1. Blood Vessels
Chapter 19

2. Blood Vessels: Overview
Structure of blood vessel wall
Tunica externa – outer covering mostly collagen
Tunica media – elastin & encircling smooth muscle
Tunica interna – endothelium
Lumen – the channel
Vasa Vasorum – in large vessels, supplies blood to the outer layers of the vessel wall
Figure 19.1b

3. Types of Blood Vessels Arteries – carry cardiac outflow.
Thicker walled & more muscular.
Repeated bifurcation (divisions): elastic arteries  muscular arteries  arterioles  then to:
Capillaries – wall has single cell thickness. Repeated anastomosis (merging) yield:
Venules which then anastomose to form:
Veins – thin wall, less muscle, more expansible, large lumen, carry venous return to heart
Figure 19.1b

4. Arteries: Types
Figure 19.1b
Elastic arteries – expand & contract passively to accommodate blood volume. Smoothes out pulsatile flow
Muscular arteries – distribution arteries. Deliver blood to organs. Less elastic / more muscle (vasoconstriction)
Arterioles – smallest; endothelium & a single layer of smooth muscle – regulate flow to capillary beds

5. Capillaries: Types
Continuous: Endothelium with occasional intercellular clefts

6. Capillaries: Types
Fenestrated: Endothelial cells full of pores. Very permeable. Absorption / filtration

7. Capillaries: Types
Sinusoids: large irregular lumen, fenestrations & intercellular clefts. Allow movement of large molecules / plasma between circulatory system & extracellular space

8. Capillary Beds True capillaries are exchange vessels
Precapillary sphincter: smooth muscle that controls blood flow between metarteriole & true capillary
Vascular Shunt: arteriole  metarteriole  venule
Pericytes: spaced along capillaries to anchor & stabilize
Figure 19.4a,b

9. Veins
Figure 19.1b
Venules: small caliber, porous; allow fluid & WBC movement out of circulation
Veins: capacitance vessels which hold 65% of blood supply. Pressure is low.
Venous valves: one way valves that inhibit retrograde flow
Small amount of smooth muscle or elastin
Venous sinuses – thin walled flattened veins supported by surrounding tissue (coronary sinuses, dural sinuses)

10. Anastomoses Anastomoses: collaterals, bypasses & shunts Arterial
Arteriovenous
Venous

11. Physiology of Circulation
Introduction to hemodynamics:
Blood flow (F)
Blood pressure (BP) &
Resistance (R)

12. Blood flow
Blood flow = volume of blood flowing through a structure; ml/min
Total blood flow = Cardiac Output
Individual structure blood flow varies
example: skin (hot vs. cold); gut (digestion)

13. Blood pressure
Blood pressure: force of blood against vessel walls (i.e. 120 mmHg systolic)
Pressure gradient keeps blood moving

14. ARTERIAL BLOOD PRESSURE
Systolic pressure
Pressure peak after ventricular systole. Ave = 120 mm Hg.
Diastolic Pressure
Pressure drop during ventricular diastole. Ave = 80 mm Hg.
BP = 120/80 mm Hg 14

15. Resistance
Resistance: opposition to flow; friction of blood moving through vessels
Blood viscosity = blood’s internal resistance to flow
Laminar flow: blood at the wall moves slower than blood in center

16. Resistance Blood vessel length: Blood vessel diameter:
increased length = increased resistance
Blood vessel diameter:
decreased diameter = increased resistance

17. Resistance Resistance varies inversely to the radius4
(i.e. 1/r4)
Doubling the radius:
Decreases resistance to R/16
Halving the radius
Increases resistance to 16R

18. Relationships: Flow, Pressure & Resistance
F = rP R
rP = Phigh - Plow
Increased rP yields:
Increased Flow
Decreased rP yields:
Decreased Flow

19. Relationships: Flow, Pressure & Resistance
F = rP R
Increased R yields:
Decreased Flow
Decreased R yields:
Increased Flow
Resistance has a greater influence than change in Pressure on Flow

20. Systemic Blood Pressure
Systemic BP
Arterial BP: depends upon distensibility of the great vessels & the volume of blood pumped into them (pulsatile)
Ventricular contraction  blood flow  to aorta  aortic stretch  pressure:

21. Systemic Blood Pressure
Systolic Pressure: peak pressure with aortic filling increases to ~120 mmHg.
Blood run off begins & flows down the pressure gradient into the systemic circulation.
Diastolic pressure: lowest pressure. As aorta recoils, pressure decreases to ~80 mmHg.

22. Systemic Blood Pressure
Pulse pressure - Difference between systolic & diastolic pressures. Felt as a pulse during systole.
PP = = 40 mm Hg 22

23. Systemic Blood Pressure
Pulse pressure = systolic - diastolic
Mean Arterial Pressure = average pressure throughout the cycle
MAP = diastolic + pulse pressure
MAP = ~90 mmHg

24. Capillary BP Capillary BP Higher pressure would destroy capillaries
~40 mmHg at the start of the capillary bed
~20 mmHg at the end
Higher pressure would destroy capillaries
Capillary permeability is high enough that exchange process occurs at low pressure

25. Venous BP / Venous Return
Venous BP (non pulsatile)
Respiratory pump: pressure changes in the thorax & abdomen b/c of breathing
Muscular pump: skeletal muscle activity

26. Maintaining BP Maintaining BP: CO = rP R rP = CO x R
Alteration of BP depends on CO & R
CO = HR x SV; a function of venous return; under neural & hormonal influences
rP = (HR x SV) x R

27. Neural Effectors of CO 27

28. Resistance: Short Term Control
Short term control by neural & chemical factors
Alters blood distribution
Maintains MAP by changes in vessel diameter
Operate via baroreceptors & chemoreceptors

29. Short Term: Neural Control
Vasomotor center (medulla): exerts vasomotor tone via vasomotor fibers that innervate smooth muscle of vessels
SNS activity  generalized vasoconstriction
Input from baroreceptors & chemoreceptors to vasomotor center modifies vasomotor output

30. Short Term: Neural Control
Baroreceptors:
Carotid sinuses (monitor blood flow to brain)
Aortic (monitor blood flow to periphery)
Detect changes in MAP
Chemoreceptors: detect [O2], [CO2] & pH (carotid & aortic bodies)

31. 31

32. MAINTAINING BLOOD PRESSURE
Short Term Mechanisms: Chemical
Epinephrine and Norepinephrine -
Enhances the sympathetic nervous system. Epi increases cardiac output; NE is a vasoconstrictor. 32

33. MAINTAINING BLOOD PRESSURE
Short Term Mechanisms: Chemical
Atrial Natriuretic Peptide (ANP) -
Antagonist of aldosterone. Causes excretion of Na+ and H2O from body
Reduces blood volume and blood pressure
When the atria of the heart encounter increased pressure they secrete ANP 33

34. MAINTAINING BLOOD PRESSURE
Short Term Mechanisms: Chemical
Antidiuretic Hormone (ADH) -
Released at high amounts when MAP drops to low levels; it acts as a vasoconstrictor (its other name is vasopressin).
It also conserves water,
but this is not an important
short-term mechanism. 34

35. MAINTAINING BLOOD PRESSURE
Short Term Mechanisms: Chemical
Angiotensin II - A potent vasoconstrictor produced within the blood.
Angiotensinogen
Also causes aldosterone and ADH release
ACE
Angiotensin I 35

36. MAINTAINING BLOOD PRESSURE
Short Term Mechanisms: Chemical
Nitric Oxide (NO) -
Promotes vasodilation, lowering MAP.
Secreted by endothelial cells in response to high flow rate 36

37. MAINTAINING BLOOD PRESSURE
Short Term Mechanisms: Chemical
Inflammatory chemicals - Histamine and other chemicals released during inflammation are vasodilators. 37

38. MAINTAINING BLOOD PRESSURE
Short Term Mechanisms: Chemical
Alcohol -
Antagonist of ADH (lowers blood volume and blood pressure)
Promotes vasodilation (thereby reducing resistance and blood pressure). 38

39. Long term control: Renal
Direct renal
Increased renal flow & BP  increased filtrate from kidney which results in decreases in volume & in pressure
Decreased renal flow & BP  decreased filtrate; conservation of volume & increases in BP
Indirect renal
Decreased BP results in renin release
  Angiotensin II (vasoconstrictor) which stimulates:
Aldosterone & ADH release which conserve Na & water

40. MAINTAINING BLOOD PRESSURE
Long Term
Mechanisms:
Renal 40

41. Alterations in BP Hypotension (low BP): systolic <100 mmHg
Hypertension (high BP) systolic >140/90
Primary HTN – no specific cause; lifestyle & heredity
Secondary HTN – identifiable cause; increased renin, arteriosclerosis, endocrine disorders

42. Alterations in BP Autoregulation; local changes in blood flow
Intrinsic: modifying diameter of local arterioles
Metabolic: endothelial response (NO, etc)
Myogenic: smooth muscle responds to increased stretch with increased tone

43. Blood Flow Through Capillaries
Fluid exchange:
Hydrostatic pressure vs. colloid osmotic pressure
Hydrostatic Pressure pushes fluid out down pressure gradient (HPc)
Interstitial Hydrostatic Pressure (HPif) pushes fluid into capillaries
Colloid Osmotic Pressure: large molecules pull H2O toward themselves. Interstitial (OPif) & Capillary (OPc)
NFP = (HPc – HPif) – (OPc – OPif)
Figure 19.16

44. Net Filtration Pressure of Capillaries
NFP = (HPc – HPif) – (OPc – OPif)
NFP at arterial end of capillary bed = 10 mmHg
Hydrostatic
NFP at venous end of capillary bed = -8 mmHg
Oncotic
Figure 19.16

45. Circulatory Shock Circulatory Shock: marked decrease in blood flow
Symptoms: increased HR, thready pulse, marked vasoconstriction;
Marked fall in BP is a late symptom

46. Circulatory Shock: Causes
Hypovolemic: inadequate volume (hemorrhage, dehydration, burns)
Vascular: normal volume but global vasodilation
Anaphylaxis: allergies (histamine)
Neurogenic: failure of autonomic nervous system
Septic: bacteria (bacterial toxins are vasodilators)
Cardiogenic pump failure