What this chapter is about
This chapter introduces the fluid connective tissues—blood and lymph—that transport materials throughout the human body, and the circulatory system that keeps these fluids in motion. You will learn the composition of blood, the types of blood cells, blood groups, and the mechanism of blood clotting. The chapter then moves to the structure of the human heart, the cardiac cycle, and how the heart generates and regulates its own beat.
Understanding circulation is essential because every cell depends on a continuous supply of oxygen, nutrients, and hormones, and must dispose of carbon dioxide and metabolic wastes. A Class 11 student meets this topic now because it builds on earlier knowledge of tissues and organ systems, and connects physiology with concepts in chemistry (gas exchange, pH buffering) and physics (pressure, flow). After studying this chapter, you should be able to describe the path blood takes through the heart and vessels, explain what happens in one cardiac cycle, read a simple ECG pattern, and distinguish between the blood and lymphatic systems.
Key ideas
- Blood is a fluid connective tissue containing plasma (about 55 %) and formed elements—erythrocytes (red blood cells), leucocytes (white blood cells) and thrombocytes (platelets).
- Erythrocytes carry haemoglobin, which binds oxygen reversibly; leucocytes defend the body; thrombocytes help in clotting.
- Human blood groups (ABO system) depend on the presence or absence of antigens A and B on RBC surfaces and the corresponding antibodies in plasma; the Rh factor adds another antigen (D).
- The human heart is a four-chambered muscular pump: two atria receive blood, two ventricles pump it out. The right side handles deoxygenated blood (to lungs), the left side oxygenated blood (to body).
- One cardiac cycle (atrial systole → ventricular systole → joint diastole) lasts about 0.8 s at rest, giving roughly 72 beats per minute; stroke volume is about 70 mL, so cardiac output ≈ 5 L/min.
- The heart is myogenic: the sinoatrial (SA) node generates impulses, which pass via the atrioventricular (AV) node and bundle of His to Purkinje fibres, ensuring coordinated contraction.
- An electrocardiograph (ECG) records the electrical activity as P wave (atrial depolarisation), QRS complex (ventricular depolarisation) and T wave (ventricular repolarisation).
- Lymph is tissue fluid that drains into lymphatic vessels, passes through lymph nodes, and returns to blood via large veins; it carries fats from the intestine and supports immunity.
Formulas and facts to remember
1. Cardiac output (CO) = Stroke volume (SV) × Heart rate (HR). At rest: CO ≈ 70 mL × 72 beats/min ≈ 5 L/min. 2. Normal adult blood volume is about 5–6 litres; plasma forms roughly 55 %, cells 45 % (haematocrit). 3. Erythrocyte count: roughly 5 million per microlitre in males, 4.5 million in females. 4. Total leucocyte count: 4 000–11 000 per microlitre; types include neutrophils, lymphocytes, monocytes, eosinophils and basophils. 5. Platelet count: 1.5–4 lakh per microlitre; essential for clot formation via fibrinogen → fibrin conversion. 6. ABO compatibility rule: a recipient should not receive blood with antigens against which they carry antibodies, else agglutination occurs. 7. Rh-negative mother carrying Rh-positive foetus can develop anti-Rh antibodies; erythroblastosis fetalis may affect later pregnancies. 8. Double circulation: blood passes through the heart twice in one complete circuit (pulmonary + systemic).
Worked examples
### Example 1 – Calculating cardiac output after exercise
A student's heart rate rises to 110 beats per minute during a brisk walk, and her stroke volume increases to 90 mL. Find her cardiac output.
Step 1: Write the relation. CO = SV × HR.
Step 2: Substitute values. CO = 90 mL × 110 per min = 9 900 mL/min.
Step 3: Convert to litres. CO = 9.9 L/min.
Interpretation: During moderate exercise, cardiac output nearly doubles compared with the resting value of about 5 L/min.
### Example 2 – Predicting transfusion compatibility
Ravi has blood group B-positive. Can he safely receive blood from a donor who is O-negative?
Step 1: Identify antigens and antibodies. Ravi's RBCs carry antigen B and Rh antigen D; his plasma has anti-A antibodies.
Step 2: Check donor cells. O-negative blood has no A, B or D antigens.
Step 3: Decide. Because the donor cells lack the antigens Ravi's plasma attacks, the transfusion is compatible.
Note: O-negative individuals are called universal donors for emergency situations, though exact cross-matching is always preferred.
### Example 3 – Interpreting a simple ECG finding
An ECG strip shows the interval from one R peak to the next (R-R interval) is 1.0 s. Estimate the heart rate.
Step 1: Heart rate = 60 s / R-R interval in seconds.
Step 2: HR = 60 / 1.0 = 60 beats per minute.
This is within the normal resting range (60–100 bpm).
Common mistakes
Thinking arteries always carry oxygenated blood → Pulmonary arteries carry deoxygenated blood from the heart to the lungs; define arteries by direction (away from heart), not oxygen content.
Confusing atrial and ventricular systole → Atrial systole comes first and lasts about 0.1 s; ventricular systole follows and lasts about 0.3 s; keep the sequence clear.
Believing the brain controls every heartbeat → The heart is myogenic; the SA node initiates beats automatically; the nervous system only modulates rate.
Mixing up antigens and antibodies in blood groups → Antigens sit on RBC surfaces; antibodies float in plasma; a person does not carry antibodies against their own antigens.
Assuming lymph and blood are identical → Lymph lacks RBCs and most proteins; it originates from tissue fluid and returns to blood via the thoracic duct.
Quick revision
- Blood = plasma + RBCs + WBCs + platelets; plasma is 55 % of blood volume.
- Heart: right side pumps to lungs, left side pumps to body; valves prevent backflow.
- SA node → AV node → bundle of His → Purkinje fibres: the conduction pathway.
- One cardiac cycle ≈ 0.8 s; CO = SV × HR; resting CO ≈ 5 L/min.
- ECG waves: P (atrial), QRS (ventricular depolarisation), T (ventricular repolarisation).
- Lymph drains tissue fluid, absorbs intestinal fats, and houses lymphocytes for immunity.