CV Physiology: Hemodynamics (Pressure, Flow, and Resistance)
A half-blown flow'ret which cold blasts amate. But this is past. .. Specimen of an Induction to a Poem. Lo! I must tell a By tenderest pressure, a faint damask mouth. To slumbery The Star-Queen's- crescent on her marriage night. Haste, haste Nay, linger not; make no resistance, sweet —. Lud. Recall this relationship from Case AP = blood flow x resistance. AP could increase because pressure in the pulmonary artery increases or because. The points of minimum 30+ so H20 o resistance, located at the left side border of whether the pressure/ flow relationship during voiding is considered to be.
On earth the good man base detraction bars From thy fair name, and waters it with tears. Written on the Day That Mr. Leigh Hunt Left Prison What though, for showing truth to flatter'd state, Kind Hunt was shut in prison, yet has he, In his immortal spirit, been as free As the sky-searching lark, and as elate. Think you he nought but prison walls did see, Till, so unwilling, thou unturn'dst the key? In Spenser's halls he strayed, and bowers fair, Culling enchanted flowers; and he flew With daring Milton through the fields of air To regions of his own his genius true Took happy flights.
Who shall his fame impair When thou art dead, and all thy wretched crew? To Hope When by my solitary hearth I sit, When no fair dreams before my " mind's eye " flit, And the bare heath of life presents no bloom; Sweet Hope, ethereal balm upon me shed, And wave thy silver pinions o'er my head. Whene'er I wander, at the fall of night, Where woven boughs shut out the moon's bright ray, Should sad Despondency my musings fright, And frown, to drive fair cheerfulness away, Peep with the moon-beams through the leafy roof, And keep that fiend Despondence far aloof.
Should Disappointment, parent of Despair, Strive for her son to seize my careless heart; When, like a cloud, he sits upon the air, Preparing on his spell-bound prey to dart Chace him away, sweet Hope, with visage bright, And fright him as the morning frightens night! Whene'er the fate of those I hold most dear Tells to my fearful breast a tale of sorrow, O bright-eyed Hope, my morbid fancy Cheer; Let me awhile thy sweetest comforts borrow Thy heaven-born radiance around me shed, And wave thy silver pinions o'er my head!
Should e'er unhappy love my bosom pain, From cruel parents, or relentless fair; O let me think it is not quite in vain To sigh out sonnets to the midnight air! Sweet Hope, ethereal balm upon me shed, And wave thy silver pinions o'er my head! In the long vista of the years to roll, Let me not see our country's honour fade O let me see our land retain her soul, Her pride, her freedom; and not freedom's shade.
From thy bright eyes unusual brightness shed — Beneath thy pinions canopy my head! Let me not see the patriot's high bequest, Great Liberty! With the base purple of a court oppress'd, Bowing her head, and ready to expire But let me see thee stoop from heaven on wings That fill the skies with silver glitterings! And as, in sparkling majesty, a star Brightening the half-veil'd face of heaven afar So, when dark thoughts my boding spirit shroud, Sweet Hope, celestial influence round me shed, Waving thy silver pinions o'er my head.
Ode to apollo In thy western halls of gold When thou sittest in thy state, Bards, that erst sublimely told Heroic deeds, and sang of fate, With fervour seize their adamantine lyres, Whose chords are solid rays, and twinkle radiant fires.
Here Homer with his nervous arms Strikes the twanging harp of war, And even the western splendour warms, While the trumpets sound afar But, what creates the most intense surprise, His soul looks out through renovated eyes.
Then, through thy temple wide, melodious swells The sweet majestic tone of Maro's lyre The soul delighted on each accent dwells, — Enraptured dwells, — not daring to respire, The while he tells of grief around a funeral pyre. Thou biddest Shakspeare wave his hand, And quickly forward spring The Passions — a terrific band — And each vibrates the string That with its tyrant temper best accords, While from their master's lips pour forth the inspiring words.
A silver trumpet Spenser blows, And, as its martial notes to silence flee, From a virgin chorus flows A hymn in praise of spotless chastity. Next thy Tasso's ardent numbers Float along the pleased air, Calling youth from idle slumbers, Rousing them from pleasure's lair — Then o'er the strings his fingers gently move, But when thou joinest with the Nine, And all the powers of song combine, We listen here on earth The dying tones that fill the air, And charm the ear of evening fair, From thee, great God of Bards, receive their heavenly birth.
To Some Ladies What though while the wonders of nature exploring, I cannot your light, mazy footsteps attend; Nor listen to accents, that almost adoring, Bless Cynthia's face, the enthusiast's friend Yet over the steep, whence the mountain stream rushes, With you, kindest friends, in idea I muse; Mark the clear tumbling crystal, its passionate gushes, Its spray that the wild flower kindly bedews.
Why linger you so, the wild labyrinth strolling? Why breathless, unable your bliss to declare? If a cherub, on pinions of silver descending, Had brought me a gem from the fret-work of heaven; And smiles with his star-cheering voice sweetly blending, The blessings of Tighe had melodiously given; It had not created a warmer emotion Than the present, fair nymphs, I was blest with from you, Than the shell, from the bright golden sands of the ocean Which the emerald waves at your feet gladly threw.
For, indeed, 'tis a sweet and peculiar pleasure, And blissful is he who such happiness finds, To possess but a sand in the hour of leisure, In elegant, pure, and aerial minds. On Receiving a Curious Shell, and a Copy of Verses, from the Same Ladies Hast thou from the caves of Golconda, a gem Bright as the humming-bird's green diadem, When it flutters in sun-beams that shine through a fountain? Hast thou a goblet for dark sparkling wine? That goblet right heavy, and massy, and gold?
And splendidly mark'd with the story divine Of Armida the fair, and Rinaldo the bold? Hast thou a steed with a mane richly flowing? Hast thou a sword that thine enemy's smart is? Hast thou a trumpet rich melodies blowing? And wear'st thou the shield of the fam'd Britomartis? What is it that hangs from thy shoulder, so brave, Embroidered with many a spring-peering flower?
- Hemodynamics (Pressure, Flow, and Resistance)
- The poems of John Keats
- Determinants of Resistance to Flow (Poiseuille's Equation)
Is it a scarf that thy fair lady gave? And hastest thou now to that fair lady's bower? I will tell thee my blisses, which richly abound In magical powers to bless, and to sooth.
On this scroll thou seest written in characters fair A sun-beamy tale of a wreath, and a chain; And, warrior, it nurtures the property rare Of charming my mind from the trammels of pain. This canopy mark 'tis the work of a fay; Beneath its rich shade did King Oberon languish, When lovely Titania was far, far away, And cruelly left him to sorrow, and anguish.
There, oft would he bring from his soft sighing lute Wild strains to which, spell-bound, the nightingales listened; The wondering spirits of heaven were mute, And tears 'mong the dewdrops of morning oft glistened.
In this little dome, all those melodies strange, Soft, plaintive, and melting, for ever will sigh; Nor e'er will the notes from their tenderness change; Nor e'er will the music of Oberon die. So, when I am in a voluptuous vein, I pillow my head on the sweets of the rose, And list to the tale of the wreath, and the chain, Till its echoes depart; then I sink to repose. Full many the glories that brighten thy youth, I too have my blisses, which richly abound In magical powers, to bless and to sooth.
O come, dearest Emma! And when thou art weary I'll find thee a bed, Of mosses and flowers to pillow thy head There, beauteous Emma, I'll sit at thy feet, While my story of love I enraptured repeat. So fondly I'll breathe, and so softly I'll sigh, Thou wilt think that some amorous zephyr is nigh Yet no — as I breathe I will press thy fair knee, And then thou wilt know that the sigh comes from me.
That mortal's a fool who such happiness misses; So smile acquiescence, and give me thy hand, With love-looking eyes, and with voice sweetly bland. From such fine pictures, heavens!
I cannot dare To turn my admiration, though unpossess'd They be of what is worthy, — though not drest In lovely modesty, and virtues rare. Yet these I leave as thoughtless as a lark; These lures I straight forget, — e'en ere I dine, Or thrice my palate moisten but when I mark Such charms with mild intelligences shine, My ear is open like a greedy shark, To catch the tunings of a voice divine.
Who can forget her half retiring sweets? Surely the All-seeing, Who joys to see us with his gifts agreeing, Will never give him pinions, who intreats Such innocence to ruin, — who vilely cheats A dove-like bosom.
In truth there is no freeing One's thoughts from such a beauty; when I hear A lay that once I saw her hand awake, Her form seems floating palpable, and near; Had I e'er seen her from an arbour take A dewy flower, oft would that hand appear, And o'er my eyes the trembling moisture shake.
But though I'll gladly trace these scenes with thee, Yet the sweet converse of an innocent mind, Whose words are images of thoughts refin'd, Almost the highest bliss of human-kind, When to thy haunts two kindred spirits flee.
To George Felton Mathew Sweet are the pleasures that to verse belong, And doubly sweet a brotherhood in song; Nor can remembrance, Mathew! The thought of this great partnership diffuses Over the genius-loving heart, a feeling Of all that's high, and great, and good, and healing. Or a white Naiad in a rippling stream; Or a rapt seraph in a moonlight beam; Or again witness what with thee I've seen, The dew by fairy feet swept from the green, After a night of some quaint jubilee Which every elf and fay had come to see When bright processions took their airy march Beneath the curved moon's triumphal arch.
But might I now each passing moment give To the coy muse, with me she would not live In this dark city, nor would condescend 'Mid contradictions her delights to lend. Should e'er the fine-eyed maid to me be kind, Ah!
There must be too a ruin dark, and gloomy, To say " joy not too much in all that's bloomy. Yet this is vain — O Mathew lend thy aid To find a place where I may greet the maid — Where we may soft humanity put on, And sit, and rhyme and think on Chatterton; And that warm-hearted Shakespeare sent to meet him Four laurell'd spirits, heaven-ward to intreat him. With reverence would we speak of all the sages Who have left streaks of light athwart their ages And thou shouldst moralize on Milton's blindness, And mourn the fearful dearth of human kindness To those who strove with the bright golden wing Of genius, to flap away each sting Thrown by the pitiless world.
We next could tell Of those who in the cause of freedom fell; Of our own Alfred, of Helvetian Tell; Of him whose name to ev'ry heart's a solace, High-minded and unbending William Wallace. While to the rugged north our musing turns We well might drop a tear for him, and Burns. I marvel much that thou hast never told How, from a flower, into a fish of gold Apollo chang'd thee; how thou next didst seem A black-eyed swan upon the widening stream; And when thou first didst in that mirror trace The placid features of a human face That thou hast never told thy travels strange, And all the wonders of the mazy range O'er pebbly crystal, and o'er golden sands; Kissing thy daily food from Naiad's pearly hands.
Had I a man's fair form, then might my sighs Had I a man's fair form, then might my sighs Be echoed swiftly through that ivory shell Thine ear, and find thy gentle heart; so well Would passion arm me for the enterprize But ah! I am no knight whose foeman dies; No cuirass glistens on my bosom's swell; I am no happy shepherd of the dell Whose lips have trembled with a maiden's eyes.
Yet must I dote upon thee, — call thee sweet, Sweeter by far than Hybla's honied roses When steep'd in dew rich to intoxication. I will taste that dew, for me 'tis meet, And when the moon her pallid face discloses, I'll gather some by spells, and incantation. Hadst thou liv'd in days of old Hadst thou liv'd in days of old, O what wonders had been told Of thy lively countenance, And thy humid eyes that dance In the midst of their own brightness; In the very fane of lightness.
Over which thine eyebrows, leaning, Picture out each lovely meaning In a dainty bend they lie, Like to streaks across the sky, Or the feathers from a crow, Fallen on a bed of snow.
Of thy dark hair that extends Into many graceful bends As the leaves of hellebore Turn to whence they sprung before. And behind each ample curl Peeps the richness of a pearl. With a glossy waviness; Full, and round like globes that rise From the censer to the skies Through sunny air. Add too, the sweetness Of thy honied voice; the neatness Of thine ankle lightly turn'd With those beauties, scarce discern'd, Kept with such sweet privacy, That they seldom meet the eye Of the little loves that fly Round about with eager pry.
Saving when, with freshening lave, Thou dipp'st them in the taintless wave; Like twin water lillies, born In the coolness of the morn. O, if thou hadst breathed then, Now the Muses had been ten. Couldst thou wish for lineage higher Than twin sister of Thalia? At least for ever, evermore, Will I call the Graces four. Hadst thou liv'd when chivalry Lifted up her lance on high, Tell me what thou wouldst have been.
I see the silver sheen Of thy broidered, floating vest Cov'ring half thine ivory breast; Which, O heavens! I should see; But that cruel destiny Has placed a golden cuirass there; Keeping secret what is fair. Like sunbeams in a cloudlet nested Thy locks in knightly casque are rested O'er which bend four milky plumes Like the gentle lilly's blooms Springing from a costly vase.
See with what a stately pace Comes thine alabaster steed; O'er his loins, his trappings glow Like the northern lights on snow. Sign of the enchanter's death; Bane of every wicked spell; Silencer of dragon's yell. I am as brisk I am as brisk As a bottle of wisk-k Ey and as nimble Give me women, wine, and snuff Give me women, wine and snuff You may do so sans objection Till the day of resurrection; For bless my beard they aye shall be My beloved trinity.
Specimen of an Induction to a Poem Lo! I must tell a tale of chivalry; For large white plumes are dancing in mine eye. Not like the formal crest of latter days But bending in a thousand graceful ways; So graceful, that it seems no mortal hand, Or e'en the touch of Archimago's wand, Could charm them into such an attitude.
We must think rather, that in playful mood, Some mountain breeze had turned its chief delight, To show this wonder of its gentle might.
I must tell a tale of chivalry; For while I muse, the lance points slantingly Athwart the morning air: And from her own pure self no joy dissembling, Wraps round her ample robe with happy trembling. Sometimes, when the good knight his rest would take, It is reflected, clearly, in a lake, With the young ashen boughs, 'gainst which it rests, And th' half seen mossiness of linnets' nests.
Or when his spirit, with more calm intent, Leaps to the honors of a tournament, And makes the gazers round about the ring Stare at the grandeur of the ballancing? How sing the splendour of the revelries, When buts of wine are drunk off to the lees? And that bright lance, against the fretted wall, Is slung with shining cuirass, sword, and shield, Where ye may see a spur in bloody field? Light-footed damsels move with gentle paces Round the wide hall, and show their happy faces; Or stand in courtly talk by fives and sevens: Like those fair stars that twinkle in the heavens.
Yet must I tell a tale of chivalry: Or wherefore comes that steed so proudly by? Wherefore more proudly does the gentle knight Rein in the swelling of his ample might?
Therefore, great bard, I not so fearfully Call on thy gentle spirit to hover nigh My daring steps: Him thou wilt hear; so I will rest in hope To see wide plains, fair trees and lawny slope The morn, the eve, the light, the shade, the flowers; Clear streams, smooth lakes, and overlooking towers.
A Fragment Young Calidore is paddling o'er the lake; His healthful spirit eager and awake To feel the beauty of a silent eve, Which seem'd full loath this happy world to leave; The light dwelt o'er the scene so lingeringly. He bares his forehead to the cool blue sky, And smiles at the far clearness all around, Until his heart is well nigh over wound, And turns for calmness to the pleasant green Of easy slopes, and shadowy trees that lean And show their blossoms trim.
Scarce can his clear and nimble eye-sight follow The freaks, and dartings of the black-wing'd swallow, Delighting much, to see it half at rest, Dip so refreshingly its wings, and breast 'Gainst the smooth surface, and to mark anon, The widening circles into nothing gone. And now the sharp keel of his little boat Comes up with ripple, and with easy float, And glides into a bed of water lillies: Broad leav'd are they and their white canopies Are upward turn'd to catch the heavens' dew.
Near to a little island's point they grew; Whence Calidore might have the goodliest view Of this sweet spot of earth. The bowery shore Went off in gentle windings to the hoar And light blue mountains: These, gentle Calidore Greeted, as he had known them long before. The sidelong view of swelling leafiness, Which the glad setting sun in gold doth dress; Whence ever and anon the jay outsprings, And scales upon the beauty of its wings.
The lonely turret, shatter'd, and outworn, Stands venerably proud; too proud to mourn Its long lost grandeur: The little chapel with the cross above Upholding wreaths of ivy; the white dove, That on the window spreads his feathers light, And seems from purple clouds to wing its flight.
Green tufted islands casting their soft shades Across the lake; sequester'd leafy glades, That through the dimness of their twilight show Large dock leaves, spiral foxgloves, or the glow Of the wild cat's eyes, or the silvery stems Of delicate birch trees, or long grass which hems These pleasant things, and heaven was bedewing The mountain flowers, when his glad senses caught A trumpet's silver voice. Friends very dear to him he soon will see; So pushes off his boat most eagerly, And soon upon the lake he skims along, Deaf to the nightingale's first under-song; Nor minds he the white swans that dream so sweetly: His spirit flies before him so completely.
While leukocytes and platelets are normally a small component of the formed elements, there are some rare conditions in which severe overproduction can impact viscosity as well. Blood vessel length is directly proportional to its resistance: As with blood volume, this makes intuitive sense, since the increased surface area of the vessel will impede the flow of blood.
Likewise, if the vessel is shortened, the resistance will decrease and flow will increase. The length of our blood vessels increases throughout childhood as we grow, of course, but is unchanging in adults under normal physiological circumstances. Further, the distribution of vessels is not the same in all tissues. Adipose tissue does not have an extensive vascular supply.
One pound of adipose tissue contains approximately miles of vessels, whereas skeletal muscle contains more than twice that. Overall, vessels decrease in length only during loss of mass or amputation. An individual weighing pounds has approximately 60, miles of vessels in the body. Gaining about 10 pounds adds from to miles of vessels, depending upon the nature of the gained tissue.
One of the great benefits of weight reduction is the reduced stress to the heart, which does not have to overcome the resistance of as many miles of vessels. In contrast to length, the blood vessel diameter changes throughout the body, according to the type of vessel, as we discussed earlier.
The diameter of any given vessel may also change frequently throughout the day in response to neural and chemical signals that trigger vasodilation and vasoconstriction. The vascular tone of the vessel is the contractile state of the smooth muscle and the primary determinant of diameter, and thus of resistance and flow.
The effect of vessel diameter on resistance is inverse: Given the same volume of blood, an increased diameter means there is less blood contacting the vessel wall, thus lower friction and lower resistance, subsequently increasing flow. A decreased diameter means more of the blood contacts the vessel wall, and resistance increases, subsequently decreasing flow. The influence of lumen diameter on resistance is dramatic: A slight increase or decrease in diameter causes a huge decrease or increase in resistance.
Arterial Blood Pressure
This means, for example, that if an artery or arteriole constricts to one-half of its original radius, the resistance to flow will increase 16 times. A Mathematical Approach to Factors Affecting Blood Flow Jean Louis Marie Poiseuille was a French physician and physiologist who devised a mathematical equation describing blood flow and its relationship to known parameters. The same equation also applies to engineering studies of the flow of fluids.
Although understanding the math behind the relationships among the factors affecting blood flow is not necessary to understand blood flow, it can help solidify an understanding of their relationships. Please note that even if the equation looks intimidating, breaking it down into its components and following the relationships will make these relationships clearer, even if you are weak in math.
Focus on the three critical variables: It may commonly be represented as 3. One of several things this equation allows us to do is calculate the resistance in the vascular system. Normally this value is extremely difficult to measure, but it can be calculated from this known relationship: The important thing to remember is this: Two of these variables, viscosity and vessel length, will change slowly in the body.
Only one of these factors, the radius, can be changed rapidly by vasoconstriction and vasodilation, thus dramatically impacting resistance and flow. Further, small changes in the radius will greatly affect flow, since it is raised to the fourth power in the equation. The Roles of Vessel Diameter and Total Area in Blood Flow and Blood Pressure Recall that we classified arterioles as resistance vessels, because given their small lumen, they dramatically slow the flow of blood from arteries.
In fact, arterioles are the site of greatest resistance in the entire vascular network. This may seem surprising, given that capillaries have a smaller size. How can this phenomenon be explained? Although the diameter of an individual capillary is significantly smaller than the diameter of an arteriole, there are vastly more capillaries in the body than there are other types of blood vessels.
Part c shows that blood pressure drops unevenly as blood travels from arteries to arterioles, capillaries, venules, and veins, and encounters greater resistance. However, the site of the most precipitous drop, and the site of greatest resistance, is the arterioles. This explains why vasodilation and vasoconstriction of arterioles play more significant roles in regulating blood pressure than do the vasodilation and vasoconstriction of other vessels.
Part d shows that the velocity speed of blood flow decreases dramatically as the blood moves from arteries to arterioles to capillaries. This slow flow rate allows more time for exchange processes to occur.
As blood flows through the veins, the rate of velocity increases, as blood is returned to the heart. The relationships among blood vessels that can be compared include a vessel diameter, b total cross-sectional area, c average blood pressure, and d velocity of blood flow. Disorders of the…Cardiovascular System: Arteriosclerosis Compliance allows an artery to expand when blood is pumped through it from the heart, and then to recoil after the surge has passed.
This helps promote blood flow. In arteriosclerosis, compliance is reduced, and pressure and resistance within the vessel increase. This is a leading cause of hypertension and coronary heart disease, as it causes the heart to work harder to generate a pressure great enough to overcome the resistance.
Arteriosclerosis begins with injury to the endothelium of an artery, which may be caused by irritation from high blood glucose, infection, tobacco use, excessive blood lipids, and other factors. Artery walls that are constantly stressed by blood flowing at high pressure are also more likely to be injured—which means that hypertension can promote arteriosclerosis, as well as result from it.
Recall that tissue injury causes inflammation. As inflammation spreads into the artery wall, it weakens and scars it, leaving it stiff sclerotic. As a result, compliance is reduced.
[OTA] The poems of John Keats
Moreover, circulating triglycerides and cholesterol can seep between the damaged lining cells and become trapped within the artery wall, where they are frequently joined by leukocytes, calcium, and cellular debris. Eventually, this buildup, called plaque, can narrow arteries enough to impair blood flow. When this happens, platelets rush to the site to clot the blood. This clot can further obstruct the artery and—if it occurs in a coronary or cerebral artery—cause a sudden heart attack or stroke.
Alternatively, plaque can break off and travel through the bloodstream as an embolus until it blocks a more distant, smaller artery. Ischemia in turn leads to hypoxia—decreased supply of oxygen to the tissues.
Hypoxia involving cardiac muscle or brain tissue can lead to cell death and severe impairment of brain or heart function. A major risk factor for both arteriosclerosis and atherosclerosis is advanced age, as the conditions tend to progress over time. However, obesity, poor nutrition, lack of physical activity, and tobacco use all are major risk factors.
Treatment includes lifestyle changes, such as weight loss, smoking cessation, regular exercise, and adoption of a diet low in sodium and saturated fats. Medications to reduce cholesterol and blood pressure may be prescribed. For blocked coronary arteries, surgery is warranted. In angioplasty, a catheter is inserted into the vessel at the point of narrowing, and a second catheter with a balloon-like tip is inflated to widen the opening.
To prevent subsequent collapse of the vessel, a small mesh tube called a stent is often inserted. In an endarterectomy, plaque is surgically removed from the walls of a vessel. This operation is typically performed on the carotid arteries of the neck, which are a prime source of oxygenated blood for the brain. In a coronary bypass procedure, a non-vital superficial vessel from another part of the body often the great saphenous vein or a synthetic vessel is inserted to create a path around the blocked area of a coronary artery.
Venous System The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart. Two factors help maintain this pressure gradient between the veins and the heart.
First, the pressure in the atria during diastole is very low, often approaching zero when the atria are relaxed atrial diastole. These physiological pumps are less obvious. Skeletal Muscle Pump In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle.
This mechanism, known as the skeletal muscle pump Figure As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves.Peripheral Resistance and Blood Flow
This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods. Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart.
Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness. The contraction of skeletal muscles surrounding a vein compresses the blood and increases the pressure in that area.
This action forces blood closer to the heart where venous pressure is lower. Note the importance of the one-way valves to assure that blood flows only in the proper direction. Respiratory Pump The respiratory pump aids blood flow through the veins of the thorax and abdomen.
During inhalation, the volume of the thorax increases, largely through the contraction of the diaphragm, which moves downward and compresses the abdominal cavity.
The elevation of the chest caused by the contraction of the external intercostal muscles also contributes to the increased volume of the thorax. The volume increase causes air pressure within the thorax to decrease, allowing us to inhale. Additionally, as air pressure within the thorax drops, blood pressure in the thoracic veins also decreases, falling below the pressure in the abdominal veins.
This causes blood to flow along its pressure gradient from veins outside the thorax, where pressure is higher, into the thoracic region, where pressure is now lower. This in turn promotes the return of blood from the thoracic veins to the atria.
During exhalation, when air pressure increases within the thoracic cavity, pressure in the thoracic veins increases, speeding blood flow into the heart while valves in the veins prevent blood from flowing backward from the thoracic and abdominal veins. The individual veins are larger in diameter than the venules, but their total number is much lower, so their total cross-sectional area is also lower. Also notice that, as blood moves from venules to veins, the average blood pressure drops see Figure This pressure gradient drives blood back toward the heart.
Again, the presence of one-way valves and the skeletal muscle and respiratory pumps contribute to this increased flow. Since approximately 64 percent of the total blood volume resides in systemic veins, any action that increases the flow of blood through the veins will increase venous return to the heart.
Maintaining vascular tone within the veins prevents the veins from merely distending, dampening the flow of blood, and as you will see, vasoconstriction actually enhances the flow. The Role of Venoconstriction in Resistance, Blood Pressure, and Flow As previously discussed, vasoconstriction of an artery or arteriole decreases the radius, increasing resistance and pressure, but decreasing flow. Venoconstriction, on the other hand, has a very different outcome. The walls of veins are thin but irregular; thus, when the smooth muscle in those walls constricts, the lumen becomes more rounded.
The more rounded the lumen, the less surface area the blood encounters, and the less resistance the vessel offers. Vasoconstriction increases pressure within a vein as it does in an artery, but in veins, the increased pressure increases flow. Recall that the pressure in the atria, into which the venous blood will flow, is very low, approaching zero for at least part of the relaxation phase of the cardiac cycle. Thus, venoconstriction increases the return of blood to the heart.