Blood Flow and Blood Pressure Control Notes

Anatomy of Blood Vessels

• Arteries:

  • Carry blood away from the heart.

  • Generally carry oxygenated blood (exception: pulmonary arteries).

  • Have thick walls with a significant layer of vascular smooth muscle.

  • Act as a pressure reservoir.

• Veins:

  • Carry blood toward the heart.

  • Generally carry deoxygenated blood (exception: pulmonary veins).

  • Have thinner walls of vascular smooth muscle compared to arteries.

  • Have a larger lumen than arteries.

  • Operate at lower pressure than arteries.

  • Act as a volume reservoir; blood pools in veins when they are dilated, leading to a loss of circulatory pressure (relevant in cases like anaphylactic shock).

Cardiovascular System Flow

• Deoxygenated blood enters the right atrium, then the right ventricle via the tricuspid valve.

• Blood flows into the pulmonary arteries for oxygenation in the lungs.

• Oxygenated blood returns to the left atrium via the pulmonary veins.

• Blood moves from the left atrium to the left ventricle through the mitral valve.

  • Both mitral and tricuspid valves have chordae tendineae to prevent backflow during ventricular contraction.

• Blood is pumped from the left ventricle into the aorta.

  • The aorta, a large and elastic vessel, helps to maintain pressure when the heart is resting.

• Blood flows into arterioles, which regulate blood flow into capillary beds and venules.

• Exchange of nutrients and gases occurs primarily at the capillaries, but some exchange can occur at the venules as well.

• Blood returns to the right atria via the venules, veins, and the superior and inferior vena cava.

Layers of Blood Vessels

• Like other luminal organs, blood vessels consist of three layers:

  • The innermost layer performs the primary function.

  • The middle layer, usually smooth muscle, controls the organ's function.

  • The outermost layer is fibrous connective tissue for protection and support.

• Tonica Intima (Endothelium):

  • The innermost layer; consists of smooth endothelium which repels platelets to prevent clot formation in healthy vessels.

  • Produces nitric oxide (a vasodilator) and prostacyclines (which also repel platelets).

• Tonica Media (Smooth Muscle Layer):

  • The middle layer; controls the size of the blood vessel and the pressure within it.

• Tonica Externa (Fibrous Connective Tissue):

  • The outermost layer; provides support to the blood vessel.

• Differences between Arteries and Veins:

  • The smooth muscle layer (tonica media) is much thinner in veins compared to arteries.

  • Arteries have higher blood pressure, smaller lumen, and a higher muscle tone compared to veins.

  • Veins have larger lumens, allowing them to hold a higher volume of blood, which contributes to the pooling of blood when they dilate.

• Layer Reduction as Vessels Descend:

  • Arteries have 3 layers

  • Arterioles lose the tonica externa, leaving only two layers: smooth endothelium and a thinner muscular layer.

  • Capillaries consist of a single layer - a thin squamous epithelium - to facilitate exchange.

  • Venules regain the fibrous layer (tonica externa) but lack the muscular layer (tonica media), allowing for some exchange.

  • Veins regain the same three-layer structure as arteries, but with a thinner smooth muscle layer.

Meta-Arterioles

• Meta-arterioles are vessels between arterioles and venules.

• Anatomical Structure:

  • Have an intermittent layer of smooth muscle; not a continuous layer like in arterioles.

  • Parts of the meta-arterioles resemble capillaries with only an endothelial lining; other parts have both the endothelial lining and smooth muscle, but not as a continuous layer.

• Functionality:

  • Meta-arterioles provide a direct route for blood to flow from arterioles to venules, bypassing the capillary bed.

  • This functions as a detour route, similar to truck routes around cities, to prevent large blood cells (e.g., white blood cells) from entering the capillary bed when not needed for exchange.

  • Valves at the entrance of the capillary bed control blood flow; when blood is not needed in the tissue, it can be detoured through the meta-arterioles directly into the venules.

Angiogenesis

• Angiogenesis is the formation of new blood vessels.

• Why is it Needed?

  • For supply and demand.

  • Occurs when there is an increased demand for blood supply, such as:

    • During growth and development to supply new tissues.

    • During wound healing to support the formation of new cells.

    • To enhance heart and skeletal muscle flow, especially during exercise.

  • Also occurs in malignancy when tumors grow rapidly and require more blood supply.

• Exercise and Angiogenesis:

  • Exercise increases demand on the heart, prompting the body to create new blood vessels (angiogenesis) as collateral circulation.

  • These additional blood vessels can provide alternate routes for blood flow if a clot or blockage occurs, reducing the severity of myocardial infarction (MI).

• Regulation of Angiogenesis:

  • Controlled by cytokines.

  • Promoted by mitogens that enhance the growth of vascular endothelium, such as:

    • Vascular Endothelial Growth Factors (VEGF)

    • Fibroblast Growth Factors (FGF)

  • Inhibited by:

    • Angiostatin (angio- = blood vessels, -statin = to stop or inhibit)

    • Endostatin (endo- = endothelial layer, -statin = to stop or inhibit)

• Coronary heart diseases can be prevented due to existing collateral circulation.

Blood Pressure

• Example Blood Pressure Reading: 120/80

  • 120 = Systolic Blood Pressure

    • Pressure exerted during ventricular contraction; primarily reflects left ventricular contraction pushing blood into the arteries.

  • 80 = Diastolic Blood Pressure

    • Pressure during ventricular relaxation.

• Pulse Pressure:

  • The difference between systolic and diastolic pressure.

  • Pulse Pressure = Systolic - Diastolic

  • In the example above, the pulse pressure is 120 - 80 = 40.

  • Valves ensure one-way flow in the veins, preventing backflow.

• MAP (Mean Arterial Pressure):

  • Represents the mean arterial pressure.

  • Is the driving force for blood flow.

  • MAP = Diastolic Pressure + 13(Pulse Pressure)

  • Using the example of 120/80, MAP = 80 + 13(40) = 93.3.

  • The MAP value should fall between the systolic and diastolic values.

• Blood Pressure Measurement:

  • Measured using a sphygmomanometer (blood pressure cuff).

  • Hypertension is a MAP level that is higher than normal, and vise versa for Hypotension.

• Understanding Systolic and Diastolic Pressure:

  • Systolic pressure (e.g., 120) is generated by ventricular contraction.

  • Diastolic pressure occurs when ventricles relax, and blood tends to flow backward.

  • Aortic and pulmonary valves prevent backflow into the ventricles.

  • The positive diastolic pressure (e.g., 80) is maintained by the elasticity of blood vessels (aorta and arteries).

  • Elastic blood vessels recoil after being stretched by ventricular contraction, which maintains pressure on the blood and pumps it forward.

  • Healthy, elastic blood vessels reduce the effort required by the heart to maintain blood pressure.

• Clinical Implications:

  • High systolic blood pressure may indicate high heart pumping pressure.

  • High diastolic blood pressure may indicate hardening of the blood vessels (atherosclerosis).

  • Systolic blood pressure is considered a physiological measure, changing with physiological demands.

  • Diastolic blood pressure is considered a pathological measure, often elevated due to underlying pathologies.

Factors Affecting Blood Flow

• Flow, Pressure, and Resistance:

  • Analogy: Milkshake in a container with a straw.

  • High resistance (small straw) results in low flow.

  • High pressure difference leads to better flow.

• Factors Controlling Resistance:

  • Radius of the blood vessel - the most frequently changing factor

  • Length of the blood vessel

  • Viscosity of the blood

• Flow vs. Velocity:

  • Flow is measured in liters or milliliters per minute (L/min or mL/min).

  • Velocity is measured in centimeters or millimeters per minute (cm/min or mm/min) and is associated with pressure; higher pressure results in higher velocity, but not necessarily higher flow.

• Primary Determinant of Velocity:

  • Total cross-sectional area of the blood vessel.

  • Pulse Pressure = Systolic - Diastolic

  • MAP = Diastolic Pressure + 13(Pulse Pressure)

Measuring Blood Pressure with a Sphygmomanometer

• The pressure in the cuff exceeds the pressure in the blood vessel.

• Blood vessel is completely closed so there is no flow and no sound.

• The blood vessel begins to open a bit, resulting in blood flowing through a tiny opening creating a turbulent flow.

• The turbulent flow is a very loud flow - Systolic

• Once the cuff pressure is lowered further, the blood vessel opens completely, and the flow returns to a smoother flow - Diastolic

Anatomy of Blood Vessels

• Arteries:

  • Carry blood away from the heart.

  • Generally carry oxygenated blood (exception: pulmonary arteries, which carry deoxygenated blood to the lungs for oxygenation).

  • Have thick walls composed of three layers: tunica intima, tunica media (significant layer of vascular smooth muscle), and tunica adventitia (externa).

  • The thick walls and elastic fibers enable arteries to withstand high pressure and maintain blood flow.

  • Act as a pressure reservoir, expanding during systole and recoiling during diastole to ensure continuous blood flow.

• Veins:

  • Carry blood toward the heart.

  • Generally carry deoxygenated blood (exception: pulmonary veins, which carry oxygenated blood from the lungs to the left atrium).

  • Have thinner walls with less vascular smooth muscle compared to arteries, making them more compliant.

  • Have a larger lumen than arteries, reducing resistance to blood flow.

  • Operate at lower pressure than arteries; often contain valves to prevent backflow of blood, especially in the limbs.

  • Act as a volume reservoir; blood pools in veins when they are dilated, leading to a loss of circulatory pressure (relevant in cases like anaphylactic shock and prolonged standing).

Cardiovascular System Flow

• Deoxygenated blood enters the right atrium, then the right ventricle via the tricuspid valve (also known as the right atrioventricular valve).

• Blood flows into the pulmonary arteries for oxygenation in the lungs; the pulmonary arteries are the only arteries that carry deoxygenated blood.

• Oxygenated blood returns to the left atrium via the pulmonary veins; the pulmonary veins are the only veins that carry oxygenated blood.

• Blood moves from the left atrium to the left ventricle through the mitral valve (also known as the bicuspid valve or left atrioventricular valve).

  • Both mitral and tricuspid valves have chordae tendineae (tendinous cords) and papillary muscles to prevent backflow during ventricular contraction, ensuring unidirectional blood flow.

• Blood is pumped from the left ventricle into the aorta through the aortic valve.

  • The aorta, a large and elastic vessel, helps to maintain pressure when the heart is resting (during diastole) due to its elastic recoil.

• Blood flows into arterioles, which regulate blood flow into capillary beds and venules.

• Exchange of nutrients and gases (oxygen, carbon dioxide) occurs primarily at the capillaries, but some exchange can occur at the venules as well.

• Blood returns to the right atria via the venules, veins, and the superior and inferior vena cava.

  • The superior vena cava drains blood from the upper body, while the inferior vena cava drains blood from the lower body.

Layers of Blood Vessels

• Like other luminal organs, blood vessels consist of three layers:

  • The innermost layer (tunica intima) performs the primary function of direct interaction with blood.

  • The middle layer (tunica media), usually smooth muscle, controls the organ's diameter and thus blood pressure and flow.

  • The outermost layer (tunica externa or adventitia) is fibrous connective tissue for protection and support, containing collagen and elastic fibers.

• Tonica Intima (Endothelium):

  • The innermost layer; consists of a single layer of smooth endothelium which repels platelets to prevent clot formation in healthy vessels.

  • Produces nitric oxide (a vasodilator) and prostacyclins (which also repel platelets), maintaining vessel patency and preventing thrombosis.

• Tonica Media (Smooth Muscle Layer):

  • The middle layer; composed of smooth muscle and elastic fibers, controls the size of the blood vessel and the pressure within it through vasoconstriction and vasodilation.

• Tonica Externa (Fibrous Connective Tissue):

  • The outermost layer; provides support to the blood vessel, anchors it to surrounding tissues, and contains nerves and vasa vasorum (small blood vessels that supply blood to the walls of larger vessels).

• Differences between Arteries and Veins:

  • The smooth muscle layer (tonica media) is much thinner in veins compared to arteries, resulting in lower blood pressure and greater compliance.

  • Arteries have higher blood pressure, smaller lumen, and a higher muscle tone compared to veins, enabling them to regulate blood flow more effectively.

  • Veins have larger lumens, allowing them to hold a higher volume of blood, which contributes to the pooling of blood when they dilate. Valves in veins prevent backflow, especially in the limbs.

• Layer Reduction as Vessels Descend:

  • Arteries have 3 layers

  • Arterioles lose the tonica externa, leaving only two layers: smooth endothelium and a thinner muscular layer, allowing for fine-tuned regulation of blood flow.

  • Capillaries consist of a single layer - a thin squamous epithelium - to facilitate exchange of gases, nutrients, and waste products between blood and tissues.

  • Venules regain the fibrous layer (tonica externa) but lack the muscular layer (tonica media), allowing for some exchange.

  • Veins regain the same three-layer structure as arteries, but with a thinner smooth muscle layer.

Meta-Arterioles

• Meta-arterioles are vessels between arterioles and venules, serving as key regulators of blood flow into capillary beds.

• Anatomical Structure:

  • Have an intermittent layer of smooth muscle; not a continuous layer like in arterioles, allowing for localized control of blood flow.

  • Parts of the meta-arterioles resemble capillaries with only an endothelial lining; other parts have both the endothelial lining and smooth muscle, but not as a continuous layer.

• Functionality:

  • Meta-arterioles provide a direct route for blood to flow from arterioles to venules, bypassing the capillary bed, when tissue perfusion is not required.

  • This functions as a detour route, similar to truck routes around cities, to prevent large blood cells (e.g., white blood cells) from entering the capillary bed when not needed for exchange.

  • Valves at the entrance of the capillary bed (precapillary sphincters) control blood flow; when blood is not needed in the tissue, it can be detoured through the meta-arterioles directly into the venules.

Angiogenesis

• Angiogenesis is the formation of new blood vessels from pre-existing vessels.

• Why is it Needed?

  • For supply and demand.

  • Occurs when there is an increased demand for blood supply, such as:

    • During growth and development to supply new tissues.

    • During wound healing to support the formation of new cells and tissue repair.

    • To enhance heart and skeletal muscle flow, especially during exercise.

  • Also occurs in malignancy when tumors grow rapidly and require more blood supply, supporting tumor growth and metastasis.

• Exercise and Angiogenesis:

  • Exercise increases demand on the heart, prompting the body to create new blood vessels (angiogenesis) as collateral circulation.

  • These additional blood vessels can provide alternate routes for blood flow if a clot or blockage occurs, reducing the severity of myocardial infarction (MI).

• Regulation of Angiogenesis:

  • Controlled by cytokines, growth factors, and inhibitors.

  • Promoted by mitogens that enhance the growth of vascular endothelium, such as:

    • Vascular Endothelial Growth Factors (VEGF): stimulate endothelial cell proliferation, migration, and survival.

    • Fibroblast Growth Factors (FGF): promote endothelial cell growth and angiogenesis.

  • Inhibited by:

    • Angiostatin (angio- = blood vessels, -statin = to stop or inhibit): inhibits endothelial cell proliferation and migration.

    • Endostatin (endo- = endothelial layer, -statin = to stop or inhibit): inhibits endothelial cell proliferation and migration.

• Coronary heart diseases can be prevented due to existing collateral circulation.

Blood Pressure

• Example Blood Pressure Reading: 120/80

  • 120 = Systolic Blood Pressure

    • Pressure exerted during ventricular contraction (systole); primarily reflects left ventricular contraction pushing blood into the arteries.

  • 80 = Diastolic Blood Pressure

    • Pressure during ventricular relaxation (diastole).

• Pulse Pressure:

  • The difference between systolic and diastolic pressure.

  • Pulse Pressure = Systolic - Diastolic

  • In the example above, the pulse pressure is 120 - 80 = 40.

  • Valves ensure one-way flow in the veins, preventing backflow.

• MAP (Mean Arterial Pressure):

  • Represents the mean arterial pressure.

  • Is the driving force for blood flow to the organs and tissues.

  • MAP = Diastolic Pressure + \frac{1}{3}(Pulse Pressure)

  • Using the example of 120/80, MAP = 80 + \frac{1}{3}(40) = 93.3.

  • The MAP value should fall between the systolic and diastolic values.

• Blood Pressure Measurement:

  • Measured using a sphygmomanometer (blood pressure cuff) and stethoscope.

  • Hypertension is a MAP level that is higher than normal, and vise versa for Hypotension.

• Understanding Systolic and Diastolic Pressure:

  • Systolic pressure (e.g., 120) is generated by ventricular contraction, reflecting the force of blood against arterial walls.

  • Diastolic pressure occurs when ventricles relax, and blood tends to flow backward.

  • Aortic and pulmonary valves prevent backflow into the ventricles, maintaining pressure during diastole.

  • The positive diastolic pressure (e.g., 80) is maintained by the elasticity of blood vessels (aorta and arteries).

  • Elastic blood vessels recoil after being stretched by ventricular contraction, which maintains pressure on the blood and pumps it forward, ensuring continuous blood flow.

  • Healthy, elastic blood vessels reduce the effort required by the heart to maintain blood pressure.

• Clinical Implications:

  • High systolic blood pressure may indicate high heart pumping pressure or increased arterial stiffness.

  • High diastolic blood pressure may indicate hardening of the blood vessels (atherosclerosis) or increased peripheral resistance.

  • Systolic blood pressure is considered a physiological measure, changing with physiological demands such as exercise or stress.

  • Diastolic blood pressure is considered a pathological measure, often elevated due to underlying pathologies such as kidney disease or hormonal imbalances.

Factors Affecting Blood Flow

• Flow, Pressure, and Resistance:

  • Analogy: Milkshake in a container with a straw.

  • High resistance (small straw) results in low flow.

  • High pressure difference leads to better flow.

• Factors Controlling Resistance:

  • Radius of the blood vessel - the most frequently changing factor due to vasoconstriction and vasodilation.

  • Length of the blood vessel - relatively constant but can change with growth and angiogenesis.

  • Viscosity of the blood - affected by hematocrit and plasma protein concentration.

• Flow vs. Velocity:

  • Flow is measured in liters or milliliters per minute (L/min or mL/min), representing the volume of blood passing a point per unit time.

  • Velocity is measured in centimeters or millimeters per minute (cm/min or mm/min) and is associated with pressure; higher pressure results in higher velocity, but not necessarily higher flow.

• Primary Determinant of Velocity:

  • Total cross-sectional area of the blood vessel. Velocity is inversely proportional to the total cross-sectional area. As the total cross-sectional area increases (e.g., in capillaries), the velocity decreases.

  • Pulse Pressure = Systolic - Diastolic

  • MAP = Diastolic Pressure + \frac{1}{3}(Pulse Pressure)

Measuring Blood Pressure with a Sphygmomanometer

• The pressure in the cuff exceeds the pressure in the blood vessel, occluding it completely.

• Blood vessel is completely closed so there is no flow and no sound.

• The blood vessel begins to open a bit as the cuff pressure is gradually released, resulting in blood flowing through a tiny opening creating a turbulent flow.

• The turbulent flow is a very loud flow - Systolic pressure, which is the first Korotkoff sound heard.

• Once the cuff pressure is lowered further, the blood vessel opens completely, and the flow returns to a smoother, laminar flow - Diastolic pressure, which is the point at which the Korotkoff sounds disappear.