Circulatory systems
Learning Objective 1: Describe the circulatory system as a system of blood vessels with a pump and valves to ensure one-way flow of blood.
A circulatory system is an internal transport network that moves substances (like oxygen, nutrients, and waste) around the body. It consists of three main components:
- The Pump: This is the heart. Its function is to contract rhythmically to generate pressure and push blood through the vessels.
- The Vessels: These are tubes that carry blood. They differ in structure based on their function:
- Arteries: Carry blood away from the heart. They have thick, muscular, and elastic walls to withstand the high pressure generated by the heart's pumping action.
- Veins: Carry blood towards the heart. They have thin walls because the blood pressure is lower here. Crucially, veins contain valves to prevent blood from flowing backwards (backflow), especially against gravity in the limbs.
- Capillaries: These are tiny vessels connecting arteries and veins. Their walls are only one cell thick to allow for rapid diffusion of substances between the blood and tissue cells.
- Valves: Structures found in the heart and in veins that ensure blood flows in one direction only. Without valves, blood would flow backwards when the heart relaxes or when gravity pulls it down.
In fish, blood passes through the heart once during each complete circuit of the body. This is called single circulation.
The path of blood flow is:
- Heart (pumps deoxygenated blood)
- Gills (blood picks up oxygen and loses carbon dioxide; pressure drops significantly here)
- Body tissues (blood delivers oxygen to cells)
- Back to the heart
Key Feature: The fish heart has only two chambers: one atrium and one ventricle. It contains only deoxygenated blood. There is no septum (wall) separating oxygenated and deoxygenated blood because there is no oxygenated blood in the heart.
In mammals, blood passes through the heart twice during each complete circuit of the body. This is called double circulation. The heart acts as two separate pumps side-by-side.
The path of blood flow involves two distinct circuits:
- Pulmonary Circuit: Heart (right side) → Lungs → Heart (left side). Blood goes to the lungs to pick up oxygen.
- Systemic Circuit: Heart (left side) → Body tissues → Heart (right side). Blood delivers oxygen to the rest of the body.
Key Feature: The mammalian heart has four chambers: two atria and two ventricles. It is divided by a septum (a muscular wall) that completely separates the left side (oxygenated blood) from the right side (deoxygenated blood). This prevents mixing.
| Mammal (Double) |
|---|
| 4 (2 atria, 2 ventricles) |
| Both oxygenated and deoxygenated blood |
| Has a septum (wall) |
| Heart → Lungs → Heart → Body → Heart |
When asked to explain why double circulation is advantageous (especially in mammals compared to fish), you must focus on pressure and efficiency.
Context: Use this explanation when comparing mammalian systems to fish or amphibian systems, or when explaining why mammals are active endotherms.
Reasoning: Examiners look for the link between the septum, pressure maintenance, and transport efficiency. In single circulation (fish), blood loses significant pressure as it passes through the narrow capillaries of the gills. This means blood flows slowly to the body tissues. In double circulation, the heart pumps blood to the lungs at low pressure (to avoid damaging lung tissue) and then re-pumps it to the body at high pressure.
Correct Phrasing:
- "Double circulation allows blood to be pumped to the body at a higher pressure than in single circulation."
- "This results in a faster rate of transport of oxygen and nutrients to tissues."
- "The septum prevents the mixing of oxygenated and deoxygenated blood, ensuring that highly oxygenated blood is delivered to active tissues."
Example Answer:
"Mammals have double circulation which allows the heart to pump blood to the body at high pressure. This ensures a rapid supply of oxygen to muscles for respiration. In fish, blood pressure drops after passing through the gills, so transport to the body is slower."
Correction: If asked to 'describe' the system, you must list the components (heart chambers, vessel types). If asked to 'explain advantages', you must mention pressure and mixing of blood. Do not just say 'it is better'; explain how it is better (higher pressure = faster transport).
Mistake: Saying veins have valves to prevent backflow in the heart.
Correction: Valves are found in veins (to stop backflow against gravity) and within the heart (between atria/ventricles and at exits). Be specific about location.
- Arteries have thick walls (or muscular walls). [1]
- Arteries have elastic walls (to stretch and recoil). [1]
- Blood passes through the heart twice (or is pumped twice). [1]
- This maintains higher blood pressure in the systemic circuit (to the body). [1]
- This allows for a faster rate of transport of oxygen/nutrients to tissues.