The Circulatory System Study Notes

Overview of the Circulatory System

• A transport network that brings nutrients and O2 to every cell and removes metabolic wastes (e.g. CO2, urea).
• Consists of two intertwined circuits:
  • Pulmonary (heart ⇄ lungs)
  • Systemic (heart ⇄ rest of body)
• Blood passes through the heart twice during one complete circulation (double circulation).

Core Functions

• Deliver O_2 and digested nutrients to tissues.
• Remove CO_2 and other waste products for excretion.
• Distribute hormones, antibodies, and heat.
• Maintain homeostasis (pH, temperature, fluid balance).

Principal Components

• Heart – muscular pump that keeps blood moving.
• Blood vessels – closed network of arteries, capillaries, and veins.
• Blood – fluid connective tissue carrying dissolved and cellular cargo.

The Heart

Location & Orientation

• Lies in thoracic cavity, just above the diaphragm, between the lungs, behind the sternum.
• Apex (tip) points slightly left of the midline.

Gross Anatomy

• Hollow, muscular organ divided internally by a complete septum ➔ right and left sides never mix blood.
• Four chambers:
  • Upper chambers: right atrium (RA) & left atrium (LA) – RECEIVE blood from veins.
  • Lower chambers: right ventricle (RV) & left ventricle (LV) – PUMP blood into arteries.
• One-way valves ensure unidirectional flow at every exit/entry between chambers and vessels.

Muscular Wall Thickness

• Left ventricle: very thick myocardium; must eject blood at high pressure through the aorta to the entire body (long distance).
• Right ventricle: thinner wall; only needs to push blood a short distance to the lungs at low pressure.

Pathway of Blood Through Heart & Body

1. Deoxygenated blood enters RA via venae cavae (main systemic veins).
2. RA contracts ➔ blood flows through tricuspid valve into RV.
3. RV contracts ➔ blood exits via pulmonary valve into pulmonary arteries → lungs.
4. In pulmonary capillaries: CO2 is released, O2 is absorbed.
5. Oxygenated blood returns via pulmonary veins into LA.
6. LA contracts ➔ blood passes bicuspid (mitral) valve into LV.
7. LV contracts ➔ blood is propelled through the aortic valve and aorta to systemic circulation.
8. Systemic capillaries exchange gases/nutrients; deoxygenated blood returns to RA and cycle repeats.

Blood Vessels

Arteries

• Carry blood away from the heart at high pressure.
• Thick, muscular, elastic walls resist and maintain pressure.
• Pulse felt here.

Capillaries

• Microscopic (one-cell-thick endothelium) to allow close contact with every cell.
• Sites of exchange: deliver O2/nutrients, pick up CO2/wastes.
• Unite to form venules, then veins.

Veins

• Carry blood back to the heart at low pressure.
• Thinner, less muscular walls; larger lumens.
• Contain semilunar valves that prevent back-flow, aiding return—especially from limbs.

Blood Composition

• Blood plasma (≈55 % of blood volume)
  • Mostly water; dissolves & transports nutrients, hormones, wastes, and gases.
• Cellular components (formed elements, ≈45 %)
  • Red blood corpuscles (erythrocytes)
  • Biconcave, anucleate discs formed in bone marrow.
  • Contain hemoglobin to bind and transport O_2.
  • White blood corpuscles (leukocytes)
  • Larger, irregular, nucleated; immune “soldiers.”
  • Destroy pathogens and mediate defense.
  • Platelets (thrombocytes)
  • Cell fragments produced in bone marrow.
  • Essential for blood clotting; initiate fibrin mesh at injury sites.

Health Issues of the Circulatory System

High Blood Pressure (Hypertension)

• Definition: chronically elevated arterial pressure (e.g., > 140/90\;\text{mmHg}).
• Consequences:
  • Increased cardiac workload ➔ heart enlargement ➔ reduced pumping efficiency.
  • Damage to arterial walls, risking aneurysm, kidney failure, heart attack, or stroke.
• Triggers: stress, smoking, obesity, inactivity, unhealthy diet, certain diseases.
• Note: pressure naturally fluctuates with activity, emotion, and time of day.

Heart Attack (Myocardial Infarction)

• Cause: stricture (atherosclerotic narrowing) or clot that blocks a coronary artery supplying the myocardium.
• Without blood, affected heart muscle dies; pumping can cease.
• Classic symptom: sharp central chest pain radiating to jaw, back, left arm.
• Risk factors: fatty diet, lack of exercise, obesity, smoking, alcohol abuse, and hereditary predisposition.

Stroke (Cerebrovascular Accident)

• Brain deprived of O_2 due to:
  • Vessel blockage (ischemic stroke) or
  • Vessel rupture (hemorrhagic stroke).
• Signs: sudden weakness, confusion, possible paralysis of body regions.

Concept Connections & Practical Notes

• Double circulation maximizes oxygen delivery efficiency compared with single-circuit systems in some animals.
• Valves in heart and veins embody a unidirectional design principle; malfunction (e.g., varicose veins) illustrates structural importance.
• Lifestyle factors influencing hypertension and coronary disease are modifiable, demonstrating the link between anatomy, physiology, and public health.
• Platelet-mediated clotting showcases positive feedback mechanisms; therapeutic anticoagulants must balance clot prevention vs. hemorrhage risk.

Key Terms & Definitions

• Atrium / Ventricle: receiving vs. pumping chambers.
• Aorta: main systemic artery from LV.
• Pulmonary artery/veins: only artery carrying deoxygenated blood & veins carrying oxygenated blood, respectively.
• Systole / Diastole: contraction / relaxation phases of the cardiac cycle, underlying the two numbers of a blood-pressure reading.
• Hypertension, Myocardial Infarction, Stroke: major cardiovascular pathologies covered above.