Chapter 2: Circulatory System

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Chapter Overview & SLOs

Why is a circulatory system necessary? The circulatory system is essential for transporting oxygen, nutrients, and hormones to body cells while removing waste products like carbon dioxide ($CO_2$). It maintains homeostasis by regulating temperature, pH, and fluid balance.

What are the main components of blood? Blood consists of four main components:

• Plasma: The liquid portion (about 55% of blood volume), composed of water, proteins, electrolytes, and dissolved nutrients. Transports blood cells, hormones, and waste products.
• Red Blood Cells (RBCs / Erythrocytes): Contain hemoglobin for oxygen transport. Biconcave shape increases surface area. No nucleus in mammals. Lifespan about 120 days.
• White Blood Cells (WBCs / Leukocytes): Part of the immune system, defend against infections and foreign invaders.
• Platelets (Thrombocytes): Cell fragments involved in blood clotting (hemostasis).

What are blood vessels and their types?

• Arteries: Carry blood away from the heart. Thick, elastic, muscular walls to withstand high pressure. Branch into arterioles and then capillaries.
• Veins: Return blood to the heart. Thinner walls than arteries, contain valves to prevent backflow due to low pressure.
• Capillaries: Microscopic vessels where exchange of gases, nutrients, and wastes occurs between blood and tissues. Walls are one cell thick.

What is the structure of the human heart?

• 4 chambers: Two atria (upper chambers receiving blood) and two ventricles (lower chambers pumping blood out).
• 3 tissue layers:
  • Endocardium: Inner lining, smooth to prevent clotting.
  • Myocardium: Thick muscular middle layer responsible for contraction. Left ventricle has the thickest wall to pump blood to the entire body.
  • Epicardium: Outer layer, part of the pericardium (protective sac).
• Septum: Muscular wall dividing the heart into left and right sides, preventing mixing of oxygenated and deoxygenated blood.

What are heart valves and their functions? Valves ensure one-way blood flow and prevent backflow.

• Atrioventricular (AV) valves:
  • Tricuspid valve: Between right atrium and right ventricle (3 flaps).
  • Bicuspid (Mitral) valve: Between left atrium and left ventricle (2 flaps).
• Semilunar valves:
  • Pulmonary valve: Between right ventricle and pulmonary artery.
  • Aortic valve: Between left ventricle and aorta.

What are the circulatory pathways?

• Pulmonary circulation: Deoxygenated blood flows from right ventricle → pulmonary artery → lungs (where $CO_2$ is released and $O_2$ is picked up) → pulmonary veins → left atrium.
• Systemic circulation: Oxygenated blood flows from left ventricle → aorta → arteries → arterioles → capillaries → venules → veins → vena cava → right atrium.
• Coronary circulation: Supplies oxygenated blood to the heart muscle itself via coronary arteries. Blockage leads to heart attack (myocardial infarction).

What is the cardiac cycle? The cardiac cycle consists of alternating contraction (systole) and relaxation (diastole) of the heart chambers.

• Systole: Contraction phase. Ventricles contract, pumping blood into arteries. AV valves close (producing "lub" sound).
• Diastole: Relaxation phase. Ventricles fill with blood from atria. Semilunar valves close (producing "dub" sound).
• Normal heart rate: 60-100 beats per minute at rest.

How does smoking impact cardiovascular health?

• Atherosclerosis: Smoking chemicals damage artery walls, leading to plaque buildup, narrowing arteries, and reducing blood flow.
• Increased heart rate: Nicotine stimulates adrenaline release, increasing heart rate and blood pressure.
• Lower oxygen-carrying capacity: Carbon monoxide in smoke binds to hemoglobin more strongly than oxygen, reducing oxygen delivery to tissues.

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we differentiate between pulmonary and systemic circulation? Pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs (where $CO_2$ is released and $O_2$ is picked up), while systemic circulation delivers oxygenated blood from the left ventricle to the rest of the body (supplying oxygen and nutrients, removing waste). The coronary circuit supplies the heart muscle itself.
• How do we identify the layers and chambers of the heart? The heart has four chambers — two atria and two ventricles — protected by three tissue layers: the Endocardium (inner lining), the thick muscular Myocardium (responsible for contraction; left ventricle has thickest wall), and the outer Epicardium (part of pericardium). The septum divides left and right sides.
• How do we explain the role of heart valves? Valves like the Tricuspid (right side) and Bicuspid/Mitral (left side) ensure a one-way flow by preventing the backflow of blood into the atria during ventricular contraction. Semilunar valves (pulmonary and aortic) prevent backflow from arteries into ventricles during diastole.
• How does smoking impact cardiovascular health? Smoking chemicals cause atherosclerosis (plaque buildup narrowing arteries), increase heart rate and blood pressure through nicotine-induced adrenaline release, and lower the oxygen-carrying capacity of blood due to carbon monoxide binding to hemoglobin.

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