The Circulatory System: Heart, Blood Vessels, and Blood
The Heart
The heart is a muscular organ that pumps blood around the body. It is part of the double circulatory system:
1. Pulmonary circuit — right side of the heart pumps deoxygenated blood to the lungs to pick up oxygen
2. Systemic circuit — left side of the heart pumps oxygenated blood to the rest of the body
Structure of the Heart
The heart has four chambers:
| Chamber | Position | Receives blood from | Pumps blood to |
|---|---|---|---|
| Right atrium | Top right | Body (via vena cava) | Right ventricle |
| Right ventricle | Bottom right | Right atrium | Lungs (via pulmonary artery) |
| Left atrium | Top left | Lungs (via pulmonary vein) | Left ventricle |
| Left ventricle | Bottom left | Left atrium | Body (via aorta) |
Key features:
- The left ventricle has a thicker muscular wall than the right ventricle because it needs to pump blood at higher pressure around the entire body (the right only pumps to the nearby lungs)
- Valves (tricuspid, bicuspid/mitral, and semi-lunar) prevent the backflow of blood
- The septum divides the left and right sides, preventing oxygenated and deoxygenated blood from mixing
- The heart has its own blood supply through the coronary arteries
- The pacemaker (sinoatrial node/SAN) in the right atrium controls the resting heart rate. Artificial pacemakers can be fitted if this is faulty.
Blood Vessels
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Direction | Away from heart | Towards heart | Connect arteries to veins |
| Blood pressure | High | Low | Low |
| Wall thickness | Thick, muscular, elastic | Thinner walls | One cell thick |
| Lumen (space inside) | Small | Large | Very small (one red blood cell wide) |
| Valves | No (except semi-lunar) | Yes (prevent backflow) | No |
| Blood type | Usually oxygenated* | Usually deoxygenated* | Both (exchange occurs here) |
*Exception: the pulmonary artery carries deoxygenated blood; the pulmonary vein carries oxygenated blood.
Capillaries and Exchange
Capillaries are adapted for exchange of substances:
- Walls are one cell thick — short diffusion distance
- Huge network provides a large surface area
- Blood flows slowly through capillaries, allowing time for exchange
- Substances exchanged include: oxygen, carbon dioxide, glucose, urea, amino acids
Blood Components
Blood is a tissue made up of:
Plasma (~55% of blood volume) — a pale yellow liquid that transports:
- Dissolved glucose, amino acids, and minerals
- Carbon dioxide (from cells to lungs)
- Urea (from liver to kidneys)
- Hormones (from glands to target organs)
- Antibodies and antitoxins
- Heat energy around the body
Red blood cells (erythrocytes):
- Contain haemoglobin which binds to oxygen → oxyhaemoglobin
- Biconcave disc shape for large surface area
- No nucleus to carry more haemoglobin
- Made in bone marrow
White blood cells (leucocytes):
- Phagocytes — engulf and digest pathogens (phagocytosis)
- Lymphocytes — produce antibodies specific to antigens on pathogens
- Have a nucleus (unlike red blood cells)
Platelets:
- Small cell fragments (no nucleus)
- Involved in blood clotting at wound sites
- They form a mesh of fibres (fibrin) that traps red blood cells, forming a scab
- This prevents blood loss and entry of pathogens
Coronary Heart Disease (CHD)
Coronary heart disease occurs when the coronary arteries become narrowed or blocked by a build-up of fatty deposits (atheroma/plaque) on their walls. This reduces blood flow to the heart muscle, which receives less oxygen and glucose.
Treatments:
- Stents — small mesh tubes inserted into the coronary artery to hold it open; effective but risk of blood clots forming
- Statins — drugs that reduce blood cholesterol levels, slowing plaque build-up; must be taken long-term; side effects possible
- Bypass surgery — a blood vessel from another part of the body is used to bypass the blocked section
- Artificial hearts — mechanical devices used as a temporary measure while waiting for a transplant, or to give the heart time to rest and repair; risk of blood clots and infection
- Heart transplants — a donor heart replaces the damaged one; risk of rejection by the immune system; donor shortage
Exam Tips
- Remember: arteries carry blood away from the heart (A for Away)
- The left ventricle wall is thicker — always explain why (higher pressure needed for whole body)
- Know the exceptions: pulmonary artery (deoxygenated) and pulmonary vein (oxygenated)