Chapter 19

Blood Vessels and Circulation

  • Typically, there are three types of blood vessels:
    • Arteries
    • Capillaries
    • Veins
  • Blood pathways flow from arteries to arterioles (smaller arteries), then to capillaries, which surround tissues, and finally to venules and veins.
  • Arterioles branch into smaller arterioles, leading to thin capillaries.
  • Veins are often shown in blue.

Direction of Blood Flow

  • Arteries carry blood away from the heart.
  • Veins carry blood towards the heart.
  • Capillaries are located near tissues, facilitating the exchange of substances.

Structure of Arteries and Veins

  • Both arteries and veins have three layers:
    • Tunica interna (intima)
    • Tunica media
    • Tunica externa
Tunica Externa
  • Similar in both veins and arteries
  • Composed of connective tissues and collagen fibers for protection.
Tunica Media
  • Thicker in arteries compared to veins to withstand higher pressure.
  • Contains smooth muscles that enable dilation and constriction for blood flow regulation.
  • Some arteries are elastic arteries with elastic fibers to stretch and recoil, aiding blood flow.
  • Smooth muscles facilitate vasoconstriction and vasodilation.
Vasoconstriction and Vasodilation
  • Vasoconstriction reduces blood vessel diameter, decreasing flow and increasing pressure.
  • Vasodilation increases blood vessel diameter, increasing flow and decreasing pressure.
  • The fluctuation of pressure regulates blood flow to organs.
Tunica Interna
  • The innermost layer.
  • Veins have larger diameters than arteries.
  • Larger diameter reduces contact with the vessel wall, decreasing resistance to flow.
  • The opening through which blood passes is called the lumen.
  • Analogy: A five-lane freeway versus a one-lane freeway represents less and more traffic congestion, respectively.

Arterioles and Capillaries

  • Arterioles lead to capillary beds and can undergo vasodilation and vasoconstriction.
  • Capillaries allow only one red blood cell (RBC) to pass at a time, facilitating material exchange with tissues.
  • Red blood cells get a "close tap", allowing exchange of materials.
  • Examples: Carbon dioxide enters, oxygen exits; nutrients like glucose and amino acids are exchanged.
  • RBCs contain hemoglobin.
  • Capillaries consist of only one cell layer; they don't have the three layers like veins and arteries, so they cannot dilate or constrict.

Pressure and Fluid Exchange in Capillaries

  • High pressure in arteries and arterioles leads to capillaries, which are thinner and more porous.
  • Water is lost due to the nature of capillaries from high to low pressure areas (arteries to veins).
  • Smaller diameter in capillaries typically increases pressure, but water loss results in a pressure drop.
  • Some water enters the lymphatic system and returns to circulation.
  • As water exits, solutes like proteins remain, creating higher solute concentration inside, which pulls water back in via osmosis.
  • Pressure drops further due to larger diameter in veins.
  • Osmosis helps maintain pressure to push blood forward.
  • Lack of protein can cause edema (tissue swelling) because water stays in tissues instead of returning to the bloodstream.
  • Edema can be caused by liver problems due to reduced production of blood proteins.

Blood Flow Control in Capillaries

  • Pre-capillary sphincters at the junction of capillaries act as valves that open and close to control blood flow based on body needs.
  • When open, blood flows freely through capillaries; when constricted, blood is shunted to other areas via a vascular shunt.
  • In cold conditions, blood is shunted away from extremities to preserve core temperature; fingers may appear pale blue due to reduced blood flow.
  • Vasomotor nerves and local chemical conditions can bypass or flood capillary beds.
  • Blood enters venules, then smaller venules, and finally veins.
  • Venules have one or two layers of smooth muscle tonica media and can constrict and dilate but are less active than arteries due to low pressure.
  • Veins have a thicker tunica externa with collagen fibers and some elastic network.

Venous Return

  • Veins need special adaptations to return blood to the heart due to low pressure.

  • Features aiding venous return:

    • Larger diameter: reduces resistance to flow.
    • Valves: prevent backflow.
    • Respiratory pump
    • Muscular pump
Muscular Pump
  • Muscles surrounding veins contract, pressing on the veins and pushing blood toward the heart.
  • Valves prevent backflow during muscle contraction.
  • This is also called "milking the blood" toward the heart.
  • Important for people with varicose veins.
  • Varicose veins occur when valves are distended and cease to work properly, causing enlarged veins.
  • More common in females due to pregnancy or prolonged standing.
  • Strengthening leg muscles and wearing tight socks can help.
Respiratory Pump
  • Pressure changes during breathing help return blood to the heart by squeezing local veins.
  • Negative pressure builds up in the lungs during inhalation, aiding venous return.
  • Blood reaches the heart via the vena cava into the right atrium and passively flows to the ventricles.

Vascular Anastomosis

  • Merging of blood vessels, more common in veins than arteries.
  • Provides alternative pathways (collateral channels) for blood to reach tissues if one pathway is blocked.
  • Example: Forearm blood vessels merging allows blood to reach tissues even if one channel is blocked.
  • Not always present everywhere: Coronary artery blockage can lead to a heart attack due to lack of alternative pathways.

Blood Pressure

  • Force per unit area exerted on blood vessel walls; measured in millimeters of mercury (mmHg).
  • Differences in blood pressure drive blood flow from high to low pressure areas.
Factors Affecting Blood Pressure
*   Viscosity
*   Total blood vessel length
*   Blood vessel diameter
Viscosity
  • Higher viscosity (e.g., too many red blood cells or proteins) increases pressure because it requires more force to move the blood.
  • Example: Honey versus water in a syringe; honey requires more pressure to expel.
  • Aspirin can be used as a blood thinner to reduce viscosity and pressure.
  • Laminar flow: Blood flows freely in the middle of the vessel without much resistance; increased viscosity causes more contact with the vessel walls, increasing resistance.
Total Blood Vessel Length
  • Longer blood vessels increase pressure because blood comes into contact with the walls more often.
  • Adults typically have higher blood pressure than children due to greater blood vessel length; Blood pressure in a giraffe vs horse.
Blood Vessel Diameter
  • Smaller diameter increases pressure.
  • Example: Blocking a hose opening halfway increases water pressure.
  • Atherosclerosis (plaque formation) reduces blood vessel diameter, increasing pressure.
  • Turbulent blood flow can result. Blood no longer flows continuously.

Blood Pressure Changes Throughout Blood Vessels

  • Aorta has the highest pressure.
  • Arteries have fluctuating pressure due to elasticity and smooth muscles.
  • Pressure drops through arterioles and capillaries.
  • Capillaries have the lowest pressure (20-40 mmHg), which is necessary for substance exchange.
  • Pressure continues to drop through venules and veins.
  • Vena cava has nearly zero pressure.
Systolic and Diastolic Pressure
  • Systolic pressure: Highest pressure during ventricular contraction.
  • Diastolic pressure: Lowest pressure when ventricles are resting.
  • Pulse pressure: Difference between systolic and diastolic pressure (e.g., 120/80, pulse pressure is 40 mmHg).
  • Mean Arterial Pressure (MAP): Average pressure.
    • MAP=DiastolicPressure+(13∗PulsePressure)MAP = Diastolic Pressure + (\frac{1}{3} * Pulse Pressure)
    • MAP=80+(13∗40)≈93.3mmHgMAP = 80 + (\frac{1}{3} * 40) \approx 93.3 mmHg
  • MAP is closer to diastolic pressure because the heart spends more time in a relaxed filling state.
Capillary Pressure
  • Low capillary pressure (20-40 mmHg) is desirable because capillaries are fragile.
  • It slows down blood flow, allowing substances to get exchanged (nutrients, gases, hormones).
Factors Aiding Venous Return
  • Same as previous notes on venous return.

Regulation and Maintenance of Blood Pressure

  • BloodPressure(BP)=CardiacOutput(CO)∗PeripheralResistance(PR)Blood Pressure (BP) = Cardiac Output (CO) * Peripheral Resistance (PR)
    • Cardiac Output CO=HeartRate(HR)∗StrokeVolume(SV)CO = Heart Rate (HR) * Stroke Volume (SV)
  • Peripheral Resistance is affected by blood vessel length, diameter, and viscosity.
  • If blood vessel diameter decreases (e.g., lipid deposits), peripheral resistance increases, increasing blood pressure.
  • Cardiac output is affected by heart rate and stroke volume.
  • Factors affecting heart rate: thyroxine, sympathetic/parasympathetic activity, epinephrine, concentration of calcium, and potassium.
  • Factors affecting stroke volume: preload, contractility, and afterload.
  • Blood pressure can be affected by blood volume; low volume leads to low pressure, and high volume leads to high pressure.
  • Other factors influencing heart rate: age, gender, body temperature, exercises.
  • Example: During exercise - venous return is high, contractility is high, stroke volume increases, increasing cardiac output, which increases blood pressure.
Neural Control Mechanism
  • Maintaining MAP requires alteration of blood vessel diameter.
Vasomotor System
  • During sympathetic activity, vasoconstriction occurs; norepinephrine is a vasoconstrictor, increasing blood pressure.
  • Baroreceptors detect pressure differences.
  • Chemoreceptors detect carbon dioxide, pH, and oxygen levels.
  • When blood pressure rises, the cardioinhibitory center is stimulated to increase blood vessel diameter and lower heart rate.
Beta Receptor Activity
  • Baroreceptors in carotid arteries and aortic arch detect rising blood pressure.
  • Send impulses to the brain.
  • Cause cardioinhibitory centers to lower heart rate, and decreases in sympathetic activity.
  • Results in less contractility and vasodilation.
  • Lowers peripheral resistance and blood pressure.
  • If blood pressure declines, beta receptors detect less force.
  • Vasomotor centers are stimulated to release more vasoconstrictive chemicals.
  • Stimulates sympathetic activity, increasing heart rate and contractility, increasing cardiac output and blood pressure.
Monitoring Circulatory Efficiency

  • By taking pulse blood pressure we want to make sure everything is normal.

  • Take vital signs & look for normal temperature.

  • Pulse: Taking regular rates is valuable.

  • Sphagnumometer: is the device used to take blood pressure, requires some practice & stethoscope.

  • Blood pressure measurements should be done over a period of time (24 hours).

  • The left arm the blood should is closers to the heart, and more accurate.

  • Extrinsic factors affecting BP: age, sex, weight, race, mood, posture, physical activity.

Abnormalities of Blood flow & Pressure
  • Hypotension: Too low of blood pressure.
  • Hypertension: To elevated blood pressure.
Blood Velocity
  • Blood velocity is inversely proportional to the cross-sectional area.
  • Capillaries have the highest cross-sectional area and the lowest velocity, facilitating substance exchange.
Autoregulation
  • Blood vessels automatically adjust to meet requirements.
  • *Dilation during exercise. 🩸
  • Constriction flow and resting.

Circulatory Pathway

  • Pulmonary circulation: The blood travels from the hearts to the lungs & back to the heart.
  • Systemic circulation: The heart sends blood to the tissues of the body, and the tissue send the blood back to the heart.
  • The right side of the heart carries oxygen poor blood, as to the left side that is rich with with oxygen.