In a way it is similar to red blood cells which are involved in transporting respiratory gases. Both types of cells have features that provide them with a large surface area. For instance, red blood cells are bi-concave shaped giving them a large surface area to volume ratio. This also applies to the flattened squamous epithelium cells; they both have a proportionally large surface area for the diffusion of substances into and out of the cells, which therefore speeds up the process of diffusion.
Red blood cells are relatively small so that the haemoglobin molecules are in close proximity to the surface which allows oxygen to be picked up and released quickly.
The red blood cells do not contain any organelles which means there is a lot more space for haemoglobin and as a result, more oxygen can be transported.
As well as having a round disc- like shape, red blood cells also have a flexible cell membrane which allows the cell to be squeezed through narrow capillaries.
The capillaries are only one cell thick and made of squamous epithelium. They are permeable to water and solutes allowing exchange of materials between the blood and the cells.
According to Fick’s law, the larger the surface area, the faster the rate of diffusion. Capillaries are extensively branched to allow the greatest surface area to volume ratio. There is no muscle or elastin in the capillary wall keeping it as thin as possible to lessen the diffusion distance and as a result speed up the rate of diffusion. By being so narrow, red blood cells can just about squeeze through the capillaries as they are of the same diameter. This slows the blood flow allowing more time for diffusion to take place.
Arteries and veins consist of the same types of tissues but with different functions.
The first layer is the endothelial tissue which contains a high proportion of squamous epithelium. This thin inner lining layer enables blood to flow smoothly through the vessels without causing friction. In capillaries however, it is the thin outer wall of squamous epithelium that is adapted for diffusion. It is permeable, moist and provides a short diffusion distance for exchange of substances.
The middle layer (tunica media) is made up of smooth muscle which contracts and relaxes increasing and reducing blood flow.
The outer layer (tunica externa) is composed of collagen, a fibrous protein which prevents the vessel from bursting in high pressure by providing strength.
Arteries have a small lumen, compared to veins. Because the oxygenated blood travels at high speed and pressure, the diameter of the lumen can be changed by stretching, regulating the blood flow. Arterioles branch out from arteries and are the control site of blood flow. Further division forms capillaries where exchange of substances occurs. Capillaries join to form venules which drain into veins.
In veins though, deoxygenated blood is travelling at low pressure and the lumen is larger so that it can hold a large volume of blood. Veins have semi lunar valves to ensure blood flows in one direction only, so they prevent backflow.
The blood vessels are interrelated in the following way:
Arteries Arterioles Capillaries Venules Veins
On the whole, squamous epithelium, erythrocytes and blood vessels correlate when it comes to working together for a function. The squamous epithelium, in particular plays a very important part in enabling exchange of materials through boundaries. They are all suited to their function and have distinctive features that permit the exchange of materials. Additionally, all three are specialised to meet the factors that affect the rate of diffusion. They erythrocytes and the squamous epithelium provide a large surface and are thin to provide a short diffusion distance which is beneficial for diffusion to occur.
Capillaries also have an extremely large surface area so that materials can be exchanged efficiently.
As they are involved in transporting blood, arteries ands veins have to be relatively strong if the blood is of high pressure like in arteries. Therefore they are adapted by having a collagen outer layer to prevent them from bursting.
All these features enable the cells, tissues and vessels to be regimented in their varied roles and functions.
