Arteries, arterioles & distribution of blood flow

what are some features of arteries

pressure tubes - act as pressure reservoirs to maintain blood flow through tissue during diastole

large diameters - provides low resistance pathway for conducting blood to organs/tissues

most of the artery consists of smooth muscle and connective tissue

why is blood flow out of the heart considered intermittent

the aortic valve (between L.V & aorta) closed upon ventricular repolarisation

however, blood flow through tissues is continuous

what is systolic blood pressure

• pressure required to stretch the arteries

• about 2/3 of blood flow goes to stretching arteries

• about 1/3 of blood flow goes to arterioles & capillaries

what is diastolic blood pressure

the pressure in your arteries when your heart relaxes between beats

• the 2/3 of blood flow that stretched the arterial walls now goes to arterioles & capillaries (thus continuous flow)

what is an aortic pressure trace

graph that displays key pressure such as:

• Systolic blood pressure

• diastolic blood pressure

• pulse pressure (SBP - DBP)

• mean arterial pressure (DBP + 1/3(PP))

what is a dicrotic notch

a small drop in pressure that represents the closure of the aortic valve upon ventricular repolarisation

how is blood pressure measured

1. inflatable cuff is pumped above systolic pressure (120mmHg), cuts off flow through the brachial artery

2. pressure in cuff is slowly released, thus causing turbulent flow (systolic)

3. once laminar flow (diastolic) is established recording is finished

what are Korotkoff sounds

sounds heard by stethoscope that indicate turbulent flow of blood through the body

what is hypertension & hypotension

hypertension - higher than normal blood pressure

• consistently above about 140/90

hypotension - lower than normal blood pressure

• consistently below about 100/60

these conditions are determined via the mean systolic/diastolic pressures

what are arterioles responsible for

controlling the proportional distribution of blood flow to organs by altering diameter of arteries

how is the diameter of blood vessel altered by arterioles

smooth muscle wraps around the arteries:

• constriction: decreases diameter = decrease blood flow

• relaxation: increase diameter = increase blood flow

what are the 2 local mechanisms for changing arteriole diameter

Autoregulation

• metabolic control

• myogenic mechanism

what is metabolic control

increase in blood flow when metabolic activity is high, occurs by relaxation of arterioles.

• ↑ adenosine

• ↑ [CO₂]

• ↓ [O₂]

• ↑ [H⁺]

results in “active hyperemia”

what is the myogenic mechanism

refers to a decrease in the pressure of an arteriole, after a sudden increase

this is cause by a constriction in the smooth muscle cells

what is the point of the myogenic mechanism

allows organs to maintain constant blood pressure even with a dramatic change in arterial pressure

how are arterioles innervated

mostly innervated by sympathetic nerves

• only genitals have significant innervation

noradrenaline acts on arteriolar muscle adreno-receptors (α) = smooth muscle constriction

what does the reflex control of arterioles consists of

some level of resting sympathetic activity, so smooth muscle is partially constricted, therefore:

• ↑ sympathetic = ↓ blood flow

• ↓ sympathetic = ↑ blood flow

how can arterioles be controlled by hormones

causes constriction by acting on α-receptors

causes dilation by acting on arterioles with β2-adrenergic adrenoreceptors (e.g. skeletal muscles)

what is microcirculation

blood flow from arteriole to venule, through the capillary beds, in which:

• O₂ + nutrients is sent

• CO₂ + waste is collected

what is some information about capillaries at rest & during exercise

rest = many capillaries are closed

during exercise:

• smooth muscle sphincters relax allowing blood to flow through metarteriole & throughfare channel

what are some details about the structure of capillaries

• thin layer of endothelial cells making exchange across the membrane easy

• narrow tube allowing blood cells to flow through in single file

what is the pressure within arterioles & capillaries

arterioles = 40mmHg

capillaries = 20mmHg

how are the different extracellular body fluids separated from each other

plasma is separated by the endothelial layer of capillaries.

• filtration = H₂O & solute transfer from plasma → interstitial fluid

• Absorption = H₂O & solute transfer from interstitial fluid → plasma

describe the movement of substances from the plasma → interstitial fluids

direct diffusion

• lipid soluble / gases (O₂, CO₂)

diffusion through pores (intercellular clefts)

• lipid insoluble = glucose, ions, A.As

Bulk flow

• H₂O

vesicular transport of large molecules

• some hormones

what are the 2 major forces driving & opposing filtration at capillaries

1. hydrostatic / blood pressure in capillaries

   • drives filtration

2. plasma osmotic (colloidal oncotic) pressure

   • caused by plasma protein

   • opposes filtration

explain how the functions of arterioles are influenced by pressure

hydrostatic pressure from blood is high at the end near arterioles, with a lower osmotic pressure

• hence the net pressure allows for the filtration of nutrients into the interstitial fluid

explain how the functions of venules are influenced by pressure

hydrostatic pressure from blood is low at the end near venules, with a higher osmotic pressure

• hence the net pressure allows for the absorption of waste from the interstitial fluid

what is important to note about the middle of the capillary bed

equal hydrostatic & osmotic pressure, hence no exchange

there is a net filtration of ~4L protein-free fluid from plasma into the interstitial fluid, how is this drained

drained by the lymphatic system, by lymphatic capillaries.

it contains blind-ended capillaries that drain fluid through lymphatic veins into subclavian vein of cardiovascular system

what occurs when drainage of extracellular fluids in the body doesn’t occur

results in oedema

• failure to maintain distribution of water thus leading to a build-up

what are some causes of oedema

increased hydrostatic pressure of blood (prevents absorption)

• heart failure, hypertension

decreased osmotic pressure (prevents filtration), caused by drop in plasma protein levels

• malnutrition, liver disease, renal disease

lymphatic drainage disruption (tumors)

disruption on the permeability of capillaries

what is some information about the veins

• low pressure

• large diameter

• low resistance

• thin-walled

• one-way valves

what is venous return & cardiac output

venous return - refers to the volume of blood that flows back to the heart

cardiac output - refers to the volume of blood pumped throughout the body per minute

venous return effects cardiac output

what is the ventricular end diastolic volume

the volume of blood in the ventricle just before a contraction

increase in end diastolic pressure = increase in length of cardiac muscle

what is stroke volume

the amount of blood pumped from the left ventricle throughout the body in one contraction

what are some equations to know regarding the cardiovascular system

cardiac output (CO) = SV x HR

stroke volume (SV) = EDV - ESV

ejection fraction (EF%) = SV/EDV x 100

what are 3 mechanisms that influence venous return

• pressure gradient

• skeletal muscle ‘pump’

• respiratory ‘pump’

how can venous return be increased using a pressure gradient

increasing the pressure of the veins compared to the right atrium will increase venous return

sympathetic activation on smooth muscle around veins stiffens the walls, thus increasing pressure

how is venous return increased by skeletal muscle ‘pump’

contraction of certain muscles within the body causes a blockage of veins, this increases the pressure of the veins driving blood back to the heart

how can venous return be increased by the respiratory ‘pump’

when we breath in (inspiration), the diaphragm move down. This increases the pressure in the abdomen & decreases pressure in the thorax

this pressure gradient drives blood through the abdominal veins back to the heart

what is the Frank-Starling mechanism

the relation between the filling of the ventricle (EDV) and stroke volume

explain the process of muscle contraction

The neurotransmitter noradrenaline from the sympathetic cardiac nerves binds to beta-1 receptors on the heart decrease the depolarisation threshold for the L-type calcium channels, thus more calcium into the cell. Also increases the calcium-induced calcium release as calcium binds to the ryanodine receptor on the surface of the sarcoplasmic reticulum. This allows for more calcium to bind to troponin, this changes the shape of troponin and tropomyosin, thus exposing actin-myosin binding sites. Myosin heads have ATP bound to it causing the formation of a cross-bridge section between the myosin head and actin-myosin binding site. Hydrolysis of the ATP molecule causes the power stroke movement shortening the sarcomere. After which ADP and phosphate are released and a new ATP molecule bind causing the detachment of the myosin head ready for another stroke. SECRA maintains calcium concentration gradient by pumping calcium back into sarcoplasmic reticulum.