Blood vessels Flashcards
The left side of the heart pumps blood through an estimated _________ km of blood vessels.
100,000 km
Function of blood vessels
Maintain homeostasis
Adjust velocity and volume of blood
Hemodynamics
The study of the mechanics involved in circulating blood within the blood vessels of the body.
What are the layers of the walls of a blood vessel?
Tunica interna
Tunica media
Tunic external
Tunica interna
Epithelial inner lining of a blood vessel.
Tunica media
Middle layer of a blood vessel wall.
Made of smooth muscle and elastic CT.
Displays greatest variation among different vessel types.
Regulates diameter of lumen, effecting blood pressure and rate of flow.
Helps limit blood loss with injury.
Allows stretch and recoil.
Separated from tunica externa by external elastic lamina.
Tunica externa.
Outer covering of a blood vessel wall.
Connective tissue. Elastin and collagen, numerous nerves and blood vessels.
Helps anchor vessel to surrounding tissue
Three components of the tunica interna
- Endothelium (inside layer). Exposed to blood in the lumen. Continuous through CV system. Composed of simple squamous epithelium.
- Basement membrane. Reticular fibres that provide support for epithelial layer.
- Internal elastic Lamina. Boundaey between tunica interna and media. Mainly elastic fibres to allow for distensibility and stretch.
Vaso vasorum
Small blood vessels that supply blood to the tissues of the vessels.
Internal elastic lamina
Boundary between tunica interna and tunica media.
Mainly elastic fibres.
External elastic lamina.
Separates tunica externa and tunica media
Arteries
Three typical layers. High compliance (adaptable to changes in volume and pressure)
Innervated by ANS
Elastic and muscular
Vasoconstriction
Decrease in diameter of artery/arteriole
Sympathetic response to need to increase BP, restrict blood flow to damaged area
Vasodilation
Caused by decrease in sympathetically (not parasympathetic activity)
Increase diameter of artery/arterioles
Elastic arteries
“Conducting arteries”
Largest diameter arteries in the body
Aorta, pulmonary trunk, major aortic branches
Walls thin relative to diameter.
Thick turnica media made of elastic fibres called elastic lamellae
Stretch and expand to accommodate surge of blood during ventricular systole.
Pressure reservoir: store mechanical energy, so can still propel blood forward when ventricles relaxes.
Muscular arteries
“Distributing arteries”.
Medium sized arteries.
The tunica media contain more smooth muscle and fewer elastic fibres than elastic arteries.
Well defined internal elastic lamina, thin externa elastic lamina.
Turnica externa has loose structure, and often thicker than turnica media.
Greater vasoconstriction and vasodilation
Responsible for control of rate of blood flow.
Cannot recoil and propel blood.
Vascular tone.
The ability of the muscle to contract and maintain a state of partial contraction.
Similar to resting muscle tone.
Important in maintaining pressure and efficient blood flow.
Anastomoses
Union of branches of two or more arteries, veins, arterioles or venules supplying the same body region.
Collateral circulation
Alternative route of blood flow to a body part through an anastomoses.
End arteries
Arteries without collateral circulation
Lead to capillary beds.
Arterioles
Small artery, “resistance vessels”
Not much bigger than capillaries.
Regulate blood flow into capillary beds.
Thin turnica interna.
Thin fenestrated internal elastic lamina (smaller than RBC).
Turnica media consists of 1-2 layers of smooth muscle
Turnica externa – loose CT containing unmyleinated sympathetic nerves. (Impact blood flow and resistance via diameter change)
Key role: regulating blood flow from arteries into capillaries by regulating resistance.
Metarteriole
Terminal end of arteriole, toward capillary bed.
Precapillary sphincter
At junction of arteriole and capillary.
Monitors blood flow.
Resistance
Opposition to blood flow.
Increased resistance –> vasoconstriction.
Decreased resistance –> vasodilation
Capillaries
Exchange vessels.
Primary function: substance exchange between blood and interstitial fluid.
Lack tunica media and tunica externa.
Walls composed of only a single layer of endothelial cells and a basement membrane.
Microcirculation
Blood circulating through capillaries.
Metarteriole – (precapillary sphincter) –> capillary bed –> postcapillary venule
Can go straight from metarteriole to post capillary venule
Capillary bed
Network of 10-100 capillaries arising from a single metarteriole
Thoroughfare channel
A capillary that connects distal metartiole to start of venules.
No smooth muscle.
Blood bypasses capillary bed.
Vasomotion
Intermittent contraction and relaxation of smooth muscles.
Occurs approximately 5-10 times/min
Due to chemicals (ie NO) released by endothelial cells.
Types of capillaries
- Continuous (classic)
- Fenestrated (windowed)
- Sinusoid. (Bill the Cat)
Continuous capillary
- continuous endothelium with intermittent breaks called Intercellular Clefts.
- brain, lungs, skeletal, smooth mm, CTz
- normal sized products.
Fenestrated capillaries.
Tiny pores present in endothelium
Larger products
Kidneys, villi of small intestine, endocrine glands.
Sinusoid capillaries.
Wider and more winding Really big fenestrations Missing basement membrane Large intercellular clefts. Found in RBM, spleen, liver, anterior pituitary, parathyroid glands.
Portal system
Allows passage of blood from one capillary bed to another through a portal vein.
Named from second capillary bed.
Two main portal system locations
- Liver (hepatic portal)
2. Pituitary gland (hypopophyseal)
Venules
Venules drain capillary blood and begins to return blood to heart.
Post capillary venules very porous and function as significant sites of exchange.
Muscular venules have 1-2 layers of smooth muscle; no exchange.
Veins
Turnica interna and media much thinner, which much less smooth muscle and elastic fibres.
Turnica externa thickest. Can increase and decrease in size to adapt to volume and pressure, but with recoil.
No elastic laminae.
Venous valves
One way
Veins and venules have such little BP that valves are necessary to prevent pooling
Blood also returns to the heart via pumping of heart and skeletal mm contractions.
Vascular (venous) sinus
A vein with a thin endothelial wall, and no smooth muscle.
Surrounding dense connective tissue replaces turnica media and externa in providing support.
Ex dural venous sinus. Or coronary sinus.
Anastomotic veins
Form “rungs” between paired veins.
Why are veins more numerous than arteries?
Veins follow same paths as muscular arteries but are often doubled.
Unaccompanied superficial veins in subcutaneous layer, which connect via anastomoses to deep vein in between skeletal muscles.
Varicose veins
Most common in lower extremities and esophagus.
Inadequate closure of valves –> pooling of blood.
Pain, hardened nodules, Edema, altered blood flow and itchiness.
In anal canal: haemorrhoids.
Angiogenesis
Growth of new blood vessel
Blood distribution at rest
64% Systemic veins and venules 12% systemic arteries and arterioles 9%. Pulmonary vessels. 7%. Systemic capillaries 7%. Heart
Capillary exchange
The movement of substances between capillaries/blood and interstitial fluid.
Three modes of capillary exchange
- Diffusion
- Transcytosis
- Bulk flow
Diffusion (capillary exchange)
Substances move passively down concentration gradient.
Important for exchange of water soluble (glucose etc) and lipid soluble (O2, CO2, steroids) solutes.
How do water soluble, lipid soluble and large particles diffuse through capillaries?
Water soluble – through intercellular clefts or fenestrations
Fat soluble – though endothelial cells
Plasma proteins and RBCs – can only pass through sinusoids
Blood Brain Barrier and capillary exchange
Continuous capillaries that are sealed together by tight junctions.
Most substances blocked.
Areas that lack barrier are: hypothalamus, pineal gland, pituitary gland.
Transcytosis
Capillary exchange by pinocytic vesicle.
Large lipid insoluble substances
Hormones (insulin) and certain large proteins (ABs)
Bulk Flow
Passive process in which large numbers of ions, molecules and particulates in a fluid move together in the same direction
Faster than diffusion alone can account for.
Involves:
1) filtration (Movement from capillaries to interstitial fluid)
2) reabsorption (movement from interstitial fluid to capillaries)
Important for regulation of the relative volumes of blood and interstitial fluid.
Filtration
Capillaries –> interstitial fluid
Involves:
A) blood hydrostatic pressure (BHP)
B) interstitial fluid osmotic pressure (IFOP)
Blood Hydrostatic Pressure
BHP
Filtration pressure.
The pressure against vessel walls generated by the pumping action of the heart
35 mmHg at arteriole
16 mmHg at venous
Interstitial Fluid Osmotic Pressure (IFOP)
Filtration pressure that pulls fluid from capillaries into interstitial fluid
1 mmHg
Reabsorption
Part of bulk flow.
Interstitial fluid –> blood
Dependent on:
A. blood colloid osmotic pressure (BCOP)
B. interstitial fluid hydrostatic pressure (IFHP)
Blood colloid osmotic pressure
BCOP
Reabsorption pressure caused by large molecules that pull fluid from interstitial fluid to blood.
26 mmHg
Interstitial fluid hydrostatic pressure
IFHP
Reabsorption pressure that pushes fluid from interstitial space into capillaries.
-1 to +1 mmHg. Effectively nada
Net filtration pressure (NFP)
The balance of pressures that dictate the direction of fluid flow.
NFP = forces of filtration - forces of reabsorption =
(BHP+IFOP) - (BCOP+IFHP)
Normally BCOP + IFHP = 26 + 0
And IFOP = 1, so only variable is BHP.
Arterial: 35+1 - 26 = +10 mmHg (–> filtration)
Venous: 16+1 - 26 = -9 mmHg (–> reabsorption)
Starling’s Law of Capillaries
The overall volume of fluids and solutes reabsorbed is the same as the volume filtered. (=> homeostasis)
Edema
Abnormal increase in interstitial fluid volume. (Due to excess filtration or inadequate reabsorption).
Usually noticeable after interstitial volume >30% above normal.
Blood flow
Volume of blood flowing through tissue in a given time.
Total blood flow = cardiac output.
mL/min
Depend on:
- Pressure difference (BP)
- Resistance
Blood Pressure difference involves:
- Systolic BP
- Diastolic BP
- Mean arterial pressure
Mean Arterial Pressure (MAP)
Average BP in the arteries
MAP = diastolic BP + 1/3(systolic BP - diastolic BP)
If 110/70 = 70 + 1/3(110-70) = 83 mmHg.
If 120/80 = 93 mmHg
Resistance (hemodynamics)
Opposition to blood flow due to friction.
Dependent on:
1 size of lumen (inverse)
2. Blood viscosity
3. Total blood vessel length.
Systemic vascular resistance
AKA total peripheral resistance (TPR)
The total resistance of the entire vascular system.
Arterioles contribute the most to SVR. Arteries are too big, venules and capillaries have no muscle.
Venous return.
The volume of blood flowing back to the heart through systemic veins.
Four mechanisms:
1. L ventricle origin of pressure. Only 16mmHg by veins, nada by R atrium
- One way valves (fill-pool-empty mechanism). Coincides with:
- Skeletal muscle contractions
- Respiratory pump.
Respiratory pump
On inhalation, decrease in thoracic cavity pressure–> increase in abdominal cavity pressure –> abdominal veins compress and move blood from abdominal to thoracic cavity
On exhalation, valves prevent backflow as pressure reversed.
Cross sectional area and velocity of blood flow
The greater the total cross sectional area, the slower the velocity
Capillaries have greatest total cross sectional area.
Syncope
Fainting, LOC due to lack of blood flow to the brain.
Orthostatic hypotension
Decrease in BP from change in body position. Can lead to syncope.
Four receptors of sensory input affecting BP and blood flow
- Baroreceptors
- Chemoreceptors
- Proprioceptors
- Limbic system and higher brain system.
Baroreceptors
Measure internal pressure changes.
Carotid sinus reflex: pressure in the brain and internal carotids . Glossopharyngeal nerves (CN IX)
Aortic reflex: systemic reflex in body and ascending and arch of aorta. Vagus nerve (CN X)
Chemoreceptors
Measure chemical changes in body. O2, CO2 and H+ concentrations.
Carotid body – measures the brain, in carotid sinus
Aortic body – measures systemics, in arch of aorta
Proprioceptors
Cells on synovial membranes, tendons, ligaments and skeletal muscle that activate when exercise is initiated.
Limbic system and higher brain centres
Emotional and psychological signals descend directly onto CV centres.
Three motor nerves involved in BP and blood flow
1) Cardiac accelerator nerve
- - thoracic spine, innervate sympathetics.
- - increase HR, contractibility and cardiac output
2) Vagus Nerve (CNX)
- - innvervates parasympathetics
- - decrease HR, contractibility and cardiac output
3) Vasomotor nerve
- - innervates sympathetics, causing vasoconstriction of blood vessels
- - run from CV centre, exit spinal cord through a T spine and L1-2 nerves, and pass into the sympathetic trunk ganglia to blood vessels.
- - constant stimulation of smooth muscles.
Vasomotor tone
The moderate state of vasoconstriction that sets the resting level of tone for most blood vessels (resting SVR)
Four hormones involved in BP/blood flow
- Renin-angiotensin-aldosterone
- Epinephrine and norepinephrine
- Anti diuretic hormone
- Atrial natiuretic peptide
- only atrial natiuretic peptide lowers BP
Renin-angiotensin-aldosterone (RAA)
Decreased BP and volume detected by juxta cells in kidneys, which secrete renin.
Renin + angiotensin-converting-emzyme (ACE) produce Angiotensin II, which raises BP by:
- Vasoconstriction
- Stimulates aldosterone which increases reabsorption of Na+ and H2O (–> increased blood volume –> increased BP).
Epinephrine and norepinephrine and BP
Secreted by adrenal medulla.
Increase rate and force of heart contraction.
Vasoconstriction of vessels to skin and abdominal organs, vasodilation of vessels to cardiac and skeletal muscle.
Antidiuretic Hormone (ADH)
AKA vasopressin
Produced by hypothalamus and released by posterior pituitary.
Vasoconstriction and thus increased BP
Also increased reabsorption of H2O by kidneys
Atrial Natiuretic Peptide (ANP)
Released by atrial cells of heart
Vasodilator. And increase H2O loss in kidneys
DECREASES systemic BP
Types of stimuli that cause autoregulatory changes in BP.
- Physical changes (temperature, myogenic response)
- Chemicals that vasodilate/constrict
Dilate: K+, H+, lactic acid, adenosine, NO
Constrict: thromboxane A2, serotonin, endothelians - Low O2 level
Low systemic - dilate
Low pulmonary – constrict.
Myogenic response
Smooth muscle contracts more forcefully when it is stretched, and relaxes when stretch lessens.
–> when blood flow decreases, vasodilation, increased blood flow.
Shock
Reaction to systemic low BP
Types of shock
- Hypovolemic (decreased total volume of blood)
- Cardiogenic (poor/absent heart function)
- Vascular (too much vasodilation/ not enough vasoconstriction). Ex. Anaphylactic or neurogenic shock.
- Obstructive. (Thrombus or embolus)
Homeostatic responses to shock
Activation of rennin-angio-tensin (RAT) system (increase BP)
Secretion of ADH
Activation of sympathetic system.
– (nor)epinephrine
Release of local vasodilators
Pulse
Travelling pressure wave generated by systole and diastole of the ventricles.
Korotkoff sounds
Sounds heard through stethoscope when measuring BP.
Systolic – initial onset of sound
Diastolic – point at which sound disappears.
Pulse pressure
Difference between systole and diastole (around 40 mmHg)
