Transport Systems in Mammals: The Human Circulatory System
Understanding the heart, blood vessels, blood composition, and how circulation maintains life
This page focuses on the human circulatory system. Learners should know the structure of the heart, the main blood vessels, the components of blood, and how circulation moves substances around the body.
1. The Human Heart
The heart is the central organ of the circulatory system, responsible for pumping blood throughout the body.
Interactive Heart Diagram
Heart Structures
External Structure
Coronary Arteries: These arteries branch off from the aorta and supply oxygen-rich blood to the heart muscle itself.
Wall Thickness
The left ventricle has the thickest walls to generate the high pressure needed to pump blood throughout the body.
Tricuspid Valve
Between right atrium and right ventricle, prevents backflow.
Bicuspid (Mitral) Valve
Between left atrium and left ventricle, prevents backflow.
Semi-lunar Valves
At base of aorta and pulmonary artery, prevent backflow.
Cardiac Cycle Simulator
Atrial Systole
Atria contract → blood pushed into ventricles
Ventricular Systole
Ventricles contract → blood pumped to lungs/body
Diastole
Heart relaxes → chambers fill with blood
Red Blood Cell Journey: Right Atrium → Lungs → Left Ventricle
Exam Tip: Be prepared for a long question (5-10 marks) tracing the path of a red blood cell!
2. Blood Vessels
Blood vessels are classified into three main types, each with distinct structures and functions:
Arteries
Structure: Thick, muscular, and elastic walls to withstand high pressure.
Function: Carry oxygenated blood away from the heart (except pulmonary arteries).
Veins
Structure: Thinner walls than arteries, with valves preventing backflow.
Function: Carry deoxygenated blood towards the heart (except pulmonary veins).
Capillaries
Structure: Microscopic vessels with walls only one cell thick.
Function: Facilitate diffusion of gases and nutrients into tissues.
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Wall thickness | Thick, muscular | Thin | Single cell layer |
| Valves present? | No | Yes | No |
| Blood pressure | High | Low | Very low |
| Direction of flow | Away from heart | Toward heart | Between arteries and veins |
| Oxygen level (usually) | Oxygenated | Deoxygenated | Exchanges gases |
3. Blood Composition
Blood Components
Liquid part, 90% water, transports nutrients and hormones
Biconcave discs with hemoglobin, transport oxygen
Fight infections, part of immune system
Cell fragments for blood clotting
4. Mechanisms of Circulation
Pulmonary & Systemic Circulation
Pulmonary Circulation (Right Side)
Systemic Circulation (Left Side)
Lymphatic System
A secondary system that drains excess tissue fluid (lymph) back into the bloodstream and plays a role in immune function.
5. Cardiovascular Diseases
Hypertension
High Blood Pressure
Causes: Poor diet, lack of exercise, genetics
Prevention: Healthy lifestyle, balanced diet, regular exercise
Atherosclerosis
Build-up of fatty deposits in arteries
Causes: High cholesterol, smoking, poor diet
Prevention: Healthy diet, no smoking, exercise
Heart Attack
Myocardial Infarction
Causes: Blockage of blood flow to heart muscle
Prevention: Manage cholesterol and hypertension
Stroke
Cerebrovascular Accident
Causes: Blockage or burst blood vessel in brain
Prevention: Control blood pressure, healthy lifestyle
Cardiovascular Risk Assessment
Age: 40 years
Exercise (hrs/week): 3 hrs
Diet (1-10, 10=healthy): 6/10
Exam Practice: Circulatory System
1. Which chamber of the heart has the thickest walls?
2. What is the function of the bicuspid valve?
3. Which blood vessels contain valves?
4. What is the main component of plasma?
5. What happens during ventricular systole?
6. Which blood cells contain hemoglobin?
Quick Check Questions
1. Trace the path of a red blood cell from the right atrium to the left ventricle.
Show Answer
Right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary veins → left atrium → bicuspid valve → left ventricle
2. What is the function of coronary arteries?
Show Answer
They branch off from the aorta and supply oxygen-rich blood to the heart muscle itself.
3. Why does the left ventricle have thicker walls than the right ventricle?
Show Answer
The left ventricle pumps blood to the entire body requiring high pressure, while the right ventricle only pumps to the lungs requiring lower pressure.
Did You Know?
Your heart beats about 100,000 times per day, pumping approximately 7,500 liters of blood through 96,000 kilometers of blood vessels - enough to circle the Earth twice!
Key Terms
What To Remember
- Heart has 4 chambers: right/left atria and ventricles; left ventricle has thickest walls
- Valves prevent backflow: tricuspid (right), bicuspid (left), semi-lunar (arteries)
- Cardiac cycle: atrial systole → ventricular systole → diastole
- Arteries carry blood away from heart (thick walls, no valves)
- Veins carry blood toward heart (thin walls, valves present)
- Capillaries are one-cell thick for gas/nutrient exchange
- Blood components: plasma (55%), red blood cells (44%), white blood cells and platelets (1%)
- `r`n Pulmonary circulation: heart → lungs; Systemic circulation: heart → body`r`n
