Mammalian transport systems
Joanna George-Johnson, Contributor

These youngsters raise a banner for peace at the PALS Peace Concert at the Urban Development Corporation car park, in downtown Kingston, on Tuesday, March 6. - Colin Hamilton/Freelance Photographer

Small organisms such as amoeba can exchange materials such as food, water and wastes easily between them and their environments by the process of simple diffusion and osmosis. This is not the case with larger animals, as their surface area to volume ratio is too small for these simple processes to adequately fill their needs. This is due to the fact that the rate of diffusion and osmosis falls off quickly as the distance over which these processes take place increases. The inner cells of the animal would therefore not receive adequate materials if these processes were used. The need thus arises for the use of specialised transport systems.

The mammalian transport system is composed of three simple parts:

• A fluid tissue known as blood
• A muscular organ to pump blood: the heart
• Vessels through which the blood flows

Blood

Blood is a tissue as it is composed of multiple cells. These include red and white blood cells (erythrocytes and leucocytes, respectively). There are also smaller cell fragments known as platelets. All of these play various roles in the functioning of blood as a whole. The cells are suspended in a fluid known as plasma (liquid part of blood), a solution of water, amino acids, ions, plasma proteins among other things.

Red blood cells (erythrocytes)

These are the most common blood cells. They are identified by their red pigmentation owing to the presence of haemoglobin, and their biconcave disc shape. This haemoglobin is the pigment responsible for the oxygen carrying capacity of the blood. They also lack nuclei. These cells are produced in the liver at a remarkable rate, as they are due to the lack of a nucleus they have very short life spans. Haemoglobin is the pigment responsible for oxygen transport.

White blood cells (leucocytes)

The purpose of white blood cells is to fight against infections, thus producing immunity against disease. Unlike red blood cells, these are colourless and have nuclei. Two types of such cells exist and they differ both in structure and function. Lymphocytes have large, round or bean-shaped nuclei, and cytoplasm without granules. They act by producing antibodies which cause pathogens to clump together. Conversely, phagocytes engulf and digest pathogenic agents. They have lobed nuclei, and cytoplasmic granules.

Platelets

These are small fragments of cells, and the endothelium of blood vessels. They aid in blood clotting.

Functions of blood

1. Transports materials, cells and fluids. Many materials are transported including:

• Oxygen - As outlined before, oxygen is transported as it binds to the iron atoms within haemoglobin, forming oxyhaemoglobin. This binding is reversible. Oxygen is added to haemoglobin in areas of high concentration (the lungs) and released in low concentration (body tissues).
  
  
• Carbon dioxide - A small amount of carbon dioxide is transported bound to haemoglobin. However, the majority of CO2 transported is in solution as hydrogencarbonate (HCO3-) ions.
  
  
• Food Materials - It is essential that all body cells have a constant supply of glucose, as it is the body's main respiratory substrate. However, cells also need fatty acids, amino acids, minerals, water and vitamins, which are absorbed from digested food in the intestines.
  
  
• Waste Materials - Urea is major nitrogenous waste in mammals. Accumulation of urea in the body would be dangerous to health, thus the blood system takes urea to the kidneys for excretion.

Hormones

2. Defends the body against diseases.

3. Regulates body temperature.

Exercise:

1. The following image is a light micrograph of human blood. See if you can identify the types of blood cells present.

2. What blood cell is normally in the highest quantity?

3. Which cells are mainly involved in the defence against diseases?

Joanna George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.