Week 8 Bioscience Flashcards
1
Q
Blood vessel structure
A
- Blood vessel walls are typically formed from three layers or tunics (coverings)
- Tunica intima – inner layer
- Tunica media – middle layer
- Tunica externa – outer layer
- Tunics surround central space = vessel lumen
Tunics provide specific physical properties that facilitate vessel function:
Structure <—–> Function
2
Q
Elastic arteries
A
- 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
3
Q
Muscular Arteries
A
- Distal to elastic arteries
- Thick tunica media
- Distributing vessels - Change diametre to control blood flow to body regions and organs
4
Q
Arterioles
A
- 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
5
Q
Capillaries
A
- 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
6
Q
Continuous Capillaries
A
- 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
7
Q
Fenestrated Capillaries
A
- 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
8
Q
Sinusoidal Capillaries
A
- 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
9
Q
Venules & Veins
A
- 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
10
Q
Blood Flow
A
- 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
11
Q
Blood Pressure
A
- 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
12
Q
Resistance
A
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
13
Q
Viscosity
A
- 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
14
Q
TPR: Vessel length
A
- Resistance to flow increases as the vessel length increases
- Relatively constant in adults
- Changes over time in children with growth
15
Q
TPR: Vessel Diametre
A
• 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
16
Q
Flow, Pressure & Resistance
A
- 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
17
Q
Systemic Blood Pressure
A
- 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
18
Q
Arterial Blood Pressure
A
- 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
19
Q
Pulse Pressure
A
- 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
20
Q
Mean Arterial Pressure
A
- 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
21
Q
Alterations in Blood Pressure
A
- 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
22
Q
Capillary Blood Pressure
A
- 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
23
Q
Venous Blood Pressure
A
- 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
24
Q
Venous Return
A
- Valves compartmentalise blood to shift it in small volumes and prevent blood backflow
- Muscular pump – skeletal muscle contraction squeezed veins and helps push blood toward the heart
- Respiratory pump – pressure changes during breathing help blood move toward the heart by squeezing abdominal veins as thoracic veins expand
- Pulsation of nearby arteries
- Venoconstriction of tunica media under sympathetic control
25
Regulation of Blood Pressure
* 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)
26
Factors that Determine Blood Pressure – Cardiac Output
* 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
27
Factors that Determine Blood Pressure - Resistance
* Peripheral resistance (TPR) = rapid, short-term regulation of BP and blood flow
- Primarily altered by changing arteriole diametre
28
Factors that Determine Blood Pressure – Blood Volume
* 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
29
Regulation of Blood Pressure
* 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
30
Autoregulation: Local Regulation of BP and Flow
* 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
31
Neural Regulation
* The cardiovascular centres in the medulla oblongata of the brainstem contain 3 centres:
1. Cardioinhibitory centre
2. Cardioacceleratory (cardiostimulatory) centre
3. Vasomotor centre
32
Neural regulation relies on the baroreceptor reflex
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
33
Baroreceptor Reflexes
Function rapidly to protect against (short-term) changes in blood pressure (and thus maintain blood flow)
34
Renal Mechanisms
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)
35
Endocrine Regulation
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
36
Capillary Dynamics
* 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)
37
Bulk Flow of Fluid
* 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
38
Hydrostatic Pressure
* 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
39
Colloid Osmotic Pressure
* 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
40
Oedema
* 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
