Tissues control blood flow in proportion to what 4 needs?
- Delivery of oxygen
- Delivery of nutrients
- Removal of CO2 and H+ and other metabolites
- Transport of various hormones and other substances to different tissues
Flow is closely related to what?
Metabolic rate of tissues
Acute control of local blood flow is determined by what structures?
- Precapillary sphincters
What 2 factors primarily alter tissue blood flow?
- 1) Tissue metabolism
- 2) Oxygen availability
What are the 2 major theories for local blood flow?
- 1) The vasodilator theory
- 2) Oxygen demand theory
How does increased tissue metabolism affect blood flow?
How does decreased oxygen concentration affect blood flow?
What law represents the relationship between pressure, flow, and resistance?
Describe the Vasodilator Theory for blood flow control
Increase in metabolism/Decrease in oxygenation leads to release of vasodilators causing decreased resistance and increased flow
Describe the Oxygen Demand Theory for blood flow control
Increase in metabolism/decrease in oxygenation leads to decrease tissue oxygen concentration causing decreased resistance and increased flow
The ability of a tissue to maintain blood flow relatively constant over a wide range of arterial pressures
List examples of vasodilators involved in the Vasodilator Theory for blood flow control
Adenosine, CO2, lactic acid, ADP compounds, histamine, K+ ions, H+ ions
What is the Metabolic Theory of autoregulation?
Decreased arterial pressure leads to decreased oxygen and nutrient delivery which releases vasodilator(s)
What is the Myogenic Theory of autoregulation?
As arterial pressure falls, arterioles have intrinsic property to dilate in response to the decrease in wall tension
Explain LaPlace's Law as it relates to the Myogenic Theory of autoregulation.
- As pressure increases, the increase in tension will cause a decrease in vessel radius
- As pressure decreases, the decrease in wall tension will cause an increase in vessel radius
Is long-term or short-term control of flow more effective?
How does long-term blood flow regulation occur?
By changing the size and number of vessels - angiogenesis
How does high altitude affect angiogenesis?
Increased vascular growth in tissues in response to decreased oxygen levels
What is the process of retrolental fibroplasia or retinopathy of prematurity?
High FiO2 decreases vessel growth or even causes degeneration. When the high oxygen concentration is removed, there is a proliferation of retinal vessels into the vitreous humor causing blindness
What is the result of chronically overactive tissue with regards to blood flow/vessels?
Increase in both number and size of arterioles and capillaries to match needs of tissue
Growth of new blood vessels
What 3 factors stimulate angiogenesis?
- Ischemic tissue
- Rapidly growing tissue
- Tissue with high metabolic rate (tumors)
What are examples of angiogenic factors?
- Vascular endothelial cell growth factor (VEGF)
- Fibroblast growth factor (FGF)
What are examples of vasoconstrictors involved in humoral regulation of blood flow?
- Angiotensin II
- Vasopressin (ADH)
What are examples of vasodilators involved in humoral regulation of blood flow?
- Nitric oxide
How do calcium ions affect blood flow?
Increased levels cause vasoconstriction through smooth muscle contraction
How do potassium ions affect blood flow?
Increased levels cause vasodilation through smooth muscle inhibition
How do magnesium ions affect blood flow?
Increased levels cause profound vasodilation through smooth muscle inhibition
How do hydrogen ions affect blood flow?
- Increased levels cause vasodilation
- Decreased levels cause vasoconstriction
How do anions like citrate and acetate affect blood flow?
Both cause mild vasodilation
How does carbon dioxide concentration affect blood flow?
Increases cause moderate vasodilation in most tissues; profound vasodilation in the brain