# Why do we have transport systems?

Organisms do not always require transport systems, and most have much simpler ones than us - so why the complexity in the mammalian transport system? The reason is our size. Because mammals are so large (increased distance from the nutrients and the cells requiring them), have a high metabolic rate and a high level of activity, we have high oxygen and nutrient requirements. We also produce a relatively large amount of waste that has to be removed - all this is achieved by our complex transport system with pump.
As you can see from the above table, some organisms do not have transport systems, and rely on diffusion alone since they are such simple creatures that diffusion is adequate. Relative to their volume, they have a large surface area for diffusion to occur. Jellyfish and sea anemones survive for the same two reasons and that they do not move around much (which requires oxygen).

# Cardiovascular System

The cardiovascular system or simply the blood system has a pump and a series of tubes, the heart and blood vessels respectively. The blood, coming from the left ventricle of the heart travels twice through the heart to make a complete circuit. Starting at the left ventricle, into the aorta, all around the body except for the lungs, back into the right side of the heart through the vena cava and then pumped out of the right ventricle into the pulmonary artery, carrying it to the lungs to be reoxygenated, then returning along the pulmonary vein to the left side of the heart. This combination makes it a double circulatory system, and since the blood never leaves it is known as a closed circulatory system.
See this picture for an extended look at the human blood vessel system:
This picture helps to describe the double circulatory system:

## Arteries

The function of the arteries is to transport blood swiftly and at high blood pressure to the tissues. Look at the diagram on the right. It is of a artery and shows three things - the tunica intima, the tunica media and the tunica externa. The tunica intima is made from endothelium, resting on a thin layer of elastic fibres as this is in contact with the blood, this is vital. Next up is the middle layer called the tunica media, containing smooth muscle, collagen and elastic fibres. Finally, the outside, the tunica externa, containing elastic and collagen fibres. The space in the middle where the blood flows through is called the lumen.
Arteries have the thickest walls of any blood vessel, this, along with the smooth muscle and elastic fibres allow the wall to stretch as pulses of blood from the heart come through at high pressure. The fact that they are elastic (i.e., that they recoil after being stretched) is vital to ensure they 'snap back' after the pulse of blood has been through, raising the pressure and giving the blood a push.
As they get further from the heart, and closer to the tissue to which they are giving blood to, they branch into smaller vessels known as arterioles - similar to arteries but smaller and with more muscle to push the blood along if required, also to enhance or restrict blood flow, such as restricting blood flow to gut during exercise.

## Veins

Veins have a relatively large lumen, a thin tunica intima and an even thinner tunica media. Their tunica externa is mostly collagen fibres, as opposed to the artery that has a lot more elasticised fibres.
Blood flows from the arterioles into capillaries, and when it leaves there it enters the veins, whose function it is to return blood to the heart. Veins have to deal with very low pressure blood, typically less than 5 mm hg - this helpfully negates the need for thick walls but how can blood be returned to the heart under such low pressure?
The answer is semi-lunar valves. See this picture: . As you can see from the picture, the semi-lunar valve only allows blood to go one way, trying to go the other simply closes the valve. Muscles in your legs also help to raise the pressure within your veins.

## Capillaries

The arterioles continue to branch from the artery, eventually forming the smallest of all blood vessels, capillaries. The capillaries function is to take blood as close as possible to cells, allowing rapid transport of substances between cells and blood - they form a network in every tissue known as a capillary.
Capillary walls are extremely thin, with walls of just a single layer of endothelial cells and each capillary is about the same size as a red blood cell, 7μm. This structure allows blood get to as close as 1μm from individual cells. Most capillary beds will have gaps in between the individual cells to allow some components of the blood to seep into the space between the cells of the body, forming tissue fluid. The blood pressure in a capillary is around 10mm Hg.

## Blood and tissue fluid

Blood is simply cells floating in a pale yellow liquid called blood plasma. Blood plasma is mostly water with solutes such as nutrients and waste products. Protein molecules that remain in the blood all the time are known as plasma proteins. As said in the previous section, plasma leaks out between the capillaries and seeps into the spaces between the cells of the tissues, and becomes known as tissue fluid.
Tissue fluid is blood plasma, without a few things that cannot fit through capillary walls- red blood cells and protein molecules. It forms the immediate environment of each individual body cell, and exchange the materials between cells - many processes in our body are designed to maintain the composition of tissue fluid, to ensure cells have an optimum environment. This is known as homeostasis, and involves the regulation of glucose concentration, water, pH, waste products and temperature.

# Lymphatic system

Of the tissue fluid discussed in the previous section, around 90% seeps back into the capillaries, and the remaining 10% becomes lymph fluid in the lymph vessels (lymphatics). These vessels are tiny 'blind-ending' vessels, and tissue fluid can flow into them but not out - and these valves are wide enough to allow large protein molecules to pass through - important since this protein cannot get into the capillaries and can't be taken away by the blood, which would cause a fatal build-up if the tissue fluid does not take it away.
The lymph vessels join up to form larger lymph vessels, which gradually transport the lymph fluid back to the large veins which run beneath the collarbone, known as the subclavian veins. Lymph nodes lie at intervals along lymph vessels and bacteria and other unwanted particles are removed from the lymph by white blood cells as the lymph passes through these nodes.

# Blood

Blood is a specialized bodily fluid composed of blood cells suspended in a liquid called blood plasma.

## Blood cells

A blood cell (also called blood corpuscle) is any cell of any type normally found in blood. In mammals, these fall into two general categories:
* Red blood cells
* White blood cells

### Red

Red blood cells (erythrocytes) are red because of the pigment haemoglobin, a globular protein, the main function of which is to transport oxygen from the lungs to respiring tissues. Red blood cells are relatively short-lived, they become more fragile as time goes on and rupture within the circulatory system but are replaced by the bone marrow. A list of their structures and functions are below.
'Red blood cells :
| Structure | Function |
| Small size (7μm) | To allow the red blood cell to get near to the cells to exchange oxygen with them - to fit into capillaries. |
| No nucleus, mitochondria or endoplasmic reticulum. | Red blood cells are highly specialized cells, and they provide as much space for haemoglobin as possible. |
| Biconcave disk shape | Large surface area for oxygen diffusion. Flexibility. |

### White

White blood cells (leucocytes) are also made in the bone marrow, but differ from red blood cells in the following ways.
* They always have a nucleus
* Larger than red blood
* Not bi-concave, usually spherical or just irregular.
There are two main groups of white blood cells - phagocytes and lymphocytes.
Phagocytes
* Destroy invading cells by phagocytosis
* Recognised by their lobed (see neutrophil) nuclei and granular cytoplasm.
Lymphocytes
* Destroy microorganisms but not by phagocytosis
* Secrete antibodies
* Smaller than phagocytes
* Large nucleus
* Small cytoplasm

# Haemoglobin

The main role of the transport system is to transport oxygen from the alveoli to where it is needed - cells around the body, and this is achieved in the protein haemoglobin. Each haemoglobin molecule can hold four oxygen molecules.

### Dissociation curve

Haemoglobin must not only pickup oxygen at the lungs (as we have seen so far) but to drop it off at the appropriate tissues. This is known as dissociation, and it is possible to measure haemoglobins dissociation and produce a graph, known as a dissociation curve.
The graph (seen here on the right), shows that at low partial pressures of oxygen, the oxygen-saturation of haemoglobin is very low. At high partial pressures of oxygen, the oxygen-saturation of haemoglobin is very high.The oxygen dissociation curve of small mammal is slightly deviated to the left because these have got many metabolic processes.

### S-Shaped

The S-Curve can be explained by the behaviour of a haemoglobin molecule as it loses and gains oxygen molecules.
A haemoglobin molecule has four haem groups, and when an oxygen molecule combines with a haem group, the haemoglobin molecule becomes slightly distorted, and this distortion increases the haemoglobins affinity for oxygen, that is, it makes it easier for the second molecule of oxygen to bind. The second oxygen molecule joins, in turn making it easier for a third oxygen molecule to bind, but this molecule makes it harder for the fourth to bind.
All this explains why the initial rise is slow, (first is hard to bind but the 25th percentile to the 75th percentile(2nd and 3rd are easy) is a steep graph, and the last quarter of the graph is slow (fourth is hard) again.

### The Bohr effect

The Bohr effect is the influence of carbon dioxide on the S-shaped graph - when carbon dioxide is released into the red blood cells, it is converted by the enzyme carbonic anhydrase, which produces excess hydrogen ions as a result. Haemoglobin readily combines with these ions, forming haemoglobinic acid, and in doing so releases the oxygen which it is carrying.
This results in two things - haemoglobin mopping up the hydrogen ions which are formed when carbon dioxide dissolves and dissociates - keeping the pH (concentration of hydrogen ions) neutral (acting as a buffer).
The fact that a high partial pressure of carbon dioxide causes haemoglobin to release oxygen - this is the Bohr effect. It is very useful in that high concentrations of carbon dioxide are found in actively respiring tissues - the ones that need oxygen the most, and cause haemoglobin to release oxygen even more readily that it would otherwise do. The Bohr Effect shifts the S curve (this time for dissociation) slightly to the right, simply meaning that haemoglobin is less saturated than it would be at a low partial pressure of carbon dioxide.

### CO$_2$ transport

There are three ways CO$_2$ can be transported in the blood.
* Carbon dioxide can be carried directly in solution. (5%)
* One of the products of carbon dioxide dissociation is hydrogencarbonate ions, which mostly diffuse into the blood plasma. (85%)
* Carbon dioxide can also combine directly with haemoglobin to form carbaminohaemoglobin. (10%)
When the blood reaches the lungs, the above first two reactions are reversed and this leaves carbon dioxide to diffuse out of the blood, and the haemoglobin free to combine with oxygen.

## Foetal haemoglobin

A developing foetus receives its oxygen across the placenta from its mother's haemoglobin. Obviously, the mother's blood has to supply her whole body as well as the foetus' needs, and thus by the time the blood reaches the placenta, the partial pressure of oxygen is relatively low.
This requires foetal haemoglobin to have a higher affinity for oxygen, and bind more readily at lower partial pressures of oxygen. The curve for foetal haemoglobin on the s-shaped curve graph is slightly to the left of the adult haemoglobin curve.
Once the foetus is born, he/she loses their foetal haemoglobin within 6 months, so that in the future a female can have a foetus inside her and that foetus will be able to use its foetal haemoglobin to attain oxygen. Also, it is necessary that the foetal haemoglobin changes to adult haemoglobin, so that the oxygen affinity is lowered sufficiently enough for the right amount of oxygen to be given up to cells and tissues. This is especially important as the child becomes more active, because its tissues will require more oxygen.

# Myoglobin

Myoglobin is a reddish pigment which combines with oxygen, just like haemoglobin. However it is mostly found in muscle tissue. It has only one polypeptide chain, one haem group and can only bind with one oxygen molecule. However, myoglobin has a very high oxygen affinity and will not release its oxygen unless the partial pressure of oxygen around it is very low.
This is useful because during the initial minutes of exercise, the heart and lungs require time to catch up with the muscles demand, and during this time the oxygen saturation drops low in the muscles as they quickly use it, and myoglobin releases its oxygen. So, myoglobin acts as an oxygen store.

# Oxygen transport difficulties

As you probably have deduced, oxygen transport in our bodies is incredibly efficient, but it can be affected by a few things.

## Carbon monoxide

Haemoglobin, for all its efficiency has a flaw—it combines, irreversibly, with carbon monoxide with an affinity 250 times that of oxygen. Carbon monoxide is inhaled from fumes from many sources, and combines with haemoglobin to form carboxyhaemoglobin.
Thus excessive concentrations of carbon monoxide, like from poorly-ventilated gas heaters, severely impact the bodies’ oxygen carrying capacity, and carbon monoxide poisoning can lead to death from asphyxiation.

## Altitude sickness

Since haemoglobin partially relies atmospheric pressure to bind readily with oxygen, humans sometimes encounter problems above heights of 6500 feet, as the air pressure becomes such that haemoglobin will at most become 70% saturated instead of the usual 92-95% saturated, causing less oxygen to be carried around the body.
This can make people feel ill, but worse it causes the arterioles in their brain to dilate, and increase the amount of blood flowing into capillaries. This causes fluid to leak from the capillaries into the brain tissues causing disorientation, and can even leak to the lungs making it difficult to breathe. This condition can be fatal.
However, given time, the body can acclimatise to the lower pressure by increasing the number of red blood cells—however this takes at least two or three weeks at a high altitude. Other changes that occur to people who live at high altitudes include broad chests (high lung capacity), larger hearts and more haemoglobin in the blood than usual.