ZM

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.