The Starling Mechanism and its Importance for Freediving
Hi there Freedivers and welcome to this brand new update from Dr Otter’s riverbank freediving laboratory.
Today we’ll be looking at an ingenious little mechanism of the heart which helps with regulating blood pressure. It also turns out to be extremely important for freedivers of all shapes and sizes. It’s called the Starling Mechanism and in order to understand completely how it works, it might be useful to brush up on some of the basics of blood pressure regulation. You can find that here in this article on age related changes in freediving which deals with it in more detail.
What is the Starling Mechanism?
The Starling Mechanism is driven by stretch in the walls of the atria (the small chambers of the heart) and the ventricles (the big chambers) which is a direct measure the amount of blood that’s entering the heart from the large vessels (the superior and inferior vena cava and the pulmonary artery) which supply them. Here’s an image showing the basic plumbing of the heart, including these vessels:
Blood enters the right atrium from the body, gets pumped by the left ventricle to the lungs and returns via the left atrium. It then passes through the mitral valve into the left ventricle which pumps it back to the body. If more blood enters the heart, than usual (we’ll see some of the ways that can happen in a minute) the walls of the chambers are stretched and this can lead to inefficiency in the way that the heart expels blood.
Imagine trying to lift a 20kg steel ball from the ground to chest height once a second - difficult enough for most of us. Now imagine that the ball is a meter in diameter - although it’s the same weight, the exercise becomes much harder because the muscles you want to use are stretched past the point where they can act most efficiently. This is what we mean when we say - often in a whining sort of voice - “it’s not heavy, it’s just awkward.” It’s the same with the heart. Stretch the muscles too much and they can’t work the way they’re meant to.
To compensate for this, the Starling Mechanism determines the force that the ventricles apply to the blood as they contract which is controlled directly by the stretch of the muscle fibres:
As the diagram suggests, a greater contractile force causes more blood to be ejected from the heart. This is called an increase in the ejection fraction (the proportion of the blood in the ventricle that gets pushed out) which in turn leads to an increase in cardiac output. Overall this results in less blood left in the ventricle so that when it refills ready for the next beat, there is less stretch and more efficient contraction. In technical language we say that overfilling of the heart causes a reflex (that is, automatic) decrease in end diastolic volume (the amount of blood filling the ventricle just before it contracts on the next beat). This is how the heart regulates its own filling, and the efficiency of its own contraction from one beat to the next.
So how is the Starling Mechanism Important for Freediving?
Those of you at the front who are paying attention might have guessed this one already. Here are some of the ways that the heart can be overfilled with blood in the first place:
- Excessive hydration - if you drink a lot in one go (think of Friday night at the pub perhaps) your blood volume is going to increase rapidly and, until your kidneys deal with the excess volume, your heart will be working harder, not that you’ll notice - because of the Starling Mechanism :)
- Poor heart function - in the later stages of heart failure the walls of the ventricles become so overstretched that they just can’t deliver enough force to get rid of the blood coming in. The Starling Mechanism compensates for a while, but in the end even that becomes ineffective. (actually you can see this if you look closely at the graph - see how the curve dips downwards at very high volumes? This is the failing point).
- Poor kidney function - If you don’t pee out what you drink, it stays in your blood and your heart has to deal with it. This is why people with renal failure have to watch the amount they drink.
- Yes, you guessed it - it’s the Mammalian Dive Reflex:
When the dive reflex kicks in, Peripheral Vasoconstriction causes blood to be shifted from the extremities (hands, feet, legs, arms, face etc) to the central organs - heart, lungs and brain. This is the equivalent of trying to fit 6L of water into a 4L container and it puts a big strain on the cardiovascular system, especially because the heart rate slows to something like 25 beats per minute at the same time.
Those of you who’ve paid attention to your heartbeat during, for example, static apnea, may be aware that it feels much stronger than usual - this is the Starling Mechanism in action, making sure that blood is pumped as efficiently as possible out of the heart so that it doesn’t fail. This ensures that all your important organs get blood at the right pressure, even though the system is overloaded. Clever or what?!
Got a burning question? Contact us at firstname.lastname@example.org and Dr Otter will get on it right away :)