Blood vessels

Blood Circulation Basics

Blood vessels, including arteries, veins, and capillaries, transport blood throughout the entire body. Blood performs several crucial functions:

  • Delivers oxygen and nutrients to tissues.

  • Removes waste products.

  • Carries hormones to various parts of the body.

The body maintains a balanced blood composition, despite constant changes and circulation.

Capillary Exchange

Capillaries are the primary sites for the exchange of substances between blood and tissues.

  • Substances exchanged include:

    • Oxygen (O2) and Carbon Dioxide (CO2).

    • Nutrients, waste products, and lipids.

  • Mechanisms of transport across capillary walls:

    • Lipid-soluble substances (e.g., O_2) move directly through the capillary cell walls.

    • Water-soluble substances (e.g., glucose) pass through pores in the capillary walls.

    • Proteins typically remain inside the capillaries, sometimes transported via vesicles.

Where Blood Goes

Blood distribution varies depending on organ function and activity level:

  • Organs involved in blood "cleaning" or processing often receive more blood than their metabolic needs:

    • Digestive organs absorb nutrients.

    • Skin aids in heat regulation.

  • Muscles receive blood based on their activity levels.

  • The brain always receives a steady and critical supply of blood.

  • All cells constantly need blood to receive O2 and remove CO2.

Blood Flow

Flow rate is the volume of blood moving through a vessel per second. It is determined by two main factors:

  1. Pressure difference (\Delta P): Blood flows from areas of high pressure to areas of low pressure.

  2. Resistance (R): The opposition to blood flow through a vessel.

  • Formula: Flow = \frac{\Delta P}{R}

  • Sympathetic activation generally decreases blood flow rate by increasing resistance or altering pressure.

Pressure and Resistance

  • A larger pressure difference leads to increased flow.

  • Higher resistance impedes flow.

  • Key factors influencing resistance:

    • Blood Thickness (Viscosity): Thicker blood increases resistance.

    • Vessel Length: Longer vessels offer more resistance (though usually constant in the body).

    • Vessel Radius (Most important):

    • Smaller radius: significantly increases resistance.

    • Larger radius: significantly decreases resistance.

Types of Blood Vessels

  • Arteries: Carry blood away from the heart.

  • Arterioles: Smaller arteries that are the primary resistance vessels, controlling blood flow into organs.

  • Capillaries: Microscopic vessels where exchange with tissues occurs.

  • Veins: Carry blood back to the heart.

Blood Pressure

Blood pressure is the force exerted by blood against vessel walls.

  • Systolic pressure: The maximum pressure during heart contraction (ventricular systole).

  • Diastolic pressure: The minimum pressure during heart relaxation (ventricular diastole).

  • Blood pressure is typically written as Systolic/Diastolic (e.g., 120/80 mmHg).

  • Pulse pressure: The difference between systolic and diastolic pressure (e.g., 120 - 80 = 40 mmHg).

    • Normal blood pressure is below 120/80 mmHg. High blood pressure is 130/80 mmHg or higher.

  • Measuring Blood Pressure (using a cuff/sphygmomanometer and stethoscope):

    1. Cuff pressure is raised above systolic pressure (no sound is heard).

    2. As cuff pressure is lowered, the first sound heard indicates systolic pressure.

    3. The point at which the sound disappears signifies diastolic pressure as normal blood flow resumes.

Arterioles and Resistance Control

Arterioles are the main regulators of peripheral resistance due to their ability to change diameter:

  • Vasoconstriction: Narrowing of arterioles, which decreases blood flow to tissues.

  • Vasodilation: Widening of arterioles, which increases blood flow to tissues.

  • Control mechanisms:

    • Nerves: Sympathetic and parasympathetic nervous systems.

    • Chemicals: Local factors like oxygen, CO_2, lactic acid, and hormones.

    • Temperature: Affects vessel diameter.

    • Stretching of vessels (Myogenic activity): Intrinsic response to pressure changes.

Vascular Tone

Arterioles maintain a baseline level of slight contraction, known as vascular tone. This allows for:

  • Fine-tuning of blood distribution to meet specific tissue needs.

  • Regulation of overall systemic blood pressure.

Bulk Flow

Bulk flow involves the movement of fluid as a group, distinct from diffusion.

  • Two types of bulk flow in capillaries:

    1. Ultrafiltration: The process where fluid is pushed out of capillaries into the interstitial fluid.

    2. Reabsorption: The process where fluid is pulled back into capillaries, primarily due to osmotic pressure created by plasma proteins.

  • Ultrafiltration and reabsorption work together to maintain fluid balance between the blood and tissues.

  • Excess interstitial fluid is collected and returned to circulation by the lymphatic system.

Veins and Blood Return

Veins act as a blood reservoir, storing approximately 60% of the body's total blood volume, and are crucial for returning blood to the heart. Blood return is aided by:

  • Valves: Prevent the backflow of blood.

  • Muscle pumps: Contraction of skeletal muscles compresses veins, pushing blood towards the heart.

  • Respiratory pump: Changes in thoracic and abdominal pressures during breathing help move blood.

  • Cardiac suction: The heart's relaxation phase creates a slight negative pressure, helping to pull blood into the atria.

Blood Pressure Control

Blood pressure is tightly regulated by various mechanisms:

  • Baroreceptors: Specialized sensory receptors located in the arteries (e.g., carotid sinus, aortic arch) that detect changes in blood pressure.

  • Adjustments are made through:

    • Alterations in sympathetic and parasympathetic nervous system activity.

    • Hormonal influences.

    • Kidney regulation of salt and water balance.

    • Other factors such as emotions and temperature.