Definition: Blood vessels are the delivery system of dynamic structures connected to the heart.
Types of Blood Vessels:
Arteries: Carry blood away from the heart; generally oxygenated except for:
Pulmonary arteries: Transport deoxygenated blood from the heart to the lungs.
Umbilical arteries: Carry deoxygenated blood in a fetus.
Capillaries: Microscopic vessels that facilitate the exchange of materials between blood and cells; they are in direct contact with individual cells.
Veins: Carry blood toward the heart; typically deoxygenated except for:
Pulmonary veins: Transport oxygenated blood from the lungs to the heart.
Umbilical veins: Carry oxygenated blood to the fetus.
Circulation Physiology
Blood Flow ()
Definition: The volume of blood flowing through a vessel, organ, or the entire circulatory system per minute, measured in milliliters per minute (ml/min).
Characteristics:
Relatively constant at rest.
Varies widely among individual organs depending on their needs.
Directly proportional to the blood pressure gradient (ΔP).
Blood flows from regions of high pressure to low pressure; a greater pressure difference stimulates a higher flow.
Systemic Blood Pressure
Definition: Blood pressure is the measure of the force exerted by blood on vessel walls.
Characteristics:
Highest: In the aorta just after being pumped from the heart.
Gradually declines through the circulatory pathway until reaching 0 mm Hg in the right atrium.
The steepest drop occurs in arterioles.
Blood Pressure Graphs
Systolic Pressure: Pressure during ventricular contraction.
Diastolic Pressure: Lowest arterial pressure.
Pulse Pressure: Difference between systolic and diastolic pressure.
Mean Arterial Pressure (MAP): The average pressure that propels blood through the circulatory system defined as: MAP=diastolicpressure+31pulsepressure
Structure of Blood Vessels
Lumen
Definition: The central cavity of blood vessels where blood flows.
Blood Vessel Walls
Tunica Intima: Layer that includes the endothelium and subendothelial layer.
Tunica Media: Composed of smooth muscle and elastic fibers, regulates blood vessel diameter.
Tunica Externa (tunica adventitia): Composed primarily of collagen fibers that protect and reinforce the vessel.
Capillaries Structure
Composed of a thin tunica intima (endothelium only). No subendothelial layer or tunica media or tunica externa to allow for maximal exchange efficiency.
Types of Arteries
Elastic (Conducting) Arteries
Example: Aorta and its major branches. Characteristics:
Large lumen to reduce resistance.
Composed of thick-walled structures with elastin in all three tunics.
Act as pressure reservoirs, expanding and recoiling as blood is ejected from the heart.
Muscular (Distributing) Arteries and Arterioles
Located distal to elastic arteries. Characteristics:
High but steady blood pressure lower than that in elastic arteries.
Thick tunica media with smooth muscle, allowing active regulation of blood flow.
Adjustments in vessel diameter directly affect blood pressure and flow.
Capillaries and Exchange Mechanisms
Capillaries
Microscopic vessels crucial for material exchange between blood and surrounding tissue.
Composed of a single layer of endothelial cells to facilitate exchanges of gases, nutrients, and wastes.
Ways Exchanges Occur
Diffusion:
Oxygen (O2) and nutrients diffuse from blood to tissues.
Carbon dioxide (CO2) and metabolic wastes diffuse from tissues into the blood.
Lipid-soluble molecules: Pass through endothelial membranes directly.
Water-soluble solutes: Pass through clefts and fenestrations.
Larger molecules (e.g., proteins): Are transported via vesicles through endocytosis or exocytosis.
Capillary Blood Pressure
Ranges from 15 to 35 mm Hg; low pressure is crucial to prevent the rupture of capillaries.
Most capillaries are highly permeable and thus require lower pressures to effectively force filtrate into interstitial spaces.
Capillary Beds
Define Capillary Beds
Networks of capillaries that form the microcirculation between arterioles and venules.
True Capillaries: Branch off metarterioles, act as exchange vessels (10-100 per capillary bed).
Blood Flow Regulation Through Capillary Beds
Precapillary Sphincters: Regulate blood flow into true capillaries, influenced by local chemical conditions and vasomotor nerves.
Venous System
Characteristics of Veins
Formed by the convergence of venules and return blood to the heart.
Low Blood Pressure: Ranges from 0-15 mm Hg; pressure changes little during the cardiac cycle.
Thinner walls with larger lumens than corresponding arteries; features a thin tunica media and a thicker tunica externa composed of collagen and elastic fibers.
Identified as Capacitance Vessels: Can hold up to 65% of the blood supply.
Mechanisms Assisting Venous Return
Large-Diameter Lumens: Offer minimal resistance to blood flow.
Valves: Prevent backflow; especially abundant in limb veins.