Why does your heart rate go up when you breathe in?

Question

(and it goes down when you breathe out)?

Just during normal breathing.

I think it has something to do with increased thoracic volume, and baroreceptors/baroreflex, but I'm not sure. How does thoracic volume affect blood pressure, which is what baroreceptors sense? Am I completely off the mark?

Answer 1

  The heart rate goes down on breathing in, & is due to stimulation of the vagus nerve ,which controls heart rate

Answer 2

i have no idea about heart rate. but when you breathe in, your blood pressure will decrease as a more negative pressure is created when the thoracic cavity expands, thus retaining more blood in the thorax. the blood pressure can drop by as much as 15mmHg if i'm not wrong. this is known as pulsus paradoxus.

however, i don't think the baroreceptors are affected by this decrease in blood pressure since they are around the aorta and carotid arteries. perhaps the higher volume of blood caused the heart to pump harder and faster?  

Answer 3

you breath in, oxygen goes into your heart, which pumps faster because before you breath in, your oxygen level was low, the lower, the less the heart pumps to distribute the oxygen throughout the body, so as more oxygen comes in, the heart pumps faster to keep up and distribute it evenly

Answer 4

Hi there,

Well its to do with the wiring of the human body. What happens is that when we breathe in, our lungs inflate and we activate receptors in the lung known as stretch receptors. These relay information to the brain that your lungs are now stretched, full with air and its about time we stop breathing in and beging breathing out - preventing overinflation of the lungs. The information from the stretch receptors goes along a nerve called the vagus nerve to the brain and the signal ends up in the pons where breathing is controlled. The pons then stops the inspiratory muscles breathing in, and tells the expiratory muscles to start working and so we exhale. However the signal from the stretch receptors also follows another path, towards centres of the brain which control signals from the vagus nerve to the heart.

The important thing about the vagus nerve it is parasympathetic, so it has the effect of decreasing heart rate. Now, the signals from the stretch receptors goes to areas in the central nervous system known as the nucleus ambiguus and dorsal motor vagal nucleus. These send signals down the vagus nerve, and as described above tell the heart to slow down. But the signal from the stretch receptors inhibits these areas of the brain, so there is less vagal stimulation on the heart and so the heart rate increases!

in simple terms:

breathe in > stretch lung > stretch receptors tell brain to stop breathing in and begin breathing out > at the same time strech receptors tell area of brain that inhibits heart to stop > heart rate goes up

Now why are we wired like that?!

Wellll....this interaction between lungs and heart is conserved throughout evolution, its widely found in birds, fish and mammals. Even in fish which dont breathe air, the flow of water over the gills is in synchronisation with the heart - pretty amazing. Basically what this is pointing to is that you are matching what is called ventilation and perfusion. If we think of the lung, it has air in it, and it has blood to take the oxygen away. If you have too much air in the lung and not enough blood, the efficiency of carrying away oxygen to the body is greatly diminished. As with too much blood and too little air, you are wasting blood which could be used in other areas of the lung to carry more oxygen away. So we aim to have perfect match between ventilation (air in the lung) and blood (perfusion).

If you have any questions please feel free to ask. I know this might appear a little complicated at first so I will gladly clarify anything.

EDIT> No I dont think thats correct. Show him my explanation see what he says. When you breathe in yes intrathoracic volume increases and decreases pressure. This sucks blood from veins and increases venous return, stroke volume would therefore go up and so would baroreceptor stimulation. So if anything on breathing in, heart rate would go down, but due to vagus inhibition it goes up. Plus if thoracic pressure decreased, this would cause an increased stretch (if any stretch!) in baroreceptors due to external suction on the baroreceptors. So everything points to heart rate going down. Like i say show him my explanation see what he says.  I've been taught this by a bunch of physiologists and is a well known reflex linked to the Hering Breuer reflex