The Cardiovascular System – Blood Vessels

Vasoconstriction and Vasodilation

Blood vessels dynamically adjust their diameter to regulate blood flow and pressure. Two key processes are:

• Vasoconstriction: The narrowing of blood vessels due to contraction of smooth muscle in the vessel wall. This decreases the lumen diameter, increases vascular resistance, reduces blood flow, and raises blood pressure.

• Vasodilation: The widening of blood vessels caused by relaxation of smooth muscle. This increases the lumen diameter, decreases vascular resistance, boosts blood flow, and lowers blood pressure.

Types of Arteries: Structure and Function

Arteries are classified based on their size, structure, and function:

Structure and Function of Veins

• Structure: Veins have all three vessel layers (tunica intima, tunica media—thinner, tunica externa—thicker). They often appear collapsed in histological sections.

• Function: Veins carry blood from capillaries back toward the heart. They are convergent vessels, collecting blood from many capillaries.

• Differences from Arteries: Veins have lower blood pressure, thinner walls, and contain more blood volume than arteries. Arteries are divergent, branching to supply tissues.

Capillary Beds: Structure and Function

• Structure: Capillaries are the smallest blood vessels, consisting only of a thin tunica intima. Red blood cells pass through in single file.

• Function: Capillaries are exchange vessels, allowing for the transfer of gases, nutrients, wastes, and hormones between blood and interstitial fluid.

• Types of Capillaries:

  • Continuous: Most common; uninterrupted lining; found in muscle, skin, brain.

  • Fenestrated: Have pores; found in kidneys, intestines, endocrine glands.

  • Sinusoid: Large gaps; found in liver, bone marrow, spleen.

Blood Flow, Blood Pressure, and Resistance

• Blood Flow (F): The volume of blood moving through a vessel, organ, or the entire circulation per unit time (ml/min). For the whole system, it equals cardiac output (CO).

• Blood Pressure (BP): The force per unit area exerted by blood on vessel walls. It creates the pressure gradient that drives blood flow from high to low pressure.

• Resistance (R): The opposition to blood flow due to friction within vessel walls. Determined by blood viscosity, vessel length, and vessel diameter.

Relationships:

• Blood flow is directly proportional to the pressure gradient and inversely proportional to resistance:

• If resistance increases (e.g., vasoconstriction), blood flow decreases (if pressure remains constant).

Factors Influencing Blood Pressure and Its Regulation

• Elasticity of arteries (compliance) and blood volume are primary determinants of blood pressure.

• Short-term regulation: Neural and reflex mechanisms (e.g., baroreceptor reflex) adjust cardiac output and vascular resistance within seconds.

• Long-term regulation: Kidneys and hormonal systems (e.g., renin-angiotensin-aldosterone system) control blood volume and vascular tone over hours to weeks.

Hypertension: Definition, Symptoms, and Consequences

• Definition: Chronic elevation of arterial blood pressure (systolic ≥ 130 mmHg or diastolic ≥ 80 mmHg).

• Symptoms: Often asymptomatic (“silent killer”).

• Consequences: Increases risk of stroke, heart attack, heart failure, dementia, and kidney damage.

Factors Increasing Mean Arterial Pressure (MAP):

• Increased cardiac output (CO): higher stroke volume and heart rate

• Increased total peripheral resistance (TPR): decreased vessel diameter, increased blood viscosity, increased vessel length

Autoregulation of Tissue Perfusion

Autoregulation ensures tissues receive adequate blood flow according to their metabolic needs.

• Myogenic Mechanism: Vessel smooth muscle contracts when stretched (high pressure) and relaxes when not stretched (low pressure) to stabilize blood flow.

• Metabolic Mechanism: Local metabolites (CO2, H+, adenosine, K+, lactic acid) cause vasodilation, increasing blood flow to active tissues.

Organ-Specific Autoregulation:

• Brain: High autoregulation; sensitive to CO2 and H+; vessels dilate if perfusion drops or acidity rises.

• Kidneys: High autoregulation; maintains stable glomerular filtration rate (GFR) via myogenic and tubuloglomerular feedback mechanisms.

• Heart: High autoregulation; local metabolic demand (adenosine release) drives vasodilation.

• Skeletal Muscle: Moderate at rest; during exercise, local metabolites override sympathetic vasoconstriction, causing vasodilation.

• Skin: Low metabolic autoregulation; blood flow mainly controlled by thermoregulatory reflexes.

Pressures Influencing Fluid Movement Across Capillaries

• Hydrostatic Pressure: The force exerted by fluid against the vessel wall, pushing fluid out of capillaries.

• Colloid Osmotic Pressure: The force exerted by plasma proteins, pulling fluid into capillaries and opposing hydrostatic pressure.

Diffusion Across Capillaries

Substances move between blood and interstitial fluid by diffusion, following concentration gradients. Four main routes:

• Lipid-soluble molecules: Diffuse directly through endothelial cell membranes (e.g., O2, CO2).

• Water-soluble molecules: Pass through intercellular clefts or fenestrations (e.g., amino acids, sugars).

• Larger molecules (proteins): Transported via vesicles by endocytosis and transcytosis.

Circulatory Shock: Definition and Causes

• Definition: A condition where blood vessels are inadequately filled, leading to insufficient circulation and tissue perfusion.

• Causes:

  • Hypovolemic shock: Low blood volume (e.g., hemorrhage, dehydration)

  • Cardiogenic shock: Heart pump failure

  • Distributive shock: Abnormal vasodilation (e.g., septic shock, anaphylaxis)

  • Obstructive shock: Physical blockage of blood flow (e.g., pulmonary embolism)