BSI: Blood Flow Distribution

No. The percentage of the cardiac output going to various organs/regions is not equal.

• Blood flow to the tissue is matched to the needs of the tissue.
• - Active tissue can require up to 20x increase in blood flow (such as exercising skeletal muscle).
• - Cardiac output only increases ~6x, therefore changes in microcirculation must take place to increase blood flow to the active tissue even more.

If blood flow exceeds the need of the tissue, regulatory mechanisms will decrease blood flow.

• The Sympathetic Nervous System (SNS) of the Autonomic Nervous System (ANS).
• Local Chemical Factors, such as the amounts of O₂, CO₂, histamine, H⁺, K⁺ and Nitric Oxide.
• Hormones, such as endothelin, bradykinin and histamine.
• Angiogenesis for long-term regulation.

The SNS of the ANS innervates blood vessels; PNS does not innervate the majority of blood vessels.

Because of this innervation, the SNS can cause generalized vasoconstriction and venoconstiction, except some dilation in skeletal muscle.

The SNS

Decreased O₂ availability causes release of vasodilator substances from tissue.

• These substances diffuse into local capillaries and act on precapillary sphincters, metarterioles, and arterioles to cause smooth muscle relaxation and vasodilation.
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It is thought that adenosine is the most important vasodilator substance.

Other vasodilators are: CO₂, histamine, H⁺, K⁺.

It is thought that adenosine is the most important vasodilator substance.

Other vasodilators are: CO₂, histamine, H⁺, K⁺.

Shearing stress of increased flow in the arteries, due to causes such as narrowing of the arteries, causes the endothelial cells to release nitric oxide.

Nitric oxide diffuses into smooth muscle in local areas and causes smooth muscle relaxation (vasodilation).

• 1) Endothelin
• Vasocontrictor in damaged vessels

• 2) Bradykinin
• Causes arteriole dilation and capillary permeability

• 3) Histamine
• Causes arteriole dilation and capillary permeability

Angiogenesis is the increase or decrease in the number of blood vessels can occur.

Angiogenesis can occur in response to ischemia (such as in frequent high intensity exercise).

When ischemia is present over the long term, tissues release angiogenic factors (VEGF, FGF, etc.).

This angiogenesis is a similar process to what we discussed in cancer lectures.

• Oxygen demand increases in various tissues
• Blood flow to those tissues increases
• Oxygen consumption (VO₂) of those tissues increases

If exercising, is really only one thing that increases blood flow to the heart:

• 1) activation of the SNS (however, recall that epi released from adrenal glands does not work on the heart!)
• 2) an increase in SNS will increase heart rate and contractility because an oxygen deficit will release vasodilator chemicals
• ??

So what is different between cardiac and skeletal muscle??

• If exercising, there are two factors that contribute to increase blood flow to skeletal muscle:
• 1) activation of the SNS via epinephrine released from adrenal glands (recall epi does not work on the heart!)
• 2) local factors cause significant vasodilation

• Let's say we are exercising...the consequences on general circulation are as follows:
• - Cardiac Output (CO) increases dramatically (from 5L/min at rest to 25L/min during exercise)
• - Stroke Volume (SV) will only increase so much and will level out at a maximum
• - Heart Rate will increase quite dramatically, but also eventually level off at a maximum (such as 120 beats per minute)
• - Oxygen consumption (VO₂) of the tissues increases dramatically and then levels off at a maximum. Note VO₂ refers to oxygen consumption of tissues, not the amount you breathe into your lungs.
• - Blood Pressure is complicated because Systolic will increase quite a bit, but diastolic will not increase at all...you may even see a decrease in DBP! Therefore, the MAP only slightly increases.

• Blood flow distribution at rest:
• CO = 5.00 L/min

• Blood flow distribution during exercise:
• CO = 25.0 L/min