19.8 short- term regulation: Neutral Controls

MAP is maintained by

1. altering blood vessel diameter, which alters resistance

• Ex: If blood volume drops, all vessels constrict (except those to heart and brain)

1. Can alter blood distribution to organs in response to specific demands

Neural controls operate via reflex arcs that involve:

• Cardiovascular center of medulla 

• Baroreceptors

• Chemoreceptors

• Higher brain centers

Cardiovascular center:

composed of clusters of sympathetic neurons in medulla 

Cardiovascular center consits of;

Cardiac centers: cardioinhibitory and cardioacceleratory centers

vasomotor center

sends steady impulses via sympathetic efferents called Vasomotor fibers to blood vessels

baroreceptor reflexes located in

carotid sinuses, aortic arch, and walls of large arteries of neck and thorax

If MAP is high…?

• Increased blood pressure stimulates baroreceptors to increase input to vasomotor center

• Inhibits vasomotor and cardioacceleratory centers

• Stimulates cardioinhibitory center

• Results in decreased blood pressure

Vasodilation

decreased output from vasomotor center causes dilation

Arteriolar vasodilation

reduces peripheral resistance, decreasing venous return and CO

Decreased cardiac output

1. Impulses to cardiac centers inhibit sympathetic activity and stimulate parasympathetic

• Reduces heart rate and contractility; CO decrease causes decrease in MAP

If MAP ia low-

• Reflex vasoconstriction is initiated that increases CO and blood pressure

• Example: upon standing, BP falls and triggers:

Cartoid sinus reflex

baroreceptors that monitor BP to ensure enough blood to brain 

aortic reflex

• maintains BP in systemic circuit

• Baroreceptors are ineffective if altered blood pressure is sustained

chemoreceptor reflexes

• Aortic arch and large arteries of neck detect increase in CO₂, or drop in pH or O₂

• Cause increased blood pressure by:

  • Signaling cardioacceleratory center to increase CO

  • Signaling vasomotor center to increase vasoconstriction

Influence of higher brain centers

• Reflexes that regulate BP are found in medulla

• Hypothalamus and cerebral cortex can modify arterial pressure via relays to medulla

• Hypothalamus increases blood pressure during stress

• Hypothalamus mediates redistribution of blood flow during exercise and changes in body temperature

• Hormones regulate BP in short term via changes in

peripheral resistance or long term via changes in blood volume

Adrenal medulla hormones

Epinephrine and norepinephrine from adrenal gland increase CO and vasoconstriction

angiotensin II

stimulates vasoconstriction

ADH -

high levels can cause vasoconstriction

atrial natriuretic peptide

decreases BP by antagonizing aldosterone, causing decreased blood volume

Baroreceptors quickly adapt to

chronic high or low BP so are ineffective for long-term regulation

kidney regulate arterial blood pressure by

1. Direct renal mechanism 

2. Indirect renal mechanism (renin-angiotensin-aldosterone)

Direct renal mechanism

• Alters blood volume independently of hormones

  • Increased BP or blood volume causes elimination of more urine, thus reducing BP

  • Decreased BP or blood volume causes kidneys to conserve water, and BP rises

Indirect mechanism

The renin-angiotensin-aldosterone system (RAAS): 

• Decreased arterial blood pressure causes release of renin from kidneys

• Renin enters blood and catalyzes conversion of angiotensinogen from liver to angiotensin I

• Angiotensin-converting enzyme, especially from lungs, converts angiotensin I to angiotensin II

• Stimulates aldosterone secretion