Why does fever cause dilation of blood vessels?

[justadream]

Isn't the purpose of fever to raise the body temperature and aid the immune system in fighting off infection?

Dilating blood vessels would cool the body.

Isn't that inefficient? The body is expending energy to raise temperature but is dilating blood vessels to cool body temperature.

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[Cawolf]

Fever and vasodilation are both results of immune system activity in the body - I have not heard of fever resulting in dilation.

Vasodilation occurs in local infection to allow for increased blood flow to the site of infection - allowing for increased perfusion and increased leukocyte counts. The dilation is due to inflammatory mediators such as bradykinins and complement proteins though - not the generalized fever.

 

[justadream]

@Cawolf

"bradykinins" - what are those? Never heard of them.

I think I had read somewhere about vasodilation helping to cool the body during a fever but your reasoning makes more sense.

Although I guess it is somewhat inefficient (energy wise) to have a fever and vasodilate. I guess fighting off infection takes precedence over energy efficiency.

 

[Czarcasm]

Isn't the purpose of fever to raise the body temperature and aid the immune system in fighting off infection?

Dilating blood vessels would cool the body.

Isn't that inefficient? The body is expending energy to raise temperature but is dilating blood vessels to cool body temperature.

It's a consequence of the inflammation response. The fever itself is due to prostaglandins action on the hypothalamus. The vasodilation, such as when someone experiences systemic shock is due to interleukins released by immune cells and also histamine (mast cells), which act to loosen the blood capillaries, so additional immune cells can come to the area of infection. This is a good way of dealing with localized infections, but problems arise when an infection travels throughout the body (such as when a bacteria escapes its immune containment). At that point, the individual undergoes systemic shock (huge loss of flood) and a huge drop in blood pressure, eventually causing death unless treated appropriately. The fever itself helps to raise metabolic activity so that the environment is less ideal for bacteria to grow (because bacteria grow in optimum temperature environments).

Also, vasodilation in general does not cool the body. Vasodilation at the skin surface helps to let latent heat escape from the skin, which as a consequence cools the body.

 

[Cawolf]

@Cawolf

"bradykinins" - what are those? Never heard of them.

Not critical for MCAT level - just a signaling chemical.

 

[justadream]

It's a consequence of the inflammation response. The fever itself is due to prostaglandins action on the hypothalamus. The vasodilation, such as when someone experiences systemic shock is due to interleukins released by immune cells and also histamine (mast cells), which act to loosen the blood capillaries, so additional immune cells can come to the area of infection. This is a good way of dealing with localized infections, but problems arise when an infection travels throughout the body (such as when a bacteria escapes its immune containment). At that point, the individual undergoes systemic shock (huge loss of flood) and a huge drop in blood pressure, eventually causing death unless treated appropriately. The fever itself helps to raise metabolic activity so that the environment is less ideal for bacteria to grow (because bacteria grow in optimum temperature environments).

Also, vasodilation in general does not cool the body. Vasodilation at the skin surface helps to let latent heat escape from the skin, which as a consequence cools the body.

Interesting, so prostaglandins are involved with fever? How do they do that? They just tell the hypothalamus to raise the temperature set point?

My only prior association with prostaglandins was that they aid smooth muscle contraction (during labor).

 

[Cawolf]

Generally yes, they signal the brain to increase the set point. This is why common drugs used to treat fever (NSAIDs) function as prostaglandin inhibitors.