AS Module 1
10.6 Tissues and organs
After this tutorial, you should be able to:
| Explain the difference between tissues and organs. |
| Describe what epithelium tissues are. |
| Explain how the features of alveolar epithelium enable efficient gaseous exchange. |
| Describe the cellular composition of blood. |
| Explain the structure and function of red blood cells. |
| Understand the relationship between size and surface area to volume ratio. |
| Describe the structure and function of blood vessels. |
Epithelial tissue
Epithelial tissue is a group of cells which lines many organs, including the alveoli in the lungs (alveolar epithelium).
Alveolar epithelium
Gaseous exchange in the human body occurs at the alveoli in the lungs. These are tiny sacs which are made up of squamous epithelial cells. The structure of these cells aids gaseous exchange by:
| Having a large surface area for increased gas contact to cell. |
| They are thin to reduce the distance the gas has to travel. |
| They are moist to ensure gases diffuse easily. |
| They have a good blood supply to allow effective gaseous exchange. |
Bloods cellular components
Blood is a specialised circulating tissue containing a number of different cell types. Red blood cells (erythrocytes) transport oxygen around the body to cells and white blood cells (leucocytes) form parts of the bodies defenses.
Leucocytes
There two types of white blood cell, granulocytes (have granules within cytoplasm) and agranulocytes (no granules).
Agranulocytes
These are formed in the bone marrow and migrate to lymph nodes. There are two kinds:
Lymphocyte
Notice the lack of granules and extremely large nucleus.
Monocyte
Notice the sickle shaped nucleus which is smaller than that of the lymphocyte.
Granulocyte
There are 3 types of this kind of cell, eosinophils, neutrophils, and basophils. They all have one important feature in common, they contain granules.
Neutrophil
All granulocytes have a much smaller nucleus than the agranulocytes.
Red blood cells
The prime function of red blood cells is to carry oxygen from the respiratory organ to the tissues and their structure is modified accordingly.
Structure of red blood cells
The structure of the red blood cell has developed into a highly efficient oxygen transporter. It has:
| No nucleus which increases the volume capable of carrying oxygen. |
| A bi-concave shape, )( , which increases surface area for oxygen to diffuse through. |
| A thin, flexible membrane which allows easy diffusion of oxygen, and also ensures that the cells can squeeze into all the regions of the body. |
Surface area to volume ratio
The surface area of a cell compared to its volume is known as the:
Surface area : volume ratio
As mentioned above, a bi-concave shape increases this ratio which allows more oxygen to be absorbed.
Blood vessels
There are various types of blood vessels which make up the circulatory system. Because they make up a system, they are organs. There are 5 types of blood vessel:
| Arteries |
| Arterioles |
| Capillaries |
| Venules |
| Veins |
Arteries
The arteries carry blood away from the heart. Arteries almost always carry oxygenated blood towards the cells in the body. The only exceptions are the pulmonary artery, which carries deoxygenated blood away from the heart to the lungs, and the umbilical artery, which carries deoxygenated blood from the fetus to the placenta.
As was stated, blood is carried away from the heart via arteries. This means that its structure has to be able to withstand high pressures created by the pumping action of the heart as well as efficiently distribute blood. It does this by (see diagram):
| Smooth endothelium(created by squamous epithelium) allows a smooth flow of blood. |
| The next layer out from the endothelium consists of elastic fibres and smooth muscle. This allows the lumen (the actual tube which blood flows down) to expand and contract with the force of the blood without damaging the vessel. |
| The contraction of the elastic fibres pushes the blood along the vessel when the heart has just finished its pumping cycle. |
Arterioles
As arteries get further from the heart they branch, and the diameter of the lumen gets smaller. The very smallest branches of the arterial system are referred to as arterioles (see picture). Arterioles are much further from the heart and so require much less elastic fibres and so therefor contain a greater proportion of smooth muscle (see picture).
Veins
Veins carry blood towards the heart. This means that most of them carry blood which is deoxygenated-it has given up its oxygen to cells. The two exception to this are as before i.e. the pulmonary and umbilical veins.
From the capillary network in the tissues, blood enters tiny venules which drain into larger and larger vessels called veins.
Structure of veins
The blood pressure in veins is relatively low – the pressure surges from the heart have been eliminated as the blood passes through the capillary beds. This means that the structure has to be different from that of the arteries because of the different purpose of the organ.
The diagram above clearly shows how the vein is adapted to transport blood under low pressure back to the heart. It has :
| A relatively thin layer of smooth muscle (orange) with few elastic fibres. These are not needed due to the blood having a low pressure. |
| Again there is a layer of smooth endothelium made up of squamous epithelium cells (red) to reduce the friction on the blood. |
| The lumen is relatively large which allows easier passage of low pressure blood. |
The veins structure also ensures that blood gets back to the heart by making use of one-way valves. Called semilunar valves (see diagram), they are formed from infoldings of the inner wall of the vein. Blood can pass through in the direction of the heart, but if it tries to flow backwards, the valves fill with blood like a parashoot, and prevent the backflow.
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