1. Bellwork: Why is a circulatory system so important to animals?

2. Circulation
Section 27.3

3. What is the difference between an open and a closed circulatory system?
In an open circulatory system blood is only partially contained within a system of blood vessels as it travels through the body
May have one or more hearts or heartlike organs pump blood through vessels that empty into sinuses or spongey cavities where blood contacts tissue
Common in arthropods and most mollusks
In a closed circulatory system blood circulates entirely within blood vessels that extend throughout the body
Heart or heartlike organ pushes blood through vessels
Due to ability to pump under higher pressure, blood can travel more efficiently

4. Are there different patterns in circulation in vertebrates?
Yes – single vs double loop circulation
Single loop common in fish with gills
Double loop – common in organisms with lungs

5. How does the heart of a mammal work?
A four chambered heart in a modern mammal is essentially two separate pumps
During chordate evolution, partitions evolved in the heart dividing the original two chambers into four
By investigating other vertebrates you can identify the other evolutionary stages
Amphibians have a three chambered heart – two atria and one ventricle
Two atria one ventricle
Left atrium receives oxygen poor rich blood, right oxygen poor
Both empty into ventricle – some mixing. Internal structure of ventricle forces most oxygen poor blood to lungs, and oxygen rich to body
Reptillian hearts have three chambers, however a partial partition in their ventricle
Less mixing of oxygen rich and oxygen poor blood

6. Key points questions
What is the difference between an open and a closed circulatory system?
Which groups of animals are more likely to gave each type of circulatory system?
Why is a closed circulatory system beneficial to a large active animal?
What different patterns of circulation are found in vertebrates?
How have hearts evolved in animals over time?