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Circulation

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The circulatory system

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 advantages of double circulation in mammals as opposed to single circulation (such as in bony fish) include the option to have a higher blood pressure and splitting oxygenated and deoxygenated blood. As blood gets oxygenated in the lungs, the diffusion process takes time, which has the blood at lower pressure. On the other hand, the already oxygenated blood can be pumped around the body at higher pressure, allowing for bigger organisms and increased metabolic activity. Splitting deoxygenated blood from oxygenated blood is key to this.

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.


Blood vessels

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.

*except for the pulmonary artery and the aorta


Control of heart rate



There are just 2 scenarios involved here. Either the heart rate must be increased or decreased.

1. The heart rate must go up because the blood pressure is low, there's a shortage of oxygen, an excess of CO2 or the pH is too low - some of these go together e.g. O2, CO2 and pH interaction during exercise, stress, etc.

The hormone that increases heart rate is noradrenaline. This can be released by the brain's medulla via the sympathetic neurons when baroreceptors which sense low blood pressure or chemoreceptors which sense excess or a lack of certain chemicals send signals via sensory neurons.

Noradrenaline then binds to the heart's sino-atrial node which results in increased contractions.

2. The heart rate must go down because the blood pressure is high, there's excess O2, low CO2 or the pH is too high

The sensing process is identical - baroreceptors and chemoreceptors do their jobs. The only difference? Of course, the released product must be different - the medulla orders acetylcholine instead of noradrenaline, and this signal follows the parasympathetic pathway

Acetylcholine makes the SA node slow contractions.




Interpreting heart function data

Heart function data can come in many forms including ECG (electrocardiogram) traces and pressure changes. The aim of collecting this data is to monitor the activity of the heart and identify any issues pertaining to the circulatory system.

ECG traces are electrical changes recorded at the skin level using electrodes. Heart beats are recorded, including the stages between them to visualise full cardiac cycle patterns over time.




The largest signal is given by the ventricular systole, with other smaller signals given by the surrounding heart cycle events. The different signals have wave terms, such as the P wave and the T wave. The spacing and duration of the signals can indicate the speed of the heart beats and their regularity, which can be used to asses various pathologies such as rapid heart rate, tachycardia, slow heart rate, bradichardya or various irregular patterns of heart beat, arrythmia.



Pressure changes in the heart are similarly representative of the different parts of the heart cycle. Just like the top ECG signal, the largest pressure increase is that of the ventricular systole, coinciding with the pressure increase in the aorta which is the vessel that carries the oxygenated blood around the body as a result of ventricular systole.


Blood

Components

Blood is fun! Blood is to body as the Thames is to London, although I sure hope slightly cleaner...

Blood is roughly split into the plasma and blood cells including erythrocytes and leucocytes (neutrophils, eosinophils, monocytes, lymphocytes). Plasma is the solution that blood cells are found in, and as such acts as their extracellular matrix. For skin cells for example, the extracellular matrix is formed of collagen, so it's different to have it essentially a liquid like plasma. Plasma is a water solution containing proteins, sugars, clotting factors (as well as platelets involved in clotting), hormones, electrolytes, carbon dioxide and oxygen.




Erythrocytes are red blood cells/RBC (and also the most common blood cells) carrying haemoglobin around the body. Haemoglobin can bind and release oxygen and is central to aerobic respiration.

Leucocytes of varying types are white blood cells/WBC, colourless, and act in defence against infection and disease.




There are many types of white blood cell. Neutrophils are the most common and, alongside monocytes, digest invading cells of bacteria and fungi by engulfing them in a process called phagocytosis. The invader is engulfed, isolated in a lysosome that contains digestive enzymes, and its remains disposed off and recycled or excreted.




Monocytes also live longer and present antigens of invaders to a type of lymphocyte called a T cell for later reference should the same invader come back later in the future. Moreover, monocytes eventually leave the bloodstream to settle in a different tissue and become macrophages in charge of clearing up cell debris and further immune function.

Eosinophils are the least common in blood, and attack larger parasites as well as modulate allergic inflammatory responses.

Finally, lymphocytes are relatively abundant in blood but much more prevalent in the lymphatic system and take part in the adaptive immune response. Phagocytosis as carried out by neutrophils and monocytes is part of the innate immune response and as such is more generic. Lymphocytes include B cells, T cells and natural killer cells (NK cells are part of the innate immune response however).

They can make antibodies against various pathogens or abnormal cells in the body such as those in tumours, present them on their surface and find target cells, and destroy any cells which do not present the expected antigens on their surface.



All in all, the function of blood covers defence, transport and formation of lymph and tissue fluid.


Coagulation (clotting)

Upon disturbance of the cells lining a blood vessel, a cascade of events leads up to the coagulation of the blood and restoration of a protective barrier between the tissue and the environment, preventing further bleeding (hemostasis). Blood turns from a liquid to a gel. Many aspects that cause coagulation also contribute to defence against pathogens. For example, as the blood clots, it traps bacteria. Some clotting components are also toxic to some bacteria.

Clotting begins with platelets forming a plug and initiating the release of clotting factors such as thromboplastin. This is a plasma protein which turns prothrombin to its active state, thrombin.




Thrombin recruits soluble fibrinogen and converts it into insoluble fibrin which covers the wound and contributes to the platelet plug by strengthening it and aiding against pathogens and bleeding.


Atherosclerosis

Atheroma, aneurysm, thrombosis, myocardial infarction?

Read on to learn what these words mean. Coronary heart disease is a major cause of death in the UK and much of the rest of the world. Risk factors associated with CHD are dietblood cholesterolcigarette smoking and high blood pressure.


Atheroma

This is the build up of fatty material in the walls of arteries. It is often the underlying cause that leads to heart disease.




As you can see, it leads to the narrowing of arteries, causing a lowering of blood supply. Atheroma is associated with an increased risk of aneurysm and thrombosis. Aneurysm is a ballooning of the artery which weakens the affected area. This requires urgent treatment, otherwise it is fatal if the balloon "pops". Thrombosis is a blood clot stuck in a vessel which results in less blood supply to a specific area, and the subsequent affected tissues may be starved of blood and die.

If the blood supply to the heart muscle is stopped, then a myocardial infarction occurs. This is the scientific name for a heart attack. The heart muscle (or part of it) dies as a result of a lack of oxygen from the blood.




These are different kinds of aneurysm (you don't need to learn the names).


Diet

Stuff like eggs and meat contain high levels of cholesterol which can lead up to atheroma. Plants on the other hand have little cholesterol (Disclaimer: I'm a vegetarian ^_^), so by far the easiest way to cut on cholesterol is to remove meat from the diet, especially fatty meats. Cholesterol levels are also genetically inherited, in which case diet is even more important in preventing CHD. There are two kinds of cholesterol, HDL and LDL. HDL has a positive impact on health by removing blood cholesterol and sending it to the liver; LDL has a negative impact by doing the exact opposite - carrying cholesterol from the liver to other cells in the body.

Plot twist however, the sugar industry might have paid scientists to publish fat-related studies to take away any emphasis on the role of sugar in disease. It turns out that excess sugar is far more troublesome than fat. Excess sugar leads to inflammation which in turn primes these tissues for malfunction and disease.


Smoking

The mechanism by which smoking causes CHD (specifically atherosclerosis = hardening of the arteries) is complex and not fully mapped out yet. However, it is known that certain substances contained in tobacco lead to artery constriction, which in turn raises blood pressure.

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