Chapter 21 - The Cardiovascular System: Blood Vessels

• Blood vessels:

  • closed system with 5 main vessel types: arteries -> arterioles -> capillaries -> venues -> veins
  • vessel types vary in length, diameter, wall thickness and tissue make-up
• Structure of Blood Vessels:

  • walls of arteries and veins have 3 layers (tunics): tunica intima/interna, tunica media and tunica externa
• 1. Tunica intima/interna - closest to lumen
  2. composed of simple squamous epithelium (endothelium), basement membrane and underlying loose CT
  3. endothelium is continuous with the endocardium layer of the heart so that cells fit close together to decrease friction
• 2. Tunica media:

  • composed of circularly arranged smooth muscle fibers; may have elastic fibers
  • smooth muscle activity is regulated by sympathetic nerves of ANS and local chemical factors
  • smooth muscle regulates the diameter of the lumen - vasoconstriction and vasodilation
• Vasoconstriction:

  • Decrease diameter of the lumen
  • Increase blood pressure 
• Vasodilation:

  • Increase diameter of the lumen 
  • Decrease blood pressure 
• 3. Tunica externa - outermost:

  • composed of CT that anchors the blood vessel to surrounding structures
• Capillaries: microscopic blood vessels that consist only of endothelium and a basement membrane

  • extremely thin vessels for gas/nutrient exchange via diffusion
• Blood pressure:

  • defined as the force exerted by the blood against the wall of a blood vessel (units = mmHg)
  • differences within the vascular system provides the driving force that keeps blood moving
  • blood flows from areas of higher pressure to areas of lower pressure
  • pumping action of the heart generates this pressure thus the blood pressure decreases as distance from the left ventricle increases
• As blood flows through vessels, friction occurs between blood and vessel wall. As blood moves farther from the heart there is no longer distance for friction to act therefore we get a progressive decrease in the blood pressure as we move away from heart.
• Hypertension (persistently high blood pressure):

  1. primary hypertension (90-95% of cases)
  2. cannot be attributed to any identifiable cause
  3. secondary hypertension (5-10% of cases)
  4. does have an identifiable underlying cause such as kidney disease, tumours of the adrenal gland or other causes
• Hypertension: known as the 'silent killer' because it can cause considerable damage to the blood vessels, heart, brain and kidneys before it causes pain or other noticeable symptoms

  Treatments:

  • decrease intake of sodium will decrease blood volume and decrease blood pressure
  • treat with diuretics to increase elimination of water and salt to decrease blood volume and decrease blood pressure
• Arterial system:

  1. Elastic arteries
  2. include the aorta (its main branches) and the pulmonary trunk
  3. tunica media has many layers of smooth muscle cells and many elastic fibers
  4. presence of elastica interna/externa
  5. acts as pressure reservoir - expands and recoil to drive blood forward; felt as a pulse
  6. BP is highest in the aorta and large systemic arteries
  7. considered conducting arteries due to large lumen size - low resistance pathways
• Elastic arteries: can detect two separate pressures, due to the pumping action of the heart:

  • systolic pressure

  1. maximum pressure against the arterial wall during ventricular contraction
  2. normally about 120 mmHg

  • diastolic pressure

  1. minimum pressure against the arterial wall during ventricular relaxation
  2. normally about 80 mmHg
  3. expressed as systolic BP/diastolic BP or 120/80

  • Differences between systolic pressure (SP) and diastolic pressure (DP) is known as the pulse pressure (PP) --> SP - DP = PP
• Muscular arteries:

  • medium-sized and branch to various organs
  • thickest tunica media with more smooth muscle cells and fewer elastic fibers; can't recoil
  • pulse pressure is gradually phased out
  • considered to be distributing arteries because they deliver blood to specific body organs
• Arterioles:

  • microscopic vessels with diameters ranging from 10 to 300 um
  • contains mostly smooth muscle with very few elastic fibers
  • smallest arterioles regulate flow into capillary beds and have a single layer of smooth muscle spiralling around the endothelium
  • dilate and constrict in response to neural, hormonal and chemical factors
  • also known as resistance vessels
  • major drop in BP (~40mmHg) occurs between arteries and arterioles
• Capillaries:

  • microscopic vessels with a diameter of only 8-10 um
  • wall has a single layer of endothelial cells and a basement membrane - NO muscular layer
  • have a large surface area in order to function in the exchange of materials between the blood and interstitial fluid via diffusion and active transport
  • BP is about 35mmHg at arteriole end and 17mmHg at venule end
  • RBC diameter is ~8um pass through capillary 1 RBC at a time
• Capillaries are organized into networks known as capillary beds:

  • a terminal arteriole leads into a metarteriole which is continuous with a thoroughfare channel
  • the thoroughfare channel in turn joins the postcapillary venule
  • about 10-100 "true" capillaries branch off the metarteriole
  • precapillary sphincter (smooth muscle) alternately relaxes/contracts to regulate flow into capillary bed
• • when precapillary sphincters are relaxes (open), blood flows into capillaries and exchanges occur
  • when the precapillary sphincters are contracted (closed), blood flow through the capillaries stops
  • an organ contains numerous capillary beds
  • amount of capillaries open varies with the metabolic needs of the organ
• Dynamics of Capillary Exchange:

  • Arterial end of the capillary gets net outward pressure for filtration = fluid exits the capillary 
  • Venous end of capillary gets a net inward pressure to have reabsorption = fluid re-enters capillary  
  • If filtration exceeds reabsorption we get edema = swelling due to abnormal accumulation of interstitial fluid 
• 3 structural types of capillaries:

  1. continuous capillaries
  2. fenestrated capillaries
  3. sinusoids
• Continuous capillaries:

  • in skin and muscles
  • endothelial cells are continuous and have intercellular clefts (gaps)**brain capillaries have no gaps between the cells - this forms the blood-brain barrier
  • held together by tight junctions
  • allow limited passage of fluids and small solutes
• Fenestrated capillaries:

  • villi of small intestine, kidneys, endocrine glands
  • endothelial cells have fenestrations (pores)
  • greater permeability to fluids and small solutes
• Sinusoids:

  • liver, bone marrow, lymphoid tissue
  • endothelial cells have large fenestrations
  • large intercellular clefts
  • large lumen; incomplete basement membrane
  • allow large molecules (blood cells and proteins) to pass between blood and tissues
• Venous system:

  1. venules
  2. veins
• Venules:

  • receive blood from capillaries
  • larger venules have several layers of smooth muscle
• Veins:

  • venules join to form veins
  • vein walls are thinner then artery walls and typically a vein looks collapsed
  • no elastic interna/externa
  • tunica media has few layers of smooth muscle cells and few elastic fibers
  • tunica externa consists of thick bundles of collagen fibers and elastic networks (often thicker than media)
  • veins are blood "reservoirs" as can hold ~60% of the blood volume
• • pressure gradient in veins is from about 15mmHg to about 0mmHg at the right atrium
  • must rely on factors other than pressure alone to help return blood back to heart (venous return)
  • gravity - above heart level is good for venous return
  • presence of valves in veins - get to checkpoint a and stays then to checkpoint b and stays
  • contraction of skeletal muscles - help squeeze blood in veins back to heart = milking the vein
  • respiratory pump - moves blood to heart as you inspire (breathe in)
  • sympathetic vasoconstriction
• Sympathetic vasoconstriction of smooth muscle decreases volume of blood in veins as it pushes blood to heart by decreasing lumen size 
• Respiratory pump moves blood to heart as you inspire (breathe in)

  • When you inspire you get a decrease pressure in chest and an increase pressure in abdomen 
  • I can increase venous return by having the blood move from high pressure in abdomen to low pressure in chest
• Leaky valves can cause veins to become dilated and twisted varicose veins 

  • Blood leaks from deep veins to superficial veins and blood pools
  • If in anal canal called hemorrhoids 
  • Compression stockings can be used to apply pressure to limbs to keep blood from pooling 
• Factors affecting circulation:

  • driving force of blood circulation is blood pressure
  • cardiovascular system strives to maintain BP(P)
  • blood flow (F) is the volume of blood flowing through a vessel = cardiac output (CO) --> F = CO
  • blood flow is directly proportional to the pressure difference (P) between 2 points in the circulation --> increase pressure difference, increase blood flow
  • blood flow is inversely proportional to the peripheral resistance in the circulation (R) --> increase resistance, decrease blood flow
  • F = P/R or CO = P/R
• Factors affecting circulation - can summarize the factors which affect BP with the equation below: mean systemic arterial BP (P) = cardiac output - heart-related factors (CO) x peripheral resistance - vessel-related factors (R)
• Resistance: amount of friction that blood encounters as it flows through vessels - most friction occurs far away from the heart

  • 3 sources of resistance:
  • vessel diameter** 
  • vessel length
  • blood viscosity

   

  **main force of friction is the narrowing of vessel diameter

  Decrease diameter, increase resistance, decrease blood flow
• Cardiac output:

  • refers to the volume of blood pumped per minute by each ventricle of the heart
  • CO = stroke volume (SV) x heart rate (HR)
  • eg. average adult cardiac output:
  • CO = 70ml/beat x 75 beats/min
  • = 5250ml/min
  • = 5.25 L of blood pumped per minute
• Volumes in ventricle:

  End Diastolic Volume (EDV) -NOTE: atria have just contracted so the ventricle will be full of blood

  • total volume of blood in ventricle at end of ventricular relaxation phase (diastole)

  End Systolic Volume (ESV) -

  • total volume of blood in ventricle after the ventricle has contracted and ejected about 60% of the blood from its chambers (40% of the blood usually remains in the ventricle)
• Stroke volume:

  • volume of blood pumped by each ventricle during each heart beat - stroke volume = EDV - ESV
  • stroke volume is affected by two things:
  • stretching of cardiac muscle fibers
  • contractility of cardiac muscle fibers
• Stretching of cardiac muscle fibers:

  • fibers are stretched by the volume of blood inside the ventricle at the end of diastole (EDV)
  • a greater degree of stretch of fibers results in a greater force of contraction (Starling's Law)
  • EDV is influenced by the venous return
• Venous return affected by:

  External factors -

  • increased respiratory movements
  • increase muscular activity to squeeze veins
  • sympathetic nervous system causing veins to constrict

  Blood volume changes - hormones such as the antidiuretic hormone (ADH) causes increase water uptake from kidneys if body gets dehydrated (increase blood volume)

  • increase venous return, increase EDV, increase stretch of fibers, increase cross bridge formation, increase contractile force, increase blood pumped by ventricle, increase stroke volume
• Contractility of cardiac muscle fibers:

  • sympathetic nervous system activity and release of hormones epinephrine and norepinephrine will increase Ca2+ entry into muscle cytoplasm and increase the force of contraction