The Autonomic Nervous System – SB

The Autonomic Nervous System

The autonomic nervous system (ANS) is a vital component of the human nervous system, responsible for regulating involuntary physiological processes that maintain homeostasis and respond to external and internal stimuli. As paramedics, understanding the intricacies of the ANS is crucial, as it influences the body’s responses during emergencies, influencing everything from heart rate to respiratory rate. In this lesson, we will delve into the detailed aspects of the ANS, exploring its subdivisions, functions, and clinical implications.

Subdivisions of the Autonomic Nervous System:

The ANS can be divided into two distinct yet interconnected subdivisions: the sympathetic nervous system and the parasympathetic nervous system. These divisions work together to modulate various bodily functions and maintain equilibrium in response to different situations.

The ANS controls a wide range of involuntary physiological processes crucial for maintaining bodily functions and adapting to changing environments. Here are some key functions:

Functions of the Autonomic Nervous System:

Heart Rate Control

Heart Rate Control
The ANS modulates heart rate to match the body’s demands. The sympathetic system increases heart rate during stress or physical activity, while the parasympathetic system slows it down during periods of rest.

Blood Pressure Regulation
The sympathetic division plays a significant role in regulating blood pressure by influencing blood vessel constriction and dilation.

Respiratory Control

Respiratory Rate Modulation
The ANS adjusts respiratory rate and depth based on factors like oxygen and carbon dioxide levels in the blood. The sympathetic system can stimulate rapid breathing, while the parasympathetic system promotes slower, controlled breathing.

Gastrointestinal Function

Digestive Processes
The parasympathetic division enhances digestion by stimulating the secretion of digestive enzymes, promoting peristalsis (intestinal contractions), and increasing blood flow to the digestive organs.

Sympathetic Inhibition
The sympathetic division temporarily inhibits digestive functions during the “fight or flight” response to redirect blood flow to other areas of the body.

Urinary and Reproductive Functions

Bladder Control
The parasympathetic system is responsible for relaxing the bladder’s smooth muscle, allowing urine to be expelled during urination.

Reproductive Processes
The ANS plays a role in sexual arousal, with both divisions contributing to different aspects of the process.

Clinical Implications for Paramedics:

As paramedics, a profound understanding of the ANS is invaluable for managing emergencies and optimizing patient care. The ANS’s influence on heart rate, blood pressure, and other vital functions underscores its importance in various medical scenarios:

Shock Management

The sympathetic response can lead to shock, characterized by decreased blood pressure. Paramedics must promptly address this by administering appropriate interventions to stabilize the patient.

Anaphylactic Reactions

In cases of severe allergic reactions, the ANS can trigger life-threatening responses. Knowledge of sympathetic and parasympathetic influences aids paramedics in administering appropriate medications, such as epinephrine, to counteract these reactions.

What’s Happening Inside

What’s Happening Inside

The body has several compensatory mechanisms in place to respond to hypovolemic shock, which is a life-threatening condition caused by a significant decrease in blood volume. These mechanisms are aimed at maintaining blood pressure, perfusion of vital organs, and overall homeostasis. However, these compensatory responses are temporary and may not be able to fully reverse the effects of hypovolemic shock. Here are some of the compensatory mechanisms the body employs

Increased Heart Rate (Tachycardia)

When blood volume drops, the heart rate increases to pump blood more rapidly and try to maintain adequate blood pressure. This helps ensure that vital organs continue to receive at least some level of oxygen and nutrients.


Blood vessels narrow (vasoconstrict) in order to maintain blood pressure by directing more blood flow to essential organs like the brain, heart, and lungs. This diversion of blood from peripheral tissues can lead to cool, pale skin and reduced blood flow to non-essential areas.

Increased Contractility

The heart’s pumping strength (contractility) can increase to improve the amount of blood being circulated to vital organs.

Fluid Redistribution

Fluid is shifted from the interstitial spaces (between cells) into the bloodstream to help increase blood volume and blood pressure. This can lead to swelling and edema in certain areas of the body.

Activation of the Renin-Angiotensin-Aldosterone System (RAAS)

In response to decreased blood volume, the kidneys release renin, an enzyme that triggers a series of hormonal responses leading to the production of angiotensin and aldosterone. These hormones cause vasoconstriction and increase the reabsorption of sodium and water by the kidneys, aiming to raise blood pressure and fluid volume.

Release of Antidiuretic Hormone (ADH)

Also known as vasopressin, ADH is released by the pituitary gland in response to low blood volume. It helps the kidneys retain water, reducing urine production and conserving fluid.

While these compensatory mechanisms initially help the body respond to hypovolemic shock, they are not sustainable in the long term. If the underlying cause of the shock, such as severe bleeding or fluid loss, is not promptly addressed and treated, the compensatory responses can become overwhelmed, leading to further deterioration of the individual’s condition and potentially irreversible organ damage. Therefore, hypovolemic shock is a medical emergency that requires immediate medical intervention to restore blood volume and stabilize the individual’s condition.