The Circulatory System - Human and Social Biology

Left Atrium Right Atrium Left Ventricle Right Ventricle

Figure 1: Overview of the human circulatory system showing heart structure and blood flow

Introduction to the Circulatory System

The circulatory system is the body's transport network, delivering oxygen and nutrients to cells while removing waste products. This double circulatory system consists of the heart, blood vessels, and blood, working together to maintain homeostasis.

Key Functions:

Structure of the Heart

The heart is a muscular pump with four chambers that ensure one-way blood flow through the circulatory system.

Left Atrium Right Atrium Left Ventricle Right Ventricle Bicuspid Valve Tricuspid Valve Aortic Valve Pulmonary Valve

Figure 2: Detailed structure of the heart showing chambers and valves

Heart Chambers and Valves:

Blood Vessels

The circulatory system contains three main types of blood vessels, each with specialized structures.

Vessel Type Structure Function Blood Pressure Example
Arteries Thick muscular walls, small lumen Carry blood away from heart High (120/80 mmHg) Aorta, coronary arteries
Veins Thin walls, large lumen, valves Carry blood to heart Low (10 mmHg) Vena cava, jugular vein
Capillaries One-cell thick walls Site of gas/nutrient exchange Medium (25 mmHg) Alveolar capillaries
Artery Capillary Network Vein Artery Capillary Vein

Figure 3: Comparison of blood vessel types with cross-sectional views

Blood Composition

Blood is a specialized connective tissue composed of cellular components suspended in plasma.

Whole Blood Plasma (55%) Buffy Coat (<1%) Erythrocytes (45%) Water (90%) Leukocytes Erythrocytes Platelets

Figure 4: Composition of blood showing separated components

Blood Components:

Double Circulation

The human circulatory system has two distinct circuits working in parallel.

1

Heart → Lungs

2

Gas Exchange

3

Lungs → Heart

4

Heart → Body

Pulmonary Circulation:

Systemic Circulation:

Lungs Body Tissues Pulmonary Circulation Systemic Circulation

Figure 5: Double circulation system showing pulmonary and systemic circuits

Cardiac Cycle

The repeating sequence of heart contraction (systole) and relaxation (diastole) that pumps blood.

Atrial Systole (0.1s) Ventricular Systole (0.3s) Diastole (0.4s) "Lub" (AV valves close) "Dub" (Semilunar valves close)

Figure 6: Cardiac cycle phases and corresponding heart sounds

Phases of the Cardiac Cycle:

  1. Atrial Systole (0.1 sec):
  2. Ventricular Systole (0.3 sec):
  3. Diastole (0.4 sec):

Blood Pressure Regulation

The body maintains optimal blood pressure through complex homeostatic mechanisms.

Baroreceptors Medulla\nOblongata Heart Nerve Signals Adjustments Blood Pressure Changes

Figure 7: Blood pressure regulation feedback loop

Regulation Mechanisms:

Common Circulatory Disorders

Disorder Causes Symptoms Prevention/Treatment
Hypertension Diet, stress, genetics High BP, headaches Reduce salt, exercise, medication
Atherosclerosis Cholesterol plaques Chest pain, fatigue Healthy diet, statins
Anemia Iron deficiency Fatigue, pallor Iron-rich foods, supplements
Leukemia Cancer of WBCs Frequent infections Chemotherapy, bone marrow transplant

Glossary of Circulatory System Terms

Artery
Blood vessel carrying blood away from the heart, usually oxygenated (except pulmonary artery).
Atrium
Upper chamber of the heart that receives blood from veins.
Capillary
Microscopic blood vessel where gas/nutrient exchange occurs between blood and tissues.
Cardiac Output
Volume of blood pumped by each ventricle per minute (CO = HR × SV).
Diastole
Relaxation phase of the cardiac cycle when chambers fill with blood.
Hemoglobin
Iron-containing protein in red blood cells that transports oxygen.
Pulse
Rhythmic throbbing of arteries as blood is pumped through them.
Systole
Contraction phase of the cardiac cycle when blood is ejected from chambers.
Vasoconstriction
Narrowing of blood vessels to increase blood pressure.
Vasodilation
Widening of blood vessels to decrease blood pressure.

Self-Assessment Questions

1. Trace the pathway of blood from the right ventricle to the left atrium.
Right Ventricle → Pulmonary Valve → Pulmonary Arteries → Lungs → Pulmonary Veins → Left Atrium
2. Explain two structural differences between arteries and veins.
1) Arteries have thicker muscular walls than veins to withstand higher pressure.
2) Veins contain valves to prevent backflow, while arteries do not need them.
3. What causes the "lub-dub" heart sounds during the cardiac cycle?
"Lub" occurs when AV valves close during ventricular contraction (systole). "Dub" occurs when semilunar valves close during ventricular relaxation (diastole).
4. How does the SA node regulate heart rate?
The sinoatrial (SA) node acts as the natural pacemaker, initiating electrical impulses (60-100/min) that spread through the heart, triggering coordinated contractions.
5. Compare oxygen transport in blood versus carbon dioxide transport.
Oxygen: 98.5% bound to hemoglobin as oxyhemoglobin, 1.5% dissolved in plasma.
Carbon dioxide: 70% as bicarbonate ions, 23% as carbaminohemoglobin, 7% dissolved in plasma.
6. Why does the left ventricle have a thicker muscular wall than the right?
The left ventricle pumps blood throughout the entire body (systemic circulation) requiring greater force, while the right ventricle only pumps to the lungs (pulmonary circulation) which is lower pressure.
7. Describe how baroreceptors help regulate blood pressure.
Baroreceptors in the aorta and carotid arteries detect pressure changes and send signals to the medulla oblongata, which adjusts heart rate and vessel diameter to maintain homeostasis.
8. What would happen if the bicuspid valve became leaky?
Blood would regurgitate into the left atrium during ventricular contraction, reducing pumping efficiency. This could lead to shortness of breath, fatigue, and pulmonary congestion (mitral valve prolapse).
9. Explain why capillary walls are only one cell thick.
The thin walls (endothelium) minimize diffusion distance for efficient exchange of oxygen, carbon dioxide, nutrients, and wastes between blood and tissues.
10. How does regular exercise benefit the circulatory system?
Strengthens heart muscle, lowers resting heart rate, improves circulation, reduces blood pressure, increases HDL cholesterol, and enhances oxygen delivery to tissues.

Summary of Key Points