Unit Two‎ > ‎

Transport Systems

We need a transport system to circulate important substances because we are relatively big animals. This means that we have a high volume to surface area ratio, therefore we cannot transport things by diffusion efficiently enough as it would take to long. An organism such as an Amoeba, a single celled organism, does not need a transport system as it has such low V:SA ratio, it can just diffuse all of its needed substances with ease.

Tools such as a 'nomogram' can be used to calculate surface area if you know the height and mass of the person. You place your points of mass and height then follow the line these points make to find your surface area. Here is a diagram of a nomogram:


There are
two types of blood circulatory systems in humans: the Pulmonary system and the Systemic system. These two systems is known as the Double Circulation system.

The Pulmonary System is the way the body can get ‘new’ oxygenated red blood cells back into circulation. It is the route which goes from the blood to the lungs and then back to the heart again. There is one unusual aspect with this system. The pulmonary artery would be expected to carry oxygenated blood, however it actually carries deoxygenated blood from the heart to the lungs to be oxygenated with oxygen from inhaled air. It is then passed back to the heart by the pulmonary vein (containing ‘new’ oxygenated blood) which will be pumped around the body to respiring cells.

This brings us onto the Systemic System: the name given to the route taken by blood as it passes from the heart to a region of body and then back to the heart again. This is possible as the aorta splits up into smaller arteries and arterioles which go to different parts of the body.

Here is a table with the names of certain arteries and veins which lead to and recede from specific bodily organs:

Name of ArteryName of OrganName of Vein
Carotid ArteryHeadJugular Vein
Renal ArteryKidneyRenal Vein
Coronary ArteryHeartCoronary Vein
Hepatic ArteryLiverHepatic Vein
Gut ArteriesGut*Hepatic Portal Vein


*The Hepatic Portal vein is a special case in which the blood is taken to the liver from the gut instead of the usual Vena Cava. This is because the newly absorbed nutrients and vitamins absorbed into the body by the small intestine needs to be filtered before it goes into complete circulation.
Simple diagram showing main arteries and veins

Coronary heart disease is caused when the coronary arteries which supply the heart its own oxygen become blocked, this blocking is known as myocardial infarction. 

Atherosclerosis is the process in which small buds of fatty material called arethomas form and block the arteries. Over time these plaques will grow by deposited lipid and cholesterol molecules and will continue to restrict the volume of blood flow to the heart.

Tissue Fluid

Blood consists of red blood cells, white blood cells, and platelets bathed in plasma. Plasma contains many dissolved substances such as glucose, amino acids, respiratory gases, plasma proteins and useful ions.

Blood arriving at the arteriole side of a capillary bed is at a higher pressure than the venule side. So as blood is squeezed into these narrow channels much of the plasma is pushed through the thin walls in a process called pressure filtration. This liquid is now called tissue fluid instead of blood plasma, the only difference is that tissue fluid contains no protein as the proteins are too large to be squeezed through the walls.

Much of the tissue fluid returns to the capillaries at the venule side by a process called osmotic return. This takes place because the water travels from a high water concentration (tissue fluid) to a region of low water concentration (blood plasma) down a concentration gradient. Carbon dioxide and other metabolic wastes enter the bloodstream by diffusion.

However not all of the tissue fluid returns back into the bloodstream. Instead it enters tny thin-walled lymphatic vessels which have blind ends and are located in the connective tissue amongst the living cells. Once it has entered the lymphatic vessels it is called lymph. The lymphatic system collects all of the lymph and eventually returns it to the blood circulatory system. This is called the Lymphatic return of tissue fluid.

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