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Mass transport in animals

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Gas exchange in large multicellular organisms is achieved by organs which have a large surface area and so are able to successfully provide the substances the organism needs in order to survive. In humans this is achieved by the lungs. But how does the oxygen acquired by the lungs actually reach every single cell of the body? A network of sorts is needed to do that. Many bigger and smaller tubes would come in handy. They would form like a... circulatory system. Oh wait, that's precisely what mammals have: a circulatory system made of arteries, veins, capillaries, etc.

Plants, too, have a vascular (tubular) system. It is made of xylems and phloems. Yes, complicated names which you will love to learn about in one of the following topics (Mass transport in plants). 

The key thing is that this circulation of a large amount of substances via a system of transportation is called mass flow, hence mass transport. Just more technical terms for you to learn, which describe something that really couldn't get any simpler. Water, gases chillin' through tubes in an organism.


Haemoglobin

Haemoglobin is a type of protein.  It is present in many varied organisms on our planet, and has a similar chemical structure in all of these organisms. In humans, haemoglobin is found in red blood cells. Haemoglobin's function is the transport of oxygen around the body. Oxygen must reach all parts of our bodies because it is required in the process of cellular respiration (to produce ATP - the main molecule involved in releasing energy for all uses). 

Haemoglobin is a big deal, so naturally, it has a quaternary structure (multiple protein chains linked together to form a greater functional unit which also includes inorganic molecules). Just stare in awe at this beauty:

Erythrocyte is a very easy to remember name for a red blood cell. As you can see, there are 4 protein chains: 2 alpha chains and 2 beta chains. At the centre of each chain there is a haem (or heme) group which is an inorganic group containing one iron ion. The million pound question, of course, is how many iron ions are there in one haemoglobin molecule? (No, you will not be lucky enough to get that question for 10 marks in any exam.)

If you think this is all nice and easy, read on to be utterly disappointed. 

Despite the fact that haemoglobins are similar in structure and function across a variety of organisms, they are adapted to different needs, as organisms span the large breadth of the biosphere. Some organisms need to be able to make use of very little oxygen available in their environment.

Since there are 4 iron ions in each haemoglobin molecule, and iron ions are directly responsible for binding the oxygen molecules (hence a lack of iron may cause anaemia), a haemoglobin molecule which has 4 oxygens bound to its heam groups is called saturated. The % saturation of haemoglobin overall in an organism is used to determine haemoglogin's ability to bind oxygen. This ability is affected by the partial pressure of oxygen (partial because it is only 21% of air).

This is a graph showing the oxygen dissociation curve:



As you can see, the higher the partial pressure of oxygen (which reflects the amount of oxygen), the higher the % saturation of haemoglobin. Haemoglobin binds oxygen in the lungs where the partial pressure of oxygen is high. Of course, more oxygen than could be taken up by haemoglobin is irrelevant because the limiting factor becomes the number of haemoglobin molecules (hence the curve plateaus at about 8 KPa).

A very important point to take from the graph is that at a low partial pressure of oxygen (for example in respiring tissues), the % saturation of haemoglobin is decreasing. What this means is that haemoglobin has the unique ability to release oxygen where it is needed most. Abundance of oxygen in the lungs? Haemoglobin FETCHES! Tissues depleted of oxygen? Haemoglobin SPITS! Of course you will use terms such as oxygen binding and releasing, and percentage saturation of haemoglobin in your exams, won't you.

Let's go back to one of the first points on this topic. I said that haemoglobin action varies slightly between different organisms, depending on their individual needs and environment. This is a crucial point because those evil examiners will put a scary looking double, or even triple curve graph in front of you and ask you to explain what is going on. Looking at the above graph, you can see that at 6 KPa the % saturation is 80. This may suit an organism fine, but others may well have shifted curves, where the points at which haemoglobin binds and releases oxygen are different. These curves may well be shifted to either the left or the right. This is what the graph might look like:




If we take, say, 80% saturation, the corresponding partial pressure of oxygen is about 35 mm Hg for high altitude deer mice, and about 50 mm Hg for low altitude deer mice. What does this mean? It means that high altitude deer mice's haemoglobin binds oxygen at a lower partial pressure than that of low altitude deer mice. Essentially, their haemoglobin is able to make better use of less oxygen. Why?

Well, let's think, what do we know of these mice? Some live at high altitudes, and others live at low altitudes. What do we know about altitudes that is relevant? We know that there is less oxygen at higher altitudes in the atmosphere. Hence the haemoglobin of high altitude deer mice has evolved to bind oxygen at lower partial pressures - where there is less of it available. This confers a clear advantage in terms of survival.

Right, onto the last bit now (phew). You will be expected to know the effect of carbon dioxide (CO2) on the oxygen dissociation curve. CO2 being acidic decreases blood pH if in increasing quantity, and it results in carbamino compounds being produced. These compounds bind to haemoglobin and shift the curve to the right. Bicarbonate ions also contribute to a shift to the right, as they release protons into the blood plasma. This is known as the Bohr effect.


Heart

In my quest to find a suitable diagram for the heart, this is what I found:


Definitely use your textbook as a guide on this. It only takes a google search to realise the ridiculous number of variations of diagrams for the heart and different annotations.

You need to be able to sketch a heart and label the main veins, valves, arteries and aorta, and the ventricles and atria. 

There are two types of circulation going on via the heart: pulmonary circulation and systemic circulation. Pulmonary circulation is a short-distance route between the heart and the lungs, where deoxygenated blood is taken to be replenished with oxygen. Although normally veins take blood away, and arteries take blood to, in the case of pulmonary circulation things are the opposite way around. The pulmonary vein brings freshly oxygenated blood into the heart - left atrium -, while the pulmonary artery takes deoxygented blood back from the right ventricle into the lungs.

Here's a quick nifty video that shows what happens as the bigger picture...


The atrioventricular valves and semilunar valves play an important role in ensuring proper heart function. The former ensure no blood flows back into the atria from the ventricles, while the latter ensure no blood flows from the ventricles into the atria.

Electrical impulses cause heart muscle contraction which creates an increased pressure of blood, resulting in it being pushed in a certain direction, with the valves opening in its way. The sequence of events in heart contraction is this:

1. Both atria contract - atrial systole
2. Both ventricles contract - ventricular systole
3. All chambers relax - diastole


The heart muscle contracts without brain stimulation - the brain only controls the speed. Electrical impulses start in the  sino-atrial node in the right atrium, travels down to the atrio-ventricular node, which then spreads it across the bundle of His, which results in the left ventricle contracting. 


Cardiac output = heart rate x stroke volume

Heart rate is measured in beats per minute, while stroke volume is measured in cm3 or ml.

Make sure you can interpret graphs showing the sequence of atria and ventricles contracting followed by diastole.

Cells in mammals require a constant supply of nutrients and oxygen, and a way to remove waste products. Blood is great, as it does all that. Blood needs a way of getting to all cells of the body, a way to... circulate. Without that, blood would just get pulled by gravity towards the centre of the earth. Not a pretty sight I'm afraid.

There are two circulations in the body:

1. The pulmonary circulation takes blood from the heart, pumping it to the lungs in order to oxygenate it.

2. The systemic circulation takes blood from the heart to everywhere else. Eyes, legs, hand, bum, you name it. 

Key point: the oxygen-rich blood vessels entering an organ are called arteries, while the oxygen-depleted blood vessels leaving an organ are called veins.

So a blood vessel entering the liver or kidneys would be an artery. A blood vessel leaving the liver or kidneys would be a vein.

The liver attribute is hepatic ( for example, the working cell unit in the liver is the hepatic cell), while the kidney attribute is renal (for example, renal failure). So what would the blood vessel entering the liver be called?


...pressing question.

...pressing on.

...

...the hepatic artery! Same principle applies to the rest: the hepatic vein, the renal artery and the renal vein.

There's a catch (welcome to biology). In the case of the blood vessels leaving or entering the lungs, the rules are reversed. The pulmonary vein carries oxygenated blood to the heart, while the pulmonary artery carries deoxygenated blood into the lungs.




You also need to learn the blood vessels entering and leaving the heart.

1. The aorta is the main artery which carries oxygen-rich blood to the rest of the body.

2. The coronary arteries supply blood to the heart itself (and they are the affected arteries in coronary heart disease).

3. The superior vena cava and the inferior vena cava bring deoxygenated blood from the upper half of the body, and the lower part of the body respectively.

It's all really logical... apart from the bit on the lungs.


Circulation

There are 4 types of blood vessels: arteriesarteriolescapillaries and veins. Each type has a different function, and therefore a different structure. Here is a diagram of how arteries branch off into arterioles, then into capillaries, and eventually into veins as the blood becomes deoxygenated.



Function

So what do they do?

Arteries must be able to counteract the pressure created by every heart beat by recoiling, so that the stream of blood is smoothened.

Arterioles are able to direct blood supply to certain parts of the body, so must be able to constrict or dilate.

Capillaries are the site of substance exchange as well as diffusion, so their walls must be thin enough for this to happen quickly.

Veins are unique as they contain valves which prevent backflow of blood.


Structure

From the above picture is it clear that there are important structural differences between arteries and veins, which reflect their different functions. Firstly, veins have valves while arteries do not*. Secondly, arteries have a narrower lumen (hollow diameter) than veins. Thirdly, arteries have a thicker wall of muscle and elastic tissue.

Arteries and arterioles are similar. The key difference is that arteries have more elastic tissue than muscle, while arterioles have more muscle than elastic tissue.

Capillaries are 1-cell thick, making them very thin and permeable.


Tissue Fluid

Tissue fluid is what surrounds all respiring cells. This is where they draw their nutrients from, and where they eliminate waste products into. Tissue fluid movement back and forth between cells and blood is directed by one of two things: either the hydrostatic pressure exerted by the blood rushing through arteries; or by osmosis caused by the proteins within blood. 

Therefore, as blood goes through the artery to respiring tissues, the tissue fluid is forced out of the blood, into the tissues. As blood passes through it loses pressure, so that the tissue fluid now enters the subsequent vein due to osmosis. This occurs because the water potential in the vein is lower than outside due to the proteins in the blood which reduce it. This sums up the circulation of tissue fluid.


*except for the pulmonary artery and the aorta

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