Cardiovascular System: Blood Vessels

Cardiovascular System: Blood Vessels

• Blood vessels form a system that starts and ends at the heart.
• Arteries carry oxygenated blood away from the heart (except pulmonary artery).
• Capillaries directly exchange substances with tissues.
• Veins carry deoxygenated blood towards the heart (except pulmonary vein).

Structure of Blood Vessel Walls

• Walls of arteries and veins have three layers:
  • Tunica intima: Innermost layer, composed of endothelium.
  • Tunica media: Middle layer, primarily smooth muscle and elastic fibers.
  • Tunica externa (adventitia): Outermost layer, mostly collagen fibers.
• Lumen: Central blood-filled space.
• Capillaries consist of a single layer of endothelium with a sparse basal lamina.

Tunics

• Tunica intima, lines the lumen of all vessels and is composed of endothelium.
  • Sympathetic vasomotor nerve fibers control vasoconstriction and vasodilation of vessels.
  • Vasoconstriction decreases lumen diameter.
• Tunica media, composed of smooth muscle and elastic fibers, regulates vasoconstriction /vasodilation.

Elastic (Conducting) Arteries

• Large, thick-walled arteries that act as pressure reservoirs.
• Expand and recoil as blood is ejected from the heart.
• Ex. Aorta and its major branches.

Muscular (Distributing) Arteries and Arterioles

• Responsible for delivering blood to specific body organs.
• Active in vasoconstriction.
• Ex. Renal artery.

Arterioles

• Smallest arteries that lead to capillary beds.
• Control blood flow into capillary beds via vasodilation and vasoconstriction.

Capillaries

• Microscopic blood vessels in all tissues except cartilage, epithelia, cornea, and lens.
• Direct exchange of substances with tissues (gases, wastes, nutrients, hormones).
• Three types: continuous, fenestrated, and sinusoidal.

Capillary Beds

• Interwoven networks between arterioles and venules.
• Microcirculation: Blood flow through capillary beds.
• Vascular shunts allow for direct connections between arterioles and venules.
• True capillaries are involved in exchange.

Veins

• Carry blood towards the heart.
• Thinner walls and larger lumens than arteries.
• Contain valves to prevent backflow.
• Act as blood reservoirs (up to 65% of blood supply).

Anastomoses

• Interconnections of blood vessels, providing alternative pathways for blood flow.
• Arterial anastomoses are common in joints, abdominal organs, brain, and heart.
• Arteriovenous anastomoses are shunts in capillaries.
• Venous anastomoses are abundant, preventing complete blockage if one vein is occluded.

Physiology of Circulation: Blood Pressure

• Blood pressure (BP) is the force exerted by blood on vessel walls.
• Expressed in mmHg (millimeters of mercury).
• Highest in the aorta and decreases throughout the system.
• Systemic arterial BP is measured in large arteries near the heart.

Resistance

• Opposition to blood flow from friction.
• Affected by blood viscosity, total blood vessel length, and blood vessel diameter.
• Higher viscosity, longer length, and smaller diameter increase resistance.

Systemic Blood Pressure

• The pumping action of the heart generates blood flow.
• Pressure declines throughout the pathway, lowest in the right atrium.
• Factors like cardiac output, peripheral resistance, and blood volume regulate blood pressure.

Arterial Blood Pressure

• Systolic pressure: Pressure during ventricular contraction.
• Diastolic pressure: Lowest pressure between contractions.
• Pulse pressure: Difference between systolic and diastolic pressure.
• Mean arterial pressure (MAP): Average pressure during a cardiac cycle.

Pulse Pressure and MAP

• Pulse pressure and MAP both decline with increasing distance from the heart.
• If a person's blood pressure is 120/80, Pulse Pressure=40 and MAP=93mm Hg.

Vital Signs

• Pulse, blood pressure, respiratory rate, and body temperature are vital signs.
• Pulse points are areas where arteries are close to the body surface.

Capillary Blood Pressure

• Ranges from 15 to 35 mmHg.
  • Low pressure is necessary to prevent rupture.

Venous Blood Pressure

• Changes little during the cardiac cycle.
• Low pressure, but several mechanisms aid in return of blood to the heart.

Regulation of Blood Pressure

• Three main factors: cardiac output, peripheral resistance, and blood volume.

Blood Vessels: Aorta

• Aorta's branches are responsible for supplying blood to various body parts.

Blood Vessels: Brain

• Internal carotid arteries, vertebral arteries, and the Circle of Willis supply blood to the brain.

Abdominal Aorta Branches

• Celiac trunk, superior mesenteric artery, and inferior mesenteric artery are major branches that supply blood to organs in the abdomen.

Arteries of the Lower Limb

• External iliac, femoral, popliteal, tibial, dorsalis pedis, and fibular arteries are important for blood supply to the lower limb.

Veins of the Head and Neck

• Superficial temporal, occipital, posterior auricular, and external jugular veins drain blood from the head and neck.

Veins Leading to the Heart

• Right and left external and internal jugular veins, subclavian veins, and brachiocephalic veins transport blood back to the heart.

Veins of the Arm

• Basilic, cephalic, and axillary veins carry blood back to the heart in the arm.

Abdominal Veins

• External and internal iliac veins, and the inferior vena cava are key veins in draining blood from the abdomen.

Hepatic Portal Vein

• Drains blood from the digestive system to the liver.

Lower Limb Veins

• Small saphenous and great saphenous veins carry blood from the lower limbs.