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