Second half of chapter 12

function of the vascular system (2)

• distribution of blood flow to the various tissues

• regulation of blood pressure

Vessels of the circulatory system

• arteries

• arterioles

• capillaries

• venules

• veins

What is the significance of branching and regional specializations of blood vessels

• to allow for an efficient matching of blood flow to metabolic demand in individual tissues

Anatomy of blood vessels

• lumen

• tunica intima

• tunica media

• tunica externa

• capillaries

lumen

internal space where the blood circulates

tunica intima

• innermost layer made of endothelium

• regulates solute exchange

tunica media

• contractile layer of the vessel

• all blood vessels except capillaries

• regulates vascular tone, blood flow and pressure

• circular layer of smooth muscle that has sympathetic innervation

• layers of elastic connective tissue

tunica externa (adventitia)

• layer of connective tissue

• all blood vessels except capillaries

• vessel reinforcement, anchoring to the surrounding structures

• collagen fibers, elastic fibers, nerve fibers and lymphatic vessels

Capillaries

• basement membrane and endothelium

elastic arteries

• largest lumens

• lots of elastic fibers

• act as pressure reservoirs

• expand when the heart pumps

• recoil when the heart relaxes

muscular arteries

• branch from the elastic arteries

• lumen diameter smaller than elastic arteries

• thick tunica media with lots of smooth muscles

  • higher contractility

• distribute and control the general blood flow to specific organs

arterioles

• branch from small muscular arteries

• smallest vessels in the arterial circut

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Types of blood vessels

• elastic arteries

• msucular arteries

• arterioles

• capillaries

  • continuous capillaries

  • fenestrated capillaries

  • sinusoid capillaries

• post capillary venules

• veins

  • venous valves

arterioles

• branch from small muscular arteries

• smallest vessels in the arterial circuit

• large arterioles → three tunics

• small arterioles → single layer of smooth muscle over the endotherlium

• high contractility

• resistance vessels

main point of control of blood pressure and flow to specific organ regions in response to different stimuli

• arterioles

• Stimuli:

  • neural → sympathetic

  • hormonal → angiotensin II, endothelia, vasopressin, epinephrine / norepinephrine

  • local chemical changes → NO, CO2, pH

capillaries

• branch from arterioles and form capillary beds

• smallest blood vessels

• thin endothelium and basement membrane

  • contractile pericytes in specific places (pre capillary spinchters) to control local flow and permeability

• exchange vessels

  • easy diffusion of solutes and gases due to their thin wall

continuous capillaries

• in skin, muscle, lungs and CNS

• presence of pericytes

• transcellular trasport of solutes

• some intercellular clefts for paracellular transport

fenestrated capillaries

• in areas with high rate of filtration or absorption (kidney or small intestine)

• fenestrations coated with glycoproteins that perforate the endothelium

• higher paracellular transport of solutes

sinusoid capillaries

• in liver, bone marrow, spleen and adrenal medulla

• irregularly shaped and larger than other capillaries

• incomplete basement membrane

• large intercellular clefts and fenestrations

• exchange of large molecules and cells

postcapillary venules

• collect blood from the capillaries

• single layer of endothelium and pericytes

  • larger venules have a thin tunica media

• very permeable

  • easily transport fluids and WBC

Veins

• large lumen diamteres

• thin walls and large lumens compared to the corresponding arteries

  • little proportion of smooth muscle and elastin in the tunica media

  • large tunica externa → high proportion of collagen and elastic fibers

• highly compliant vessels

  • can expand to fit more volume → capacitance vessels

special structure related to the veins

• venous valvues

  • prolongations of the tunica intima (similar to the semilunar valves)

  • abundant in the veins of the limbs

  • absent in the veins of the abdominal and thoracic cavity

  • prevent back flow of blood in the vein

why do the veins require valves and the arteries dont

• arteries have high pressure driven by the heart and almost forces the blood to move in one direction

• veins move very slowly and do not have as much pressure, therefore it is more likely for it to slide back and thus the valves help prevent backflow

what is required to circulate the blood through the blood vessels

a differential of pressure is required

3 mechanisms the veins use to compensate the low pressure to return the blood to the heart

• muscular pump

• respiratory pump

• sympathetic vasoconstriction

muscular pump

• skeletal muscle contracting and squeezing the blood through the valves

• values prevent backflow when the muscle relax

respiratory pump

• during inspiration

  • pressure in the abdominal cavity rises

  • pressure in the thoracic cavity drops

Sympathetic vasoconstriction

• sympathetic stimulation

  • vein smooth muscle contraction → blood gets squeezed back to the heart

what region of the vascular network is the most important to sustain body functions? why?

Capillaries

• they require low BP, close to constant blood flow and slow flow velocities to not burst and to ensure a proper exchange of solutes

• BP decreases and flow becomes constant as we move away from the heart

where does the biggest drop in pressure and velocity occur and due to what factor

• at the arterioles

• due to high vascular resistance

systolic pressure

• normally 120 mmHg

diastolic pressure

• normally 80 mmHg

Pulse pressure

systolic pressure - diastolic pressure = 40 mmHg

make the elastic arteries expand and recoil

Mean arterial pressure

• calculated pressure that propel the blood through the vessels

• MAP = diastolic pressure + (pulse of pressure / 3)

• Normal MAP = 80 mmHg + (40 mmHg / 3) = 93 mmHg

what does it mean for blood pressure to be pulsatile

it will go up and down in waves rather than flowing in a flat, steady stream

Why is blood pressure no longer pulsatile in the capillaries?

the viscoelastic properties of the arteries progressively muffle the blood pulsatility

as we move away form the heart the pulsatility decreases

why does the pressure decrease along the vascular tree

as blood moves through the vessels, the fluid particles experience friction against the internal walls of the tubes

the friction wears down the PE of the pressure generated by the heart, causing the pressure to drop steadily as blood travels father away from the heart

As a tube gets longer, why does the pressure drop?

• the liquid is experiencing friction against the walls and a dissipation of energy

What happens when we replace the water in a tube for something more viscous and thick?

• there is a much higher resistance to flow

• experience more friction

• PE dissipates faster

• flow decreases and could stop before the end of the tube

factors that determine the flow of a hydraulic system

• the differential of pressure between two points of the circuit

• length of the circuit (length of blood vessels)

• viscosity of the fluid (blood)

• internal diameter of the pipes (luminal radius of the blood vessels)

Ohms law in relation to blood flow

Blood flow = blood pressure / total peripheral resistance

Cardiac output equation

Cardiac output = Mean systemic arterial pressure (MAP) / Total peripheral Resistance (TRP)

How can we change the blood pressure

• change the total peripheral resistance

  • this mainly determined by the radius of the blood vessels

• change the cardiac output

Of the factors that determine blood flow, which drives the flow and relates to MAP and which is resistance against the flow and related to TPR

MAP:

• differential of pressure

TRP:

• Length of the blood vessels

• blood viscosity

• luminal radius of the blood vessels

Which blood vessels are ideal to control the systemic blood pressure and the distribution of blood flow in specific regions of the different organs?

• arterioles

  • still have at least one layer of smooth muscle → contractile

  • have the latest total cross sectional area in the arterial dise

  • smallest cimultaneous contraction of all the arterioles causes a massive change in the total peripheral resistance

  • contraction of arterioles in specific vascular niches within an organ increase the resistance and decreases the flow to specific areas

what are the two factors that affect cardiac output

• stoke volume

• heart rate

what are the three factors that effect total peripheral resistance

• diameter of blood vessels

• blood viscosity

• blood vessel length

neural control

short-term regulation of the blood pressure ho

hormonal control

short term regulation of the blood pressure

explain the hormonal pathway when you are under stress

• the adrenal gas will release epinphrine

• it can cause vasoconstriction through alpha adrenergic receptors

• this will cause vasoconstriction

• then increased TPR

• which will increase the blood pressure

explain the hormonal pathway when blood pressure / volume is low

• the low blood pressure will rigger the kidney to release renin

• this will cause angiotensinogen to convert to angiotensin I

• causing ACE to converted into angiotensin II

• this will cause vasoconstriction

• increased TPR

• increased blood pressure

explain the hormonal pathway when blood pressure / volume I high

• atrial natriuretic pepride is release by the heart when the blood pressure is too high

• this promotes vasodilator to bring the pressure back down

renal control

long term regulation of blood pressure

List the renal control: direct renal mechanism

• low arterial pressure

• low pressure in the glomerulus

• low kidney filatation

• low urine formation

• increased blood volume

• increased mean arterial pressure

list the renal control: indirect renal mechanism

• blood pressure is low

• kidneys release renin into the bloodstream

• angiotensinogen is converted into angiotensin I

• ACE is converted into angiotensin II which triggers the adrenal lads to release aldosterone

• The aldosterone tells the kidneys to pump up their Na+ / K+ ATPase pumps, forcing the kidneys to reabsorb more sodium and water

• this increases the blood volume instead of peeing them out

• reabsorbing more water increases overall blood volume, which results in a higher blood pressure

Hypertension: prevalence, effects and types

sustained increase of the systolic or diastolic pressure

• prevalence:

  • 30-40% of people over 50 years olf

• effects

  • LV hypertrophy → heart failure

  • vasular disease

  • renal failure

  • stroke

• types:

  • primary (essential) hypertension

  • secondary hypertension

causes of primary hypertension

• genetic predispotion

• diet

• obesity

• age

• diabetes mellitus

• stress

• smoking

causes of secondary hypertension

• obstruction of renal arteries

• kidney disease

• endocrine disorders

How can we re-direct blood flow to the organs / tissues that need it?

• by contracting or relaxing specific distributing arteries or arterioles in the circulate system

• flow always follows the pathway of less resistance

How the vessels know when to contact or dilate?

• extrinsic controls of regional blood flow

  • neural and hormonal

  • redirect blood flow to specific organs

• intrinsic controls

  • signals produced by the organ itself to contract or relax arterioles within the organ to distribute blood flow into specific regions

vasodilator signals of intrinsic controls

• low O2

• high CO2

• low pH

• prostaglandins

• adenosine

• nitric oxide

• bradykinin

vasoconstrictor signals of intrinsic controls

• blood vessel stretch

• endothelin’s

cerebral auto regulation pathways

• visual stimulation

• activation of neurons of the visual cortex

• increased neuronal metabolism

• increased CO2 production

• lower pH

• local vasodilation

• increase blood flow in the visual cortex

Once in the capillaries, how do the solutes and fluids move across the vessel walls? depends on:

• type of capillary

• gradient of concentration

• chemical properties

• size of the solute particle