The Circulatory System
What Is the Circulatory System?
The circulatory system (also called the cardiovascular system) is the body's transport network. It delivers oxygen, nutrients, and hormones to every cell and removes waste products such as carbon dioxide and urea.
It consists of three main parts:
- The heart — a muscular pump
- Blood vessels — tubes carrying blood around the body
- Blood — the transport medium
The Heart
The heart is a muscular pump about the size of a fist, located in the chest slightly to the left. It beats about 70 times per minute at rest, pumping blood continuously.
Four Chambers
| Chamber | Location | Function |
|---|---|---|
| Right atrium | Upper right | Receives deoxygenated blood from the body via the vena cava |
| Right ventricle | Lower right | Pumps deoxygenated blood to the lungs via the pulmonary artery |
| Left atrium | Upper left | Receives oxygenated blood from the lungs via the pulmonary vein |
| Left ventricle | Lower left | Pumps oxygenated blood to the entire body via the aorta |
The left ventricle has the thickest, most muscular wall because it must pump blood to the whole body — a much greater distance than the right ventricle, which only pumps to the lungs.
Heart Valves
Valves inside the heart prevent backflow of blood, ensuring it flows in one direction:
- Atrioventricular valves — between atria and ventricles
- Semilunar valves — at the exit of each ventricle (pulmonary and aortic valves)
When valves open and close, they create the "lub-dub" heartbeat sound.
Double Circulation
Humans have double circulation — blood passes through the heart twice per complete circuit of the body:
Circuit 1 — Pulmonary circulation: Right side of heart → lungs → left side of heart (Blood picks up oxygen and releases CO2 at the lungs)
Circuit 2 — Systemic circulation: Left side of heart → all body tissues → right side of heart (Blood delivers oxygen and picks up CO2 and waste)
This is more efficient than single circulation (found in fish) because oxygenated and deoxygenated blood are kept separate, and blood pressure can be maintained at high levels for the systemic circuit.
Blood Vessels
There are three main types of blood vessel:
| Vessel | Direction | Wall structure | Pressure | Special features |
|---|---|---|---|---|
| Arteries | Away from heart | Thick, muscular, elastic walls | High | Pulse felt in arteries |
| Veins | Towards heart | Thinner walls | Lower | Valves prevent backflow |
| Capillaries | Between arteries and veins | One cell thick | Very low | Huge surface area |
Capillaries are where the real work happens — oxygen, glucose, and other nutrients diffuse out of the blood into tissues, and CO2 and waste diffuse in.
Why Do Arteries Have Thick Walls?
Arteries carry blood at high pressure directly from the heart. Thick, muscular, elastic walls allow them to withstand and smooth out the surges of pressure from each heartbeat.
Components of Blood
Blood is a mixture of cells and liquid:
| Component | % of blood | Function |
|---|---|---|
| Red blood cells (erythrocytes) | ~45% | Transport oxygen using haemoglobin |
| White blood cells (leukocytes) | <1% | Immune defence against pathogens |
| Platelets (thrombocytes) | <1% | Blood clotting and wound healing |
| Plasma | ~55% | Liquid carrier of nutrients, hormones, CO2, waste, heat |
Red Blood Cells
Adaptations of red blood cells for carrying oxygen:
- Biconcave disc shape — large surface area for oxygen diffusion
- No nucleus — more space to pack haemoglobin
- Contain haemoglobin — protein that binds to oxygen to form oxyhaemoglobin
- Flexible — can squeeze through narrow capillaries
White Blood Cells
Two main types:
- Phagocytes — engulf and destroy pathogens (phagocytosis)
- Lymphocytes — produce antibodies specific to antigens on pathogens
Plasma
Plasma is a pale yellow liquid (about 90% water) that carries:
- Dissolved nutrients (glucose, amino acids)
- Hormones
- Carbon dioxide (as dissolved CO2 and bicarbonate ions)
- Urea (waste from protein breakdown)
- Heat (distributes body temperature)
The Cardiac Cycle
The heart muscle contracts and relaxes in a rhythmic cycle:
- Diastole — heart relaxes; blood fills the atria from veins
- Atrial systole — atria contract; blood pushed into ventricles
- Ventricular systole — ventricles contract; blood pumped out to lungs and body
The heart's rhythm is controlled by a natural pacemaker called the sinoatrial node (SAN) in the right atrium.
Key Terms
| Term | Definition |
|---|---|
| Artery | Blood vessel carrying blood away from the heart |
| Vein | Blood vessel carrying blood towards the heart |
| Capillary | Tiny vessel where exchange of substances occurs |
| Haemoglobin | Protein in red blood cells that carries oxygen |
| Double circulation | System where blood passes through the heart twice per circuit |
| Plasma | Liquid component of blood |
| Valve | Structure preventing backflow of blood |
| Systole | Contraction phase of the heart |
| Diastole | Relaxation phase of the heart |
Worked Example
Q: Explain why the left ventricle has a thicker muscular wall than the right ventricle.
The left ventricle pumps oxygenated blood around the entire body (systemic circulation), which requires high pressure to reach distant organs like the legs and brain.
The right ventricle only pumps blood to the lungs (pulmonary circulation) — a much shorter distance requiring less pressure.
Therefore, the left ventricle needs a thicker, more muscular wall to generate the greater force needed for systemic circulation.
Common Mistakes
- Saying arteries always carry oxygenated blood — the pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.
- Saying veins always carry deoxygenated blood — the pulmonary vein carries oxygenated blood from the lungs to the left atrium.
- Confusing atria and ventricles — atria (upper chambers) receive blood; ventricles (lower chambers) pump blood out.
- Forgetting valves in veins — veins have valves to prevent backflow; arteries do not need them because blood flows under high pressure.
Tips and Tricks
- Mnemonic for heart chambers: "Arteries Away" — arteries always carry blood away from the heart.
- The right side of the heart deals with deoxygenated blood going to the lungs; the left side deals with oxygenated blood going to the body.
- Red blood cells have no nucleus to maximise space for haemoglobin — this is a classic exam adaptation question.
- Blood type questions often link to plasma (which carries antibodies and antigens).