The Challenges of Size

Chemical Exchange

The circulatory system is the bodies main bulk transport system for moving chemicals around the body. It carries, oxygen, carbon dioxide, the products of digestion, waste products like urea, water, and hormones.

The main components of the circulatory system are:

Heart - to pump blood.

Arteries - to carry blood away from the heart.

Veins- to carry blood towards the heart.

Capillaries - to carry blood to and from the body tissues.


  1. Red blood cells to carry oxygen
  2. White blood cells for defence against pathogens
  3. Platelets to clot the blood when cut.
  4. Plasma, the liquid part of the blood that carries everything except oxygen.

The Challenges of Size, figure 1

The human circulatory system is divided into two linked systems via the heart.

Main circulation: This takes oxygenated blood from the left side of the heart to the body tissues and returns deoxygenated blood to the right side of the heart.

Pulmonary circulation: This part of the system connects the heart to the lungs. Deoxygenated blood is pumped from the right side of the heart to the lungs and then oxygenated blood returns to the left side of the heart, before reentering the main circulatory system.

Circulatory System

The heart is the central pump of the circulatory system, it is made of cardiac muscle that does not get tired, unlike the other muscles of the body. The heart is actually two pumps side by side, the right side pumps deoxygenated blood returning from the body to the lungs. The left side pumps oxygenated blood from the lungs back to the body.

Notice on the diagram that the left side has a thicker layer of muscle than the right. This allows the left side to generate more force to push the blood around the whole body. This is the reason your heart beat is stronger on the left side and so is your pulse, it is not because your heart is on the left side of the chest.

The Challenges of Size, figure 1

The flow of blood through the heart follows a one-way path. To ensure this is true there are one-way values between each of the chambers of the heart and at each entrance and exit. As the heart muscle contracts to push blood from one chamber to the other, the values only allow blood to flow forwards round the heart, in the direction shown in the diagram.

The route for the blood is:

Vena Cava - large vein bringing blood from the upper and lower body to the Right Atrium

Right Atrium - contracts to push the blood down into the Right Ventricle

Right Ventricle - contracts to push the blood into the two sections of the Pulmonary Artery

Pulmonary Arteries - carry blood to the lungs.

Pulmonary Veins - carry blood from the lungs to the Left Atrium

Left Atrium - contracts to push the blood down into the Left Ventricle

Left Ventricle - contracts to push blood into the Aorta

Aorta - carries blood to the body

There are three types of blood vessels in the circulatory system.

Arteries to carry blood away from the heart.

Veins to carry blood towards the heart.

Capillaries that carry blood from the arteries to the body tissues and then back to the veins.


The Challenges of Size, figure 2

Arteries have a thick outer protection layer which is thickened with connective tissue to give it strength.

Below this is a thick layer of muscle fibres and elastic connective tissue, to allow the artery to stretch and contract.

The lumen is narrow.

Arteries carry high pressure blood direct from the heart, they need to be thick to withstand this pressure. The thick layer of muscle and elastic tissue allows the artery to squeeze the blood to help maintain the high pressure needed to transport the blood throughout the whole body.


The Challenges of Size, figure 3

Veins have a thinner outer layer and thinner layer of muscle with some elastic tissue underneath.

The lumen is much larger than in an artery.

Along the length of the vein are a series of one-way values.

The blood pressure in the venous part of the circulatory system is much lower, the veins do not need as much muscle or elastic tissue for this reason. The lumen is much larger to allow a fast flow of blood so that it can return to the heart efficiently. As the blood is under low pressure and has to flow upwards towards the heart in most of the body, there are a set of one-way valves to prevent the blood flowing backwards and away from the heart.


The Challenges of Size, figure 4

Capillaries are only one cell thick and have a small lumen. Capillaries are very small compared to both veins and arteries.

Chemicals are exchanged between the body tissues and the blood via the capillaries. This happens by diffusion, the single cell thickness of the walls means there is only a small distance for the chemicals to diffuse.

Blood Vessels

Transpiration is the process within a plant that pulls water up from the root and delivers it to all the parts of the plant via the xylem vessels in the roots, stems and leaves.

To move the water, a pumping force is required and this comes from the constant evaporation of water from the small holes on the underside of the surface of the leaves. As the water evaporates, this creates a lower water pressure in the leaf’s cells than in the xylem vessels in the veins of the leaf. This means water flows from the xylem vessels into the cells of the leaf. This in turn reduces the water pressure in the top end of the xylem pulling water up from the high water pressure area of the roots. This means that, to uptake and move water, a plant must constantly lose some of that water as evaporation into the air.

The Challenges of Size, figure 1

The evaporation of the water from the leaves drives the process of transpiration. Any environmental factors that affect the rate of evaporation will also affect the rate of transpiration.

Temperature: Water evaporators faster in hotter condition. Plants transpire more in hot conditions, meaning that they use more water.

Air movement: Wind pushes the water vapour away from the leaves leading to faster evaporation and, therefore to faster transpiration.

Light intensity: High levels of light lead to more photosynthesis which takes up water. This along with the heating effect increase the rate of transpiration in lighter condition.

Controlling Transpiration

Although environmental factors, such as temperature, can affect the rate of transpiration, a plant can also control transpiration. There maybe times when a plant needs a lot of water and others when the rate is too high and the plant is in danger of wilting. In both situation, the plant needs to control the flow of water. To do this it controls the evaporation of water from its leaves by opening and closing small holes in the lower surface of the leaf called stomata.

When the stomata is open, the water can evaporate freely increasing the supply of water to the plant. If they are closed, the evaporation will stop and so will the flow of water around the plant. By controlling how open they are and how many stomata are open, the plant can have control the flow of water and the loss of water too.

The Challenges of Size, figure 2

Plant Roots

Translocation is the movement of the products of photosynthesis around the plant. Although glucose is made during photosynthesis, plants then make other compounds, such as amino acids to make proteins from the glucose.As this happens in the leaves, the plant must transport these compound to other part of the plant, like the roots, stems, flowers, and fruits or seeds.

This is done via the phloem, the living transport tubes in the veins of leaves, stems, and the roots. These cells use active transport to allow chemicals to be moved between any two parts of the plant at will.

What process in a plant is responsible for the movement of water?