Circulatory System
Components
- Heart: Muscular pumping organ.
- Blood vessels: Arteries, veins, capillaries.
- Blood: Fluid connective tissue.
Types of Circulation
- Pulmonary circulation: Right ventricle -> lungs -> left atrium (deoxygenated blood to lungs, oxygenated back).
- Systemic circulation: Left ventricle -> body -> right atrium. (oxygenated blood to body, deoxygenated back).
- Coronary circulation: Blood supply to heart muscle itself.
The Heart
Structure
- Four-chambered: 2 atria (upper) + 2 ventricles (lower).
- Walls: Pericardium (outer), Myocardium (muscular middle), Endocardium (inner lining).
- Septum divides left and right sides.
Left side: Oxygenated blood (thicker wall, higher pressure). Right side: Deoxygenated blood.
Valves
- Atrioventricular valves: Bicuspid (mitral, left), Tricuspid (right).
- Semilunar valves: Pulmonary (right), Aortic (left).
- Prevent backflow of blood.
Cardiac Cycle
| Phase | Atria | Ventricles | AV Valves | SL Valves | Blood Flow |
|---|---|---|---|---|---|
| Atrial systole | Contract | Relaxed | Open | Closed | Atria -> Ventricles |
| Ventricular systole | Relaxed | Contract | Closed | Open | Ventricles -> Arteries |
| Diastole | Relaxed | Relaxed | Open | Closed | Veins -> Atria |
Heart sounds:
- 'Lub' (first sound): Closure of AV valves (start of ventricular systole).
- 'Dup' (second sound): Closure of semilunar valves (end of ventricular systole).
Cardiac Output
Volume of blood pumped by each ventricle per minute.
CO = Stroke volume x Heart rate
Normal: ~5 L/min (can increase to 20-25 L/min during exercise).
Blood Vessels
Arteries
- Carry blood AWAY from heart.
- Thick, elastic walls (handle high pressure).
- Branch into arterioles -> capillaries.
Veins
- Carry blood TOWARDS the heart.
- Thin walls, larger lumen, valves to prevent backflow.
- Venules collect from capillaries.
Capillaries
- One cell thick. Site of exchange (O2, CO2, nutrients, waste).
- Connect arteries and veins.
Blood
Composition
- Plasma (55%): Water (90%), proteins (albumin, globulin, fibrinogen), nutrients, hormones, waste.
- Formed elements (45%): RBCs, WBCs, Platelets.
Red Blood Cells (Erythrocytes)
- Biconcave, anucleate (mammals).
- Contain haemoglobin (oxygen transport).
- Lifespan: ~120 days.
- Produced in bone marrow (erythropoiesis).
- Destroyed in spleen (haemolysis).
White Blood Cells (Leukocytes)
- Nucleated, involved in immune defence.
| Type | % | Function |
|---|---|---|
| Neutrophils | 60-65% | Phagocytosis (bacteria) |
| Lymphocytes | 20-25% | Antibody production (B cells), Cell-mediated immunity (T cells) |
| Monocytes | 3-8% | Phagocytosis (macrophages) |
| Eosinophils | 2-3% | Allergic reactions, parasites |
| Basophils | 0.5-1% | Histamine release (inflammation) |
Platelets (Thrombocytes)
- Cell fragments. Involved in blood clotting (clotting factors + fibrinogen -> fibrin clot).
Blood Groups (ABO System)
| Blood Group | Antigen | Antibody | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A | anti-B | A, AB | A, O |
| B | B | anti-A | B, AB | B, O |
| AB | A, B | None | AB | A, B, AB, O (Universal recipient) |
| O | None | anti-A, anti-B | A, B, AB, O (Universal donor) | O |
Rh factor: Rh+ individuals have Rh antigen. Rh- individuals lack it. Rh incompatibility can cause erythroblastosis fetalis.
Lymphatic System
- Lymph: Fluid from interstitial spaces, similar to plasma but less protein.
- Lymph nodes: Filter lymph, house lymphocytes.
- Functions: Drainage, immune defence, fat absorption (lacteals in intestine).
Electrocardiogram (ECG)
- Graphic recording of heart's electrical activity.
- P wave: Atrial depolarisation (systole).
- QRS complex: Ventricular depolarisation.
- T wave: Ventricular repolarisation (diastole).
Excretory System
Components
- Kidneys (2): Filter blood, produce urine.
- Ureters (2): Transport urine to bladder.
- Urinary bladder (1): Stores urine.
- Urethra (1): Excretes urine.
Kidney Structure
- Outer: Cortex (convoluted tubules, glomeruli).
- Inner: Medulla (loops of Henle, collecting ducts).
- Functional unit: Nephron (about 1 million per kidney).
Nephron Structure
- Bowman's capsule + Glomerulus = Renal corpuscle (Malpighian body).
- Proximal Convoluted Tubule (PCT).
- Loop of Henle (descending and ascending limbs).
- Distal Convoluted Tubule (DCT).
- Collecting duct.
Urine Formation (Three Steps)
1. Glomerular Filtration:
- Blood pressure forces filtrate (water, glucose, amino acids, salts, urea, creatinine) from glomerulus into Bowman's capsule.
- Large molecules (proteins, RBCs) are retained.
- GFR (Glomerular Filtration Rate) = ~125 mL/min.
2. Reabsorption (Tubular Reabsorption):
- PCT: Reabsorbs glucose, amino acids, 70% water, Na+, Cl-, HCO3-.
- Loop of Henle: Countercurrent mechanism (creates medullary concentration gradient).
- DCT: Reabsorbs Na+, Cl-, Ca2+ (regulated by hormones).
3. Secretion (Tubular Secretion):
- PCT: Secretes H+, NH3, drugs.
- DCT and Collecting duct: Secretes K+, H+.
Hormonal Regulation
- ADH (Vasopressin): Increases water reabsorption (collecting duct). Decreases urine volume.
- Aldosterone: Increases Na+ reabsorption, K+ secretion. Increases water retention.
- ANF (Atrial Natriuretic Factor): Opposite to aldosterone. Decreases Na+ reabsorption. Increases urine output.
Composition of Urine
- Water (96%), Urea (2%), Uric acid, Creatinine, Na+, K+, Cl-, NH3, pH about 6.
Disorders
- Uremia: Excess urea in blood.
- Renal calculi: Kidney stones (calcium oxalate).
- Glomerulonephritis: Inflammation of glomeruli.
- Diabetes insipidus: ADH deficiency (excessive dilute urine).
- Dialysis: Artificial kidney (haemodialysis or peritoneal dialysis).
Worked Examples
Example 1: If a person has heart rate 72/min and stroke volume 70 mL, find cardiac output. Solution: CO = 72 x 70 = 5040 mL/min = 5.04 L/min.
Example 2: A person with blood group B needs a blood transfusion. Which blood groups can they receive? Solution: B and O (B has anti-A antibodies, cannot receive A or AB).
Common Mistakes
- Arteries vs veins: Arteries carry blood AWAY from heart (usually oxygenated). Veins carry blood TOWARDS heart (usually deoxygenated). Exception: Pulmonary artery carries deoxygenated blood.
- Universal donor is O-: O negative is universal donor. O positive contains Rh antigen.
- ADH vs Aldosterone: ADH increases water permeability (collecting duct). Aldosterone increases Na+ reabsorption.
- Nephron parts: Know the sequence of filtration, reabsorption, and secretion along the nephron.
ISC Exam Focus
- Theory (70%): Heart structure, cardiac cycle, blood components, urine formation, kidney function.
- Application (30%): Cardiac output calculation, blood group compatibility, diagram labelling.
- ISC frequently asks: "Describe the structure of nephron" and "Explain urine formation."
Self-Test Questions
Q1: Draw and label the internal structure of the human heart. Answer: Four chambers, valves (bicuspid, tricuspid, pulmonary, aortic), septum, major vessels.
Q2: Define cardiac output. How is it calculated? Answer: Blood pumped by each ventricle per minute. CO = Stroke volume x Heart rate.
Q3: Differentiate between blood groups A and B. Answer: Group A: A antigen, anti-B antibodies. Group B: B antigen, anti-A antibodies.
Q4: Name the parts of a nephron. Answer: Bowman's capsule, PCT, Loop of Henle (descending + ascending), DCT, Collecting duct.
Q5: Describe the process of glomerular filtration. Answer: Blood pressure forces filtrate (water, salts, glucose, urea) through glomerular capillaries into Bowman's capsule. Large molecules retained.
Q6: What is the role of ADH in urine formation? Answer: ADH increases water reabsorption from the collecting duct, reducing urine volume and concentrating urine.
