blood vessels lecture

Overview of the Circulatory System

• The circulatory system consists of the heart, blood vessels, and blood. It functions to transport nutrients, gases, hormones, and waste products throughout the body.

Heart Function and Blood Flow

• Blood pumped from the heart first enters large elastic arteries, which expand slightly due to generated pressure, helping to move blood further along the system.

• As blood travels, it moves into muscular arteries, which contain smooth muscle that contracts or relaxes to regulate blood pressure and flow.

• Arterioles are smaller branches that connect arteries to capillary networks, with the ability to influence blood flow into specific capillaries.

Blood Vessels and Their Characteristics

• Capillaries are the sites of exchange between blood and tissues:

  • Exchange of gases, nutrients, and waste products occurs exclusively at capillaries.

• Venules and veins return blood to the heart:

  • These vessels tend to have thinner walls and lack the muscular structure of arteries.

  • Valves in veins help maintain blood flow back to the heart against gravity.

Blood Volume Distribution

• Approximately 60% of blood volume is located in the veins, making them a significant reservoir, while 30-40% is in arteries, and a small percentage in capillaries.

Lymphatic System

• The lymphatic system is associated with the circulatory system, consisting of thin vessels that pick up excess fluid from tissues and transport it back to the bloodstream:

  • Lymph nodes filter the lymph and play a role in the immune response by monitoring for pathogens.

  • The lymphatic system helps prevent fluid accumulation in tissues by returning excess fluid to the bloodstream.

Anatomy of Blood Vessels

• Blood vessels have three main layers:

  • Tunica Intima: Innermost layer, includes the endothelium that regulates blood flow and interacts with blood components.

  • Tunica Media: Middle layer composed of smooth muscle and elastic tissue; varies in thickness between arteries and veins.

  • Tunica Externa: Outermost layer, provides structural support, contains collagen, and houses the vasa vasorum, blood vessels that supply larger vessels.

• Differences in structure:

  • Arteries: Thicker walls, more smooth muscle, and more elastic tissue compared to veins.

  • Veins: Thinner walls, more connective tissue, and larger lumens.

Blood Pressure Dynamics

• Blood pressure is highest in the aorta and decreases as it moves through arteries to arterioles, and capillaries, to the vena cava:

  • Mean arterial pressure (MAP) is the average pressure that drives blood flow; key for assessing blood flow adequacy.

  • The difference between systolic and diastolic pressures represents the pressure changes during heart contraction and relaxation.

  • Bulk flow drives blood movement down the pressure gradient from areas of high pressure (aorta) to low pressure (vena cava).

Resistance and Blood Flow Regulation

• Total Peripheral Resistance (TPR) affects blood pressure and is influenced by:

  • Length of blood vessels: Longer vessels increase resistance.

  • Viscosity of blood: Thicker blood increases resistance (e.g., dehydration raises viscosity).

  • Radius of arterioles: Smaller radius greatly increases resistance due to the fourth power exponent in Poiseuille's Law.

Vasodilation and Vasoconstriction

• Blood vessel radius is dynamically adjusted through:

  • Vasodilation: Relaxation of smooth muscle decreases resistance and lowers blood pressure.

  • Vasoconstriction: Contraction of smooth muscle increases resistance and raises blood pressure.

• Controlled by the sympathetic nervous system through norepinephrine which binds to alpha-1 adrenergic receptors on smooth muscle.

Precapillary Sphincters and Tissue Perfusion

• Precapillary sphincters regulate blood flow into capillary beds:

  • Open during increased metabolic activity, allowing blood to perfuse active tissues.

  • Close during metabolic inactivity, redirecting blood flow to areas of higher demand.

  • Factors such as low oxygen and high carbon dioxide trigger sphincter opening, while high oxygen and low waste levels lead to closure.

Conclusion

• Understanding these components and their interactions in the circulatory system is crucial for comprehending how blood circulation supports bodily functions, including the delivery of nutrients and removal of waste.