The circulatory system is concerned with the transportation of blood throughout the body. The blood has the following functions:
- The carriage of oxygen and the carriage of carbon dioxide.
- The carriage of food.
- The carriage of nitrogenous waste.
- The carriage of hormones or chemical messengers.
- The protection of the body against disease.
- Regulation of body temperature.
The circulatory system centres on a muscular pump - the heart.
The heart is a hollow organ with a wall made of three layers:
- The Pericardium: The outer layer
- The Myocardium: The middle layer
- The Endocardium: The inner lining of the heart cavities
This heart is made up of four chambers; Two atria which are thin walled, the suction chambers, and two ventricles which are thick walled, the discharge chambers.
The left ventricle, which pumps blood around the body, has a much thicker wall than the right ventricle, which only pumps blood to the lungs
Separation of the Atria and the Ventricles
The atria and ventricles are separated by the atrio-ventricular valves:
- Tricuspid Valve: Separates the right atrium from the right ventricle.
- Mitral Valve: Separates the left atrium from the left ventricle.
Right Atrium Two veins enter the right atrium, the inferior vena cava and the superior vena cava. These veins bring blood back to the heart from all of the body except the lungs. Blood from the right atrium passes into the right ventricle and then into the pulmonary artery to the lungs
Left Atrium Blood from the four pulmonary veins runs into the left atrium. This blood is passed into the left ventricle which is connected to the main artery which passes blood to all parts of the body except the lungs. This main artery is known as the Aorta.
The blood is circulated around the body by a network of flexible tubes, the blood vessels.
Arteries Strong, muscular and elastic walled vessels, arteries carry mainly oxygenated blood. All arteries flow away from the heart. The exception is the pulmonary artery which carries de-oxygenated blood from the heart to the lung.
Veins Thin walled vessels, with one way valves, veins carry mainly de-oxygenated blood back to the heart. The exception is the pulmonary vein which carries oxygenated blood from the lungs to the heart.
Arteries sub-divide to form a dense network of fine thin-walled blood vessels known as capillaries. The thin capillary walls allow the exchange of gases and other material between the cells of the body and the blood. The capillaries eventually rejoin passing through the tissues to become veins.
Blood is a complex tissue made of different kinds of cells, free proteins, other chemicals and factors and water.
The average adult has about 6 litres of blood circulating in the body. Blood consists of a clear yellow fluid (plasma) and solids. Approximately 90% of the plasma is water, in which other substances are dissolved or suspended. The most important solids in suspension are:
- Red blood cells: The red blood cells are formed in the bone marrow and contain a red pigment, haemoglobin. This is also the protein that carries oxygen to the tissues. Haemoglobin is an iron-containing compound. The iron that is in the aemoglobin molecule is responsible for the chemical affinity of haemoglobin for Oxygen and Carbon Monoxide.
- White Blood cells: Several kinds of cells found in the blood are colourless or white in appearance. All of these cells play a role in protecting the body from disease. The white blood cells are formed from "stem cells" found in the bone marrow. These cells mature into the specialized forms that protect the body from infection. Although these white cells are located in the blood, they function as part of the body's immune system.
- Platelets: Platelets help the blood clot. When a blood vessel is severed or torn the damaged ends constrict and retract in order to minimize blood loss. Almost immediately the blood that is escaping from the damaged vessel begins to clot. Platelets congregate at the site of the injury and release clotting factors. These clotting factors start to convert one of the blood substances, fibrinogen, into the protein, fibrin. Fibrin forms a dense weblike structure that in turn traps more platelets. This forms into a jelly like clot taking about 10 minutes. As the clot hardens it begins to shrink, releasing a watery substance, serum. The serum carries antibodies to combat infection and specialized cells that begin the process of repair.
Together the above cells account for 45% of the blood's total volume the remainder is called plasma.
Plasma Plasma is a yellow, slightly alkaline fluid consisting of 90% water and 10% solid matter. The composition of the plasma is controlled mainly by the kidneys, these solids include:
- Amino acids
- Urea and other nitrogenous waste
The cycle of blood flow through the body is as follows:
- Blood from the right atrium is pumped into the right ventricle.
- From the right ventricle the blood goes into the pulmonary artery which carries blood to the lungs.
- In the capillaries of the lungs, gaseous exchange occurs:
- Oxygen is taken into the blood.
- Carbon dioxide is passed into the lungs.
- The freshly oxygenated blood returns to the left atrium of the heart via the pulmonary veins.
- The left atrium empties into the left ventricle which is connected to the aorta.
- Contraction of the left ventricle forces blood into the aorta, the major artery which is connected to the rest of the body save the lungs.
- The aorta divides into arteries that carry the blood to the tissues. These arteries divide into capillaries which give off the oxygen and take up carbon dioxide before the blood returns to the heart.
- All blood returning to the heart collects in the superior or inferior vena cava which feed directly into the right atrium.
Further Uses of Blood Circulation
As the blood passes through the body the following organs carry out the following functions:
- Stomach: Nutrition from food is picked up and carried along to the tissues.
- Spleen: Old blood cells are taken out of circulation.
- Liver: Removes toxins and adds proteins to the blood.
- Kidneys: Adjust the water content and remove waste products./li>
- Bone Marrow: Helps renew white blood cells.
Your respiratory system is made up of the organs in your body that help you to breathe. Remember, that Respiration = Breathing. The goal of breathing is to deliver oxygen to the body and to take away carbon dioxide.
The lungs are the main organs of the respiratory system. In the lungs oxygen is taken into the body and carbon dioxide is breathed out. The red blood cells are responsible for picking up the oxygen in the lungs and carrying the oxygen to all the body cells that need it. The red blood cells drop off the oxygen to the body cells, then pick up the carbon dioxide which is a waste gas product produced by our cells. The red blood cells transport the carbon dioxide back to the lungs and we breathe it out when we exhale.
The trachea (TRAY-kee-uh} is sometimes called the windpipe. The trachea filters the air we breathe and branches into the bronchi.
The bronchi (BRAHN-ky) are two air tubes that branch off of the trachea and carry air directly into the lungs.
Breathing starts with a dome-shaped muscle at the bottom of the lungs called the diaphragm (DY-uh-fram). When you breathe in, the diaphragm contracts. When it contracts it flattens out and pulls downward. This movement enlarges the space that the lungs are in. This larger space pulls air into the lungs. When you breathe out, the diaphragm expands reducing the amount of space for the lungs and forcing air out. The diaphragm is the main muscle used in breathing.