adminAssuming that your heart beats 70 times each minute of the 70 years that the average person lives, your heart will beat 2,577,204,000 times. Two and a half billion! Even that is an underestimate! So what is this amazing organ?Structure of the heart
The adult human heart weighs about a kilogramme and has four cavities inside it. These “chambers” are divided into two at the top called the atria (each is called an atrium) and two at the bottom, called the ventricles.
The diagram (above) shows that the heart is in fact divided neatly in two down its middle so that each side has one atrium and one ventricle. Both the left and right sides of the heart pump blood. The only difference between them is where the blood arrives from and to where it is pumped.
The muscle that makes up the heart is unusual as it doesn’t get tired while it pumps. But it can be damaged by drugs and poor diet leading to heart disease.
The heart in action
Blood fills the atria and the bicuspid and tricuspid valves are closed.
The atria gently contract and push their blood into the two ventricles.
The ventricles start to contract, this closes the atrioventricular valves (the bicuspid and tricuspid valves) and pushes the blood out through the semi-lunar valves into the pulmonary artery and the aorta.
Both sides of the heart beat in sequence together but the blood they pump goes to different locations. The left ventricle pumps blood to all of the body whereas the right ventricle pumps blood to the lungs from where it returns to the left side of the heart.
Each time the ventricles pump you can feel a great wave of blood moving along the arteries. This can be felt and gives an idea about how the heart acts.
If you listen to the heart with a stethoscope you hear two sounds which are described as being “lub” and “dub”. The first is the sound made by the two atrial valves closing, whilst the second is caused by the two semi-lunar valves closing.
The Blood
The heart and blood vessels carry out a transport function. They carry food molecules, water and oxygen to cells and remove waste products such as carbon dioxide. They form the circulatory system.
A double circulation
Instead of just being a single loop the circulation has two interconnected loops, in a sort of figure of eight.
Circulatory system
Blood returns from the body to the right atrium. The blood has lost most of the oxygen it carries and is now deoxygenated.
The right ventricle pumps the blood along the pulmonary artery to the lungs where it picks up fresh oxygen. It is now oxygenated. The oxygenated blood enters the left side of the heart and is pumped out through the aorta to the body. Once it reaches the capillaries around the body, oxygen diffuses out to the surrounding cells.
The deoxygenated blood is carried back towards the heart in the veins. These join up to form the vena cava which is the largest vein.
Useful tip: One sneaky exam fact is that veins only carry deoxygenated blood except for the pulmonary vein. This is the only one that carries oxygenated blood because it takes blood from the lungs to the left heart ready to get pumped round the body.
The hepatic circulation
This is a special part of the circulation system.
Normally the circulation system takes blood straight back from the capillaries in each organ or tissue.
But the blood from the digestive system carries all sorts of molecules that have been absorbed there. In order to stop the rest of the blood system getting clogged up there is a special detour. It is called the hepatic portal system.
The diagram above shows this system. Hepatic means to do with the liver. The liver is the factory organ of the body. It deals with all sorts of chemicals, breaking them down and rearranging them. The hepatic portal vein carries blood to the liver. Then the blood can leave the liver for the heart.
The Circulatory System
Blood Vessels
What is the blood? It isn’t just a red liquid. It is five litres of a careful mixture of plasma and blood cells. These cells come in three varieties: red, white and platelets. The plasma makes up most of the blood. It is mainly water but carries lots of other essential ingredients.
Plasma
The following substances are carried in the plasma:
Dissolved carbon dioxide: This is the waste gas produced by respiration in cells
Dissolved glucose and amino acids: Food molecules for respiration, building and repairing cells
Urea: Waste product of digestion, this is lost from the kidney
Antibodies and antitoxins: Chemicals that protect us from disease and poisons
Hormones: Chemicals that control some of our body functions
Plasma has a yellowish appearance. It sometimes oozes out of blisters. Nice!
Red blood cells
The best known of the cells are the red blood cells, correctly called erythrocytes.
Erythrocytes contain the oxygen carrying molecule haemoglobin; this is a special pigment that gives blood its red colour. Iron is needed in the production of haemoglobin; if your diet lacks this mineral you can develop the condition anaemia. Red blood cells are unlike other cells in that they do not contain a nucleus. They are really just a bag containing the haemoglobin. The cells have a doughnut-shape with a flattened centre instead of a hole.
In the diagram above you can see a whole cell on the left and another cut in half on the right.
When blood picks up oxygen we say that it has been oxygenated. This happens because haemoglobin molecules form weak bonds with oxygen to make a new complex molecule called haemoglobin.
White blood cells
Although much less common than red blood cells, the white cells come in many different varieties.
There are thousands of different types each with a specific job in defending the body against disease. They form part of the immune system.
When a disease is detected the corresponding white blood cell is copied until there are millions produced which attack the invading foreign cell. Some white blood cells “eat” invading cells. Others produce chemicals to attack them.
Platelets
These are not really cells in their own right. They are fragments of larger cells. Their job is to form part of a clot so that they plug a wound and stop too much blood being lost. Without that clotting mechanism you would cut your finger and bleed to death!
So hooray for the platelets! — s-cool.co.uk
Online Reporter