chapter 21rnrausch.com/25x/pdf/253/chapter_21_2x.pdf · Capillary blood ﬂow is variable! ... Factors Affecting Blood Flow Table 21-1 (bottom)! 28 ... Water moves in response to

Chapter 21 – Blood Vessels and Circulation!

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Chapter 21!Blood Vessels and Circulation!

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SECTION 21-1!Blood vessels differ in size, structure, and functional properties!


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Major Vessel Types!

Arteries - carry blood away from the heart•  Higher pressure vessels!•  Lumen typically round on microscope slides!•  Thick wall compared to lumen size!

Veins - carry blood towards the heart•  Lower pressure vessels!•  Typically flattened on microscope slides!•  Thin wall compared to lumen size!

Capillaries - exchange of gases, nutrients, wastes, etc. between blood and ECF!

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Vessel Structure – 1!

1.  Tunica intima (tunic = “coat”)!•  a.k.a. tunica interna!•  Simple squamous endothelium!•  Basement membrane!•  Internal elastic lamina (arteries only)!

2.  Tunica media!•  Smooth muscle and CT!•  Thicker in arteries!•  External elastic membrane (lamina)!

Arteries only!


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Vessel Structure – 2 Figure 21-1!

3.  Tunica externa (adventitia)!•  Arteries - collagen and elastin fibers!•  Veins - collagen, elastin, smooth muscle!

ü  T. externa thicker than tunica media!

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Artery vs. Veins – Typical Structure Figure 21-1!


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Arteries – General Properties!

A. Elasticity - Stretch and recoil!B. Contractility!

•  Vasoconstriction = ↓ vessel diameter!•  Vasodilation (vasodilatation) = ↑ vessel

diameter!•  Contractility affects peripheral resistance

(PR) and therefore:!1.  Blood pressure!2.  Afterload on the heart!3.  Blood flow through capillaries!

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Artery Types – 1!

A.  Elastic arteries!•  a.k.a. conducting arteries!•  e.g. aorta, pulmonary trunk, brachiocephalic,

common carotid, subclavian, common iliac (i.e. first branches off of aorta and heart)!

•  Relatively thin walls!•  Tunica media mostly elastic fibers, little

smooth muscle!Tolerate pressure changes!Cushion pressure increases!Maintain blood flow away from the heart!


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Artery Types – 2!

B.  Muscular arteries!•  a.k.a. medium-sized or distribution arteries!•  e.g. external carotids, brachials, mesenterics!

Distribute blood to skeletal muscle and internal organs!

Most arteries are this typeTunica media mostly smooth muscle!

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Artery Types – 3!

C. Arterioles!•  a.k.a. pressure storers or resistance vessels!•  Small diameter vessels!•  One or two layers of smooth muscle!

Smallest ones have an incomplete layer!•  Regulate blood flow to capillaries!•  Contract or relax in response to:!

a)  Local conditions (e.g. ↑ CO2 or ↓ O2, ↓ pH)!b)  Sympathetic innervation!c)  Hormones!


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•  Connect arterioles to venules!•  No tunica media or externa!

Endothelium and basal lamina only!•  Function = diffusion or active transport between

blood and ECF. (Filtration is discussed later.)!!Diffusion rate = Flux = D • A • ΔC !

L•  Wall thin so diffusion distance (L) is small!•  Blood velocity is low!•  Large total surface area (A) for exchange –

called a “safety factor”!

Capillaries (Exchange Vessels) – General!

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Continuous Capillaries Figure 21-3a!

•  Found in most regions of the body!(but not in epithelia or cartilage!)!

•  Endothelium complete (i.e. continuous)!Openings between endothelial cells only!

•  Relatively low permeability!


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Fenestrated Capillaries Figure 21-3b!

Fenestra = “window”!•  Have pores through endothelial cells!•  High permeability!•  Rapid exchange of water and solutes!

Small peptides can pass through pores!For example: choroid plexus, some endocrine

organs, intestinal tract, kidneys (glomerulus)!

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Sinusoids Figure 21-3b!

Large, leaky capillaries!•  Large:!

Blood moves very slowly!•  Leaky:!

Large gaps between endothelial cells!Basal lamina thin or absent!Plasma proteins can pass through wall!

e.g. liver (lab histology), bone marrow, spleen, adrenal medulla!


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Capillary Beds (Plexuses, Networks)!

Entrance regulated by precapillary sphincter!•  More direct A-V connections = metarterioles!

Smooth muscle opens or closes metarteriole!•  Flow sent through capillaries and/or

thoroughfare channels!Anastomoses (singular = anastomosis)!•  Connection between two vessels!•  Arterial anastomosis!

Alternative route to same capillary bed!•  Arteriovenous (AV) anastomosis!

Bypass a particular capillary bed!

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Organization of a Capillary Bed Figure 21-4!

Tissue’s cells not shown!


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Vasomotion!

Capillary blood flow is variable!•  Precapillary sphincters open/close!•  Blood flow in a single capillary is not constant!•  Perfusion of different parts of a capillary bed

at different times is normal!Autoregulation!•  Local conditions (e.g. O2, pH) determine

which capillaries are open within a capillary bed!

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Veins!

•  a.k.a. volume storers or capacitance vessels!•  Take blood to heart, act as a blood reservoir!•  Venules → medium-sized veins!

Collect blood from several capillaries!Have only tunica interna and externa near

capillaries!Add tunica media nearer heart!

•  Large veins!Have all three tunics!

e.g. SVC and IVC!


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Venous Return and Valves!

•  Valves prevent backflow into capillaries (gravity), especially in limbs!

•  Venous return must be maintained so the heart has something to pump!!1. Skeletal muscle pump!•  Blood forced in one direction towards heart!•  Can’t flow backwards due to valves!

2. Respiratory pump!•  Inspiration → ↓ thoracic pressure → blood

pulled into IVC, R. Atrium!

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Venous Valves Figure 21-5!

https://www.youtube.com/watch?v=63pmtlMPHkE


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Blood Distribution!

•  About 35% in heart, arteries, capillaries!•  About 65% in veins - 1/3 of this in liver,

bone marrow, skin!•  Veins have high capacitance!

↑ Volume → not much ↑ in pressure (compared to arteries)!

•  Hemorrhage results in:!Constriction of systemic veins →!

↑ Venous return!Constriction of veins in liver, skin, lungs →!

↑ Blood volume in general circulation!

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The Distribution of Blood Figure 21-6!


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Goal is to maintain adequate perfusion of tissues, organs!

Capillary exchange:!•  Deliver oxygen and nutrients!•  Remove carbon dioxide and other

metabolic wastes!

SECTION 21-2!Pressure and resistance determine blood flow and affect rates of capillary exchange!

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Cardiovascular Physiology Overview!From 9th edition!

This is the point!!


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Pressures!

Blood pressure is hydrostatic (“water”) pressure!= pressure exerted by a liquid in response to an

applied force!•  e.g. heart → pumps blood → hydrostatic

pressure on walls of vessel!•  e.g. gravity → pulls on blood → hydrostatic

pressure on walls of vessel!Fluid moves from higher to lower pressure!

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(Total) Circulatory Pressure!

Is the difference between pressure at base of ascending aorta and pressure in right atrium!

1.  Mean (average) arterial pressure!•  MAP ≈ 100 mmHg!•  Capillary blood flow is proportional to MAP!

2.  Capillary hydrostatic pressure (CHP)!•  a.k.a. capillary pressure!•  Range about 35–18 mmHg (arterial to venous end)!

3.  Venous pressure!•  Pressure difference between venules and R. atrial

pressure (≈ 18 mmHg)!


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Factors Affecting Blood Flow Table 21-1 (bottom)!

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Flow and Resistance to Flow Through a Tube!

Resistance reduces flow through a tube. OR…!Flow is inversely proportional to resistance.!!Flow through a tube α 1 OR Flow = ΔPπr4 !

R 8ηL! At constant pressure:!

R = Resistance to flow!ΔP = pressure difference along a tube!r = radius (to the 4th power in the formula)!η= (eta) = the viscosity of the fluid!L = the length of the tube!


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Friction Affects Resistance and Flow!

Vessel length vs. internal surface area!

Vessel radius and friction!

Note laminar flow pattern!

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Vessel Diameter vs. Resistance and Flow!

Flow α 1/Resistance•  Flow α radius4

•  Resistance α 1/radius4 Affects BP;

calculation later!

Vessel diameter vs. resistance and flow **


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Turbulence Affects Blood Flow!

Turbulence increases resistance to blood flow!Turbulence increases with:!•  High flow rate (e.g. aorta)!•  Rough vessel walls (e.g. injury, plaques)!•  (Sudden) changes in vessel diameter!

Laminar flow

Turbulent flow

Lower laminarflow rate

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Review of Key Terms Table 21-1 (top)!


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Changes Along the Circuit Figure 21-8!

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Pressures Along the Systemic Circuit Figure 21-9!


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Arterial Blood Pressure!

Blood pressure (BP) is the pressure exerted by blood against the wall of a vessel.!

Systolic (blood) pressure = maximum pressure generated during ventricular contraction!

Diastolic (blood) pressure = minimum pressure at end of ventricular relaxation!

Pulse pressure = systolic minus diastolic pressure!•  Disappears (by small arterioles) due to elastic

rebound in arteries!BP reported as: systolic pressure ≈ 120 mmHg !

diastolic pressure 80 mmHg!

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Mean Arterial Pressure (MAP)!

MAP is an index of “average” blood pressure!Calculated as: !

diastolic pressure + 1/3 pulse pressure!Factors affecting MAP:!!

MAP = CO x PR!!

CO = cardiac output!PR = total peripheral resistance (to blood flow)!PR α 1/flow (PR is the same as R on slide #28)!


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Peripheral Resistance Affects MAP!!Total peripheral resistance PR = 8ηL!

∆Pπr4!

The major factor determining PR is vessel radius (r)!Example:!

If r = 1, then r4 = 1!If r = 2, then r4 = 16!Doubling radius decreases resistance by 16X!Decreasing radius by 1/2 increases resistance by

16X!Small change in radius → big change in PR →

big change in MAP!

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Venous Pressures and Venous Return!

Venous pressures:!Much lower than arterial pressures!

(Otherwise blood wouldn’t flow in the direction that it does!!!)!Range about 18 mmHg at venules to about 2 mmHg at vena cavae (i.e., net gradient = 16 mmHg)!Pressure is low, but resistance is low, so flow is adequate (flow = 1/R)!

Venous return:!Affects Cardiac Output (Frank-Starling Principle)!


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Capillary Exchange – General Features!

Depends upon processes of:!1.  Diffusion!2.  Filtration!3.  Reabsorption!!

Slightly more fluid leaves the capillaries for the tissues than re-enters the capillaries from the tissues!

The excess tissue fluid produced is returned to the circulation by the lymphatic system!

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1. Capillary Exchange – Diffusion!

Fick’s equation: flux = (D • A • ΔC) / L!1. Diffusion between endothelial cells!

Ions, small organic molecules!2. Diffusion through membrane leak channels!

Ions!3. Diffusion through fenestrations!

Large water-soluble organics!4. Diffusion through cell membranes!

Lipid-soluble molecules!5. Diffusion through sinusoid fenestrations!

Plasma proteins (e.g. in liver)!


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2. Capillary Exchange – Filtration Figure 21-10!

•  Filtration = movement of water and solutes across a semipermeable membrane in response to hydrostatic pressure!

•  Solute movement depends on solute size compared to pore (or fenestration) size!

•  Think of making coffee with a drip coffee maker!

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3. Capillary Exchange – Reabsorption!

Reabsorption is the result of osmosis!Water moves in response to osmotic pressure!

Osmotic pressure is proportional to the number of particles in solution!•  Blood colloid osmotic pressure (a.k.a.

oncotic pressure)!•  Interstitial colloid osmotic pressure!


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Figure 21-11 is a very important figure!Hydrostatic (blood) pressure and osmotic pressure can both be measured in mmHg so the different pressures acting across the capillary wall can be added or subtracted to calculate a NET FORCE.!

Forces:!A.  Capillary (Blood) Hydrostatic Pressure (CHP)!B.  Interstitial Hydrostatic Pressure (IHP)!C.  Blood Colloid Osmotic Pressure (BCOP)!D.  Interstitial Colloid Osmotic Pressure (ICOP)!

Forces Acting Across Capillary Walls!

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Forces Acting Across Capillary Wall !Figure 21-11!


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Forces and Approximate Values!

A. CHP (Capillary Hydrostatic Pressure) (blood pressure)!•  About 35 mmHg at Arterial end!•  About 18 mmHg at Venous end!•  Tending to force fluid OUT of capillary!

!B. IHP (Interstitial Hydrostatic Pressure)!•  Hydrostatic pressure in tissues!•  Small: assume 0 mmHg!

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Forces and Approximate Values – 2!

C. BCOP (Blood Colloid Osmotic Pressure)!•  Due mostly to plasma proteins that are too big

to leave the capillary!•  Tends to pull water back INTO blood!•  About 25 mmHg!

!D. ICOP (Interstitial Colloid Osmotic Pressure)!•  Due to small number of proteins in ECF

(interstitium) that are too big to enter capillary!•  Tends to suck water OUT of capillary!•  Small: assume 0 mmHg!


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Net Filtration Pressure (NFP)!

NFP = ! (Net hydrostatic pressure) - (Net colloid osmotic pressure)!At arterial end…NFP = (CHP - IHP) - (BCOP - ICOP)!

= (35 - 0) - (25 - 0) = 10 mmHg !(i.e. OUT of capillary)!

At venous end…!NFP = (CHP - IHP) - (BCOP - ICOP)!

= (18 - 0) - (25 - 0) = -7 mmHg !(i.e. INTO capillary)!

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Net Filtration and Reabsorption!

About 24 l/day moves out of capillaries to tissues!About 20.4 l/day returns from tissues to capillaries!

•  About 3.6 l/day must be returned to circulation via lymphatic vessels!

Lymph formation and return:!•  Allows communication between plasma and ECF!•  Speeds distribution of nutrients, hormones,

gases in tissues (bulk flow in lymph in addition to diffusion)!

•  Transports lipids and proteins into circulation!•  Immune functions (e.g. bacteria → lymph

nodes)!


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Edema!

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Causes of Edema – 1 !

Edema = accumulation of interstitial fluid!1.  Capillary damage!

•  Plasma proteins leak out into ECF → ↓ BCOP and ↑ ICOP → ↓ osmotic pressure to “pull” fluid back into capillary, but more to pull it out → fluid accumulates in interstitium!

2. Starvation (e.g. Kwashiorkor)!•  Body eventually starts using protein for

energy → ↓ plasma proteins → ↓ BCOP → fluid not “pulled” back into capillaries!


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Causes of Edema – 2!

3.  Hypertension = increased arterial pressure!•  ↑CHP → more fluid forced out of capillary but

same BCOP to pull it back in!4. Blockage of lymphatic vessels!•  e.g. filariasis - parasitic worm blocks vessels!•  Decreased return of tissue fluid to circulation!

Called elephantiasis!5.  Kidney dysfunction!•  ↓ urine production → ↑ blood volume →

↑ blood pressure → ↑ CHP → ↑ fluid loss to interstitium!

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SECTION 21-3!Cardiovascular regulatory mechanisms involve autoregulation, neural mechanisms, and endocrine responses!


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Cardiovascular Regulation!

Tissue perfusion depends upon interrelated factors:!•  Cardiac output!•  Peripheral resistance!•  Blood pressure!

MAP = CO x PR!Mechanisms to maintain perfusion:!•  Autoregulation!•  Neural mechanisms!•  Hormonal mechanisms!

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Decreased local blood pressure and blood

flow

Start

Longer-term regulation

Shorter-term Regulation

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2.

3.

(Figure 21-12)!

Response to a Decrease in BP and Blood Flow!


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Figure 21-12 From Current Edition!

Decreased blood flow

or BP!

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Autoregulation!

Local vasodilators act on precapillary sphincters•  ↓ O2 or ↑ CO2!•  ↓ pH!•  ↑ Nitric oxide!•  ↑ [K+]!•  Inflammation (histamine)!•  ↑ local temperature!


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Neural Mechanisms – 1!

Review Chapter 20 about Cardiovascular Centers!1.  Vasomotor centers in medulla!•  Small group of neurons causes vasodilation!•  Large group of neurons causes

vasoconstriction!Vasoconstrictor group is chronically active

Produces vasomotor tone!Small adjustments → large change in

BP and blood flow!

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2.  Cardiovascular reflexes (See also Chapter 20)!A. Baroreceptor reflexes!

Pressure (stretch) monitored by:!•  Carotid sinuses!•  Aortic sinuses!•  Right atrial wall!

Example:!•  ↑ BP → ↓ CO (mostly ↓ HR) and ↓ PR!•  ↓ BP → opposite!

Receptors respond immediately, adapt rapidly!


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Baroreceptor Reflexes Figure 21-13!

Decrease in BP

Increase in BP

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B. Chemoreceptor reflexes!Chemicals sensed by receptors in:!•  Carotid bodies, aortic bodies and medulla!

Sense CO2, O2, pH in blood and CSF (medulla)!Example:!•  ↑ CO2 or ↓ pH or ↓ O2 (What would cause this?)!•  Receptors signal cardioacceleratory and

vasomotor centers → ↑ HR and ↑ PR → ↑ CO and ↑ BP!

•  Also stimulate respiratory centers!


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Chemoreceptor Reflexes Figure 21-14!

Start

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Hormones and Cardiovascular Regulation!

1.  Antidiuretic hormone (ADH - from the ? gland.)!a.k.a. argenine vasopressin!Released in response to:!•  ↓ blood volume!•  ↑ blood osmotic pressure!•  Angiotensin II!

Effects:!•  Vasoconstriction!•  Reabsorption of water by kidneys

(Chapter 26)!


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Hormones – 2!

2.  Angiotensin II (You know this from the endocrine chapter.)

Mechanism of release:!Decreased renal blood pressure causes:!•  Renin release!•  Renin converts angiotensinogen to

angiotensin I!•  Angiotensin converting enzyme (lungs)

converts angiotensin I → angiotensin II!

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Hormones – 3!

2.  Angiotensin II (continued)!ANG II effects:!•  Causes aldosterone release from adrenal

cortex!(Aldosterone causes Na+ and water

reabsorption, and K+ secretion at kidneys.)!

•  Stimulates ADH secretion•  Stimulates thirst!•  Causes vasoconstriction (angiotensin)!


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Hormones – 4!

3.  Erythropoietin!Released by kidneys if:!•  ↓ renal BP!•  ↓ renal blood O2 content (hypoxia)!

Effects:!Increased production and maturation of RBCs!•  ↑ blood volume and viscosity →!•  ↑ BP and O2 carrying capacity!

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Hormones – 5!

4.  Natriuretic peptides!•  Atrial and brain natriuretic peptides (ANP; BNP)!•  ANP released in response to atrial stretch!•  BNP release in response to ventricular stretch!A. Decreases blood volume:!•  ↑ Na+ loss by kidney →!•  ↑ water loss by kidney!•  ↓ thirst!

B. Decreases blood pressure:!•  ↓ vasoconstriction!•  ↓ release of ADH, aldosterone, NE, E!


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Pressure and Volume Regulation Figure 21-15a!

↓ BP or blood volume

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Pressure and Volume Regulation Figure 21-15b!

↑ BP or blood volume


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SECTION 21-4!The cardiovascular system adapts to physiological stress and maintains a special vascular supply to the brain, heart and lungs!

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Blood Distribution During Exercise!

Do the directions of the Rest vs. Strenuous exercise make sense to you?

+ 10.5X

+ 3X

+ 3.8X

- 1.8X

- 2.3X

No change

Rest vs. Strenuous

+ 3X

(Modified from Table 21-2)!

* *


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Training Effects Table 21-3!

Subject!Heart weight

(g)!

Stroke volume

(ml)!Heart rate

(bpm)!Cardiac output (l/

min)!

Blood pressure (systolic/diastolic)!

Cardiac Reserve!

Non-athlete (rest)! 300! 60! 83! 5.0! 120/80! -!

Non-athlete (maximum)! -! 104! 192! 19.9! 187/75! About 4

(19.9/5.0)!

Athlete (rest)! 500! 100! 53! 5.3! 120/80! -!

Athlete (maximum)! -! 167! 182! 30.4! 200/90*! About 5.75

(30.4/5.3)!

!* indicates that diastolic pressure was not measured.

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Cardiovascular Responses to Hemorrhage !

Assume that hemostasis has failed (chapter 19)!•  Cardiovascular responses to hemorrhage

include both short- and long-term mechanisms to restore homeostasis!

!Big picture:!•  Short-term responses produce a rapid

elevation of blood pressure.!•  Long-term responses restore blood volume.!


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Short-term Responses Restore Blood Pressure

If blood pressure is low…!Increase blood pressure by:!

1. Carotid and aortic reflexes (baroreceptors and chemoreceptors)!↑ CO and ↑ PR → ↑ BP!

2. Anxiety → sympathetic stimulation → !↑ vasoconstriction → ↑ BP!

3. Hormones!NE, E, ADH, Angiotensin II!

(↑ vasoconstriction → ↑ BP)!

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Long-term Responses Restore Blood Volume

If blood volume is low…!1.  ↓ Capillary hydrostatic pressure → favors

uptake of fluid from interstitium (effect on Net Filtration Pressure – see slide #44)!

2.  Aldosterone and ADH (kidney effects)!3.  ↑ Thirst (ADH, ANG II)!4.  Erythropoietin from kidneys!


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Responses to Hemorrhage Figure 21-16!

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Shock (Not all of this information is from the text.)!

Shock = cardiovascular system not delivering sufficient O2 and nutrients to tissues!•  i.e. inadequate perfusion of tissues!•  Cells switch from aerobic to anaerobic

metabolism!ü  Lactic acid accumulates!ü  Blood pH decreases (not necessarily

because of lactic acid)!ü  ATP production inadequate!ü  Cells die


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Types of Shock!

1.  Hypovolemic shock!2.  Cardiogenic shock!3.  Vascular shock!

•  Anaphylactic shock!•  Neurogenic shock!•  Septic shock!

4.  Obstructive shock!These are pathological conditions, not homeostatic

responses! (Origin of anaphylactic = “against” + “protection”)!

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1. Hypovolemic Shock – Low Blood Volume!

Common cause = hemorrhage!Also:!•  Diabetes mellitus (excessive urine

production)!•  Diarrhea!•  Low fluid intake!•  Excessive sweating (A&P exams L)!

Big picture:!•  ↓ fluid volume → ↓ venous return → ↓ stroke volume → ↓ cardiac output → ↓ tissue perfusion!


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2. Cardiogenic Shock!

Inadequate cardiac pumping!•  Common cause = myocardial infarction (heart

attack)!•  Also other cardiac malfunctions:!

ü  Ischemia (low blood flow to heart)!ü  Valve problems !ü  High BP (high afterload), etc., etc.!

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Vascular Shock and Obstructive Shock!

3. Vascular shock: CO and blood volume normal, but ↓↓ PR (inappropriate vasodilation)!•  Allergic reaction - massive histamine release → ↑ vasodilation → anaphylactic shock!

•  Trauma to head damages medullary cardiovascular centers → ↑ vasodilation → neurogenic shock!

•  Bacterial infection and release of toxins → ↑ vasodilation → toxic shock!

4. Obstructive shock: blood flow blocked by obstruction!•  Most common = blood clot in lungs = pulmonary

embolism!


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Symptoms of Hypovolemic Shock!

Appear when fluid loss reaches about 30%!1.  Hypotension (systolic pressure < 90 mmHg)!2.  Skin pale and cool (vasoconstriction) and moist

(sympathetic activation of sweat glands)!3.  Mental confusion (↓ blood flow to brain)!4.  ↑ HR and weak pulse!5.  ↓ Urine output (↓ blood flow to kidneys)!6.  ↓ pH (acidosis: ↓ O2 delivery → ↑ lactic acid

production)!

Documents

chapter 21rnrausch.com/25x/pdf/253/chapter_21_2x.pdf · Capillary blood ﬂow is variable! ... Factors Affecting Blood Flow Table 21-1 (bottom)! 28 ... Water moves in response to