The Quiet Scholar

The circulatory system is responsible for the movement of important substances within the bodies of living organisms. These substances include oxygen, nutrients, hormones and metabolic wastes. However, circulation does not occur in exactly the same way in all organisms. Depending on how the circulatory fluid moves within the body, circulatory systems are generally classified as open circulatory systems and closed circulatory systems . To better understand the general patterns of circulation found in different organisms, you may also read our previous article on Types of Circulation and Their Organisms . What Is an Open Circulatory System? An open circulatory system is a type of circulatory system in which the circulatory fluid is not always enclosed within blood vessels. The heart pumps the circulatory fluid into body spaces called sinuses or haemocoels . The fluid then comes into direct contact with the tissues and organs before eventually returning to the heart. In many animals wi...

Yawning is one of the most common human behaviours, yet many people do not know exactly why it happens. A yawn usually involves opening the mouth widely, taking in a deep breath and then slowly breathing out. We yawn when we are tired, sleepy or bored, but yawning can also occur when we have just woken up, feel stressed or even when we see another person yawn. But why exactly do human beings yawn?                                                      What Happens During a Yawn? When a person yawns, several muscles in the face and jaw become active. The mouth opens widely, the chest expands and a deep inhalation may occur. This is usually followed by a slower exhalation. Yawning is an involuntary action, meaning that it often happens without conscious planning. Although we can sometimes suppress a yawn, the urge to yawn may become ...