Week 8 Bioscience

Question 1

Blood vessel structure

Answer 1

• Blood vessel walls are typically formed from three layers or tunics (coverings)
  
  1. Tunica intima – inner layer
  2. Tunica media – middle layer
  3. Tunica externa – outer layer
• Tunics surround central space = vessel lumen
  Tunics provide specific physical properties that facilitate vessel function:
  Structure <—–> Function

Question 2

Elastic arteries

Answer 2

• Thick-walls
• Located near the heart
• Diametre: large (1 – 2.5 cm)
• Elastin preset in all tunics
• Conducting vessels – conduct blood away form the heart

Question 3

Muscular Arteries

Answer 3

• Distal to elastic arteries
• Thick tunica media
• Distributing vessels - Change diametre to control blood flow to body regions and organs

Question 4

Arterioles

Answer 4

• Smallest arteries
• Diametre 10 µm – 0.3 mm
• Predominantly tunica media
• Resistance vessels: change their diametre to control resistance to blood flow - controls flow into capillary beds within specific tissues/organs
• Vasoconstriction - decreases blood flow
• Vasodilation - increases blood flow

Question 5

Capillaries

Answer 5

• Microscopic vessels
• Average length 1 mm, diametre 8-10 µm
• Thin walls of tunica intima and a supportive basement membrane
• There are ~ 40 billion capillaries in an adult body
• Exchange vessels - exchange of nutrients, wastes, gases, hormones etc. with interstitial fluid and thus with cells
• Rich supply in most tissues – but there are exceptions
• Three structurally and functionally distinct types of capillaries
  1. Continuous capillaries
  2. Fenestrated capillaries
  3. Sinusoidal capillaries

Question 6

Continuous Capillaries

Answer 6

• Endothelial cells joined by tight junctions to form a smooth, lining
• Intercellular clefts - some gaps between endothelial cells allow limited passage of fluids and small solutes
• Pinocytotic vesicles ferry fluids and larger solutes across the capillary wall

Question 7

Fenestrated Capillaries

Answer 7

• Endothelial cells contain pores (fenestrations)
• Pores increase permeability to allow rapid exchange of fluids and small solutes
• Found in areas of active filtrations (kidneys), absorption (small intestine) or in endocrine glands

Question 8

Sinusoidal Capillaries

Answer 8

• Most leaky capillaries
• Large spaces between endothelial cells (sinusoids) and large fenestrations, incomplete basement membrane
• Slow blood flow allows large molecules and cells to pass between the blood and tissues
• Found in the liver, lymphoid organs, adrenal medulla

Question 9

Venules & Veins

Answer 9

• Capillaries unite to form venules
• Venules unite to form veins
• Large lumen - easy blood flow
• Tunica intima folds to form valves
• Little smooth muscle or elastin
• Thick tunica externa of collagen fibres
• Capacitance vessels: thick tunica externa provides support for accommodating a large blood volume

Question 10

Blood Flow

Answer 10

• Blood flow is the volume of blood flowing through a vessel, organ or the entire circulation (= cardiac output) in a given time period.
• Measured in ml/min
• Relatively constant when at rest
• Varies in individual tissues/organs, depending on need
• Blood flow is determined by
• Blood pressure
• Resistance The goal of the cardiovascular system is to maintain adequate blood flow

Question 11

Blood Pressure

Answer 11

• Blood pressure is the force exerted on a vessel wall by the blood in that vessel.
• Expressed in mmHg
• Measured as systemic arterial blood pressure in large arteries near the heart
• Force, generated by the pumping action of the heart, that keeps blood moving (i.e. maintains blood flow) - blood moves from an area of high pressure to lower pressure, i.e. down a pressure gradient

Question 12

Resistance

Answer 12

Resistance is the opposition to blood flow. Resistance is a measure of the amount of friction blood encounters as it flows through a vessel.
Three primary sources of total peripheral resistance (TPR):
1. Blood viscosity
2. Total blood vessel length
3. Blood vessel diametre

Question 13

Viscosity

Answer 13

• Thickness or stickiness of a fluid
• Due to the concentration of blood cells and plasma proteins
• Normally fairly constant but alters with changes in:
• Blood cell numbers (i.e. changes in RBC)
• increased haematocrit (e.g. polycythaemia) leads to decreased­ viscosity
• decreased haematocrit (e.g. anaemia) leads to increased viscosity
• Plasma volume, e.g. dehydration leads to increased viscosity

Question 14

TPR: Vessel length

Answer 14

• Resistance to flow increases as the vessel length increases
• Relatively constant in adults
• Changes over time in children with growth

Question 15

TPR: Vessel Diametre

Answer 15

• Changes in blood vessel diametre are
• Frequent
• Significantly alter resistance and blood flow
• Small diametre arterioles act as resistance vessels
• Vasoconstriction → decreases diametre → increases
resistance → decreases blood flow
• Vasodilation → increases diametre → decreases
resistance → increases blood flow
• Changes in resistance (via changes in arteriole diametre) are the primary means of altering local blood flow

Question 16

Flow, Pressure & Resistance

Answer 16

• Blood flow (F) is determined by:
• The difference in blood pressure between two points in the circulation (i.e. the pressure gradient)
• Resistance (R)

F= P/R

Question 17

Systemic Blood Pressure

Answer 17

• The pumping action of the heart generates pressure, which in turn, drives blood flow
• Blood flows down a pressure gradient – from an area of high pressure to an area of low pressure
• Blood flow is opposed by resistance
• Blood pressure decreases with distance from the heart as it overcomes resistance to drive blood flow

Question 18

Arterial Blood Pressure

Answer 18

• Arterial pressure reflects two factors:
  1. How much elastic arteries can be stretched, i.e. COMPLIANCE
  2. The volume of blood forced into the elastic arteries by ventricular contraction, i.e. STROKE VOLUME (e.g. increased SV leads to increased pressure as more blood moves into the arteries and pushes on the artery walls)
• Blood pressure in elastic arteries near the heart is pulsatile (i.e. not constant) – due to the pumping action of the heart leads to gives rise to two extremes of pressure

Question 19

Pulse Pressure

Answer 19

• Pulse pressure = systolic pressure minus diastolic pressure i.e. 120 – 80 = 40 mmHg
• Felt as a throbbing pulsation in an artery (a pulse)
• Pulse points - superficial arteries, often overlying bone
• Declines with increasing distance from the heart

Question 20

Mean Arterial Pressure

Answer 20

• Mean arterial pressure (MAP) = the pressure that propels blood through the vessels
• Average between systolic and diastolic pressure (as diastole lasts longer than systole, MAP is not halfway between these two pressures – see next slide)
• Declines with increasing distance from the heart
• Defined as: MAP = diastolic pressure + (1/3 x pulse pressure) = 80 + (1/3 x 40) = 93 mmHg
• MAP can also be calculated as follows:
  1. If blood Flow F = P/R or F = MAP/R
  2. and total blood flow = cardiac output à CO = MAP/R
  3. Then rearrange the equation à MAP = CO x R
• Anything that ­ CO or R will ­ BP
  -­ R via vasoconstriction à there is less room in the vessel for the blood (¯ lumen volume), thus blood pushes harder against the vessel walls - ­ BP and
• Vasoconstriction will ­ venous return - ­ CO - ­ MAP
  -­ CO (i.e. via ­ HR or SV) - ­ MAP

Question 21

Alterations in Blood Pressure

Answer 21

• Hypotension
  
  • Systolic pressure is below 90 mmHg
  • May result in dizziness and fainting
• Hypertension
  
  • Transient elevation due to exercise, illness, emotions
  • Chronic hypertension
  • Sustained systolic pressure > 140 mmHg
    
    • Major cause of heart failure, vascular disease, stroke
    • Risk factors: smoking, stress, diet, obesity, age, health

Question 22

Capillary Blood Pressure

Answer 22

• Ranges from
• 35 mmHg at the arterial end of the capillary bed to
• 15 mmHg at the venous end of the capillary bed
• Low capillary pressure is required because
  1. High pressure would damage thin-walled, fragile capillaries
  2. Most capillaries are very permeable so low pressure is adequate for fluid exchange with tissues

Question 23

Venous Blood Pressure

Answer 23

• Fairly constant at ~ 15 mmHg
• Does not change significantly with cardiac cycle
• Very small pressure gradient, too low to provide adequate venous return to the heart

Question 24

Venous Return

Answer 24

1. Valves compartmentalise blood to shift it in small volumes and prevent blood backflow
2. Muscular pump – skeletal muscle contraction squeezed veins and helps push blood toward the heart
3. Respiratory pump – pressure changes during breathing help blood move toward the heart by squeezing abdominal veins as thoracic veins expand
4. Pulsation of nearby arteries
5. Venoconstriction of tunica media under sympathetic control

Question 25

Regulation of Blood Pressure

Answer 25

* Regulation of blood pressure is essential to ensure adequate blood flow to vital organs - the brain must co-ordinate the activities of the heart, blood vessels and kidneys * Factors that determine blood pressure (and thus blood flow) include: 1. Cardiac output (rapid, short-term regulation) 2. Peripheral resistance (rapid, short-term regulation) 3. Blood volume (slower, long-term regulation)

Question 26

Factors that Determine Blood Pressure – Cardiac Output

Answer 26

* Cardiac output = rapid, short-term regulation of BP and blood flow * CO = stroke volume x heart rate * Determined by EDV (preload), blood volume, venous return, ESV, contractility, ANS, hormones, plasma electrolytes * SV or HR - ­ CO (= an ­ in the volume of blood moving from the heart into the arteries) - ­ BP * Cardiac output directly determines blood pressure

Question 27

Factors that Determine Blood Pressure - Resistance

Answer 27

* Peripheral resistance (TPR) = rapid, short-term regulation of BP and blood flow - Primarily altered by changing arteriole diametre

Question 28

Factors that Determine Blood Pressure – Blood Volume

Answer 28

* Blood volume (BV) = slower, long-term regulation of BP * Controlled by renal and endocrine mechanisms * Changes in blood volume alter venous return, EDV and preload, and thus SV, CO and BP * Small increases in blood volume offset by vessel compliance (stretch) but compliance decreases with age/atherosclerosis

Question 29

Regulation of Blood Pressure

Answer 29

* Blood pressure, and thus blood flow, are regulated at multiple levels: 1. Autoregulation – occurs within tissues and is dependent on local conditions 2. Neural regulation – involves the cardiovascular centres, the autonomic nervous system and the baroreceptor reflex 3. Renal mechanisms 4. Endocrine regulation The above mechanisms alter cardiac output, peripheral resistance and/or blood volume in order to regulate blood pressure and thus maintain adequate blood flow

Question 30

Autoregulation: Local Regulation of BP and Flow

Answer 30

* Tissues regulate their own blood pressure and flow in response to local conditions by altering arteriole diameter to regulate blood flow into capillary beds (a) Arterioles dilated—blood flows through capillaries. Arteriole (b) Arterioles constricted—no blood flows through capillaries. * Intrinsic regulation: (control from within) * Metabolic control

Question 31

Neural Regulation

Answer 31

* The cardiovascular centres in the medulla oblongata of the brainstem contain 3 centres: 1. Cardioinhibitory centre 2. Cardioacceleratory (cardiostimulatory) centre 3. Vasomotor centre

Question 32

Neural regulation relies on the baroreceptor reflex

Answer 32

Baroreceptors: * Stretch receptors * Detect changes in pressure * Inform the medullary cardiovascular centres * Locations: - Carotid artery sinuses - Aortic arch - Walls of most large arteries in the neck and thorax * Initiate the baroreceptor reflex

Question 33

Baroreceptor Reflexes

Answer 33

Function rapidly to protect against (short-term) changes in blood pressure (and thus maintain blood flow)

Question 34

Renal Mechanisms

Answer 34

1. Direct mechanism * Blood pressure directly determines the rate of urine formation - alters blood volume - alters blood pressure 2. Indirect mechanism – involves hormones: Renin-angiotensin-aldosterone system (RAAS)

Question 35

Endocrine Regulation

Answer 35

Hormones that increase BP: * Adrenalin and noradrenalin - rapid ­ in CO (­ both SV & HR) and peripheral vasoconstriction * Angiotensin II - vasoconstriction, thirst, and promotes secretion of: - Aldosterone - renal sodium ion and water reabsorption - ADH - peripheral vasoconstriction and ­ renal water reabsorption Hormones that decrease BP: * Atrial natriuretic peptide (ANP) * Produced by the heart in response to high BP

Question 36

Capillary Dynamics

Answer 36

* Blood flow through capillaries is slow and intermittent – controlled by arteriole diamtere in response to local conditions (e.g. low O2 or high CO2 levels) * When blood flows through capillary beds exchange occurs * Capillary exchange involves: 1. Exchange of solutes - respiratory gases, nutrients and wastes 2. Bulk flow of fluid (which carries solutes)

Question 37

Bulk Flow of Fluid

Answer 37

* Fluid moves across capillary walls by bulk flow - out of the capillary at the arterial end - into the capillary at venous end · Fluid moves through: - Intercellular clefts (between endothelial cells in all capillaries) - Fenestrations (pores within endothelial cells in some capillaries) - Sinusoids (big gaps between endothelial cells in some capillaries) · Bulk fluid flow determines the relative fluid volumes of the blood and ISF · Direction and volume of fluid movement is determined by two opposing forces: hydrostatic and colloid osmotic pressures

Question 38

Hydrostatic Pressure

Answer 38

* Force exerted by fluid pushing against a tissue wall * Capillary hydrostatic pressure (HPc) - Force of the blood plasma on the capillary walls = blood pressure - Pushes fluid and solutes out of capillaries through intercellular clefts/fenestrations/sinusoids at the arterial end of bed - Some fluid, cells and most proteins remain in the capillary

Question 39

Colloid Osmotic Pressure

Answer 39

* Force related to the tonicity (solute concentration) of a solution - pulling force * Capillary colloid osmotic pressure (OPc) - Due to the presence of solutes within the plasma that are unable to diffuse out of the capillary, e.g. proteins - These solutes pull fluid back into the capillaries at the venous end of the bed

Question 40

Oedema

Answer 40

* Oedema is an abnormal increase in the volume of interstitial fluid * Due to an increased push, or decreased pull, force: * Increased capillary hydrostatic pressure (i.e. BP) * High blood volume, local vessel blockage, incompetent venous valves * Inflammation - causes increased capillary permeability * Decreased colloid osmotic pressure * Low levels of plasma proteins due to liver disease or malnutrition * Blockage of lymphatic vessels - preventing fluid drainage from tissues * Parasites, surgical removal