Blood Vessels

The Cardiovascular System: Blood Vessels and Hemodynamics

Anatomy of Blood Vessels

• Closed system of tubes that carries blood.
• Types of Blood Vessels:
• Arteries: Carry blood from heart to tissues.
  • Types:
  • Elastic Arteries: Large, e.g., pulmonary, aorta; composed of elastic tissue to withstand high pressure.
  • Muscular Arteries: Distribute blood to specific regions; thicker smooth muscle layer.
  • Arterioles: Smallest arteries leading to capillaries.
• Capillaries: Small, thin-walled vessels enabling nutrient and gas exchange.
• Venules: Collect blood from capillaries and merge to form veins.
• Veins: Return blood to the heart; have thinner walls than arteries and contain valves to prevent backflow.

Structure of Blood Vessels

• Tunica Externa (Adventitia): Outermost layer made of loose connective tissue.
• Tunica Media: Middle layer, generally the thickest, consists of smooth muscle and elastic fibers for vasoconstriction and vasodilation.
• Tunica Interna (Intima): Smooth inner layer made of simple squamous epithelium, repelling blood cells and platelets.

Regulation of Blood Vessel Diameter

• Sympathetic Innervation:
• Increases muscle contraction (vasoconstriction) or relaxation (vasodilation vitals in systemic circulation).
• Aids in reducing blood loss following injuries (vasospasm).
• Chemical signals (NO, K+, H+) induce vasodilation.

Capillaries and Microcirculation

• Types of Capillaries:
• Continuous Capillaries: Basic structure; found in muscles, brain, lungs.
• Fenestrated Capillaries: Have pores; located in kidneys and endocrine glands.
• Sinusoids: Very leaky; found in liver, bone marrow, and spleen.
• Capillary Function: Exchange of nutrients and wastes occurs due to their thin walls.
• Control of Perfusion: Precapillary sphincters control blood flow in capillary beds.

Blood Flow and Hemodynamics

• Blood Pressure Dynamics:
• Arterial pressure highest during ventricular contraction (systole) and lowest during relaxation (diastole).
• Normal values: 120/80 mmHg; hypertension indicated by chronic BP > 140/90 mmHg.
• Factors Influencing Blood Flow:
• Cardiac Output (CO): CO = Stroke Volume (SV) × Heart Rate (HR).
• Peripheral Resistance: Influenced by vessel radius, blood viscosity, and vessel length.

Venous Return Mechanisms

• Venous pressure pushes blood towards the heart (7-13 mmHg).
• Mechanisms include:
• Valves: Prevent backflow in veins.
• Skeletal Muscle Pump: Muscle contractions push blood through veins.
• Respiratory Pump: Changes in thoracic pressure during breathing aid blood flow.

Shock and Homeostasis

• Shock: Inadequate perfusion leading to cellular damage; types include hypovolemic, cardiogenic, and obstructive shock.
• Signs of Shock: Rapid heart rate, clammy skin, altered mental status, reduced urine output.
• Homeostatic Responses:
• Activation of sympathetic nervous system, renin-angiotensin-aldosterone system, and baroreflex help restore BP and blood flow.

Aging and the Cardiovascular System

• Age-Related Changes:
• Decreased aortic compliance and cardiac output.
• Increased total cholesterol levels and risks of cardiovascular diseases.
• Systolic pressure rises; diastolic pressure decreases with widening of pulse pressure.

Clinical Considerations

• Edema: An increase in interstitial fluid from excess filtration or inadequate reabsorption; can be caused by various medical conditions (e.g., heart failure, liver disease).
• Capillary Exchange Dynamics: Regulated by hydrostatic and osmotic pressures; Starling's law explains fluid movements across capillaries.
• Local and Hormonal Regulation: Local factors can cause vasodilation or constriction depending on the metabolic state of tissues; Hormones like angiotensin II influence BP significantly.