Vessels & Hemodynamics Flashcards
How do vessels contribute to homeostasis?
VESSELS:
- Provide flow to and from heart
- Provide tissue exchange
- Adjust velocity & volume of blood flow
What are the three turnicas (layers) of blood vessels?
- Tunica Interna (endothelium)
- Turnica Media (smooth muscles)
- Turnica Externa - Elastic and contains numerous nerves and helps anchor vessels to surrounding tissue.
Turnica Interna is the inner most blood vessel layer - what is it made up of?
Turnica Interna is made up of epithelial cells, collagen fibers, elastic fibers. In direct contact with blood as it flows through lumen
Turnica Media is the middle blood vessel layer - what is it made up of?
Turnica Media is made up of muscular & connective tissue, smooth muscle cells, elastic fibers
Turnica Externa is the outer most blood vessel layer - what is it made up of?
Turnica Externa is made up of numerous nerves, elastic and collagen fibres.
Large arteries are termed:
Large arteries are termed:
Elastic (conducting) arteries
Medium- sizes arteries are called:
Medium- sizes arteries are called:
Muscular (distributing) arteries.
Arteries
- Consists of the three typical layers: Thick muscular and elastic tunica media
- High Compliance: Great stretch and recoil capacity in response to blood pressure
- Arteries must be able to stretch and recoil as they control the flow of blood.
What is an arteries turnica media like?
The tunica media of arteries is innervated by sympathetic fibres of ANS
- Increased SNS stimulation or damage to BV > smooth muscle contraction > narrowing of lumen = vasoconstriction
- Decreased SNS stimulation or presence of certain chemicals > smooth muscle relaxation > increase in lumen diameter = vasodilation
What are the largest arteries in the body?
Elastic arteries - the aorta and pulmonary trunk (they both exit the heart and are the size of a garden hose - with smaller finger size arteries off them).
What am I?
- Medium-sized arteries.
- Tunica media contains more smooth muscle fibers – hence named muscular artery
- Capable of greater vasoconstriction and vasodilation to adjust rate of blood flow
- 3 to 40 layers of circumferentially arranged smooth muscle cells depending on size of artery
- Branch off from larger elastic arteries
- Vessel wall approx 25% of total vessel diameter (thicker than elastic arteries)
- Femoral and axillary artery - pencil-sized
Arteries that enter organs - string-sized
- Have well-defined internal elastic lamina, but thin external elastic lamina
- Tunica interna
Not significantly different
- Tunica media
contains more smooth muscle fibres than elastic arteries
Ability to maintain a state of partial contraction = vascular tone
contains fewer elastic fibers than elastic arteries.
Less ability to recoil to help propel blood
- Tunica externa
Often thicker than tunica media
loosely structured permits diameter change but prevents shortening or retraction if cut
Muscular (Distributing) Arteries
- Medium-sized arteries.
- Tunica media contains more smooth muscle fibers – hence named muscular artery.
- Capable of greater vasoconstriction and vasodilation to adjust rate of blood flow.
- 3 to 40 layers of circumferentially arranged smooth muscle cells depending on size of artery.
R & L Axillary arteries
- armpit
R & L Brachial arteries
- arm
R & L Radial arteries
- forearm
R & L Femoral arteries
– femur of leg
Anastomoses
Union of branches of two or more vessels supplying same body region. Arteries that do not anastomose called end arteries.
Arterioles = Regulate blood flow into capillary networks, what is their structure?
Arterioles = small vessels
Wall thickness 50% of total vessel diameter
Arterioles = small vessels - what is their function?
Contraction of smooth muscles > vasoconstriction > increased resistance > decreased blood flow into capillaries
Arterioles regulate blood flow from arteries to capillaries by regulating resistance
Resistance – the opposition to blood flow, mainly due to friction between blood and inner wall of BV’s. The narrower the lumen, the greater the friction and therefore the greater the resistance.
Why is the venous system is the body’s reservoir.
Blood pressure goes up when blood rushes up. Digestion put on hold when we run/use muscles. The arterioles are involved/responsible for this. They determine where the blood goes, depending on what activity we are undertaking.
Every organ, every cell is part of a capillary bed. Blood shifts to where it is needed (eg to lungs and muscles when running, to venous system + spleen/liver, when resting).
Sphincter & arterioles
If the sympathetic nervous system gives the signal, then the sphincters contract and blood flows.
Precapillary sphincter – composed of most distal muscle cell. Regulates blood flow into and out of the capillary network
What are capillaries?
Capillaries:
- Smallest blood vessel
- Most have diameter of 5 -10mm
- Connect arterial outflow to venous return
- Form extensive network (20billion approx) of short, branched, interconnecting vessels that reach all individual cells of body
What is the function of capillaries?
The primary function of capillaries is exchange of substances between the blood and interstitial fluid. Hence also called ‘exchange vessels’
Structure of capillaries
Structure of capillaries:
No tunica media or externa
Single layer of epithelial cells & basement membrane only
Provides optimal flow of substances from blood to interstitial fluid
Note: Exchange of materials only occurs through the walls of capillaries and the beginning of venules
Vasomotion
Typical intermittent contraction/relaxation occuring 5-10 times per min.
Vasomotion is typical for blood flow through capillaries.
Types of capillaries
Continuous – wont allow much through
Fenestration – openings/windows
Sinusoids – have complete paths missing so larger things can go through.
Continuous capillaries
- Most capillaries are continuous
- Plasma membranes of neighbouring endothelial cells are packed tightly > forms a continuous tube except for the slight gaps between the endothelial cells called intercellular clefts
- Found in: brain, lungs, skeletal & smooth muscle, connective tissues
Fenestrated capillaries
- Plasma membranes of endothelial cells have many ‘fenestrations’
- Small pores ranging from 70 – 100 nm in diameter
- Found in kidneys (as they must filter the blood), villi of the small intestine, cilliary processes of the eye, and endocrine glands
Sinusoids (capillaries)
- Wider and more winding
- Display large fenestrations
- Absent or incomplete basement membrane
- Very large intercellular clefts
allowing large molecules (e.g. proteins and blood cells) to pass
Venules
= Venule=little vein
= Venules display much thinner walls
and do not readily maintain their shape
= Venules drain capillary blood to begin blood return to heart
Post Capillary Venules
Smallest venules
10mm to 50 mm in diameter
- Weakest endothelial contacts of the entire vascular tree
- Loose intercellular junctions
- Very porous
- Form part of microcirculatory exchange: Significant sites of exchange of nutrients, wastes, and WBC emigration
Veins made up of… (as compared to arteries)
- Tunica interna : thinner than arteries
- Tunica media: thinner than arteries– little smooth muscle & elastic tissue
- Tunica externa: thick (similar size to arteries) – collagen and elastic fibres
Veins cant deal with high pressures - low elasticity and recoil
- Internal and external elastic laminae are absent and walls are not as strong as arteries.
- Veins are able to accommodate changes in blood volume and pressure to certain extent - however cannot withstand high pressure.
- Note: lumen in veins is larger than arteries.
Venous valves
- Thin folds of tunica interna forming flaplike cusps
- Valve cusps project into lumen, pointing towards the heart
- Low pressure in veins – blood can slow and tries to flow back (gravity) > valves prevent back flow
Vascular (venous) Sinus
Vascular (venous) sinus
- Vein with thin endothelial wall
- Lacks smooth muscle cells
- Relies on surrounding tissues (dense connective) to provide support, eg. dural venous sinuses & coronary sinus
Blood distribution at rest:
- 64% in veins and venules
- 7% in capillaries
- 13% in arteries and arterioles
- 7% in heart
- 9% in pulmonary vessels
Veins that have the blood sitting there instananeously constrict – so to divert blood to where/when it is required. Not much in the heart nor in pulmonary vessels (as we don’t need much oxygen). Not much taking part in the exchange of capillaries.
Substances enter and leave capillaries via 3 basic mechanisms
- Diffusion: molecules across a semi-permeable membrane from a region of high concentration to low concentration until equilibrium is reached
- Transcytosis: Endocytosis & exocytosis (Mainly for large, lipid-insoluble molecules e.g. insulin)
- Bulk Flow
Substances enter and leave capillaries via DIFFUSION
“Passive movement of molecules across a semi-permeable membrane from a region of high concentration to low concentration until equilibrium is reached”
Many substances are able to diffuse across capillary BV walls.
Substances can cross capillary walls by diffusion through:
1. intercellular clefts & fenestrations: Water soluble substances (glucose, amino acids)
2. Endothelial cells: Fat soluble substances (O2, CO2, steroid hormones) – can cross lipid bilayer of cell membranes
Substances enter and leave capillaries via TRANSCYTOSIS
Transcytosis is the process of endocytosis followed by exocytosis
- Substances in blood are enclosed in vesicles that enter endothelial cells via endocytosis, then exit on other side via exocytosis
- Mainly for large, lipid-insoluble molecules (e.g. insulin)
Substances enter and leave capillaries via BULK FLOW
Passive process
- Large number of substances (ions, molecules, particles) in a fluid move in same direction
- Occurs from an area of higher pressure to lower pressure. NB: pressures differs from the arterial end to the venous end of capillaries.
- Pressure driven movement of fluids & solutes through:
1. Filtration: From blood capillaries into interstitial fluid
2. Reabsorption: From interstitial fluid into blood
Blood Hydrostatic Pressure (BHP) as a pressure driving bulk flow
The pressure generated by the pumping action of the heart due to pressure that water in blood plasma exerts against blood vessel walls
Interstitial Fluid Hydrostatic Pressure (IFHP)
Pressure of water in interstitial fluid on capillaries. Pushes fluid from interstitial fluid back into capillaries.
However IFHP is close to 0mmHg so it does not factor into net filtration
Blood colloidal osmotic pressure (BCOP)
Caused by colloidal suspension of large proteins in plasma.
Draws water into blood stream in capillary from interstitial fluid.
Interstitial Fluid Osmotic Pressure (IFOP)
Very few large proteins are in interstitial fluid.
Pulls fluid out of capillaries into the interstitial fluid.
Hormones Regulation BP
- Renin-Angiotensin-Aldosterone (RAA) System
- Epinephrine and norepinephrine, AKA adrenaline and noradrenaline
- Antidiuretic hormone (ADH)
- Atrial natriuretic peptide (ANP)
Renin-Angiotensin-Aldosterone (RAA) System
- Blood pressure falls
- Detected by kidneys as reduction in blood flow
- Juxtaglomerular cells in the kidneys secrete renin into blood stream
- Angiotensin 2 raises BP by two mechanisms:
- Vasoconstrictor of arterioles > increased SVR (2)
- Stimulates secretion of aldosterone - Aldosterone makes its way via blood stream to kidneys and increases reabsorption of Na+ in kidneys
> H2O follows Na+
> more H20 retained & less lost in urine
> increases blood volume
> BP returns to normal
Adrenalin & Noradrenalin
Adrenalin & Noradrenalin released by adrenal medulla:
- Increases HR and force of ventricular contraction, therefore increasing cardiac output
- Redistributes blood flow
- Cause vasoconstriction of arterioles & veins in skin and abdominal organs.
- Causes vasodilation of arterioles in cardiac & skeletal muscle
Oedema = swelling
Oedema = swelling Results from either 1. Excess filtration Increased capillary blood pressure Increased permeability of capillaries 2. Inadequate reabsorption Decreased concentrations of plasma proteins
Blood Flow – Basic Principles
- Volume of blood that flows through any tissue in a given time period (mL/min)
- Flows from regions of higher pressure to regions of lower pressure
- The greater the pressure difference > the greater the blood flow
- The higher the resistance to blood flow > the smaller the blood flow
- Total blood flow = cardiac output (CO)
Volume of blood circulating through systemic or pulmonary BV’s each minute
CO = SV x HR - Distribution of CO into circulatory routes depends on two factors
Pressure difference driving blood flow though a tissue
Resistance to flow in specific vessels
Blood Pressure
- Blood Pressure = hydrostatic pressure exerted by blood on blood vessel walls
- Contraction of the ventricles generates blood pressure. Highest in the aorta and large systemic arteries
- Systolic blood pressure: highest pressure obtained in arteries during ventricular systole
- Diastolic blood pressure: lowest arterial pressure during ventricular diastole
Factors determining blood pressure
Blood Volume
Cardiac output
Vascular Resistance
Vascular resistance
Vascular resistance = opposition to blood flow due to friction between blood and blood vessel walls Depends on: 1. Size of the lumen 2. Blood viscosity 3. Total blood vessel length
How size of lumen affects blood pressure
The smaller the BV lumen, the greater the resistance to blood flow
Vasoconstriction > increases resistance (e.g. stress hormones, SNS) > increase in BP
Vasodilation > decreases resistance (PSNS) > decrease in BP
How viscosity affects blood pressure
Viscosity = thickness of blood Depends on ratio of RBCs to plasma Higher blood viscosity = higher resistance E.g. dehydration, polycythemia Lower blood viscosity = lower resistance E.g. Hemolytic anemia, hemorrhage
How blood vessel length affects blood pressure
Resistance to blood flow is directly proportional to the length of blood vessel
the longer the vessel, the greater the resistance.
If you gain weight you will grow blood vessel (ie lengthening when gain weight, more resistence).
Systemic Vascular Resistance (SVR)
Aka Total Peripheral Resistance (TPR) - all the vascular resistances by all the systemic blood vessels.
- Arteries & veins: very little blood comes into contact with walls – their diameter is very large
- Arterioles, capillaries & venules: most of the contribution to resistance is from these smaller vessels
Venous return depends on…
Venous return depends on 1. Pressure differences between venules & right ventricle Maintained by 2. Skeletal muscle pump 3. Respiratory pump
Filtration (opposite of reabsorption) is promoted by…
Filtration is promoted by :
- Blood Hydrostatic Pressure (BHP): Pushes fluid from capillaries to interstitial fluid
- Interstitial Fluid Osmotic Pressure (IFOP): Pulls fluid out of capillaries into interstitial fluid
Reabsorption (opposite of filtration) is promoted by…
Promoted by
Blood Colloidal Osmotic Pressure (BCOP)
“Pulls fluid from interstitial spaces into the capillaries”
Net Filtration Pressure (NFP)
- The balance of these pressures is called net filtration pressure (NFP) determines whether the volumes of blood and interstitial fluid remain steady or change
- The NFP indicates the direction of fluid movement into or out of capillaries
- If pressures that push fluid out of capillaries exceed pressures that pull fluid into capillaries > fluid will move from capillaries into interstitial spaces = FILTRATION
- If pressures that push fluid into capillaries exceed pressures that pull fluid out of capillaries > fluid moves from interstitial spaces into capillaries = REABSORPTION
Reabsorption & Filtration - how does it work?
85% of fluid filtered out of capillaries is reabsorbed
15% of fluid enters lymphatic capillaries and is reabsorbed back in to blood stream
What is a blood reservoir?
Veins and venules act as blood reservoirs - 64% of blood is held there (at rest) and it can be diverted quickly as required (for example during muscular activity the cardiovasvular centre in the brain stem sends sympathetic nerve impulses to the veins. The result is venocontriction, which reduces the volume in the reservoirs and allows greater blood volume to the skeletal muscles.
Is the venous side of capillaries filtering or reabsorption?
The venous side of capillaries is reabsorption only - from interstitial fluid.
Is the arteriole side of capillaries filtering or reabsorption?
The arteriole side of capillaries is filtering only - into interstitial fluid.
What happens to veins during a hemorrhage?
During a hemorrhage blood reservoirs assist to counteract the drop in blood volume and blood pressure. Venocontriction occurs, narrowing the lumen of the veins - which helps to counteract the drop in blood pressure.
What role do baroreceptors play in blood pressure?
Baroreceptors are located in the aorta, neck arteries and large arteries of the chest. Baroreceptors send impulses to the cardiovascular system to regulate blood pressure.
