Understanding Circulation

Circulation is the process of transporting essential substances like nutrients and oxygen to cells within an organism, and removing waste products they produce.
Single circulation occurs in fish where blood passes through the heart once in one complete cycle of the body. In contrast, double circulation occurs in mammals where blood passes through the heart twice.
Closed circulatory systems, found in vertebrates and some invertebrates, have blood confined to vessels. In open circulatory systems, found in insects, blood is not always confined to vessels.
Transportation of blood, containing many substances such as glucose, oxygen, hormones and waste products like carbon dioxide and urea, is essential for cellular function and homeostasis.

The Mammalian Heart

The mammalian heart is a muscular organ responsible for pumping blood around the body. It is divided into four chambers: two atria and two ventricles.
Blood is collected in atria and pumped out of the heart from ventricles.
The right side of the heart pumps deoxygenated blood to lungs for gaseous exchange, returning to the left side of the heart. The left side then pumps oxygenated blood to the rest of the body.
Muscle contractions in the heart, or myogenic contractions, are controlled by the sinoatrial node (SAN), also known as the heart’s natural pacemaker.
Blood flows in one direction due to the presence of valves which prevent backflow. Damage or malfunction of these valves can disrupt normal blood flow, leading to medical conditions such as heart murmurs or circulatory disease.

Blood is a connective tissue composed of plasma, erythrocytes (red blood cells), leukocytes (white blood cells) and platelets.
Erythrocytes transport oxygen, and their unique shape maximises oxygen carrying capacity.
Leukocytes are crucial for immune response, whereas platelets help in clotting to prevent excessive blood loss from injury.
Three types of blood vessels exist: arteries carry blood away from the heart, veins carry blood to the heart, and capillaries provide exchange between blood and tissues.

The autonomic nervous system regulates the heart rate through neurotransmitters, responding to various body conditions.
Circulatory diseases include hypertension (high blood pressure), atherosclerosis (build up of fatty material in arteries) and strokes. Each of these conditions disrupt the normal flow of blood and can lead to significant damage or death.
Lifestyle factors, including diet, exercise, stress and smoking, can significantly influence circulatory health.