MCAT Content / Circulatory System / Nervous And Endocrine Control

Nervous and endocrine control

Topic: Circulatory System

The cardiovascular center forms part of the autonomic nervous system and is responsible for the regulation of the cardiac output. Hormones help to regulate the thickness of blood in collaboration with the kidneys.

The cardioaccelerator center stimulates cardiac function by regulating heart rate and stroke volume via sympathetic stimulation from the cardiac accelerator nerve. The cardioinhibitor center slows cardiac function by decreasing heart rate and stroke volume via parasympathetic stimulation from the vagus nerve.

The majority of these neurons act via the release of the neurotransmitter norepinephrine from sympathetic neurons. Although each center functions independently, they are not anatomically distinct. The cardiovascular center can respond to numerous stimuli. Hormones such as epinephrine and norepinephrine or changes in pH such as acidification due to carbon dioxide accumulation in tissue during exercise are detected by chemoreceptors.

Baroreceptors that detect stretch can also signal to the cardiovascular center to alter heart rate. Baroreceptors are specialized stretch receptors located within thin areas of blood vessels and heart chambers that respond to the degree of stretch caused by the presence of blood. They send impulses to the cardiovascular center to regulate blood pressure.

Generalized vasoconstriction usually results in an increase in systemic blood pressure, but may also occur in specific tissues, causing a localized reduction in blood flow.

A key hormonal modulator of blood viscosity is the renin-angiotensin system (RAS) or the renin-angiotensin-aldosterone system (RAAS), a hormone system that regulates blood pressure and water balance. When blood volume is low, the kidneys secrete renin directly into circulation. Plasma renin then carries out the conversion of angiotensinogen released by the liver to angiotensin I. Angiotensin I is subsequently converted to angiotensin II by the enzyme angiotensin-converting enzyme found in the lungs. Angiotensin II is a potent vasoactive peptide that causes blood vessels to constrict, resulting in increased blood pressure.

 


Practice Questions

 


Khan Academy

 

MCAT Official Prep (AAMC)

Practice Exam 4 B/B Section Passage 8 Question 43

Biology Question Pack, Vol 2. Passage 7 Question 47

 


Key Points

• The cardioaccelerator center, the cardioinhibitor center, and the vasomotor center form the cardiovascular center, a cluster of neurons that function independently to regulate blood pressure, cardiac output and blood flow. The cardiovascular center can alter heart rate and stroke volume to increase blood pressure and flow.

• The release of the neurotransmitter norepinephrine from sympathetic neurons directs the majority of neurons associated with the cardiovascular center.

• Baroreceptors respond to the degree of stretch caused by the presence of blood; this stimulates impulses to be sent to the cardiovascular center to regulate blood pressure to achieve homeostasis when needed.

• Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues, causing a localized reduction in blood flow.

• When blood volume is low, renin, excreted by the kidneys, stimulates the production of angiotensin I, which is converted into angiotensin II. This substance has many effects, including an increase in blood pressure due to its vasoconstrictive properties.


Key Terms

Cardiovascular center: A region of the brain responsible for nervous control of the cardiac output

Autonomic nervous system: The part of the nervous system that regulates the involuntary activity of the heart, intestines, and glands. These activities include digestion, respiration, perspiration, metabolism, and blood pressure modulation

Norepinephrine: A catecholamine with multiple roles including as a hormone and neurotransmitter. Areas of the body that produce or are affected by this substance are described as noradrenergic

Sympathetic: related to the part of the autonomic nervous system that under stress raises blood pressure and heart rate, constricts blood vessels and dilates the pupils

Baroreceptor: A nerve ending that is sensitive to changes in blood pressure.

Parasympathetic: relating to the part of the autonomic nervous system that inhibits or opposes the effects of the sympathetic nervous system

Vasoconstriction: The constriction (narrowing) of a blood vessel

Chemoreceptors: detect changes in chemical concentrations

Cardioinhibitor: an area of the brain that slows down the cardiac system

Cardioaccelerator: an area of the brain that speeds up the cardiac system

Stroke volume: the volume of blood ejected from each ventricle due to the contraction of the heart muscle



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