Key Points

Blood is a connective tissue composed of a fluid matrix called plasma (55%) and formed elements (45%), which include erythrocytes, leucocytes, and platelets.

Plasma is a straw-coloured fluid, 90-92% water, with proteins like fibrinogen (for clotting), globulins (defense), and albumins (osmotic balance). Plasma without clotting factors is called serum.

RBCs are the most abundant cells (5 to 5.5 million per $\text{mm}^3$), lack a nucleus in mammals, and contain haemoglobin for transporting respiratory gases like $O_2$. They have a lifespan of 120 days.

WBCs are part of the immune system and are categorized as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Their average count is 6000-8000 per $\text{mm}^3$.

Platelets are cell fragments from megakaryocytes involved in blood coagulation. A normal count is 1,500,00 to 3,500,00 per $\text{mm}^3$.

This system is based on the presence of A and B antigens on RBCs. Blood group 'O' are universal donors, while group 'AB' are universal recipients.

Based on the presence (Rh+ve) or absence (Rh-ve) of the Rh antigen. An Rh-ve mother with an Rh+ve foetus can develop antibodies, leading to erythroblastosis foetalis in subsequent pregnancies.

A cascade process where the enzyme thrombin converts inactive fibrinogen into fibrin, forming a network of threads that traps formed elements to create a clot. Calcium ions ($Ca^{++}$) are essential for this process.

Lymph is the fluid released from blood capillaries into tissue spaces. It has the same mineral distribution as plasma but fewer proteins and facilitates the exchange of nutrients and gases.

In an open system (arthropods), blood is pumped into body cavities (sinuses). In a closed system (vertebrates), blood is confined within a network of vessels, allowing for more efficient flow.

The human heart has four chambers: two atria and two ventricles. Valves like the tricuspid and bicuspid (mitral) ensure unidirectional blood flow.

The Sino-atrial node (SAN) is the pacemaker, generating 70-75 action potentials per minute. The impulse spreads through the atria to the Atrio-ventricular node (AVN) and then to the ventricles via the AV bundle and Purkinje fibres.

This is the sequential event of contraction (systole) and relaxation (diastole) of the atria and ventricles. A single cardiac cycle lasts about 0.8 seconds.

Cardiac output is the volume of blood pumped by each ventricle per minute. It is calculated as Stroke Volume ($\approx 70 \text{ mL}$) multiplied by Heart Rate ($\approx 72 \text{ beats/min}$), which averages 5 litres per minute.

An ECG is a graphical representation of the heart's electrical activity. The P-wave indicates atrial depolarisation, the QRS complex indicates ventricular depolarisation, and the T-wave indicates ventricular repolarisation.

Humans have two separate circulatory pathways: pulmonary circulation (heart to lungs and back) and systemic circulation (heart to the rest of the body and back). This system prevents the mixing of oxygenated and deoxygenated blood.

The heart is myogenic (auto-regulated by nodal tissue), but its function can be moderated by the autonomic nervous system (ANS) and adrenal medullary hormones.

Key disorders include Hypertension (high blood pressure, above 140/90), Coronary Artery Disease (CAD or atherosclerosis), Angina (chest pain due to low oxygen to heart muscle), and Heart Failure.