Chapter 49 – Circulatory Systems
49.1 Why Do Animals Need a Circulatory System?
- Circulatory/cardiovascular system = a muscular pump (heart), a fluid (blood), a series of conduits (blood
- Function of the system = transport heat, hormones, respiratory gases, blood cells, platelets,
nutrients, & waste products.
- Not all animals have circulatory systems because their cells are close enough to the external
environment for such transport to occur w/o one; SA = , diffusion path length = .
- Sometimes acquatic invertebrates have highly branched central cavities called gastrovascular
systems that bring the external environment into the animals. Problem: inactive, slow, sedentary
animals. Ex: Sponges.
- All cell nutrients come from extracellular fluid and all waste is disposed of there.
- Open system = vessels of these animals empty their fluid directly into the tissues and later the
extracellular fluid flows back into the circulatory system to be pumped back out again.
- Insects have open system but can achieve high levels of metabolic output because they do not depend
on their circulatory systems for respiratory gas exchange.
- Closed system = completely contain the circulating fluid, blood, in a continuous system of
vessels BUT liquid and solutes are still exchanged b/w the blood and the extracellular fluids
surrounding the cells of the body.
- The term extracellular fluid refers to both the fluid in the circulatory system and the fluid b/w
the cells of the body. to distinguish, the fluid in the circulatory system = blood plasma, and
the fluid around the cells = interstitial fluid.
Open circulatory systems move extracellular fluid
- In open circulatory systems, excracellular fluid squeezes through intercellular spaces as the animal moves and the
muscular pump assists with the distribution of the fluid. The contraction of the pump propels the extracellular fluid
through vessels leading to different regions of the body, but the fluid leaves the vessels and trickles through the tissues and
eventually returns back to the heart.
- Such systems found in arthropods (fluid returns to the heart through valved openings called ostia), mollusks.
Closed circulatory systems circulate blood through a system of blood vessels. Ex: worms.
1) Fluid flows more rapidly and therefore so does nutrients and wastes
2) By changing resistance in the vessels, can be more selective in directing blood to specific tissues.
3) Specialized cells and large molecules stay in the vessels, but can drop their cargo in the tissue where it is needed.
49.2 How Have Vertebrate Circulatory Systems Evolved?
- Vertebrates have circulatory systems with heat of 2 or more chambers.
- Arterioles = smaller vessels than arteries which feed blood into capillary beds.
- Capillaries = thin-walled vessels where materials are exchanged b/w the blood and the tissue fluid.
- Venules = small vessels that drain capillary beds.
- Veins = venules join together to form larger vessels called veins, which deliver blood back to heart.
- Pulmonary circuit – blood is pumped from the heart to the lungs and back to the heart.
- Systemic circuit – From the heart, blood is pumped to the rest of the body and back to the heart.
Fish have two-chambered hearts
- Fish heart has two chambers: an atrium (receives blood from the body and pumps it into a more muscular chamber), the
- The ventricle pumps deoxygenated blood to the gills, where gases are exchanged.
- Blood leaving the gills collects in the large dorsal artery, the aorta, which distributed blood to smaller arteries and
arterioles leading to organs and tissues.
- In tissues, blood flows through capillaries, collected in veins and venules, and eventually returns to atrium of the heart.
Amphibians have three-chambered hearts
- Pulmonary and systemic circulation are partly separated in adult amphibians.
- A single ventricle pumps blood to the lungs and to the rest of the body.
- 2 atria receive blood returning to the heart. One receives oxygenated blood from the lungs and the other receives
deoxygenated blood from the body. Because both atria deliver blood to same ventricle, the oxygenated and deoxygenated
blood can mix and the blood delivered to the tissues would not carry a full load of oxygen.
- This mixing is somewhat minimized because of anatomic features of the ventricle which direct deoxygenated blood to
lungs and oxygenated blood to aorta.
Reptiles have exquisite control of pulmonary and systemic circulation
- Turtles, snakes, and lizards have 3 chambered hearts because ventricles are not completely separated into left and right
- Crocodilians (crocodiles and alligators), have to completely separated ventricles.
- For many reptiles and crocodilians, when the animals are not breathing, it is a waste of energy for them to pump blood
through their lungs they have evolved the capability to bypass the pulmonary circuit and pump all the blood to the body
when they are not breathing.
- For reptiles:
- Reptiles have two aortas – a left and right. The left aorta receives oxygenated blood from the left side of the
ventricle and carries it to the tissues of the body. The right aorta can receive blood from either the right side of the
left side of the ventricle.
- The two sides of the ventricle are partially divided by a septum.
- When breathing, the resistance in the pulmonary circuit is lower than the resistance in the systemic circuit, so
blood from the right side of the ventricle tends to flow into the pulmonary artery rather than the right aorta.
- When not breathing, pulmonary vessels constrict, resistance in the pulmonary circuit goes up, and therefore
blood from the right side of the ventricle tend to flow into the right aorta. As a result, blood from both sides of the
ventricle flows through aortas to the systemic circuit.
Birds and mammals have fully separated pulmonary and systemic circuits
- Birds and mammals have four-chambered hearts and completely separated pulmonary and systemic circuits.
- Advantages of separate circuits:
- Oxygenated and deoxygenated blood cannot mix
- Respiratory gas exchanged if maximized because the blood with the lowest oxygen content and higher CO2
content is sent to the lungs.
- Systemic and pulmonary circuits can operate at different pressures.
- Many vessels present high resistance to the flow of blood high pressure is required in the systemic circuits.
- Pulmonary circuits have fewer capillaries and lower resistance than their systemic circuits, the pulmonary circuit can
function at lower pressures.
Circulatory/cardiovascular system = a muscular pump (heart), a fluid (blood), a series of conduits (blood vessels). Function of the system = transport heat, hormones, respiratory gases, blood cells, platelets, nutrients, & waste products. Not all animals have circulatory systems because their cells are close enough to the external environment for such transport to occur w/o one; sa = Sometimes acquatic invertebrates have highly branched central cavities called gastrovascular systems that bring the external environment into the animals. All cell nutrients come from extracellular fluid and all waste is disposed of there. Open system = vessels of these animals empty their fluid directly into the tissues and later the extracellular fluid flows back into the circulatory system to be pumped back out again. Insects on their circulatory systems for respiratory gas exchange. have open system but can achieve high levels of metabolic output because they do not depend.