Lec59[1]
-
Upload
mbbs-ims-msu -
Category
Documents
-
view
811 -
download
0
Transcript of Lec59[1]
Local Control of Blood Flow
Each tissue controls its own blood flow in proportion to its needs.
Tissue needs include:1) delivery of oxygen to tissues2) delivery of nutrients such as glucose, amino
acids, etc.3) removal of carbon dioxide hydrogen and other
metabolites from the tissues4) transport various hormones and other
substances to different tissues
Flow is closely related to metabolic rate of tissues.
Variations in Tissue Blood Flow
Brain 14 700 50Heart 4 200 70Bronchi 2 100 25Kidneys 22 1100 360Liver 27 1350 95 Portal (21) (1050) Arterial (6) (300)Muscle (inactive state) 15 750 4Bone 5 250 3Skin (cool weather) 6 300 3 Thyroid gland 1 50 160 Adrenal glands 0 .525 300 Other tissues 3.5 175 1.3
Total 100.0 5000 ---
Percent ml/min
ml/min/100 gm
Control of Blood Flow by Tissues
Local control In most tissues, blood flow is
proportional to metabolic needs of tissues
Nervous System Most important for regulating blood flow
and maintaining blood pressure Hormonal Control
Sympathetic action potentials stimulate epinephrine and norepinephrine
Tissue Metabolic Activity Is the Main Factor in Acute Control of Local Blood Flow
One of the most fundamental principles of circulatory function is the ability of each tissue to control its own local blood flow in proportion to its metabolic needs
Metabolic Mechanism Any intervention that results in an
inadequate oxygen (nutrient) supply for the metabolic requirements of the tissues results in the formation of vasodilator substances which increase blood flow to the tissues.
Acute Control of Local Blood Flow
Increases in tissue metabolism lead to increases in blood flow.
Decreases in oxygen availability to tissues increases tissue blood flow.
Two major theories for local blood flow are:1) The vasodilator theory2) Oxygen demand theory
Effect of Tissue Metabolic Rate on Tissue Blood Flow
Effect of Tissue Oxygen Concentration on Blood Flow
Acute Local Feedback Control of Blood Flow
Lack of oxygen? Formation of vasodilators?
Combination of both??
Metarteriole
Precapillary Sphincter
Capillary
Relaxation of smooth muscle
Increased Blood Flow
Metabolic Mechanisms
Hypoxia
Tissue metabolites and ions Adenosine Potassium ions Carbon dioxide Hydrogen ion Lactic acid
Metabolic Control of Local Blood Flow
Autoregulation of blood flow when arterial pressure changes from normal
Intrinsic ability of an organ to maintain a constant blood flow despite changes in pressure
Possible explanations for Autoregulation: Myogenic Mechanism Metabolic Mechanism
Theories to Explain Autoregulation:Metabolic Mechanism
When the pressure increases to a tissue, the flow increases, and excess oxygen and nutrients are provided to the tissues. These excess nutrients cause the blood vessels to constrict and the flow to return nearly to normal despite the increased pressure.
Theories to Explain Autoregulation:Myogenic Mechanism
When the lumen of a blood vessel is suddenly expanded, the smooth muscles respond by contracting in order to restore the vessel diameter and resistance. The converse is also true.
Vascular smooth muscle cells depolarize when stretched.
Proposed mechanism is stretch of vascular smooth muscle causes activation of membrane calcium channels.
Theories to Explain Autoregulation:Myogenic Mechanism
P1
F1
↑P
↑F
↑P
F1
Long-term Regulation of Blood Flow
Long-term regulatory mechanisms which control blood flow are more effective than acute mechanism
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
Short and long term Regulation of Blood Flow
Angiogenesis
Angiogenesis is the growth of new blood vessels
Angiogenesis occurs in response to angiogenic factors released from:1) ischemic tissue2) rapidly growing tissue3) tissue with high metabolic rates
Most angiogenic factors are small peptides such as vascular endothelial cell growth factors (VEGF), fibroblast growth factor (FGF), and angiogenin
vascular endothelial growth factor (VEGF)
The endothelium plays an active role in regulating blood flow
Endothelium is a source of substances that elicit contraction or relaxation of the vascular smooth muscle
Vasoactive substances released from endothelium: Nitric Oxide (NO)
Endothelium-derived relaxing factor Prostacyclin Endothelin
Control of blood flow from endothelial cells
Originally called Endothelium-derived relaxing factor (EDRF), which is now identified as nitric oxide
Rapid flow of blood through the arteries and arterioles causes significant increase in the release of nitric oxide
The nitric oxide relaxes the blood vessels
L-Arginine is converted to NO by the enzyme nitric oxide synthase (NOS)
Prostacyclin
Prostacyclin (PGI2)
Strong vasodilator
Inhibits platelet adhesion to the vascular endothelium
Endothelin
Potent vasoconstrictor
Increased aldosterone secretion
Decreased renal blood flow and GFR
When Damage to Endothelium Occurs
Damage to endothelial cells will lead to: Decreased Nitric Oxide and Prostacyclin
production Increased Endothelin production
This will lead to: Vasoconstriction Vasospasm Thrombosis