Integrative Physiology: Hemodynamics I & II

Integrative Physiology Notes

Key Concepts in Hemodynamics
Stroke Volume

  • Definition: The volume of blood pumped from the left ventricle of the heart with each beat.

Key Terms and Definitions

  • End-Diastolic Ventricular Volume: The amount of blood in a ventricle at the end of diastole.

  • Mean Systemic Arterial Pressure: The average arterial pressure during one cardiac cycle.

  • Cardiac Output: The volume of blood the heart pumps per minute, calculated as:
    extCardiacOutput=extStrokeVolumeimesextHeartRateext{Cardiac Output} = ext{Stroke Volume} imes ext{Heart Rate}

  • Total Peripheral Resistance: The resistance to blood flow offered by all of the systemic vasculature.

  • Activity of Sympathetic Nerves to the Heart: Increases heart rate and contractility.

  • Activity of Parasympathetic Nerves to the Heart: Decreases heart rate.

  • Plasma Epinephrine: Hormone that increases heart rate and force of contraction.

  • Atrial Pressure: Pressure in the atria of the heart during different phases of the cardiac cycle.

  • Venous Return: The flow of blood back to the heart.

  • Intrathoracic Pressure: Pressure within the thoracic cavity which affects venous return.

  • Skeletal Muscle 'Pump': Mechanism aiding venous return during muscle contraction.

  • Blood Viscosity: Thickness and stickiness of blood which can also affect circulation.

  • Sympathetic Vasodilator and Vasoconstrictor Nerves: Different types of sympathetic nerves affecting arterial diameter.

  • Local Controls: Factors that alter blood flow locally, such as metabolites and injury agents.

  • Hematocrit: The volume percentage of red blood cells in blood.

Blood Vessel Arrangement and Morphology

  • Overview of blood vessel types: Arteries, arterioles, capillaries, venules, and veins.

  • Comparison between types:

    • Arteries:

    • Diameter: 4.0 mm (Elastic arteries)

    • Provides high-pressure blood flow from the heart.

    • Arterioles:

    • Diameter: 30.0 µm

    • Regulate blood flow into capillary beds.

    • Capillaries:

    • Diameter: 8.0 µm

    • Site of exchange between blood and tissues.

    • Venules and Veins:

    • Venules: 20.0 µm diameter

    • Veins: 5.0 mm diameter, return blood to the heart with lower pressure than arteries.

Capillary Function

  • Capillaries lack smooth muscle and elastic tissue, facilitating material exchange with tissues.

  • Metarterioles: Regulate blood flow into capillary beds and can act as bypass channels.

  • Exchange at Capillaries:

    • Bulk Flow: Governed by hydrostatic and osmotic pressure gradients.

    • Filtration: Occurs at the arterial end due to higher hydrostatic pressure.

    • Absorption: Occurs at the venous end when osmotic pressure pulls fluid back into the capillaries.

Blood Pressure Regulation

  • Mean Arterial Pressure (MAP): Calculated as:
    extMAP=extDiastolicPressure+rac13(extSystolicPressure−extDiastolicPressure)ext{MAP} = ext{Diastolic Pressure} + rac{1}{3}( ext{Systolic Pressure} - ext{Diastolic Pressure})

  • MAP is influenced by:

    • Cardiac Output

    • Total Peripheral Resistance

  • Regulation includes both rapid cardiovascular responses and slower kidney mechanisms.

Factors Influencing Mean Arterial Pressure

  • Blood Volume

  • Heart Effectiveness: How well the heart pumps (Cardiac Output)

  • Resistance to Blood Flow: Primarily influenced by arteriolar diameter and fluid dynamics.

Baroreceptor Reflex

  • Involved in short-term regulation of blood pressure.

  • Senses stretch in arteries:

    • Decreased blood pressure leads to less stretch and lower action potential frequency in baroreceptors, increasing sympathetic activity.

    • Increased blood pressure causes increased baroreceptor firing leading to decreased sympathetic output.

Hormonal Regulation of Blood Pressure

  • Renin–Angiotensin–Aldosterone System (RAAS): Increases blood pressure.

  • Epinephrine and Norepinephrine: Also increase blood pressure through vasoconstriction and increased heart rate.

  • Antidiuretic Hormone (ADH): Increases blood volume by promoting water retention.

  • Atrial Natriuretic Peptide (ANP): Reduces blood pressure by promoting sodium and water excretion.

Blood Distribution in the Body

  • At rest, blood distribution varies significantly among organs:

    • Brain: 0.7 L/min

    • Heart: 0.2 L/min

    • Liver & Digestive Tract: 1.35 L/min

    • Kidneys: 1.0 L/min

    • Skeletal Muscle: 1.05 L/min

    • Skin: 0.25 L/min

Cardiovascular Disease (CVD): Risk Factors

  • Uncontrollable: Age, sex, family history.

  • Controllable: Smoking, obesity, sedentary lifestyle, untreated hypertension.

  • Blood Lipids: Relation to atherosclerosis and different lipoprotein levels (LDL and HDL).

  • Hypertension: Risk increases with each incremental increase in blood pressure.

Clinical Tests for Blood Health

  • Hematocrit: Ratio of red cells to plasma volume. Standard ranges:

    • Males: 40% - 54%

    • Females: 37% - 47%

  • Red cell count, white cell count, differential white cell count are also important metrics.

Hematopoiesis (Blood Cell Formation)

  • Process occurs in bone marrow, regulated by cytokines.

  • Three main blood cell types:

    • Erythrocytes (Red Blood Cells): Transport oxygen; live approximately 120 days.

    • Thrombocytes (Platelets): Involved in clotting, derive from megakaryocytes.

    • Leukocytes (White Blood Cells): Immune system function (including lymphocytes, monocytes, and granulocytes).

Coagulation Cascade and Blood Clotting

  • Coagulation Reaction Steps:

    1. Injury exposes tissue factors.

    2. Platelet adhesion and release of factors.

    3. Activation of clotting factors through intrinsic and extrinsic pathways.

    4. Fibrinogen converted to fibrin, forming a clot.

  • Fibrinolysis: Process that removes the clot after healing is complete, involving the conversion of plasminogen to plasmin.

Blood Types and Transfusion Considerations

  • Antigens on red blood cells define blood types (A, B, AB, O).

  • Blood transfusions require careful matching to prevent agglutination and hemolysis.

  • Rh factor significance, particularly in prenatal care to prevent hemolytic disease of the newborn.

Conclusion

  • Understanding hemodynamics is crucial for recognizing cardiovascular function, pathology, and treatment approaches in clinical settings.