The closed circulatory system consists of a heart, blood, and blood vessels, allowing for unidirectional blood flow. This system is present in all vertebrates as well as some invertebrates, with variations like single and double circulation observed in different species. Specific adaptations in blood flow are seen in amphibians, reptiles, and fish, each employing unique cardiac structures for effective circulation.

1. The Closed Circulatory
System

2. The closed circulatory system is composed
primarily of a muscular pump (heart), a fluid
(blood), and a series of channels (blood vessels)
that allow the fluid to be circulated.

3. Unidirectional Flow
● A characteristic of the closed
circulatory system is that it
has unidirectional blood flow.
● This means that the blood
flows in only one direction,
and no backflow occurs.
An earthworm has a closed
circulatory system.

4. Closed Circulation: Organisms
● All vertebrates have a
closed circulatory system.
● Other organisms with a
closed circulatory system
include the cephalopods
and the annelids.
Cephalopods have a closed
circulatory system.

5. Single vs Double Circulation
● In single circulation, blood only passes the heart once
to make a complete circulation cycle.
● In double circulation, blood passes the heart twice,
once before oxygenation and once more after
oxygenation.

6. Blood Flow in Amphibians
1. Oxygenated blood from
the pulmonary vein
enters the left atrium,
and deoxygenated
blood enters the right
atrium at the same
time.
The three-chambered
amphibian heart

7. Blood Flow in Amphibians
2. Both oxygenated and
deoxygenated blood from
the atrium flows into the
single ventricle.
3. Blood flows out of the heart.
One path goes to the lungs
while the other goes to the
body tissues.
The three-chambered
amphibian heart

8. Blood Flow in Amphibians
5. Oxygenated blood and
deoxygenated blood flow
back into the heart.
6. The cycle repeats.
The three-chambered
amphibian heart

9. Blood Flow in Reptiles
● Most reptiles, like turtles,
lizards, and snakes have a
three-chambered heart.
● Crocodiles have a four-
chambered heart.
Reptile either have three- or four-
chambered hearts.

10. Blood Flow in Reptiles
1. Blood from the atria flows
into the left side of the
partially divided ventricle.
2. Deoxygenated blood flows
to the right side of this
ventricle.
Reptile either have three- or four-
chambered hearts.

11. Blood Flow in Reptiles
3. From the right ventricle,
deoxygenated blood is
pumped to the lungs via
pulmonary artery.
4. Simultaneously, oxygenated
blood is pumped through the
aorta and to the tissues. Reptile either have three- or four-
chambered hearts.

12. Blood Flow in Fish
● The heart of a fish is also
simple, consisting of an
atrium and a ventricle.
● The gill circuit leads to the
gills for the oxygenation
of blood.
Blood flow from the gills of fish
to the capillaries in organs.

13. Blood Flow in Fish
1. The heart pumps out
deoxygenated blood.
2. The blood from the heart
goes to the gills for
oxygenation, and gas
exchange happens via
respiratory capillaries.
Blood flow from the gills of fish
to the capillaries in organs.

14. Blood Flow in Fish
3. Oxygenated blood from the
gills flows to the different
body tissues for gas and
nutrient exchange.
4. The deoxygenated blood in
the different body tissues
flows back into circulation
and back into the heart.
Blood flow from the gills of fish
to the capillaries in organs.

15. Blood Flow in Fish
5. Blood is received by the
sinus venosus, and flows
into the atrium, ventricle,
and conus arteriosus,
respectively.
6. The cycle repeats. Blood flow from the gills of fish
to the capillaries in organs.

16. In a closed circulatory system, the circulatory fluid
or blood is contained inside the blood vessels.
1
The closed circulatory system has unidirectional
blood flow. This means that the flow of blood is only
in one direction, and no backflow occurs.
2
All vertebrates have a closed circulatory system.
Some invertebrates also have this.
3

17. Identify if the statements are true or false.
1. Cephalopods have a closed circulatory system.
2. Frogs have a three-chambered heart.
3. The atria of frogs contain either oxygenated or
deoxygenated blood.
4. Fish have a closed circulatory system.
5. In fish, oxygenated blood does not pass through the
heart again until it becomes deoxygenated.

18. What are the
advantages of a
closed circulatory
system?