1.0 The Circulatory Medium: Blood
Blood is a fluid connective tissue that circulates through the heart and blood vessels. It consists of a liquid part called Plasma and cellular structures called Formed Elements.
The Formed Elements
Blood Coagulation: A complex cascade involving Thromboplastin, Prothrombin, and Fibrinogen to form a Fibrin net that traps blood cells.
Serum vs. Plasma
Serum is Plasma from which the clotting factor Fibrinogen has been removed. Serum does not clot.
Mature human RBCs are enucleated (lack a nucleus). This provides more space for hemoglobin and increases the surface area for oxygen absorption.
The life span of an RBC is about 120 days. After this, they are destroyed in the spleen, often called the "graveyard of RBCs."
2.0 Anatomy of the Human Heart
The human heart is a muscular organ about the size of a closed fist. It is protected by a double-walled membranous sac called the Pericardium, which contains pericardial fluid to reduce friction during heartbeats.
Chambers and Valves
- Four Chambers: Two upper Atria (receiving chambers) and two lower Ventricles (pumping chambers).
- Tricuspid Valve: Between Right Atrium and Right Ventricle.
- Bicuspid (Mitral) Valve: Between Left Atrium and Left Ventricle.
- Semilunar Valves: At the base of the Pulmonary Artery and Aorta.
3.0 Blood Vessels
Our circulatory system is "closed," meaning blood flows through a continuous network of vessels. There are three distinct types of blood vessels.
In ICSE exams, never forget: All arteries carry oxygenated blood except the Pulmonary Artery. All veins carry deoxygenated blood except the Pulmonary Veins.
These are microscopic vessels with walls only one cell thick (Endothelium). This extreme thinness allows for the diffusion of gases and nutrients between the blood and tissues.
4.0 Double Circulation
In humans, blood passes through the heart twice to complete one full circuit of the body. This is known as Double Circulation, consisting of two distinct pathways.
Pulmonary Circulation
Blood flows from the Right Ventricle to the Lungs (for oxygenation) and back to the Left Atrium.
Systemic Circulation
Oxygenated blood flows from the Left Ventricle to all body parts and returns to the Right Atrium.
5.0 The Cardiac Cycle
The cardiac cycle is the sequence of events that occurs during one complete heartbeat, lasting approximately 0.8 seconds. It involves two main phases:
- 💓 Systole: The phase of contraction (Atrial or Ventricular).
- 💓 Diastole: The phase of relaxation where chambers fill with blood.
The Hepatic Portal System
This is a unique venous connection where blood from the digestive tract (intestines) is carried to the Liver by the Hepatic Portal Vein before reaching the heart. This allows the liver to process nutrients and detoxify the blood.
The Left Ventricle has the thickest muscular walls. Why? Because it must pump blood with enough pressure to reach the entire body, unlike the Right Ventricle which only pumps to the nearby lungs.
The "Lubb-Dupp" sound of the heart is caused by the closing of valves. "Lubb" is the closing of atrioventricular valves, and "Dupp" is the closing of semilunar valves.
6.0 The Lymphatic System
Not all the fluid that leaves the capillaries returns to them. Some of it enters a parallel system of vessels called the Lymphatic System. This fluid is known as Lymph.
- Nutritive: Supplies nutrition to cells where blood cannot reach.
- Protective: Lymph nodes produce lymphocytes (WBCs) to fight infection.
- Absorption: Absorbs fats from the intestine through specialized vessels called Lacteals.
7.0 The Spleen and Blood Pressure
The Spleen is the largest lymphatic organ in the body, located behind the stomach. It acts as a reservoir for blood and produces lymphocytes.
Blood Pressure (BP)
The pressure exerted by blood on the walls of the arteries. It is measured using a Sphygmomanometer.
- Systolic Pressure: ~120 mm Hg (During ventricular contraction).
- Diastolic Pressure: ~80 mm Hg (During ventricular relaxation).
Blood vs. Lymph
In the ICSE paper, if you are asked to identify a cross-section of a vessel: Arteries have small lumens and thick walls, Veins have large lumens and thin walls, and Capillaries have walls composed of only one layer of squamous cells.
The Spleen is often called the "Blood Bank" of the body because it can release stored blood into the circulation during emergencies like a heavy hemorrhage!