Body Fluids and Circulation NEET Notes

Body Fluids and Circulation

Overview

This lecture covers body fluids and circulation, particularly focusing on blood, lymph, circulatory pathways, the heart, its regulation, and related disorders. The content is geared towards the NEET 2025 exam, with an expected 2-3 questions from this topic.

Body Fluids

Function

Body fluids transport nutrients, gases, and wastes.

Types

1. Blood
2. Lymph

Blood

Definition

Blood is a specialized connective tissue.

Composition

• Plasma (Fluid)
  • Water: 90-92%
  • Proteins:
    • Fibrinogen: Involved in clotting.
    • Albumins: Maintain osmotic balance.
    • Globulins: Defense mechanisms.
  • Minerals: $\text{Na}^+$, $\text{Ca}^{2+}$, $\text{Mg}^{2+}$, $\text{HCO}_3^-$, Cl
  • Nutrients: Glucose, amino acids, lipids
• Formed Elements (45%)
  • Erythrocytes (Red Blood Cells)
  • Leucocytes (White Blood Cells)
  • Platelets (Thrombocytes)

Erythrocytes (RBCs)

• Most abundant cells in blood.
• Produced in the red bone marrow.
• Count: 5-5.5 million/mm^3
• Lifespan: 120 days
• Characteristics:
  • Biconcave
  • Enucleated (no nucleus)
• Function: Transport of gases
• Spleen: Graveyard of RBCs

Leucocytes (WBCs)

• Count: 6,000-8,000/mm^3
• Generally short-lived.
• Function: Defense.
• Types:
  • Granulocytes
    • Neutrophils: 60-65%, phagocytic, resist infections
    • Eosinophils: 2-3%, involved in inflammatory and allergic reactions
    • Basophils: 0.5-1%, secrete histamine (vasodilator), serotonin etc.
  • Agranulocytes
    • Lymphocytes: 20-25%, immune responses.
    • Monocytes: 6-8%, phagocytic.

Platelets (Thrombocytes)

• Cell fragments of megakaryocytes.
• Short-lived.
• Function: Coagulation of blood.
• If the number drops, it can lead to loss of blood from the body.

Blood Groups

ABO Blood Grouping

• O is the universal donor.
• AB is the universal recipient.

Rh Grouping

• Based on the presence of Rh factor (Rhesus antigen) on RBCs, identified in Rhesus monkeys.
• Rh+ : Rh factor is present.
• Rh- : Rh factor is absent.
• 80% of humans have the Rh antigen.

Rh Incompatibility

• Special Case:
  • Mother is Rh- and the fetus is Rh+.
  • First pregnancy: Baby is normal because during pregnancy, no mixing of blood due to separation by the placenta.
  • During delivery, there are chances of mixing of blood.
  • The Rh- mother makes antibodies against the Rh antigen.
  • Subsequent pregnancy: Rh antibodies cross the placenta.
  • Severe anemia and jaundice in baby (Erythroblastosis foetalis).
  • Prevention: Administer anti-Rh antibodies to the mother immediately after the first delivery to save the baby.

Blood Coagulation

• Process to prevent excessive blood loss after injury.
• Involves a cascade of reactions.
• \text{Ca}^{2+} is essential.

Steps

1. Injury or removal of blood from vessels.
2. Release of thrombokinase.
3. Prothrombin \xrightarrow{\text{Thrombokinase}, \text{Ca}^{2+}} Thrombin
4. Fibrinogen \xrightarrow{\text{Thrombin}} Fibrin threads
5. Blood cells + Fibrin threads = Clot

Lymph (Tissue Fluid)

Formation

As blood passes through capillaries in tissues, some water along with small water-soluble substances move out into the spaces between the cells, forming the interstitial fluid or tissue fluid.

Lymphatic System

An elaborate network of vessels collects this fluid and drains it back into the major veins. The fluid present in the lymphatic system is called lymph.

Function

• Lymph is a colorless fluid containing specialized lymphocytes responsible for the immune responses.
• Important carrier for nutrients and hormones.
• Fats are absorbed through lymph in the lacteals present in the intestinal villi.

Circulatory Pathways

Open vs. Closed Circulatory Systems

• Open Circulatory System: Blood flows in open spaces called sinuses without vessels (e.g., arthropods and mollusks).
• Closed Circulatory System: Blood flows in closed vessels (e.g., annelids and chordates).

Circulation in Vertebrates

Arrangement based on chambers of heart:

Single-Loop Circulatory System

Seen in fishes: Heart → Gills → Body → Heart

Double Circulation

• Pulmonary Circulation: Heart → Lungs → Heart
• Systemic Circulation: Heart → Body → Heart

Human Circulatory System

Components

• Heart
• Blood vessels
• Blood

Heart

• Mesodermally derived organ located between the lungs, tilted to the left.
• Present in the thoracic cavity.
• Protected by a double-walled membranous bag called the pericardium, filled with pericardial fluid to reduce friction on lungs.
• Four-chambered: Two atria (thin-walled) and two ventricles (thick-walled).
• Inter-atrial septum separates the atria.
• Inter-ventricular septum separates the ventricles.
• Atrio-ventricular septum separates the atria from the ventricles (thick fibrous).

Cardiac Valves

Ensure unidirectional blood flow.

• Atrio-Ventricular Valves
  • Tricuspid valve: Between the right atrium and right ventricle (3 cusps).
  • Bicuspid/Mitral valve: Between the left atrium and left ventricle (2 cusps).
• Semilunar Valves
  • At the base of the pulmonary artery.
  • At the base of the aorta.

Nodal Tissue

Specialized cardiac musculature.

• Sino-Atrial Node (SAN): Located in the right upper corner of the right atrium; acts as the pacemaker; initiates and maintains the rhythmic contractile activity of the heart.
• Atrio-Ventricular Node (AVN): Located in the left lower corner of the right atrium.
• AV Bundle (Bundle of His): Divides into right and left bundles that pass through the interventricular septum and then into the ventricles via Purkinje fibers.
• Purkinje Fibers: Spread throughout the ventricular myocardium.

All are auto excitable.

Blood Vessels

• Arteries: Carry blood away from the heart; thick walls; high pressure.
• Veins: Carry blood towards the heart; thin walls; low pressure; have valves to prevent backflow.
• Capillaries: Smallest vessels; thin walls (one-cell thick); facilitate exchange of materials between blood and tissues.

Types of Circulation

• Pulmonary Circulation: Deoxygenated blood flows from the right ventricle to the pulmonary artery, goes to the lungs for oxygenation, and then oxygenated blood returns to the left atrium via the pulmonary veins.
• Systemic Circulation: Oxygenated blood flows from the left ventricle to the aorta, goes to systemic arteries, capillaries (tissues) for nutrient/gas exchange, then deoxygenated blood returns to the right auricle via systemic veins.
• Hepatic Portal Circulation: Deoxygenated blood from digestive tract goes to liver via hepatic portal vein.
• Coronary Circulation: Blood flow to and from the cardiac musculature, mostly deoxygenated blood

Differences between Vessels

Cardiac Cycle

Definition

The sequential events in the heart which are cyclically repeated.

• Duration: 0.8 seconds.
• Involves systole (contraction) and diastole (relaxation) of both atria and ventricles.

Phases

Heart Sounds

• Lub: Closure of tricuspid and bicuspid valves.
• Dub: Closure of semilunar valves.
• These sounds have clinical significance and can be heard using a stethoscope.

Cardiac Output

• Volume of blood pumped out by each ventricle per minute.
• CO = SV \times HR
  • SV (Stroke Volume) = Volume of blood pumped out by each ventricle during a cardiac cycle (approx. 70 ml).
  • HR (Heart Rate) = Number of heartbeats per minute (approx. 72).
• Normal Cardiac Output = 70 \text{ ml} \times 72 \approx 5000 \text{ ml} = 5 \text{L}$$
• The body has the ability to alter stroke volume or heart rate to alter cardiac output.
• Athletes have a much higher cardiac output than ordinary individuals.

ECG

• Electrocardiogram: A graphical representation of the electrical activity of the heart during a cardiac cycle.
• 3 leads are connected to monitor heart activity: Right wrist, left wrist, and left ankle.
• The number of QRS complexes in a given time period determines the heart beat rate of an individual.
• The end of the T-wave marks the end of systole.

ECG Waves

• P wave: Depolarization of atria → Contraction of atria.
• QRS complex: Depolarization of ventricles → Contraction of ventricles.
• T wave: Repolarization of ventricles → Relaxation of ventricles.

Regulation of Heart Activity

Neural Regulation

• Medulla Oblongata (Cardiac Center) regulates heart activity.
• Sympathetic nervous system:
  • Increases heart rate, heart beat, ventricular contraction strength, and cardiac output (through cardioaccelerator).
• Parasympathetic nervous system:
  • Decreases heart rate, heart beat, ventricular contraction strength, and cardiac output (through cardiac inhibitor).

Disorders of the Circulatory System