Blood (haema) is a fluid that circulates through the bodies of all humans and most animals. Thus, it represents the circulatory system. It has numerous important functions, but the main one is delivering vital substances like oxygen and various important nutrients to the body’s cells and transporting metabolic waste products away from the body’s cells. Blood cannot be artificially synthesized, and the only way patients in need can get blood is through blood donations.
Blood can come in several colors. However, the two most common are red, which all vertebrates (mammals, amphibians, reptiles, and birds) have, because of an iron-containing substance called hemoglobin; and blue, which all mollusks (e.g., squids, octopuses, snails, slugs, and horseshoe crabs) have, because of a copper-containing substance called hemocyanin.
Vertebrates have a closed circulatory system, as the blood circulates around the body through blood vessels. Mollusks have an open circulatory system, as blood is contained in a cavity where it surrounds the internal organs. Furthermore, flatworms, nematodes, and cnidarians (jellyfish, sea anemones, and corals) do not have a circulatory system and thus do not have blood. They obtain nutrients directly from the environment they live in.
Blood is composed of four main components:
Plasma is a yellowish liquid. It is composed mainly out of water, but it also contains proteins, sugars, hormones and salts. Its main function is to transport water and nutrients to the body’s tissues. About 55% of blood is plasma.
Red blood cells (erythrocytes) live in the body for about 120 days. However, every hour or so, the bone marrow will generate approximately five billion red blood cells. About 40% to 45% of the blood volume is comprised of RBC.
White blood cells (leukocytes) are also generated from the bone marrow. They are very important, as they fight off infections, illnesses, and diseases. About 1% of the blood volume is comprised of WBC.
Platelets are called platelets because they look like small plates in their non-active form. Their main function is to control bleeding. When an injury occurs, the blood vessels receive signals which they redirect to the platelets. The platelets then "activate" and attach themselves to the vessels in order to seal the wound until it heals.
Blood circulates around the body through blood vessels with the help of the heart. As we inhale, the blood in the arteries carries oxygen to the tissues of the body, and as we exhale, the blood in the veins carries carbon dioxide from the tissues to the lungs.
The heart (cardia) is a muscular organ which sits in the middle of the chest cavity, slightly to the left of the sternum. As it contracts, the heart pumps approximately 7000–10000 litters of blood. Blood is transported from the heart to the rest of the body through a complex network of arteries, arterioles, and capillaries. Blood is returned to the heart through venules and veins. The heart and the circulatory system make up the cardiovascular system.
The heart is composed of four chambers: the two upper chambers which receive the blood are called atria, and the two lower chamber which release the blood are called ventricles. They are separated by a wall of muscle called the septum. The heart and the roots of the great blood vessels are surrounded by a thin, protective, double-layered membrane called the pericardium, which contains three layers of tissue: epicardium (a layer mostly made of connective tissue which lines the outside of the heart), myocardium (the muscles of the heart), and the endocardium (a layer that lines the inside of the heart and protects the chambers).
The heart beats around 116,000 times a day.
There are 11 body systems: circulatory, respiratory, nervous, muscular, skeletal, physical actions, renal system (urinary system and excretory system), digestive, endocrine (hormones), lymphatic (or immune system), reproductive, and integumentary (skin, hair). All of these organ systems are interconnected and work in conjunction with one another. Each system relies on the others to work well.
For instance, the circulatory system is closely connected with the respiratory system, as the air in our lungs is used to oxygenate our blood. The circulatory system also carries hormones from the endocrine system. The largest organ in the body, the skin, gets nutrition from the circulatory system, and in turn, it protects the heart. The nervous system is very important to the heart, as it provides the stimulus to keep the heart beating. The digestive system gives nutrients and eliminates waste products from the blood. All of the body systems work together in order to maintain the organism in a perfect balance, which is known as homeostasis.
You can find more information on how the organ systems work with each other and complement the functions of the blood and the heart here.