10.1 Types of Circulatory System
Multicellular organisms require a circulatory system to transport respiratory gases, nutrients, and waste products across longer distances where simple diffusion is insufficient.
Open vs. Closed Circulatory Systems
- Open Circulatory System: Hemolymph flows directly into body cavities (hemocoel) and bathes tissues directly (e.g., insects).
- Closed Circulatory System: Blood is enclosed within continuous blood vessels at all times (e.g., earthworms, vertebrates).
Circulatory Systems in Vertebrates
- Fish: Single circulatory system — heart has 2 chambers (1 atrium, 1 ventricle). Blood flows through the heart once per complete circuit ($1\text{ atrium} \rightarrow 1\text{ ventricle} \rightarrow \text{gills} \rightarrow \text{body tissues}$).
- Amphibians: Double incomplete circulatory system — heart has 3 chambers (2 atria, 1 ventricle). Oxygenated and deoxygenated blood mix partially in the single ventricle.
- Humans / Mammals / Birds: Double complete circulatory system — heart has 4 chambers (2 atria, 2 ventricles). Complete separation of oxygenated and deoxygenated blood.
- Pulmonary Circulation: Right ventricle $\rightarrow$ Pulmonary artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary vein $\rightarrow$ Left atrium.
- Systemic Circulation: Left ventricle $\rightarrow$ Aorta $\rightarrow$ Body tissues $\rightarrow$ Vena cava $\rightarrow$ Right atrium.
10.2 Circulatory System of Humans
Structure of the Human Heart
- Right Atrium & Ventricle: Receive deoxygenated blood from the body and pump it to the lungs. Separated by the tricuspid valve.
- Left Atrium & Ventricle: Receive oxygenated blood from the lungs and pump it to the rest of the body. Separated by the bicuspid (mitral) valve.
- Semilunar Valves: Located at the bases of the pulmonary artery and aorta to prevent backflow of blood into the ventricles.
- Thick Ventricular Wall: The left ventricle has a much thicker muscular wall (myocardium) than the right ventricle to generate higher pressure to pump blood to the entire body.
Mechanism of Heartbeat Initiation and Regulation
- Heart muscle is myogenic (initiates its own contractions without external nerve impulses).
- Sinoatrial (SA) Node: Natural pacemaker located in the right atrium; generates electrical impulses initiating atrial contraction.
- Atrioventricular (AV) Node: Spreads impulses down the Bundle of His, Purkyne fibers, and through the ventricular walls, causing ventricular contraction from the apex upward.
10.3 Mechanism of Blood Clotting
Blood clotting prevents excessive blood loss and prevents the entry of pathogens through wounds.
The Clotting Cascade
- Damaged tissue and blood platelets release an enzyme called thrombokinase.
- Thrombokinase, in the presence of Calcium ions ($\text{Ca}^{2+}$) and Vitamin K, converts inactive prothrombin into active thrombin.
- Thrombin converts soluble fibrinogen (plasma protein) into insoluble fibrin threads.
- Fibrin forms a mesh structure that traps red blood cells to form a clot and seal the wound.
10.4 Lymphatic System of Humans
Formation of Tissue Fluid and Lymph
- High hydrostatic pressure at the arterial end of capillaries forces fluid out into intercellular spaces, forming interstitial (tissue) fluid.
- About 85% of this fluid re-enters capillaries at the venous end. The remaining 15% drains into lymphatic capillaries and is called lymph.
Comparison Between Blood, Tissue Fluid, and Lymph
- Blood: Contains red blood cells, white blood cells, platelets, and plasma proteins.
- Tissue Fluid: Similar to plasma but lacks red blood cells, platelets, and large plasma proteins.
- Lymph: Similar composition to tissue fluid, but contains higher concentrations of lymphocytes and lipid droplets (chylomicrons from lacteals).
Return of Lymph to the Blood Circulation
- Lymph vessels empty into two main ducts:
- Thoracic Duct: Drains lymph from the left side of the head, neck, chest, and lower body into the left subclavian vein.
- Right Lymphatic Duct: Drains lymph from the upper right side of the body into the right subclavian vein.
10.5 Health Issues of the Human Circulatory & Lymphatic Systems
- Atherosclerosis: Accumulation of fatty deposits (cholesterol/plaques) inside lumen of arteries, narrowing the passage.
- Arteriosclerosis: Hardening and loss of elasticity in arterial walls due to calcium deposition.
- Coronary Heart Disease (CHD): Blockage of coronary arteries supplying heart muscle, leading to angina or myocardial infarction (heart attack).
- Hypertension: Consistently elevated blood pressure ($>140/90\text{ mmHg}$).
- Stroke: Interruption of blood supply to brain tissues due to a clot (thrombosis) or ruptured blood vessel (hemorrhage).
- Lymphatic Filariasis (Elephantiasis): Parasitic worm infection blocking lymphatic vessels, causing severe swelling (edema) of tissues.