (D antigen) on RBCs, discovered by Landsteiner and Wiener in Rhesus monkeys.

• Rh positive (Rh+): RBCs carry the Rh antigen.

• Rh negative (Rh-): RBCs lack the Rh antigen.

Erythroblastosis Fetalis

This condition occurs with an Rh-negative mother carrying an Rh-positive fetus.

• First Pregnancy: Usually normal. During delivery, fetal Rh+ blood may enter the mother's circulation, prompting her immune system to produce anti-Rh antibodies.

• Second Pregnancy: If the mother conceives another Rh+ fetus, her pre-formed anti-Rh antibodies can cross the placenta, attacking fetal RBCs. This causes severe anaemia, jaundice, and can be fatal.

• Prevention: Rh-negative mothers receive anti-Rh antibodies (RhoGAM) after the first delivery to destroy fetal Rh+ RBCs, preventing maternal antibody formation.

Blood Vessels

Three main types of blood vessels:

1. Arteries:

• Function: Carry blood away from the heart to organs.

• Structure: Thick, elastic, muscular walls to withstand high pressure. Narrow lumen.

• Exception: Pulmonary artery carries deoxygenated blood.

2. Veins:

• Function: Carry blood towards the heart from organs.

• Structure: Thinner, less elastic, less muscular walls. Wider lumen. Carry blood under low pressure.

• Valves: Contain internal valves to prevent backflow.

• Exception: Pulmonary vein carries oxygenated blood.

3. Capillaries:

• Function: Network connecting arterioles and venules; primary site for exchange of gases, nutrients, waste, and water between blood and tissues.

• Structure: Extremely thin-walled, single layer of endothelium.

• Diapedesis: Thin walls allow WBCs to move into tissues.

Lymph / Interstitial Fluid

• Formation: Plasma and WBCs filter from capillaries into interstitial spaces, forming interstitial fluid (tissue fluid). Lacks RBCs and large proteins.

• Composition: Similar to plasma but with fewer proteins; contains WBCs.

• Function:

• Medium for Exchange: Facilitates exchange between blood and tissue cells.

• Immunity: WBCs in lymph (lymphocytes) fight infections.

• Fat Absorption: Lacteals in intestinal villi absorb fats.

• Lymphatic System: Collects interstitial fluid and returns it to circulation via vessels and lymph nodes (sites of lymphocyte production and pathogen filtration).

Circulatory Pathways

Two main types:

1. Open Circulatory System:

• Mechanism: Heart pumps blood into open spaces (sinuses) bathing tissues. No closed vessels.

• Efficiency: Less efficient.

• Examples: Arthropods, most molluscs.

2. Closed Circulatory System:

• Mechanism: Blood is always confined within blood vessels. Exchange occurs across capillary walls.

• Efficiency: More efficient, allows controlled blood flow.

• Examples: Vertebrates, annelids.

The Heart

The heart is a mesodermally derived organ responsible for pumping blood.

Evolution of Heart Chambers

• Two-Chambered Heart (Single Circulation): One atrium, one ventricle. Deoxygenated blood from the body to the heart, then to the gills for oxygenation, then directly to the body. (e.g., Fish)

• Three-Chambered Heart (Incomplete Double Circulation): Two atria, one ventricle. Oxygenated and deoxygenated blood mix in the single ventricle. (e.g., Amphibians, most reptiles)

• Four-Chambered Heart (Complete Double Circulation): Two atria, two ventricles. Complete separation of oxygenated and deoxygenated blood, highly efficient. (e.g., Mammals, birds, crocodiles)

Human Heart Structure

• Location: Between lungs, slightly tilted left in the mediastinum.

• Size: Roughly a clenched fist.

• Protective Covering: Double-walled pericardium.

• Chambers:

• Right Atrium (RA): Receives deoxygenated blood from vena cava.

• Left Atrium (LA): Receives oxygenated blood from pulmonary veins.

• Right Ventricle (RV): Pumps deoxygenated blood to lungs via pulmonary artery.

• Left Ventricle (LV): Pumps oxygenated blood to body via aorta. Thicker wall for higher pressure.

• Septa: Inter-atrial, inter-ventricular, and atrioventricular septa separate chambers.

• Valves: Prevent backflow.

• Tricuspid Valve: Between RA and RV (three cusps).

• Bicuspid Valve / Mitral Valve: Between LA and LV (two cusps).

• Semilunar Valves: At pulmonary artery and aorta openings.

• Support Structures: Chordae Tendineae connect valve cusps to Papillary Muscles to prevent inversion.

Blood Flow Through the Heart

1. Deoxygenated blood from body → Right Atrium → Tricuspid Valve → Right Ventricle.

2. Right Ventricle → Pulmonary Semilunar Valve → Pulmonary Artery → Lungs.

3. Oxygenated blood from lungs → Pulmonary Veins → Left Atrium.

4. Left Atrium → Bicuspid/Mitral Valve → Left Ventricle.

5. Left Ventricle → Aortic Semilunar Valve → Aorta → Body.

Cardiac Conduction System

The human heart is myogenic, meaning it generates its own contractions.

• Sinoatrial (SA) Node: In upper right RA; heart's natural pacemaker, initiates electrical impulses.

• Atrioventricular (AV) Node: In lower left RA; receives impulse from SA node, delays it briefly.

• Bundle of His: Extends from AV node into interventricular septum.

• Purkinje Fibres: Branch from Bundle of His into ventricular walls, spreading impulse.

Double Circulation

Blood passes through the heart twice per complete body cycle:

1. Pulmonary Circulation: Deoxygenated blood from the right ventricle to the lungs for oxygenation, then oxygenated blood returns to the left atrium.

2. Systemic Circulation: Oxygenated blood from the left ventricle to body tissues, then deoxygenated blood returns to the right atrium.
   This ensures complete separation of blood types for high efficiency.

Cardiac Cycle

The sequence of events in one heartbeat. Normal heart rate is 72 beats per minute, with one cycle lasting approximately 0.8 seconds.

Phases of the Cardiac Cycle

1. Joint Diastole (0.4 sec): All chambers relaxed. Blood flows passively from atria to ventricles. Semilunar valves closed.

2. Atrial Systole (0.1 sec): Atria contract, pushing remaining blood into ventricles.

3. Ventricular Systole (0.3 sec): Ventricles contract.

• Isovolumetric Contraction: Tricuspid and bicuspid valves close, producing the first heart sound, "Lub".

• Ejection Phase: Semilunar valves open, blood ejected into pulmonary artery and aorta.

Heart Sounds

• "Lub": First sound; closure of tricuspid and bicuspid valves during ventricular systole. Longer, lower-pitched.

• "Dub": Second sound; closure of semilunar valves during ventricular diastole. Shorter, higher-pitched.

Cardiac Output

• Stroke Volume: Blood volume pumped by each ventricle per beat (~70 mL).

• Cardiac Output: Blood volume pumped by each ventricle per minute.

• Cardiac Output = Stroke Volume × Heart Rate

• Approximately 5 liters/minute (70 mL/beat × 72 beats/min).

Electrocardiogram (ECG)

An ECG is a graphical representation of the heart's electrical activity. The machine is an Electrocardiograph.

• P-wave: Represents atrial depolarisation (atrial contraction).

• QRS complex: Represents ventricular depolarisation (ventricular contraction).

• T-wave: Represents ventricular repolarisation (ventricles relaxing).

Blood Pressure

Blood pressure is the force exerted by blood from the aorta into arteries.

• Systolic Pressure: Pressure during heart contraction (~120 mmHg).

• Diastolic Pressure: Pressure when the heart relaxes (~80 mmHg).

• Normal Blood Pressure: 120/80 mmHg.

• Measurement Instrument: Sphygmomanometer.

Regulation of Heartbeat

The Medulla Oblongata in the hindbrain regulates the involuntary heartbeat, acting as a cardiac centre and respiratory centre.

Cardiac Disorders

Cardiac Disorders are diseases that affect the heart's structure or function, reducing its efficiency in pumping blood. Common disorders include hypertension, coronary artery disease, heart failure, and heart attacks.

Atherosclerosis

Caused by calcium, fat, and cholesterol deposits in blood vessels, narrowing the lumen, increasing resistance, and leading to increased blood pressure.

Angina Pectoris

Chest pain due to insufficient oxygen supply to heart muscles.

Heart Attack (Myocardial Infarction)

Death or damage of heart muscles (myocardial infarction) due to a blockage or lack of blood supply and oxygen. A specific area is damaged, impairing local function.

Heart Failure

Heart fails to pump blood effectively to meet body needs; pumping action is significantly compromised.

ECG Interpretation for Disease Diagnosis

Variations in ECG waves indicate cardiac conditions:

• P wave Enlargement: Suggests atrial enlargement.

• PR Interval Lengthening: Indicates inflammation in atria or AV node.

• QT Interval Enlargement or Increased Space between Q and R: Suggests myocardial infarction.

• ST Segment Elevation: Indicates an acute myocardial infarction.

• ST Segment Depression: Indicates insufficient oxygen supply to heart muscles.

• T wave Flattening: Indicates insufficient oxygen to the heart, potentially leading to angina pectoris, myocardial infarction, or heart failure.

Understanding Body Fluids and Circulation is essential for building a strong foundation in biology and excelling in the NSEJS exam. 

Mastering topics such as blood composition, heart function, circulation, cardiac cycle, and related disorders helps students answer conceptual questions confidently while developing a deeper understanding of the human body's transport and regulatory systems.