Circulatory System Overview

Components of the Circulatory System

The circulatory system consists of three main components working together:

1. Vessels - Tubes that carry blood throughout the body
2. Pump - Heart that pushes blood through vessels
3. Valves - Ensure one-way flow of blood

Types of Circulation

Single Circulation (Fish)

• Blood passes through heart once per complete circuit
• Path: Heart → gills → body → heart
• Less efficient - blood pressure drops significantly after gills

Double Circulation (Mammals)

• Blood passes through heart twice per complete circuit
• Pulmonary circulation: Heart → lungs → heart
• Systemic circulation: Heart → body → heart
• More efficient - maintains higher blood pressure to body tissues

Advantages of Double Circulation

1. Higher blood pressure to body tissues
2. Separate oxygenated and deoxygenated blood flow
3. More efficient delivery of oxygen and nutrients
4. Supports higher metabolic rates needed for active lifestyles

Heart Structure and Function

Heart Structure

The heart is a muscular organ with four chambers and internal structures:

Chambers

• Atria (plural of atrium): Upper chambers receiving blood
• Ventricles: Lower chambers pumping blood out

Other Structures

• Muscular wall: Thick outer layer pumping blood
• Septum: Wall dividing left and right sides
• Valves: Ensure one-way blood flow
• Coronary arteries: Supply blood to heart muscle itself

Blood Flow Direction

Specific Examples

• Aorta: Main artery from left ventricle to body
• Pulmonary artery: From right ventricle to lungs
• Pulmonary veins: From lungs to left atrium
• Vena cava: From body to right atrium

Blood Vessels

Arteries

Structure

• Thick muscular walls: Withstand high pressure
• Small lumen diameter: Maintains pressure
• Elastic: Stretch and recoil with pressure changes
• No valves: Blood flows under pressure

Function

• Carry blood away from heart
• Transport oxygenated blood (except pulmonary artery)
• High pressure for rapid delivery

Veins

Structure

• Thinner walls: Lower pressure
• Larger lumen diameter: Accommodates more blood
• Valves: Prevent backflow of blood
• Less elastic: Lower pressure system

Function

• Carry blood towards heart
• Transport deoxygenated blood (except pulmonary veins)
• Return blood from body tissues

Capillaries

Structure

• Very thin walls: One cell thick
• Small lumen: Red blood cells pass single file
• No valves
• Short length: Quick diffusion

Function

• Site of exchange between blood and tissues
• Oxygen, nutrients, water diffuse out
• Carbon dioxide, waste diffuse in

Blood Components

List of Blood Components

Blood consists of four main components:

1. Red blood cells (erythrocytes)
2. White blood cells (leukocytes)
3. Platelets (thrombocytes)
4. Plasma

Red Blood Cells

Structure and Appearance

• Biconcave disc shape (thicker edges, thinner centre)
• No nucleus in adult mammals
• Flexible and can squeeze through small capillaries

Function

• Oxygen transport from lungs to body tissues
• Haemoglobin protein binds oxygen
• Each cell contains ~270 million haemoglobin molecules

Haemoglobin

• Protein with iron-containing groups
• Binds oxygen in lungs where concentration is high
• Releases oxygen in tissues where concentration is lower

White Blood Cells

Structure and Appearance

• Variable shapes and sizes
• Contain nucleus
• Smaller numbers than red blood cells

Functions

• Phagocytosis: Some white blood cells engulf and destroy pathogens
• Antibodies: Others produce antibodies for specific immune defence

Types

• Phagocytes: Engulf bacteria and debris
• Lymphocytes: Produce antibodies for specific pathogens

Platelets

Structure and Appearance

• Small cell fragments (not complete cells)
• Irregular shape
• Smaller than white blood cells

Function

• Blood clotting (coagulation)
• Clump together at injury site
• Release clotting factors
• Form fibrin mesh to seal wounds

Plasma

Composition and Appearance

• Yellow liquid (~55% of blood volume)
• Water-based with dissolved substances

Function

• Transport of:
  • Nutrients from digestion
  • Hormones from glands
  • Carbon dioxide to lungs
  • Waste products to kidneys
  • Heat to help regulate body temperature

Heart Monitoring and Health

Heart Monitoring Methods

ECG (Electrocardiogram)

• Records electrical activity of heart
• Shows heart rate and rhythm
• Can detect heart problems

Pulse Rate

• Beats per minute measured at wrist or neck
• Normal resting: 60-100 beats per minute
• Increases with exercise, stress, illness

Valve Sounds

• “Lub-dub” sounds from valves closing
• Abnormal sounds indicate valve problems
• Used in medical diagnosis

Effect of Physical Activity on Heart Rate

Resting Heart Rate

• Lower heart rate indicates efficient heart
• Trained athletes have lower resting heart rates

During Exercise

• Heart rate increases significantly
• Blood pumped to muscles increases
• Oxygen delivery to tissues increases

Recovery

• Heart rate returns to normal after exercise
• Faster recovery indicates better cardiovascular fitness

Coronary Heart Disease

Cause

• Blockage of coronary arteries by fatty deposits
• Reduces blood supply to heart muscle
• Can lead to heart attack

Risk Factors

Prevention

• Healthy diet: Low fat, high fibre
• Regular exercise: Strengthens heart
• Avoid smoking: Protects artery walls
• Manage stress: Reduces blood pressure
• Medical monitoring: Early detection

Vascular System and Liver

Vessels to/from Liver

Hepatic Portal Vein

• Collects blood from small intestine
• Rich in nutrients absorbed from food
• Delivers to liver for processing before reaching rest of body

Vessels to/from Lungs

Vessels to/from Kidney

Immune Defence via Blood

White Blood Cell Types and Functions

Lymphocytes (B cells)

• Produce antibodies (proteins)
• Antibodies bind to specific antigens on pathogens
• Help destroy invaders

Phagocytes (Neutrophils, Macrophages)

• Phagocytosis: Engulf entire pathogen
• Digest pathogen using enzymes
• Release chemicals to destroy bacteria

Blood Clotting

Purpose

• Prevent blood loss from wounds
• Prevent pathogen entry through breaks in skin

Process

1. Platelets clump at injury site
2. Release clotting factors
3. Convert fibrinogen to fibrin
4. Fibrin forms mesh trapping blood cells
5. Forms clot to seal wound

Glossary