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Local Control of Blood Flow
each tissue controls its own blood flow in proportion to its needs
- tissue needs:
- 1. delivery of O2 to tissues
- 2. delilvery of nutrients such as glucose, amino acids, ect
- 3. removals of CO2 , H and other metabolites from the tissues
- 4. Transport various hormones and other substances to different tissues
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increase in tissue metabolism leads to
increase in blood flow
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decrease is oxygen availability to tissue...
increases tissue blood flow
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Flow (Q) through a blood vessel is determined by
- 1. the pressure difference (ΔP) between 2 ends of the vessel
- 2. resistance (R) of the vessel
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autoregulation
ability of a tissue to maintain blood flow relatively constant over a wide range of arterial pressure
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Metabolic Theory of Blood Flow Autoregulation
as arterial pressure is decrease, O2 or nutrient delivery is decreased resulting in a release of a vasodilator
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Myogenic Theory of Blood Flow Autoregulation
as arterial pressure falls the arterioles have an intrinsic property to dilate in response to decreases in wall tensions
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Laplace's Law of Myogenic Mechanism
Tension = Pressure x radius
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Long-term Local Blood Flow Regulation
- occurs by changing the degree of vascularity of tissues (size and number of vessels)
- oxygen is an important stimulus for regulating tissue vascularity
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Angiogenesis
- the growth of new blood vessels
- occurs in response to angiogenic factors
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Anigogenic factors are released from:
- 1. ischemic tissues
- 2. rapidly growing tissue (embryology)
- 3. tissue with high metabolic rates (ovary, cancer)
- most are small peptides
- VEGF: vascular endothelial cell growth factors
- FGF: fibroblast growth factor
- angiogen
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Vasoconstrictors
- NE and EPI
- angiotensin
- vasopressin
- endothelin
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Vasodilator
- bradykinin
- serotonin
- histamine
- prostaglandins
- nitric oxide
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NE and EPI vasoconstrictors
- Catecholamine, non-peptide NT, that are secreted by the adrenal medulla
- bind to α and β GPCR andwork through second messengers (Gs- cAMP) affecting K and Ca
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Angiotensin (II)
- important in kidney control of BP
- peptide hormone more potent then NE/ Epi
- mainly acts on the arterioles to increase total peripheral resistance
- AT1 and AT2 receptors
- activate PLC IP3 / DAG
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function of renin
cleaves angiotensiogen into angiotensin I
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ACE
- angiotensin converting enzyme
- converts Angiotensin I to II
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Effects of Angiotensin II
- vasoconstrictor
- Acts on Adrenal Cortex, Pituitary, Cardiac and Vascular hypertrophy, systemic vasoconstriction
- increased blood volume
- renal sodium and fluid retention
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Vasopressin
- vasocontrictor
- ADH: anti-diuretic hormone
- formed from nerve cells of hypothalamus
- secreted by posterior pituitary gland into blood
- increases water reabsorption in the kidneys
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serotonin
- vasodilator
- 5HT2
- important in localized vasoconstriction
- dilated cardiac blood vessels
- secreted by platelets
- Increases PLC, increase IP3, and increases intracellular calcium
- involved in wound healing
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histamine
- vasodilator
- occurs in every tissue
- released by immune cells in respinse to damage, allergy, and inflammation
- Gs
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Nitric oxide (NO)
- vasodilator
- released from healthy, endothelial cells
- acts locally on vascular SM cells
- cGTP converted to cGMP by guanylate cyclase enzyme to cause local SM relaxation
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