Autoregulation• The Renin-angiotensin-aldosterone (RAA)

system is an important endocrine component of autoregulation.• Renin is released by kidneys when blood

volume falls or blood flow decreases.• It is subsequently converted into the active

hormone angiotensin II which raises BP by vasoconstriction and by stimulating secretion of aldosterone from the adrenal glands.

Autoregulation• Epinephrine and norepinephrine are also

released from the adrenal medulla as an endocrine autoregulatory response to sympathetic stimulation.• They increase cardiac output by increasing rate and

force of heart contractions.• Antidiuretic hormone (ADH) is released from the

posterior pituitary gland in response to dehydration or decreased blood volume.

Autoregulation• Atrial Naturetic Peptide (ANP) is a natural

diuretic polypeptide hormone released by cells of the cardiac atria.• ANP participates in autoregulation by:• Lowering blood pressure (it causes a direct vasodilation) • Reducing blood volume (by promoting loss of salt and

water as urine)

Circulation• In an autoregulatory response, important

differences exist between the pulmonary and systemic circulations:• Systemic blood vessel walls dilate in response to

hypoxia (low O2) or acidosis to increase blood flow.• The walls of the pulmonary blood vessels constrict

to a hypoxic or acidosis stimulus to ensure that most blood flow is diverted to better ventilated areas of the lung.

Circulation• A measure of peripheral circulation can be done

by checking the pulse. The pulse is a result of the alternate expansion and recoil of elastic arteries after each systole.– It is strongest in arteries closest to the heart and

becomes weaker further out.– Normally the pulse

is the same as the heart rate.

Circulation• Blood pressure is the pressure in arteries generated by

the left ventricle during systole and the pressure

remaining in the arteries when the ventricle is in diastole.

Alterations Of Blood Pressure• About 50 million Americans have hypertension

(HTN).• It is the most common disorder

affecting the CV system and is a major cause of atherosclerotic vasculardisease (ASVD), heart failure, kidney disease and stroke.

Alterations Of Blood Pressure• Hypertension is defined as an elevated systolic

blood pressure (SBP), an elevated diastolic blood pressure (DBP), or both. Depending on severity, it is classified as pre-hypertension, Stage 1 HTN, or stage 2 HTN.

Alterations Of Blood Pressure• Hypotension is defined as any blood pressure

too low to allow sufficient blood flow (hypo-perfusion) to meet the body's metabolic demands (to maintain homeostasis).

• Many persons, especially some thin, young women, have very low BP, yet experience no dizziness, fatigue, or other symptoms – they are not hypotensive, and in fact are probably very healthy (cardiovascular wise).

• Hypotension leading to hypo-perfusion (pressure and flow are related) of critical organs results in shock

Shock And Homeostasis• The 4 basic types of shock are:• Hypovolemic shock, due to decreased blood volume• Cardiogenic shock, due to poor heart function• Obstructive shock, due to obstruction of blood flow• Vascular shock, due to excess vasodilation - as seen

in cases of a massive allergy (anaphylaxis) or sepsis. In the U.S., septic shock causes >100,000 deaths/yr. and is the most common cause of death in hospital critical care units.

Shock And Homeostasis• The same negative feedback mechanism

discussed in autoregulation of blood pressure/flow is activated to restore blood and nutrient flow in cases of shock.• Heart will respond with rate and force of

contraction.• Selective tissue beds will vasoconstrict to shunt

blood flow to those tissue most necessary to life (brain).

• The other neural, hormonal, and chemical pathways will be recruited to restore balance.

Shock and Homeostasis

• Heart rate & force increase• Vasoconstriction or vasodilation

depending on type of shock• ADH released conserve water• Renin released Angiotensin II• Aldosterone released conserve Na+• ANP inhibited

The body responds via negative feedback to restore homeostasis