Cardiovascular Dynamics Part 2 Biology 260

Maintaining Blood Pressure Requires – Cooperation of the heart, blood vessels, and kidneys – Supervision by the brain

Maintaining Blood Pressure The main factors influencing blood pressure: – Cardiac output (CO) – Peripheral resistance (PR) – Blood volume

Maintaining Blood Pressure F =  P/PR and CO =  P/PR Blood pressure = CO x PR (and CO depends on blood volume) Blood pressure varies directly with CO, PR, and blood volume Changes in one variable are quickly compensated for by changes in the other variables

Cardiac Output (CO) Determined by venous return and neural and hormonal controls Resting heart rate is maintained by the cardioinhibitory center via the parasympathetic vagus nerves Stroke volume is controlled by venous return (EDV)

Cardiac Output (CO) During stress, the cardioacceleratory center increases heart rate and stroke volume via sympathetic stimulation – ESV decreases and MAP increases

Figure 19.8 Venous return Exercise Contractility of cardiac muscle Sympathetic activity Parasympathetic activity Epinephrine in blood EDVESV Stroke volume (SV) Heart rate (HR) Cardiac output (CO = SV x HR Activity of respiratory pump (ventral body cavity pressure) Activity of muscular pump (skeletal muscles) Sympathetic venoconstriction BP activates cardiac centers in medulla Initial stimulus Result Physiological response

Control of Blood Pressure Short-term neural and hormonal controls – Counteract fluctuations in blood pressure by altering peripheral resistance Long-term renal regulation – Counteracts fluctuations in blood pressure by altering blood volume

Short-Term Mechanisms: Neural Controls Neural controls of peripheral resistance – Maintain MAP by altering blood vessel diameter – Alter blood distribution in response to specific demands

Short-Term Mechanisms: Neural Controls Neural controls operate via reflex arcs that involve – Baroreceptors – Vasomotor centers and vasomotor fibers – Vascular smooth muscle

The Vasomotor Center A cluster of sympathetic neurons in the medulla that oversee changes in blood vessel diameter Part of the cardiovascular center, along with the cardiac centers Maintains vasomotor tone (moderate constriction of arterioles) Receives inputs from baroreceptors, chemoreceptors, and higher brain centers

Short-Term Mechanisms: Baroreceptor-Initiated Reflexes Baroreceptors are located in – Carotid sinuses – Aortic arch – Walls of large arteries of the neck and thorax

Short-Term Mechanisms: Baroreceptor-Initiated Reflexes Increased blood pressure stimulates baroreceptors to increase input to the vasomotor center – Inhibits the vasomotor center, causing arteriole dilation and venodilation – Stimulates the cardioinhibitory center

Figure 19.9 Baroreceptors in carotid sinuses and aortic arch are stimulated. Baroreceptors in carotid sinuses and aortic arch are inhibited. Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. Impulses from baroreceptors stimulate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. CO and R return blood pressure to homeostatic range. CO and R return blood pressure to homeostatic range. Rate of vasomotor impulses allows vasodilation, causing R Vasomotor fibers stimulate vasoconstriction, causing R Sympathetic impulses to heart cause HR, contractility, and CO. Sympathetic impulses to heart cause HR, contractility, and CO. Stimulus: Blood pressure (arterial blood pressure falls below normal range). Stimulus: Blood pressure (arterial blood pressure rises above normal range) a 4b Homeostasis: Blood pressure in normal range 4b a

Figure 19.9 step 1 Stimulus: Blood pressure (arterial blood pressure rises above normal range). 1 Homeostasis: Blood pressure in normal range

Figure 19.9 step 2 Baroreceptors in carotid sinuses and aortic arch are stimulated. Stimulus: Blood pressure (arterial blood pressure rises above normal range). 2 1 Homeostasis: Blood pressure in normal range

Figure 19.9 step 3 Baroreceptors in carotid sinuses and aortic arch are stimulated. Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. Stimulus: Blood pressure (arterial blood pressure rises above normal range) Homeostasis: Blood pressure in normal range

Figure 19.9 step 4a Sympathetic impulses to heart cause HR, contractility, and CO. Baroreceptors in carotid sinuses and aortic arch are stimulated. Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. Stimulus: Blood pressure (arterial blood pressure rises above normal range) a Homeostasis: Blood pressure in normal range

Figure 19.9 step 4b Sympathetic impulses to heart cause HR, contractility, and CO. Rate of vasomotor impulses allows vasodilation, causing R Baroreceptors in carotid sinuses and aortic arch are stimulated. Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. Stimulus: Blood pressure (arterial blood pressure rises above normal range) b 4a Homeostasis: Blood pressure in normal range

Figure 19.9 step 5 Sympathetic impulses to heart cause HR, contractility, and CO. CO and R return blood pressure to homeostatic range. Rate of vasomotor impulses allows vasodilation, causing R Baroreceptors in carotid sinuses and aortic arch are stimulated. Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. Stimulus: Blood pressure (arterial blood pressure rises above normal range) b 4a 5 Homeostasis: Blood pressure in normal range

Figure 19.9 step 1 Stimulus: Blood pressure (arterial blood pressure falls below normal range). 1 Homeostasis: Blood pressure in normal range

Figure 19.9 step 2 Baroreceptors in carotid sinuses and aortic arch are inhibited. Stimulus: Blood pressure (arterial blood pressure falls below normal range). 2 1 Homeostasis: Blood pressure in normal range

Figure 19.9 step 3 Baroreceptors in carotid sinuses and aortic arch are inhibited. Impulses from baroreceptors stimulate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. Stimulus: Blood pressure (arterial blood pressure falls below normal range) Homeostasis: Blood pressure in normal range

Figure 19.9 step 4a Baroreceptors in carotid sinuses and aortic arch are inhibited. Impulses from baroreceptors stimulate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. Sympathetic impulses to heart cause HR, contractility, and CO. Stimulus: Blood pressure (arterial blood pressure falls below normal range) a Homeostasis: Blood pressure in normal range

Figure 19.9 step 4b Baroreceptors in carotid sinuses and aortic arch are inhibited. Impulses from baroreceptors stimulate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. Vasomotor fibers stimulate vasoconstriction, causing R Sympathetic impulses to heart cause HR, contractility, and CO. Stimulus: Blood pressure (arterial blood pressure falls below normal range) b 4a Homeostasis: Blood pressure in normal range

Figure 19.9 step 5 Baroreceptors in carotid sinuses and aortic arch are inhibited. Impulses from baroreceptors stimulate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. CO and R return blood pressure to homeostatic range. Vasomotor fibers stimulate vasoconstriction, causing R Sympathetic impulses to heart cause HR, contractility, and CO. Stimulus: Blood pressure (arterial blood pressure falls below normal range) b 4a 5 Homeostasis: Blood pressure in normal range

Figure 19.9 Baroreceptors in carotid sinuses and aortic arch are stimulated. Baroreceptors in carotid sinuses and aortic arch are inhibited. Impulses from baroreceptors stimulate cardioinhibitory center (and inhibit cardioacceleratory center) and inhibit vasomotor center. Impulses from baroreceptors stimulate cardioacceleratory center (and inhibit cardioinhibitory center) and stimulate vasomotor center. CO and R return blood pressure to homeostatic range. CO and R return blood pressure to homeostatic range. Rate of vasomotor impulses allows vasodilation, causing R Vasomotor fibers stimulate vasoconstriction, causing R Sympathetic impulses to heart cause HR, contractility, and CO. Sympathetic impulses to heart cause HR, contractility, and CO. Stimulus: Blood pressure (arterial blood pressure falls below normal range). Stimulus: Blood pressure (arterial blood pressure rises above normal range) a 4b Homeostasis: Blood pressure in normal range 4b a

Short-Term Mechanisms: Baroreceptor-Initiated Reflexes Baroreceptors taking part in the carotid sinus reflex protect the blood supply to the brain Baroreceptors taking part in the aortic reflex help maintain adequate blood pressure in the systemic circuit

Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes Chemoreceptors are located in the – Carotid sinus – Aortic arch – Large arteries of the neck

Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes Chemoreceptors respond to rise in CO 2, drop in pH or O 2 – Increase blood pressure via the vasomotor center and the cardioacceleratory center Are more important in the regulation of respiratory rate (Chapter 22)

Influence of Higher Brain Centers Reflexes that regulate BP are integrated in the medulla Higher brain centers (cortex and hypothalamus) can modify BP via relays to medullary centers

Short-Term Mechanisms: Hormonal Controls Adrenal medulla hormones norepinephrine (NE) and epinephrine cause generalized vasoconstriction and increase cardiac output Angiotensin II, generated by kidney release of renin, causes vasoconstriction

Short-Term Mechanisms: Hormonal Controls Atrial natriuretic peptide causes blood volume and blood pressure to decline, causes generalized vasodilation Antidiuretic hormone (ADH)(vasopressin) causes intense vasoconstriction in cases of extremely low BP

Long-Term Mechanisms: Renal Regulation Baroreceptors quickly adapt to chronic high or low BP Long-term mechanisms step in to control BP by altering blood volume Kidneys act directly and indirectly to regulate arterial blood pressure 1.Direct renal mechanism 2.Indirect renal (renin-angiotensin) mechanism

Direct Renal Mechanism Alters blood volume independently of hormones – Increased BP or blood volume causes the kidneys to eliminate more urine, thus reducing BP – Decreased BP or blood volume causes the kidneys to conserve water, and BP rises

Indirect Mechanism The renin-angiotensin mechanism –  Arterial blood pressure  release of renin – Renin  production of angiotensin II – Angiotensin II is a potent vasoconstrictor – Angiotensin II  aldosterone secretion Aldosterone  renal reabsorption of Na + and  urine formation – Angiotensin II stimulates ADH release

Figure Arterial pressure Baroreceptors Indirect renal mechanism (hormonal) Direct renal mechanism Sympathetic stimulation promotes renin release Kidney Renin release catalyzes cascade, resulting in formation of ADH release by posterior pituitary Aldosterone secretion by adrenal cortex Water reabsorption by kidneys Blood volume Filtration Arterial pressure Angiotensin II Vasoconstriction ( diameter of blood vessels) Sodium reabsorption by kidneys Initial stimulus Physiological response Result

Figure Activity of muscular pump and respiratory pump Release of ANP Fluid loss from hemorrhage, excessive sweating Crisis stressors: exercise, trauma, body temperature Bloodborne chemicals: epinephrine, NE, ADH, angiotensin II; ANP release Body size Conservation of Na + and water by kidney Blood volume Blood pressure Blood pH, O 2, CO 2 Dehydration, high hematocrit Blood volume BaroreceptorsChemoreceptors Venous return Activation of vasomotor and cardiac acceleration centers in brain stem Heart rate Stroke volume Diameter of blood vessels Cardiac output Initial stimulus Result Physiological response Mean systemic arterial blood pressure Blood viscosity Peripheral resistance Blood vessel length