Circulation in Animals
1. What is circulation?
Circulation is the movement of substances such as nutrients and gases within blood vessels and cavities throughout the body.
2. Do all animals have a circulatory system?
Not all animals have a circulatory system.
Poriferans, cnidarians, platyhelminthes and nematodes (nematodes have pseudocoelom fluid but no vessels) are avascular animals. Echinoderms do not have true circulatory systems either.
3. What is the alternative means of substance transport in animals without a circulatory system? Why is blood important for larger animals?
In animals that do not contain a circulatory system, the transport of substances occurs by cell to cell diffusion.
Blood is a fundamental means of substance transport for larger animals since, in these animals, tissues are distant from each other and from the environment thus making diffusion impossible.
Open and Closed Circulatory Systems
4. What are the two types of circulatory systems?
Circulatory systems can be classified into open circulatory systems and closed circulatory systems.
5. What is an open circulatory system?
An open circulatory system is one in which blood does not circulate only inside blood vessels but also flows into cavities that irrigate tissues. In open circulatory systems, blood pressure is low and generally the blood (called hemolymph) has a low level of cellularity.
Arthropods, molluscs (cephalopods are exception) and protochordates have open circulatory systems.
6. What is a closed circulatory system?
A closed circulatory system is one in which blood circulates only inside blood vessels. For this reason, the blood pressure is higher in animals with closed circulatory systems. The cellularity of the blood is also higher, with many specific blood cells.
Closed circulatory systems are a feature of annelids, cephalopod molluscs and vertebrates.
7. What are the advantages of a closed circulatory system over an open circulatory system?
A closed circulatory system is more efficient. Since blood circulates only inside blood vessels, it has a higher pressure and, as a result, can travel greater distances to the organs where hematosis happens and to peripheral tissues. In addition, the higher circulatory speed increases the animal’s capacity to distribute large supplies of oxygen to tissues that consume it in large amounts, such as muscle tissues, which can then perform faster movements. Animals with an open circulatory system (with the exception of insects, which carry out gas exchange separately from circulation) are generally slower and have a low metabolic rate.
8. What is the difference between octopuses and mussels regarding their circulatory systems? How does that difference have an effect on the mobility of these animals?
Cephalopod molluscs, such as octopuses and squids, have a closed circulatory system with blood pumped under pressure flowing within vessels. Bivalve molluscs, such as mussels and oysters, have an open circulatory system (also known as a lacunar circulatory system) where blood flows under low pressure, since it falls into cavities in the body and does not only circulate within blood vessels. Molluscs with closed circulatory systems are larger, agile and can actively move; molluscs with open circulatory systems are smaller, slow and some are practically sessile.
9. Why can flying insects such as flies beat their wings at a great speed despite having an open circulatory system?
In insects, the circulatory system is open but this system does not participate in the gas exchange process or in oxygen supply to tissues. Gases enter and exit through the independent tracheal system, which allows for the direct contact of cells with the ambient air. Therefore, an insect can supply the large oxygen demand of its fast-beating wing muscles even though it has an open circulatory system.
The Components of the Circulatory System
10. What are the typical components of a closed circulatory system?
The typical components of a closed circulatory system are blood vessels within which blood circulates (arteries, veins and capillaries), a pumping organ (heart) and blood or bloodlike fluid.
11. How does the heart pump blood?
The heart is a muscular organ that contains chambers (right atrium and right ventricle and left atrium and left ventricle) through which blood passes. The blood enters the heart in the atria, goes to the ventricles and then leaves the organ.
The blood is pumped out of the heart by the contraction of the muscle fibers that form the ventricular walls. The contraction reduces the volume of ventricle, thus increasing the internal pressure and forcing the blood to flow to the exit vessels (the pulmonary artery for the right ventricle and the aorta for the left ventricle). When ventricular muscle fibers expand, the ventricles regain their original size and receive new blood flow from the atria.
12. What is the difference between systole and diastole?
Systole and diastole are the two stages into which the cardiac cycle is divided. Systole is the stage when the contraction of ventricular muscle fibers occurs and the ventricles are emptied. Diastole is the stage of the cardiac cycle when the ventricular muscle fibers expand and the ventricles are filled with blood.
13. What are arterial vessels, arteries and arterioles?
Arterial vessels are every blood vessel that carries blood from the heart to tissues. Arteries and arterioles are arterial vessels. Arterioles are thin arteries that end in capillaries.
However, not all arteries contain arterial blood (highly oxygenated blood). The pulmonary artery and its branches, arteries that carry blood from the right heart ventricle to the lungs, contain venous blood.
14. What are venous vessels, veins and venules?
Venous vessels are every blood vessel that carries blood from tissues to the heart. Veins and venules are venous vessels. Venules are thin veins connected to capillaries.
In general, venous vessels carry venous blood. However, the pulmonary veins that carry blood from the lungs to the left atrium of the heart contain arterial blood.
15. What are the capillaries of the vascular system?
Capillaries are small blood vessels that carry out the exchange of substances between the blood and body tissues. Capillaries are neither arteries nor veins since they have distinct features. In capillaries, the wall is made of a single layer of endothelial cells through which substances are exchanged. These vessels receive blood from arterioles and drain into venules.
16. What part of the vascular system carries out the exchange of gases and other substances between tissues?
Only capillaries carry out the exchange of gases and other substances between tissues.
17. Which contain more muscle tissue, arteries or veins? How different are the walls of these two types of blood vessels?
The arterial system has thicker muscle walls, since within arteries blood circulates under higher pressure. Veins are more flaccid than arteries.
From the lumen to the external layer, both types of vessels are made of endothelium, muscle tissue and connective tissue. The endothelium of both is made up of a single layer of cells. In arteries, the muscle tissue portion is thicker than in veins whereas, in veins, the external connective tissue is thicker than in arteries.
Arteries are pulsating blood vessels. The arterial pulse can be felt during a medical examination, for example, through the palpation of the radial artery in the internal-lateral face of the wrist near the base of the thumb.
18. What are the valves of the venous system? What is their function?
The valves of the venous system are structures inside veins that make it so that blood only flows in the right direction (from tissues to the heart). preventing it from going backwards in favor of gravity. The valves close when the pressure of the fluid column above (afterwards, in terms of normal flow) is higher than the fluid pressure below them. Valves are therefore necessary for the process of blood returning to the heart.
19. How do the muscles of the legs and feet contribute to venous return?
The muscles of the legs, and mainly the muscles of the calves, contract and compress the deep veins of the legs, pushing blood towards the heart.
The plantar portion of the foot holds blood and, when it is pushed against the ground, it pushes its blood volume back towards the heart and therefore aids in venous return.
20. What are varices? Why are they more common on the lower limbs?
A varix means is an abnormal enlargement of a vein. Varices occur when excessive pressure against the normal blood flow enlarges the vein and, as a result, causes its valves to stop working properly (venous insufficiency).
Varices are more common in the veins of the lower limbs since the fluid column above these vessels is higher. This is the reason why people who spend a large amount of time standing (e.g., surgeons) are more likely to develop varices.
In general, varices are not the superficial veins that can be seen on the legs of varix patients. These superficial veins are the result of internal varices (venous insufficiency) in the deep internal veins of the legs. These outer veins appear this way because blood flow is diverted from the internal veins to superficial ones. (However, superficial veins with this appearance are often called varices.)