Blood Vessel Lab: Hemostasis

• Blood Vessel Lab is separated into 3 parts: locations, histology and blood pressure, heart sounds, and cardiac output
• Hemostasis is the reduction and stoppage of blood loss from a damaged vessel

Three steps in hemostasis

• Vascular spasm, platelet plug formation, and coagulation or clotting are the 3 main steps

Vascular spasm

• Smooth muscle contracts, causing vasoconstriction which temporarily stops bleeding for about 30 minutes
• Helps until longer-term mechanisms start to have an effect
• Caused by substances released by damaged endothelial cells and platelets

Platelet Plug Formation:

• Platelets stick to exposed collagen in connective tissue of the damaged vessel
• Platelets extend processes and release substances like vasoconstrictors and clotting factors
• Platelets stick to each other and form a plug

Blood Clotting (Coagulation)

• Blood coagulation is a multi-step cascade involving many clotting factors (proteins) that produces a meshwork of insoluble protein fibers, called fibrin.
• The meshwork traps formed elements and is called a clot
• Blood is converted from liquid to a gel

Blood Clotting Phases

• Phase 1 includes an extrinsic and intrinsic pathway
• Extrinsic clotting occurs more rapidly and requires an outside factor
• Intrinsic clotting occurs slowly and requires inside factors
• Phase 2 & 3 common pathway. Prothrombin converts to Thrombin, Thrombin converts fibrinogen(soluble) to fibrin(insoluble)

Mechanisms that regulate blood clotting

• Vitamin K is involved in clotting factor production in the liver from gut bacteria and diet
• Rapid iniation and progression of clotting are due to positive feedback loop with thrombin
• Anticoagulants, like antithrombin and heparin, are present in the blood
• Clotting factors are washed out by circulation while thrombin is trapped in the clot by fibrin for specific localization of blood clotting
• Clot dissolution (fibrinolysis) has 3 steps:
  • Endothelium releases tissue plasminogen activator (t-PA)
  • t-PA converts plasminogen into plasmin
  • Plasmin dissolves the clot

Hemostasis Disorders

• Thrombus: A blood clot in an uninjured vessel that can decrease or block blood flow
• Embolus: A dislodged blood clot or other material that flows freely in a blood vessel
• Thrombus or embolus blocks the passage of blood to a tissue, leading to tissue death
• Hemophilia: Inherited deficiency in blood clot formation due to a deficiency in specific clotting factors. Most common type is hemophilia A due to factor VIII deficiency.

Blood Vessels

• Blood vessels are part of the cardiovascular system and include arteries, veins, and capillaries

Arteries vs. Veins

• Arteries carry blood away from the heart (usually oxygenated blood), while veins carry blood to the heart (usually deoxygenated)
• Arterioles are small arteries, while venules are small veins
• Arteries have thicker walls with smooth muscle and elastic tissue to withstand high pressure from the heart
• Veins have thinner muscle walls with less elastic tissue due to lower blood pressure and contain valves to prevent backflow blood

Capillaries

• Capillaries are very thin in structure
• They are composed of the tunica intima only
• Exchange between the blood plasma and cells occurs in capillaries
• They vary in diameter and permeability, depending on the location

Blood Pressure

• Blood pressure is the pressure exerted by blood on the inside walls of arteries due to the contraction of ventricles

Factors Affecting Blood Pressure:

• Cardiac output: Volume of blood pumped each minute
• Peripheral resistance: Friction between blood and walls of blood vessels
• Heart rate: Rate of contraction
• Stroke volume: Volume of blood pumped in each contraction
• Size of the lumen of vessels: Vasoconstriction/vasodilation
• Viscosity of the blood: Red blood cells + albumin

Systolic vs Distolic Pressure

• Systolic pressure is the pressure exerted on blood vessels when the ventricles contract
• Diastolic pressure: Pressure still exerted when the ventricles are relaxed
• Normal Blood Pressure Values: Systolic Blood Pressure ~ 120 mm Hg, Diastolic Blood Pressure ~ 80 mm Hg (8-10 mm Hg lower for females)

Blood pressure abnormalities

• Hypotension is blood pressure lower than the average, with Systolic < 100 mm Hg
• Hypertension is blood pressure higher than the average, with Systolic/Diastolic > 140/90 mm Hg

Regulation of blood pressure

• Autonomic Nervous System affects blood pressure indirectly by changing heart rate or the diameter of blood vessels
• Autoregulation of capillaries' active tissue cells can change blood pressure indirectly by releasing chemicals that change the diameter of blood vessels
  • Examples of vasodilating substances are K+ and H+
  • Examples of vasoconstricting substances is O2
• Endocrine system hormones change blood viscosity or ion levels
  • An increase in aldosterone and ADH leads to water retention and then an increase in BP
  • Epinephrine leads to vasoconstriction and an increase in BP
• When blood pressure increases, impulses from baroreceptors are received by the cardiovascular center of the brainstem, resulting in decreased sympathetic impulses to the heart and vasodilation
• When blood pressure falls, decreased impulses from baroreceptors are received by the cardiovascular center of the brainstem, which leads to vasoconstriction and increases sympathetic impulses to the heart

Major Arteries

• Main arteries from the heart to the upper body: arch of aorta, brachiocephalic trunk, right subclavian (right shoulder), right common carotid (neck), left common carotid (neck), left subclavian (left shoulder)
• Main arteries from the heart to the lower body: descending aorta, hepatic artery (liver), gastric artery (stomach), mesenteric artery (intestines), renal artery (kidney), right and left common iliac artery (pelvic), and right and left femoral artery (thigh)
• Arterial Supply to the Heart: Ascending aorta > right and left coronary artery

Major Veins

• Main veins (upper body to heart): superior vena cava, right and left brachiocephalic, right and left subclavian (shoulder), right and left jugular (neck)
• Main veins (lower body to heart): inferior vena cava, hepatic portal vein (liver), hepatic vein (liver), gastric vein (stomach), mesenteric vein (intestines). renal vein (kidney), and right and left common iliac and femoral vein (thigh)