Human Physiology | Module 2 Unit 2 | Calicut University First Semester BSc. Zoology Notes PDF |

Human Physiology: Calicut University Free Study Material 

Hello everyone! In this study material, I've put together notes from unit 2 circulation, from the module physiology of respiration and circulation. This unit deals with some of the most interesting topics in physiology such as the origin of blood, how clots are formed, and common cardiovascular problems. I've tried my best to make the notes clear, structured, and exam-friendly. 

 If you do find these notes useful feel free to share it among your friends and classmates. Also feel free to leave a comment or suggestion if you would like me to add more explanation, diagrams, or anything else.

Summary of what’s covered in this study material

❤️ Pacemaker and Conducting System of the Heart

The human heart is myogenic, meaning it can initiate its own rhythmic contractions without direct nervous stimulation. This intrinsic rhythmicity is due to a specialised conduction system composed of modified cardiac muscle cells.

The sinoatrial (SA) node, located in the right atrium near the opening of the superior vena cava, acts as the natural pacemaker of the heart. It generates electrical impulses at a rate of approximately 70–80 beats per minute. These impulses spread across the atria, causing atrial contraction.

The impulse then reaches the atrioventricular (AV) node situated in the interatrial septum. From here, it passes into the Bundle of His, which divides into right and left bundle branches along the interventricular septum. Finally, the impulses are distributed through Purkinje fibres, ensuring coordinated ventricular contraction.

🩸 Blood: Composition and Functional Significance

Blood is a specialized connective tissue that performs transport, regulatory, and protective functions. In adults, it constitutes nearly 8% of body weight. On average, males possess about 5–6 litres of blood, while females have about 4–5 litres.

Blood consists of plasma and formed elements. Plasma forms about 55% of total blood volume and is primarily composed of water along with proteins, electrolytes, nutrients, hormones, gases, and metabolic wastes. The major plasma proteins  such as albumin, globulins, and fibrinogen which regulate osmotic balance, immune defence, transport, and coagulation.

The formed elements account for roughly 45% of blood volume and include erythrocytes, leukocytes, and platelets.

🔴 Erythrocytes (Red Blood Cells)

Erythrocytes are the most abundant blood cells and are specialised for respiratory gas transport. They are biconcave, anucleate cells packed with haemoglobin, allowing efficient oxygen binding and release. Their shape provides a high surface area-to-volume ratio, enhancing diffusion efficiency.

Red blood cells are produced in the red bone marrow in adults and have an average lifespan of 120 days. Their production is regulated by oxygen levels in the blood; reduced oxygen stimulates increased erythropoiesis.

⚪ Leukocytes (White Blood Cells)

Leukocytes are nucleated cells primarily involved in defence against infections. They are classified into granulocytes and agranulocytes.

Granulocytes include neutrophils, eosinophils, and basophils which contain cytoplasmic granules and are formed in the bone marrow. Neutrophils act as first-line phagocytes, eosinophils combat parasitic infections and participate in allergic responses, and basophils release inflammatory mediators such as heparin and histamine.

Agranulocytes include lymphocytes and monocytes. Lymphocytes are central to adaptive immunity, with B-lymphocytes producing antibodies and T-lymphocytes mediating cellular immunity. Monocytes differentiate into macrophages in tissues and function as powerful phagocytes.

🟡 Platelets and Haemostasis

Platelets are small cytoplasmic fragments derived from megakaryocytes in bone marrow. Though not complete cells, they play a crucial role in haemostasis where they prevent excessive blood loss.

Haemostasis occurs in three stages: vascular spasm, platelet plug formation, and coagulation. Following vessel injury, vasoconstriction reduces blood flow. Platelets adhere to exposed collagen fibres and aggregate to form a temporary plug. This is reinforced by the coagulation cascade, where fibrinogen is converted to fibrin, forming a stable clot.

🧪 Blood Coagulation Mechanism

Blood coagulation is a complex enzyme cascade involving clotting factors synthesized mainly in the liver. It proceeds through intrinsic and extrinsic pathways, both converging into a common pathway leading to fibrin formation.

The extrinsic pathway is triggered by tissue damage, whereas the intrinsic pathway is activated by contact with exposed collagen. Both pathways ultimately convert prothrombin to thrombin, which transforms fibrinogen into fibrin strands that stabilise the clot.

⚠️ Disorders of Clotting

Clotting disorders may result in either excessive bleeding or abnormal clot formation.

Haemophilia is a genetic bleeding disorder caused by deficiency of clotting factors, most commonly Factor VIII (Haemophilia A) or Factor IX (Haemophilia B). It is inherited as an X-linked recessive trait and predominantly affects males.

Thrombosis refers to the formation of clots within blood vessels, obstructing blood flow. Coronary thrombosis can result in myocardial infarction, while cerebral thrombosis may lead to stroke.

🧬 Haemopoiesis

Haemopoiesis is the formation of blood cells from pluripotent stem cells in the bone marrow. In embryos, it initially occurs in the yolk sac and liver before shifting to bone marrow in adults.

All formed elements such as erythrocytes, leukocytes, and platelets originate from haemocytoblasts, which differentiate into specialised precursor cells and mature into functional blood components.

📊 Erythrocyte Sedimentation Rate (ESR)

ESR measures the rate at which red blood cells settle in anticoagulated blood over one hour. It serves as a non-specific indicator of inflammation or infection.

Elevated ESR values are observed in conditions such as tuberculosis, anaemia, and chronic inflammatory disorders.

🧬 Haemoglobinopathies

Haemoglobinopathies are inherited disorders affecting haemoglobin structure or production.

Sickle cell anaemia results from a mutation in the β-globin chain, producing abnormal haemoglobin (HbS). Under low oxygen tension, RBCs assume a sickle shape, leading to haemolysis and vascular blockage.

Thalassemia is caused by reduced synthesis of α- or β-globin chains, leading to ineffective erythropoiesis and chronic anaemia.

🩸 Human Blood Groups

Blood grouping is based on specific antigens present on red blood cells.

The ABO system, discovered by Karl Landsteiner, classifies blood into A, B, AB, and O groups based on antigen presence. The Rh system depends on the presence or absence of the D antigen. Rh incompatibility during pregnancy may lead to erythroblastosis fetalis

⚙️ Electrocardiography (ECG)

Electrocardiography is a non-invasive technique used to record the electrical activity of the heart. A normal ECG consists of the P wave, QRS complex, and T wave, representing atrial depolarization, ventricular depolarization, and ventricular repolarization respectively.

⚠️ Common Cardiovascular Disorders

• Hypotension and hypertension represent abnormal blood pressure conditions. Chronic hypertension increases the risk of organ damage.
  
  
• Arteriosclerosis and atherosclerosis involve thickening and narrowing of arterial walls due to plaque deposition. Severe blockage in coronary arteries may cause myocardial infarction (heart attack).
  
  
• Bradycardia and tachycardia refer to abnormally slow and fast heart rates respectively, often linked to conduction system abnormalities.
  
  
• Angina pectoris is characterised by chest pain due to reduced coronary blood supply, while cardiac arrest involves sudden cessation of effective heart function.
  
  

📝 Important Exam Questions

• Define systemic and pulmonary circulation.
• What is the function of the SA node?
• Define haemostasis.
• What is ESR?
• State the cause of sickle cell anaemia.Describe the mechanism of blood coagulation.
• Explain the composition and functions of blood.
• Discuss cardiovascular disorders with causes and consequences.

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