Lecture 12 Outline: Endocrine Regulation of Blood Pressure

 

I. Physiology of Circulation

 

A. Blood pressure (BP)

1. Force per unit area along the vascular system

2. Pressure gradient between two points in the vascular system determines blood flow

3. Blood flow is opposed by resistance

4. Sources of resistance

a. Blood viscosity

i. Thickness related to formed elements

ii. Not a significant factor except at altitude

b. Total blood vessels length

i. Longer the vessels, the greater the resistance

c. Blood vessel diameter

i. Flow is inversely related to diameter

ii. Larger the diameter, the less resistance (1/r4)

iii. In healthy humans, diameter is the greatest source of resistance

5. Relationship between flow, pressure and resistance

 

Blood Flow (F) = ∆P/PR (Difference in blood pressure between two points/peripheral resistance

 

II. Systemic Blood Pressure

 

A. Background

1. Heart pumping generates blood flow

2. Pressure results when flow is opposed by resistance

3. Blood flows along a pressure gradient

a. From higher to lower pressure

i. Highest in aorta

ii. Lowest in right atrium

 

B. Arterial blood pressure

 

1. Factors affecting arterial pressure

a. Stretching of arteries near heart

i. Compliance

ii. Distensibility

b. Volume of blood forced into the arteries near heart

2. Changes associated with systole

a. Aorta is stretched by blood leaving left ventricle

i. Kinetic energy

b. Blood moves toward periphery because peripheral pressure is lower than aortic pressure

i. Systolic pressure: 120 mm Hg

3. Changes associated with diastole

a. Semilunar valve closes

b. Aorta recoils

c. Pressure is maintained by reducing volume

i. Diastolic pressure: 70 - 80 mm Hg

 

C. Venous blood pressure

1. Characteristics

a. Relatively steady throughout cardiac cycle

b. Gradient from venules to vena cava

i. 20 mm HG (60 from aorta to arterioles)

2. Venous return

a. Venous pressure is too low to promote adequate return

b. Need additional functional modifications

3. Functional modification

a. Respiratory pump

i. Abdominal (ventral body cavity) pressure increases squeeze local veins

ii. Backflow is prevented by valves

iii. Blood is forced toward the heart

iv. Chest cavity pressure decreases

v. Thoracic veins expand

vi. Blood enters right atrium

b. Muscular pump (more important)

i. Contraction of skeletal muscle surrounding veins compress vein

ii. Backflow is prevented by valves

iii. Blood moves in direction of heart

 

III. Regulation of Blood Pressure

 

A. Factors influences blood pressure

1. Cardiac output (CO)

2. Peripheral resistance (PR)

3. Blood volume (BV)

 

B. BP = CO * PR

1. Cardiac output is directly related to blood volume

a. CO = SV * HR

b. SV = EDV ESV

c. As BV increases, EDV increases

2. Blood pressure is directly related to CO, BV and PR

 

C. Factors that affect cardiac output

1. Reduce parasympathetic control

a. Reduce inhibitory effect of the vagus nerve

i. HR increases

2. Increase sympathetic activity

a. Increases contractility of heart

i. Reduces ESV

ii. Increases stroke volume

b. Releases Epi into blood stream from adrenal medulla

i. Increases heart rate

3. Increase activity of respiratory and muscular pumps

a. Increases venous return

i. Increases EDV

ii. Increases stroke volume

 

D. Neural control of blood pressure

1. Short-term mechanisms

2. Nervous control of peripheral resistance

a. Alter blood distribution

b. Alter blood vessel diameter

3. Vasomotor center

 

 

a. Regulation of blood vessel diameter

b. Vasomotor fibers

i. Sympathetic efferents

ii. Innervate smooth muscle of blood vessels

iii. Primarily arterioles

iv. Release NE

v. Vasoconstrictor

c. Vasomotor tone

i. Tonic vasoconstriction

 

4. Baroreceptors

 

a. Detect changes in arterial blood pressure

i. Pressure sensitive mechanoreceptors

ii. When BP rises, receptors are stretched

b. Located in carotid sinuses, aortic arch and walls of all large vessels

c. Stretching increases signaling to vasomotor center

i. Inhibits vasomotor center

ii. Causes dilation of arteries and veins (i.e., decreases vasoconstriction)

d. Arteriole dilation reduces peripheral resistance

e. Venodilation shifts blood to venous reservoirs

i. Venous return decreases

ii. Cardiac output declines

f. Baroreceptors also send efferent signals to cardiac centers in the medulla

i. Inhibit sympathetic NS

ii. Stimulate parasympathetic NS

iii. HR and contractile force decrease

g. Respond to acute changes in blood pressure

i. Carotid sinus reflex protects blood supply to brain

ii. Aortic reflex maintains supply to systemic circuit

 

5. Chemoreceptors

 

a. Respond to changes in O2 and CO2 concentrations and pH

b. Located in carotid and aortic arch and carotid sinus

c. Primarily involved in control of respiratory rate and depth

 

F. Chemical control of blood pressure

1. Short-term

2. Levels of O2 and CO2

3. Blood-borne chemicals

 

a. Adrenal medulla hormones

i. NE and EPI (nicotine is a monoamine agonist)

ii. NE is a vasoconstrictor

iii. EPI increase cardiac output by increasing cardiac muscle contractility

b. Atrial natriuretic peptide (ANP)

i. Atrial peptide hormone

ii. Reduces blood pressure by antagonizing aldosterone

iii. Increases water excretion from kidney

c. Antidiuretic hormone (AVP)

i. Posterior pituitary hormone

ii. Increases blood pressure by increasing water absorption by distal tubule

iii. At high concentrations, causes vasoconstriction

d. Angiotensin II

i. Mediated by release of renin by JGA of kidney tubule

ii. When amount of blood entering kidney tubule is too low, renin is released

iii. Renin catalyzes the conversion of angiotensinogen into angiotensin II

iv. Angiotensin II causes vasoconstriction of systemic arterioles

v. Increases BP

vi. Angiotensin II also causes release of aldosterone from adrenal cortex

vii. Aldosterone increases Na+ absorption osmolarity AVP release

e. Endothelium-derived factors

i. Endothelin-vasoconstrictor

ii. Prostaglandin-derived growth factor (PDGF)-vasoconstrictor

iii. Nitrous oxide (NO)-fast acting, local vasodilator

f. Inflammatory chemicals-vasodilators

i. Histamine, etc. (see Immune Lecture)

ii. Increase capillary permeability

g. Alcohol

i. Reduces blood pressure

ii. Inhibits AVP release-increases loss of water in urine

iii. Increases vasodilation (skin) by depressing vasomotor center

 

G. Renal regulation of blood pressure

1. Long-term mechanisms for blood pressure regulation

2. Kidney controls blood volume by regulating water loss in urine

3. Blood volume affects cardiac output via:

a. Venous pressure

b. Venous return

c. EDV

d. Stroke volume

4. Blood pressure change parallels change in blood volume

a. Increase in volume increases BP

i. Kidney responds by eliminating water to reduce volume

b. Decrease in volume decreases BP

i. Kidney responds by absorbing water to increase volume

5. Direct action of the kidney

a. Alteration to rate of fluid filtration from blood stream to kidney tubules

i. Increased BP increases amount of filtrate entering tubules

ii. Filtrate entering is greater than the amount that can be processed

iii. Fluid leaves body in the form of urine

iv. Blood volume decreases and therefore BP

b. Indirect renal mechanisms

 

i. Renin-angiotensin mechanism (see above)

ii. Aldosterone also causes Post. Pituitary to release AVP

iii. AVP promotes water reabsorption from by kidney