Blood Pressure and Blood Composition

Blood Pressure

Mean arterial blood pressure is determined by two factors:

  • Cardiac output: The amount of blood pumped into the arteries.
  • Total peripheral resistance: The resistance to blood flow in the arteries.

Factors Affecting Resistance

Resistance is primarily influenced by arterioles, which can change their radius due to intrinsic or extrinsic factors.

Cardiac Output

Cardiac output is determined by heart rate and stroke volume.

  • Heart rate: Can be altered by parasympathetic or sympathetic nervous system activity.
  • Stroke volume: Impacted by the nervous system and venous return. Venous return is influenced by various factors.

Blood Viscosity

Blood viscosity is how thick the blood is. Higher viscosity increases resistance. It is determined by:

  • Hematocrit: The ratio of blood cells to plasma. More blood cells increase viscosity, while more plasma decreases it.

Baroreceptors

Baroreceptors are receptors that monitor blood pressure and are located in:

  • Aorta: Provides pressure readings directly from the heart.
  • Carotid artery: Ensures the brain receives adequate blood supply. The brain prioritizes its blood supply to maintain function and avoid vessel rupture due to high pressure.

What baroreceptors measure:

  • Mean blood pressure.
  • Pulse pressure: The difference between systolic and diastolic pressure.

Frequency of Action Potentials:

  • High blood pressure: Leads to a higher frequency of action potentials.
  • Low blood pressure: Results in a lower frequency of action potentials.

Negative Feedback Loop

The purpose of having receptors is to maintain homeostasis through negative feedback loops by adjusting blood pressure.

Parasympathetic vs. Sympathetic Stimulation

  • Parasympathetic stimulation: Primarily affects the heart via the vagus nerve, reducing heart rate, cardiac output, and blood pressure.
  • Sympathetic stimulation: Affects the heart, arterioles, and veins. It increases heart rate and stroke volume, leading to increased cardiac output and blood pressure. It also vasoconstricts arterioles to increase resistance and veins to increase venous return.

The sympathetic system is more complex because it is associated with survival responses like freeze, flee, fight, or fright.

Sympathetic Effects on Heart

  • Can increase heart rate.
  • Can increase the inotropic state of the heart which increases stroke volume.

Blood Pressure Regulation

When blood pressure is above normal:

  1. Carotid and aortic baroreceptors detect the increase and fire faster.
  2. The cardiovascular center in the brain decreases sympathetic activity and increases parasympathetic activity.
    • The result is Heart rate and stroke volume decrease.
    • Arterioles dilate, reducing total peripheral resistance.

When blood pressure falls below normal:

  1. Baroreceptors fire slower.
  2. The cardiovascular center activates the sympathetic and deactivates the parasympathetic nervous system.
    • The result is increased heart rate and vasoconstriction in veins and arterioles.
    • Increased stroke volume due to increased venous return and inotropic state of the heart.

The cardiovascular center is also involved in exercise responses. Exercise can help alleviate anxiety by aligning the physiological response with the perceived threat.

Hypertension

Hypertension (high blood pressure) can cause significant damage to the body. It is primarily detected using a blood pressure cuff. Consequences include lung damage, stroke, blood clots, and retinal damage leading to blindness.

  • Primary hypertension: The cause is unknown and requires direct treatment of the high blood pressure.
  • Secondary hypertension: The underlying cause is identified and treated to resolve the high blood pressure.

Treatment often involves reducing salt intake to decrease water retention and blood volume, which lowers pressure. Sugar also attracts water but is less impactful than salt.

Hypotension

Hypotension (low blood pressure) has two main types:

  • Orthostatic hypotension: Related to body position. Occurs when transitioning from a horizontal to a vertical position. The cardiovascular system may not adjust quickly enough to counteract gravity, leading to insufficient blood flow to the brain.

    • Countermeasures: People are advised to sit up slowly. Tilt tables are used to gradually acclimate patients in a coma to vertical positions.

  • Circulatory shock: A more dangerous condition.

    • Cardiogenic shock: Results from a weakened heart that cannot pump enough blood, leading to decreased stroke volume, cardiac output, and blood pressure.
    • Neurogenic shock: Triggered by extreme pain or fright, leading to the shutdown of the sympathetic nervous system and widespread vasodilation.
    • Vasogenic shock: Related to blood vessels. Septic shock is caused by an infection that releases vasodilators and anaphylactic shock results from a widespread histamine release, both leading to widespread vasodilation.
    • Hypovolemic shock: Caused by reduced blood volume due to severe hemorrhage or loss of fluids through urination, diarrhea, or vomiting.

Compensatory Reactions to Low Blood Volume

  • Thirst increases to encourage fluid intake.
  • Vasopressin and renin-angiotensin-aldosterone are released to decrease urine output.
  • The cardiovascular system activates the sympathetic and deactivates the parasympathetic nervous systems to increase heart rate.

Hematocrit

A hematocrit is a blood test that measures the proportions of blood components after centrifugation. The components are:

  1. Plasma (top layer, lightest).
  2. Buffy coat (platelets and white blood cells).
  3. Red blood cells (bottom layer, heaviest).

Normal hematocrit is 55% plasma and 45% red blood cells.

Plasma

Plasma is mainly water that maintains body temperature, and helps to dump excess heat and transport hydrophilic substances or substances with binding agents. It also contains:

  • Electrolytes: Such as ions.

  • Nutrients and waste: For delivery and pickup by the cardiovascular system.

  • Plasma proteins: Which helps with ultrafiltration and reabsorption bulk flow of water.

    • Albumin: Large proteins contributing to colloidal osmotic pressure.
    • Globulins: Important for the immune system.
    • Fibrinogen: Aids in blood clotting.

Cellular Elements

  • Erythrocytes: Red blood cells involved in oxygen transport.
  • Leukocytes: White blood cells involved in immune responses.
  • Platelets: Cell fragments from megakaryocytes that help with blood clotting.

Erythrocytes

Erythrocytes (red blood cells) are essential for gas exchange. They transport oxygen bound to hemoglobin molecules. Erythrocytes are produced in bone marrow. Production is triggered by erythropoietin released from the kidney in response to low blood oxygen levels. Mature erythrocytes lack organelles and have a lifespan of about ninety days.

Athletes and Erythrocytes:

  • Endurance athletes often train at high altitudes to stimulate erythropoiesis and increase red blood cell count.
  • Blood doping includes blood transfusions and shots of artificial erythropoietin, which is dangerous because they increase blood viscosity.

Anemia

Anemia is the deficiency of red blood cells, resulting in a low hematocrit. It can be caused by iron deficiency (treatable with iron supplements or cooking on cast iron). Reticulocytes (immature red blood cells) in the blood indicate recent blood loss.

Polycythemia

Polycythemia is an excess of red blood cells, resulting in more viscous blood. It can be caused by altitude training or blood doping. Dehydration can cause relative polycythemia due to decreased plasma volume.

Blood Cell Production

Stem cells in bone marrow differentiate into erythrocyte precursors, white blood cells (granulocytes and monocytes), lymphoid stem cells (T cells), and megakaryocytes (platelet producers).

White Blood Cells

White blood cells (leukocytes) are divided into polymorphonuclear granulocytes (multiple nuclei with granules) and mononuclear agranulocytes (single nucleus without granules).

Neutrophils

Neutrophils are part of the innate, nonspecific immune system. They phagocytize pathogens and produce neutrophil extracellular traps (NETs).

Eosinophils

Eosinophils fight parasitic worms and are involved in autoimmune issues. Helminthian therapy involves introducing good worms to calm eosinophils.

Basophils

Basophils release heparin and histamine to increase blood flow to the area for immune response.

Monocytes

Monocytes become macrophages and help clean up infections and injuries though they contribute to atherosclerosis.

Lymphocytes

Lymphocytes are part of the acquired adaptive immune system (the T cells and B cells) . They learn to recognize specific invaders. The invader has to be introduced either by:

  • A Vaccine either dead or attenuated.
  • Wild type which puts the person in contact with the live virus.
    breastfeeding is important for children. Breastfeeding is important for the children's acquired adaptive immune system to pass on antibodies to the child for the first two years of their life.