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Mammalian circulation

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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 contractatrial systole
2. Both ventricles contractventricular systole
3. All chambers relaxdiastole


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 Purkyne (a.k.a. Purkinje) fibres, 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: arteries, arterioles, capillaries 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.




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.


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.





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