Blood Vessels and Circulation

• Blood Vessels

  • Arteries
  • Capillaries
  • Veins
• Arteries


  Carry blood away from the heart, oxygenated except for pulmonary circulation and umbilical vessels of a fetus
• Capillaries


  Contact tissue cells and directly serve cellular needs
• Veins


  Carry blood toward the heart, have valves to help direct the flow
• Layers of Arteries


  • Tunica intima (covered by endothelium)
  • Tunica media (muscular layer)
  • Tunica externa
• Types of Arteries


  • Elastic (Conduction) (allows for expansion and recoil)
  • Muscular (Distributing) (actively change diameter)
  • Arterioles (small arteries, control blood flow into tissues)
• Types of Capillaries

  • Continuous (tight junctions) (skin and muscles)
  • Fenestrated (contain pores) (kidney)
  • Sinusoidal (allow for large molecules to pass) (liver, bone marrow, spleen)
• Capillary Beds


  Connect terminal end of arteriole to venule
• Veins


  Thin walls, valves help keep blood flowing to the heart, venules are the smallest veins
• Blood Flow through Capillaries (Microcirculation)

  1. Slow and Intermittent (due to low pressure gradient along the vessel, opening and closing of sphincters)

  2. Site of all exchange between blood and tissues

  3. Simple Diffusion (high to low) (O2 into tissue and CO2 out into blood)

  4. Lipid soluble substances diffuse directly through endothelial membranes

  5. Water soluble substances pass through clefts between cells

  6. Water and larger ions move in and out of capillaries via bulk flow
• Frank Starling Law of Capillaries


  The main forces determining direction are hydrostatic and colloid osmotic pressure
• Hydrostatic Pressure

  • Capillary hydrostatic pressure (pushes fluid into the tissues)
  • Hydrostatic pressure of the interstitial fluid (pushes fluid back into the capillary)
• Colloid Pressure "oncotic pressure"


  Caused by plasma proteins that do not move out of the capillary into the interstitial space, draw water back to the vessel
• Net Filtration Pressure (NFP)

  All of the forces (pressures) taken together create this, decides whether water will move in or out of the capillary

  -NFP Positive

  -at the arteriole end of the capillary

  -water moves out of the vessel

  
  

  -NFP Negative

  -venous end of capillary

  -water will move into the vessel
• A small amount of fluid (3L/ day) is not returned to the capillary, but will be drained by the lymphatic system
• Blood Pressure


  Force per unit area exerted on the wall of a blood vessel by the blood, expressed in mm Hg
• Pressure Gradient


  What causes blood to flow
• Factors influencing blood pressure

  • Cardiac output (CO)
  • Peripheral resistance
  • Blood volume
• Types of Arterial Blood Pressure

  • Systolic Pressure (ventricles contract, highest pressure)
  • Diastolic Pressure (ventricle relax, lower pressure)
  • Pulse Pressure (difference between systolic and diastolic pressures)
  • Mean Arterial Pressure (pressure that pushes blood along) MAP= diastolic pressure + 1/3 pulse pressure
• Systemic pressure is what health professionals use, normal value = 120/80, highest in the aorta and declines throughout the pathway
• Capillary Blood Pressure


  Ranges from 15 to 35 mm Hg, low capillary pressure is desirable
• Venous Blood Pressure

  Changes little during the cardiac cycle, small pressure gradient, about 15 mm Hg, low pressure due to cumulative effects of peripheral resistance
• Mechanisms for Maintaining and Controlling Blood Pressure

  • Short-Term Neural Control
  • Short-Term Hormonal Control
  • Long-Term Renal Control
• Short Term Neural Control


  • Baroreceptors in carotid sinus and aortic arch (maintain systemic circulation)
  • Chemoreceptors in same locations (Respiratory System)
  • Baroreceptors -> Vasomotor Control Center -> Vascular Muscles (Dilation/Constriction)
• Short Term Hormonal Control

  • Epinephrine
  • Norepinephrine
  • Angiotensin II
  • Antidiuretic Hormone (ADH) - vasopressin
  • Atrial Natriuretic Peptide (ANP)
• Long-Term Renal Regulation


  • Direct renal mechanism (alters blood volume independently of hormones)
  • Indirect renal (renin-angiotensin) mechanism
• Direct Renal Mechanism

  1. Increased BP or blood volume causes the kidneys to eliminate more urine, thus reducing BP

  2. Decreased BP or blood volume causes the kidneys to conserve water, and BP rises
• Indirect Renin-Angiotensin Mechanism

  Arterial blood pressure -> release of renin -> production of angiotensin II -> increased blood pressure (vasoconstriction, aldosterone secretion, ADH release)
• Measuring Systemic Arterial BP

  Indirectly by the auscultatory method using a sphygmomanometer, pressure is increased in the cuff until it exceeds systolic pressure, then released slowly while listening for Korotkoff sounds
• Blood Flow


  Volume of blood flowing through a vessel, an organ, or the entire circulation in a given period, measured as ml/min, equivalent to cardiac output for entire vascular system
• Resistance (Peripheral Resistance)

  Opposition to flow, measure of the amount of friction blood encounters, generally encountered in the peripheral systemic circulation, determined by blood viscosity, total blood vessel length, and blood vessel diameter
• Blood flow is directly proportional to the blood pressure gradient


  If pressure increases, blood flow speeds up
• Blood flow is inversely proportional to peripheral resistance


  If resistance increases, blood flow decreases
• Resistance is more important in influencing local blood flow, easily changed by altering blood vessel diameter
• Perfusion


  The flow of blood through a tissue or an organ, purpose is to deliver oxygen and nutrients, and to remove carbon dioxide and waste products
• Blood Flow regulation

  The velocity of the blood flow changes, fastest in aorta, slower in small arteries and arterioles, slowest in capillaries, picks up speed again in venules and veins, regulated via autoregulation to adjust the diameter of the arteries feeding that particular tissue
• Parts of the Aorta

  • Ascending Aorta
  • Aortic Arch
  • Descending Aorta
  • Abdominal Aorta
• Branches of the Aortic Arch

  • Brachiocephalic
  • Left common carotid artery
  • Left subclavian artery
• Branches of the Abdominal Aorta

  • Celiac Trunk (splits into common hepatic, splenic, and left gastric arteries)
  • Superior and inferior mesentery arteries
  • Suprarenal
  • Renal
  • Testicular/Ovarian arteries
• Head and Neck Arteries

  • Common Carotids
  • Vertebral Artery
  • Thyrocervical Trunk
  • Costocervical Trunk