Re: The four-chambered heart

Evelyn Barbour-Hill — Mon, 15 Apr 2013 14:00:30 GMT

Thank you. That is very well explained.

Re: The four-chambered heart

David Mills — Mon, 15 Apr 2013 09:54:46 GMT

[quote user="Neil Williams"]

I think the main advantage of 4 chambers is the atria act as reservoirs, allowing the ventricles to rapidly fill during diastole - if the veins connected directly to the ventricles they wouldn't have enough time to fill before the next contraction.

I stand to be corrected though

[/quote]

I think this is about the size of it. The heart starts off as two single chambers but as the foetus grows it requires a more efficient and voluminous pumping mechanism. The atria fill with blood during ventricular systole so that filling can start at ventricular relaxation. When more blood is required a primary mechanism is to increase HR which occurs by shortening the diastolic phase, that is per heart beat less blood is pumped but this is overwhelmed (upto a level - around 180-90 in dogs, difficult to say in cats but likely >200) by the higher rate to cause increase in CO. In normal circumstances, atrial contraction adds 20% of the volume: for most resting activities this is not required (e.g. cats can live with 3rd degree AV block quite happily). When increased HR is required, atrial contraction adds proportionally more to the ventricular volume so a contractile chamber close to the ventricle is very useful.

Imagine the one chambered system. The diastolic phase would be very long whilst the chamber filled as rapid voluminous filling could not occur from veins (low pressure) - it would trickle in. This would mean a low HR and low CO, and, more so, an inability to alter CO by increasing HR because of the severe limits of rapid filling. Atrial contraction absence could not add a 'top up' significant volume either. An alternative would be to contract the veins and actively fill the ventricle in a sequential manner - but this would need to start fairly proximal to the heart and there would likely be insufficient distance before lungs were encountered. This long lebgth of vein would then need to refill rapidly during systole. So you'd be left with a low HR with low filling volumes. This may be fine for small organisms but for animals that move, hunt, run etc, the CV system needs to be highly responsive with plenty of 'reserve'.

The one-chambered model is also very energy-demanding for the moderate output. In a four-chambered model much of the 'energy' of the system is contained in ventricular contraction - this alters pressures which allows/encourages blood to flow down pressure gradients. Rapid ventricular filling is a passive process; similarly atrial filling where blood rapidly moves into lower-pressure chambers. A small atrial contraction also demands energy, though a low amount. This is surprisingly energy efficient. In the one chambered scheme both ventricular filling and emptying would be energy-demanding processes. Whilst ventricular filling could occur due to pressure, the lack of an atrial reservoir both reduces the gradient and disallows any rapid filling.

In this scheme, it is obvious that organisms without a four-chambered heart have a size and indeed activity limit.

Re: The four-chambered heart

Neil Williams — Sun, 14 Apr 2013 19:55:31 GMT

I stand to be corrected though

Re: The four-chambered heart

Evelyn Barbour-Hill — Sun, 14 Apr 2013 18:55:21 GMT

Yeah, with due respect, that answers nothing. The question is, why two chambers on each side? By all means get quite scientific and even mathematical if you wish.

Re: The four-chambered heart

An On MRCVS — Sun, 14 Apr 2013 17:55:42 GMT

http://www.sharecare.com/question/why-hearts-have-four-chambers