Why Do We Need a Circulatory System?

Every cell in our body needs to receive a variety of molecules on a regular basis so they can stay alive and continue to metabolize.
Every cell in our body needs to have waste products removed on a regular basis.
Cells need to be able to communicate with each other through the release and reception of different chemicals.
The circulatory system provides the infrastructure for this to happen.

The United States needs a system of roads to move things and people from one place to another.
Roads provide the infrastructure that allows transportation of things and people from one place to another.
- Our circulatory system is analogous to a system of roads.
Our roads have different kinds of transportation vehicles: trucks, cars, motorcycles, bikes, etc.
- Our circulatory system has different “transportation vehicles”: red blood cells, white blood cells, plasma proteins, etc.

Open vs. Closed Circulatory Systems

Open Circulatory System
- Circulatory system is “open” to tissues (exchanges materials with them directly).
- Allows hemolymph to bathe the tissues.
Closed Circulatory System
- Circulatory system is “closed” to the tissues.
- Blood circulates within the system.
- Nutrients and wastes must move across the membrane to enter and leave tissues.

Figure 42.2, page 813, Campbell's Biology, 5th Edition

Heart (two chambers):
- One atrium (from Latin “hall” or “entrance”)
- One ventricle (from Latin “little belly” or chamber”)
Blood pressure is dramatically reduced after it goes through the capillaries in the gill: this is not an efficient system.

Figure 42.3(a), page 814, Campbell's Biology, 5th Edition

Heart (three chambers):
- Two atria: left and right
- One ventricle
Double circulation (an evolutionary breakthrough)
- Pulmocutaneous circuit: absorbs oxygen from the atmosphere and releases carbon dioxide
- Systemic circuit: provides tissues with oxygen and nutrients, and removes wastes from them.
This system is more efficient than the fish's, because there is a separation of oxygenated from deoxygenated blood with the two atria.
It is not a perfect system, however, since there is mixing of oxygenated and deoxygenated blood in the ventricle.

Figure 42.3(b), page 814, Campbell's Biology, 5th Edition

Heart (four chambers):
- Two atria: left and right
- Two ventricles: left and right
The “perfection” of double circulation
- Pulmonary circuit: lungs
- Systemic circuit: tissues
- The two circulatory systems are completely separate, so there is no mixing of oxygenated and deoxygenated blood.
- Maximum efficiency

Figure 42.3(c), page 814, Campbell's Biology, 5th Edition

All these structures allow blood to flow through them in the following ways:

Anterior and posterior vena cavae: deoxygenated blood returning from the body enters the right atrium of the heart.
Pulmonary artery: deoxygenated blood goes to the lungs from the right ventricle.
Pulmonary vein: oxygenated blood returning from the lungs enters the left atrium.
Aorta: oxygenated blood goes to the body from the left ventricle.

Figure 42.4, page 815, Campbell's Biology, 5th Edition

Valve function:
- To prevent backflow of blood in the heart when the heart contracts.
- This facilitates the one-way flow of blood.
Atrioventricular (AV) valves
- Separate the atria from ventricles.
- Two AV valves:
  - Tricuspid valve: between the right atrium and ventricle
  - Bicuspid valve: between the left atrium and ventricle
    - Also called the “mitral valve” since it looks like a bishop's mitre.
Semilunar valves
- Valves at the exit of the heart
- Two of them: right and left ventricles

Figure 42.5, page 815, Campbell's Biology, 5th Edition

If you listen to someone's heart you will hear two distinct sounds.
- First sound: lub
- Second sound: dub (or "dup," depending on what book you read)
The sounds come from the valves closing and the resulting recoil of blood against the closed valves.
- Lub: recoil of blood against the closed AV valves.
- Dub: recoil of blood against the closed semilunar valves.