Control of blood flow and special circulation Flashcards
What are the different controls of blood flow?
1) Local control
2) Global control (ANS + Reflexes)
What is meant by local control?
When there is an increased metabolic activity:
1) Acute
- Acts within seconds or minutes
- It includes reactive hyperemia, active hyperemia, and autoregulation, where there is a constriction or relaxation of the arterioles, metarterioles, and precapillary sphincters based on the rate of metabolism and oxygen availability
When there is a sustained increase in blood flow demand:
2) Chronic
- Acts within days or weeks
- It results in physical changes like the size and number of BV supplying the tissue
What is meant by active hyperemia?
- It is a local increase of blood flow to tissue when the tissue becomes active (like during exercise), it increases by 20 folds approximately
What is meant by reactive hyperemia?
- It is a transient increase in blood flow (4-7 times) after a brief period of arterial occlusion
- Temporary occlusion followed by a surge in blood flow once it is removed
What is meant by autoregulation?
It is the intrinsic ability of an organ to maintain a constant blood flow despite changes in the arterial pressure
There are two theories behind it:
1) Metabolic theory
2) Myogenic theory
- Thanks to autoregulation blood flow remains constant even if the BP changes, which is essential in the brain as it is highly sensitive to oxygen levels
What is the metabolic theory of autoregulation?
It is a theory based on two different mechanisms:
1) Vasodilation:
- Vasodilation will occur whenever there is an increase in metabolic activity, increasing the blood flow to meet the demands
2) Oxygen lack theory:
- Vasomotion, where precapillary sphincters and metarterioles open and close several times per minute based on the availability of oxygen and nutrients, when they are not available these two structures will relax and open
What is the myogenic theory?
It is an intrinsic property of the smooth muscle cells where a stretch of the vascular smooth muscles due to high BP will lead to the contraction of the arterioles and thus an increase in the vascular resistance, maintaining a constant blood flow
Autoregulation is the main mechanism that controls BP in which circulations?
1) Cerebral
2) Coronary
3) Renal
4) Skeletal muscles during exercise
What are some other substances in the blood that control blood flow?
- Vasoconstriction agents:
1) Norepinephrine and Epinephrine
2) Endothelin
3) Angiotensin-II
4) Serotonin
5) Vasopressin/ADH
6) Increased Calcium
- Vasodilating agents:
1) Nitric oxide
2) Bradykinin
3) Histamine
4) Increased potassium, magnesium, hydrogen, & carbon dioxide
What is the long-term regulation of blood flow?
Due to the increased metabolic demand for a long period, new blood vessels will form to keep up with the demand by angiogenic factors like (vascular endothelial growth factor “VEGF”, fibroblast growth factor, and angiogenin)
What is meant by global control?
- It is based on the needs of the whole body, including:
1) Predominantly neural:
- a1-receptors
- b2-receptors
- Muscarinic cholinergic receptors
2) Humoral:
- Catecholamines
- Angiotensin-II
- ADH
- Aldosterone
- ANP
- Erythropoietin
3) Reflexes (combination of both)
Which factors induce angiogenesis?
1) Vascular endothelial growth factor
2) Fibroblast growth factor
3) Angiogenin
What are the different reflexes involved in the global control that controls blood flow?
1) Arterial baroreceptor reflex
2) Volume reflexes (atria stretch receptors, “ADH, ANP”)
3) cHEMORECEPTOR REFLEX (CENTRAL AND PERIPHERAL)
4) Central ischemic response and Cushing’s reflex
What are some examples of organs with special circulation?
1) Skin & Kidneys need blood flow for both nutrition and excretion
2) Control of blood flow in the brain and heart for example is minimal
3) Blood can be shunted from the liver and splanchnic (abdomen) circulation during exercise
What are the blood flow changes that occur during exercise?
1) Blood flow to skeletal muscles increases by 1066% via shunts
2) Blood flow to the brain remains unchanged
Describe the circulation in skeletal muscles
- Blood flow depends on metabolic demands
- Its arteries have a dual control:
1) Neural mainly during rest
- a1-receptor, B2-receptor
2) Local control during exercise
What is the effect of exercise on muscle blood flow?
- It is associated with reactive hyperemia where when the muscle contracts it is temporarily occluded and blood flow increases upon relaxation
- Like a milking action
Describe the GI (splanchnic) circulation
It is predominantly controlled neurally where the a1 receptor dominates, there are also local and hormonal controls
- Extravascular compression is Important during exercise and aids in venous return
Describe the coronary circulation
- Neural control is only for fine-tuning
Local control:
- It is controlled predominantly by the metabolic factors like adenosine and oxygen levels
-Important notice is that the heart has the highest extraction rate of oxygen from the blood which is 65-75% at rest and 90% during vigorous exercise, making it very vulnerable to blocks in case of hypoxia
Describe the phasic blood flow dueing systole and diastole in the coronary arteries
The left coronary artery has a higher reactive hyperemia as the left ventricle contracts harder causing a greater transient occlusion
Describe the control of the cerebral circulation
- BF is relatively constant
1) Neural control is of minor importance
2) Local control predominated like CO2 or H+, where if they increased there will be vasodilation
- Extravascular compression is important in pathological cases like hemorrhage and cerebral edema
Describe the control mechanisms of the cutaneous circulation
Neural tone dominates like a1-receptors, sympathetic cholinergic stimulation, bradykinin where it causes vasodilation
Local controls like temp have a direct effect to
What controls the pulmonary circulation?
Local controls dominates, especially oxygen levels, but in this circulation, hypoxia will cause vasoconstriction unlike the systemic circulation where it causes vasodilation
