# Lecture 25: Integration — Response to Exercise and Stress

## Anatomy and Physiology II

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## Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the integrated physiological responses to acute exercise across multiple organ systems
2. Explain how the cardiovascular, respiratory, muscular, and metabolic systems coordinate during exercise
3. Distinguish between the body's responses to aerobic vs. anaerobic exercise
4. Define stress and describe the general adaptation syndrome (GAS)
5. Explain the roles of the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis in the stress response
6. Describe the physiological consequences of chronic stress on body systems
7. Integrate knowledge from previous lectures to explain multi-system homeostatic responses

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## Lecture Content

### I. Overview of Exercise Physiology

Exercise represents one of the greatest challenges to homeostasis, demanding a coordinated, multi-system response. The body must simultaneously increase oxygen delivery to working muscles, increase removal of carbon dioxide and metabolic wastes, increase fuel supply to muscles, dissipate excess heat, and maintain blood pressure and adequate perfusion of vital organs. Meeting these demands requires the nervous system, endocrine system, cardiovascular system, respiratory system, and metabolic pathways to act in concert.

### II. Cardiovascular Responses to Exercise

#### Heart Rate and Cardiac Output

Cardiac output (CO) equals heart rate (HR) multiplied by stroke volume (SV). At rest, CO is approximately 5 L/min (HR around 72 bpm, SV around 70 mL). During maximal exercise, CO can increase to **20-25 L/min** (and up to 35-40 L/min in trained athletes). Heart rate increases to 180-200 bpm through sympathetic stimulation and parasympathetic withdrawal, while stroke volume increases to 100-130 mL through enhanced venous return, increased contractility via the Frank-Starling mechanism, and sympathetic inotropic effects. **Sympathetic activation** increases HR (positive chronotropy) and contractility (positive inotropy), while **parasympathetic withdrawal** (removal of vagal tone on the SA node) contributes to the initial rise in heart rate.

#### Blood Flow Redistribution

At rest, skeletal muscles receive only about 15-20% of cardiac output. During exercise, they can receive **up to 80-85% of CO**. This dramatic redistribution occurs through several mechanisms. **Local (metabolic) vasodilation** in active muscles results from accumulation of carbon dioxide, hydrogen ions, potassium, adenosine, and nitric oxide, which dilate arterioles and increase blood flow (functional or exercise hyperemia). **Sympathetic vasoconstriction** of arterioles in non-essential vascular beds (GI tract, kidneys, inactive muscles, and initially the skin) redirects blood to active muscles. Skin blood flow initially decreases but then increases as exercise continues and core temperature rises, facilitating thermoregulation through cutaneous vasodilation and sweating.

**Venous return** is enhanced by the skeletal muscle pump (rhythmic compression of veins during contraction), the respiratory pump (decreased intrathoracic pressure during inspiration), and venoconstriction (sympathetic tone on veins reduces venous compliance, increasing preload).

#### Blood Pressure During Exercise

**Systolic blood pressure** increases to approximately 180-200 mmHg during intense exercise due to increased CO. **Diastolic blood pressure** remains relatively constant or decreases slightly because vasodilation in muscles reduces total peripheral resistance. **Mean arterial pressure** increases moderately. During resistance or isometric exercise, both systolic and diastolic blood pressure rise significantly due to the Valsalva-like effect.

### III. Respiratory Responses to Exercise

Minute ventilation (tidal volume multiplied by respiratory rate) increases dramatically during exercise. At rest, it is approximately 6 L/min (tidal volume around 500 mL, respiratory rate around 12/min). During maximal exercise, it can increase to **100-150 L/min** (up to 200 L/min in trained athletes), with tidal volume increasing to 2-3 L and respiratory rate to 40-50 breaths per minute.

Multiple mechanisms drive increased ventilation. Anticipatory responses from cortical motor centers activate respiratory centers even before exercise begins. Neural input from proprioceptors in joints and muscles stimulates the respiratory center. Chemical stimuli include rising carbon dioxide, hydrogen ions, and decreasing oxygen (during intense exercise) acting on central and peripheral chemoreceptors. Rising body temperature and circulating epinephrine further stimulate ventilation.

**Oxygen consumption (VO2)** increases linearly with exercise intensity up to VO2 max, the maximum rate of oxygen consumption during maximal exercise. VO2 max serves as the gold-standard measure of cardiorespiratory fitness, depending on maximum cardiac output, pulmonary diffusion, oxygen-carrying capacity of blood, and muscle oxidative capacity. Average values for untrained individuals are approximately 35-45 mL/kg/min, while elite endurance athletes achieve approximately 70-85 mL/kg/min.

**Oxygen debt (excess post-exercise oxygen consumption, or EPOC)** refers to the elevated oxygen consumption after exercise, which serves to replenish ATP and creatine phosphate stores, convert lactate to glucose (Cori cycle), restore myoglobin oxygen stores, and support the elevated body temperature and heart rate during recovery.

<image>A multi-panel figure showing cardiovascular and respiratory responses to graded exercise. Panel A: A graph plotting cardiac output, heart rate, and stroke volume against exercise intensity (% VO2 max). Cardiac output rises linearly. Heart rate rises linearly throughout. Stroke volume rises initially then plateaus at ~40–50% VO2 max. Panel B: A graph plotting minute ventilation against exercise intensity, showing a linear increase up to the ventilatory threshold, then a steeper exponential rise at higher intensities. VO2 and VCO2 curves are shown, with the ventilatory threshold (anaerobic threshold) marked where VCO2 begins to rise disproportionately. Panel C: A diagram of blood flow redistribution — two human figures (one at rest, one exercising) with pie charts showing the percentage of cardiac output going to different organs. At rest: muscles 15–20%, kidneys 20%, GI 20–25%, brain 15%, skin 5%, heart 5%, other 10–15%. During maximal exercise: muscles 80–85%, kidneys 3–5%, GI 3–5%, brain 3–4%, skin 2–5% (variable with heat), heart 4–5%. Arrows indicate the magnitude of change.</image>

### IV. Metabolic Responses to Exercise

#### Energy Systems

The body draws on three energy systems depending on exercise duration and intensity. The **phosphagen system (ATP-PC system)** provides immediate energy for the first 10-15 seconds. Creatine phosphate donates a phosphate group to ADP via creatine kinase to regenerate ATP. This system is extremely rapid but has very limited supply, serving explosive activities such as sprinting starts, throws, and jumps.

**Anaerobic glycolysis** provides short-term energy from approximately 30 seconds to 2 minutes. Glucose is converted to pyruvate and then to lactate in the absence of sufficient oxygen, yielding a net of 2 ATP per glucose molecule. While fast, it produces lactate and hydrogen ions that contribute to muscle fatigue and metabolic acidosis. This system dominates during high-intensity, short-duration activities.

**Oxidative (aerobic) metabolism** provides long-term energy for activities lasting more than 2 minutes. It involves complete oxidation of glucose, fatty acids, and (to a lesser extent) amino acids, yielding approximately 30-32 ATP per glucose molecule and approximately 106 ATP per palmitate molecule. Although slower, it is sustainable and dominates during moderate-intensity, prolonged activities. At rest and low intensity, fatty acids serve as the primary fuel (providing approximately 60% of energy). As intensity increases, glucose (from muscle glycogen and blood glucose) becomes the primary fuel. At very high intensity, anaerobic glycolysis contributes significantly.

#### Hormonal Responses to Exercise

**Sympathetic activation** increases epinephrine and norepinephrine, which increase HR and contractility, produce bronchodilation, stimulate glycogenolysis in liver and muscle and lipolysis in adipose tissue, and redirect blood flow. **Glucagon** increases to promote hepatic glucose output. **Insulin** decreases to allow blood glucose to remain elevated for muscle use and prevent glucose storage. **Cortisol** increases to promote gluconeogenesis, protein catabolism for amino acid mobilization, and anti-inflammatory effects. **Growth hormone** increases to promote lipolysis, protein synthesis, and tissue repair. **Endorphins** increase to modulate pain and produce the "runner's high."

#### Lactate Threshold and Fatigue

The **lactate threshold (anaerobic threshold)** is the exercise intensity at which blood lactate begins to accumulate exponentially. Below this threshold, lactate production equals removal. Above it, production exceeds removal, leading to progressive accumulation. Training shifts the lactate threshold to a higher percentage of VO2 max.

**Muscle fatigue** is multifactorial, involving depletion of glycogen and creatine phosphate stores, accumulation of hydrogen ions (metabolic acidosis) that inhibit enzyme function (particularly PFK) and contractile proteins, accumulation of inorganic phosphate that impairs cross-bridge cycling, depletion of calcium release from the SR, and central fatigue (reduced neural drive from the CNS).

### V. Thermoregulatory Response During Exercise

Exercise generates substantial heat, with up to 75-80% of metabolic energy released as heat. Core temperature rises during exercise to approximately 38-40 degrees Celsius during prolonged activity. Heat dissipation mechanisms are activated in response: cutaneous vasodilation (after initial vasoconstriction) increases radiative and convective heat loss, sweating (which can reach 1-2 L per hour) provides evaporative cooling, and increased ventilation contributes some respiratory heat loss.

A fundamental challenge during intense exercise is that blood must serve both active muscles and skin for cooling simultaneously, creating competing demands that can lead to heat illness if the balance is lost. Sweat contains water, sodium, chloride, and potassium, so significant fluid and electrolyte losses can produce dehydration and electrolyte imbalance, impairing performance and risking heat exhaustion or heat stroke. Adequate hydration before, during, and after exercise is essential.

### VI. The Stress Response

**Stress** is any real or perceived threat to homeostasis (called a stressor). Physical stressors include trauma, surgery, extreme temperature, infection, and intense exercise. Psychological stressors include anxiety, fear, grief, and work pressure. Hans Selye described the body's three-stage response to prolonged stress as the **General Adaptation Syndrome (GAS)**.

#### Stage 1: Alarm Reaction (Fight-or-Flight)

The alarm reaction is the immediate response mediated by the **sympathetic nervous system** and **adrenal medulla** (the sympathoadrenal response). The hypothalamus activates the sympathetic nervous system, and preganglionic neurons stimulate the adrenal medulla to release **epinephrine (approximately 80%)** and **norepinephrine (approximately 20%)** into the blood. Within seconds, this produces increased heart rate and contractility (increased CO), bronchodilation (increased airflow), vasoconstriction in skin and GI tract with vasodilation in skeletal muscles and heart, glycogenolysis and lipolysis (elevated blood glucose and fatty acids), pupil dilation, increased alertness and arousal, inhibition of digestion and urinary output, and sweating. The overall effect is to prepare the body for immediate physical action.

#### Stage 2: Resistance (Adaptation)

If the stressor persists, the body attempts to adapt and maintain homeostasis through the **hypothalamic-pituitary-adrenal (HPA) axis**. The hypothalamus releases **CRH (corticotropin-releasing hormone)**, which stimulates the anterior pituitary to release **ACTH (adrenocorticotropic hormone)**, which in turn stimulates the **adrenal cortex** to release **cortisol** (the primary stress hormone).

Cortisol produces wide-ranging effects: gluconeogenesis in the liver (mobilizing amino acids from muscle protein for conversion to glucose), lipolysis (mobilizing fatty acids from adipose tissue), raising blood glucose to ensure fuel for brain and muscles, protein catabolism in non-essential tissues (muscle, bone, connective tissue, lymphoid tissue), anti-inflammatory and immunosuppressive effects (inhibiting prostaglandins, cytokines, and immune cell activity, which is beneficial short-term but harmful long-term), and permissive effects enhancing the actions of catecholamines on blood vessels.

Other hormones active during the resistance stage include **aldosterone** (for sodium and water retention to maintain blood volume and pressure), **ADH** (for water retention), **growth hormone** (for tissue repair and fuel mobilization), and **thyroid hormones** (for increased metabolic rate during prolonged stress).

<image>A multi-panel diagram of the stress response. Panel A (Alarm Reaction — Sympathoadrenal Response): A flowchart starting with a stressor perceived by the cerebral cortex/limbic system, signaling the hypothalamus, which activates the sympathetic nervous system. Preganglionic sympathetic neurons stimulate the adrenal medulla to release epinephrine and norepinephrine into the blood. Target organ effects are listed: heart (increased HR and contractility), lungs (bronchodilation), blood vessels (selective vasoconstriction and vasodilation), liver (glycogenolysis → elevated blood glucose), adipose tissue (lipolysis → fatty acids), pupils (dilation), GI tract (decreased motility). The timeline indicates this response occurs within seconds and lasts minutes to hours. Panel B (Resistance — HPA Axis): A flowchart showing the stressor activating the hypothalamus to release CRH, which stimulates the anterior pituitary to release ACTH, which stimulates the adrenal cortex to release cortisol. Cortisol effects are shown: gluconeogenesis, lipolysis, protein catabolism, anti-inflammatory/immunosuppressive effects, and elevated blood glucose. A negative feedback loop shows cortisol inhibiting CRH and ACTH release. The timeline indicates this response takes minutes to begin and can persist for weeks. Panel C (Exhaustion): A depiction of the consequences of chronic stress — adrenal exhaustion, immune suppression (with a weakened shield icon), muscle wasting, osteoporosis, visceral fat accumulation, hypertension, and depression.</image>

#### Stage 3: Exhaustion

Exhaustion occurs when the stressor is severe and prolonged beyond the body's ability to adapt. Resources become depleted and homeostatic mechanisms fail. Consequences include adrenal insufficiency (cortisol depletion), immune system collapse with increased susceptibility to infection and disease, organ damage and failure, and potentially death if unresolved.

### VII. Consequences of Chronic Stress

Chronic stress damages virtually every organ system. **Cardiovascular** effects include sustained elevation of heart rate and blood pressure, leading to hypertension, atherosclerosis, and increased risk of myocardial infarction and stroke. **Immune** consequences include chronic cortisol suppression of immune function, increased susceptibility to infections, impaired wound healing, and possible promotion of cancer progression. Paradoxically, chronic stress can also promote chronic inflammation through dysregulated immune responses. **Metabolic** effects include chronic cortisol causing insulin resistance, hyperglycemia, increased risk of type 2 diabetes, and visceral fat accumulation producing Cushing-like features. **Musculoskeletal** effects include protein catabolism causing muscle wasting and weakness, and bone resorption leading to osteoporosis. **GI** effects include decreased blood flow and motility producing ulcers and irritable bowel syndrome. **Reproductive** effects occur because cortisol suppresses GnRH, leading to decreased FSH and LH, amenorrhea in women, decreased testosterone in men, decreased libido, and infertility. **Neurological and psychological** effects include hippocampal atrophy from cortisol neurotoxicity (impairing memory and learning) and increased risk of anxiety, depression, and insomnia. **Skin** effects include impaired collagen synthesis causing poor wound healing and thinning skin.

### VIII. Integration: Multi-System Response to Hemorrhage (Case Example)

A hemorrhage provides an excellent integrative example of how multiple organ systems coordinate to maintain homeostasis. Blood loss decreases blood volume, which decreases venous return, which decreases cardiac output and blood pressure. The **baroreceptor reflex** detects decreased firing from carotid sinus and aortic arch baroreceptors, signaling the cardiovascular center in the medulla to increase sympathetic output and decrease parasympathetic output. This increases heart rate and contractility, produces arteriolar vasoconstriction (increasing TPR), and causes venoconstriction (increasing venous return). **Chemoreceptors** stimulate increased heart rate and ventilation if oxygen delivery drops significantly. **RAAS activation** occurs as decreased renal perfusion triggers renin release, leading to angiotensin II production (causing vasoconstriction) and aldosterone secretion (promoting sodium and water retention). **ADH release** from the posterior pituitary promotes water reabsorption to conserve volume. The **sympathoadrenal response** enhances cardiac output, redirects blood flow, and mobilizes glucose. **Erythropoietin** released by the kidneys (a delayed response over days) stimulates erythropoiesis to restore oxygen-carrying capacity. **Thirst** is stimulated by angiotensin II and elevated osmolarity to encourage fluid intake. Finally, the **capillary fluid shift** occurs as decreased capillary hydrostatic pressure drives net absorption of interstitial fluid into capillaries, partially restoring plasma volume.

<image>A comprehensive integrative diagram showing the multi-system response to acute hemorrhage. Center: a bleeding wound with an arrow indicating blood loss and initial consequences (decreased blood volume, decreased venous return, decreased cardiac output, decreased blood pressure). Radiating outward are the compensatory responses organized by system. Cardiovascular: baroreceptor reflex activating the sympathetic nervous system — increased HR (heart icon), vasoconstriction (arteriole icon), venoconstriction. Renal: RAAS cascade (renin → angiotensin II → aldosterone → Na+/H2O retention), ADH release (water reabsorption), and EPO secretion (red blood cell icon). Respiratory: increased ventilation rate and depth. Endocrine: epinephrine from the adrenal medulla (glucose mobilization). Fluid shifts: arrows showing interstitial fluid moving into capillaries due to decreased hydrostatic pressure. Behavioral: thirst drive icon. All arrows converge on a central goal: restoration of blood volume, blood pressure, and oxygen delivery. A timeline along the bottom shows the time course of each response: immediate (baroreceptor reflex, sympathetic activation), minutes-hours (RAAS, ADH, fluid shifts), hours-days (thirst/fluid intake, EPO and erythropoiesis).</image>

### IX. Clinical Correlations

**Exercise-induced asthma** involves bronchospasm triggered by exercise, especially in cold, dry air, and is treated with inhaled beta-2 agonists before exercise. **Rhabdomyolysis** results from extreme exercise causing muscle fiber breakdown, releasing myoglobin that can injure the kidneys (presenting with dark urine and elevated CK). **Cushing syndrome** results from chronic cortisol excess (endogenous or exogenous) producing central obesity, moon face, hypertension, hyperglycemia, osteoporosis, and immunosuppression. **Addison disease (adrenal insufficiency)** involves insufficient cortisol and aldosterone, causing hypotension, hypoglycemia, hyperkalemia, hyperpigmentation, and fatigue. **Post-traumatic stress disorder (PTSD)** involves a dysregulated stress response with chronic hyperactivation of the sympathoadrenal system and HPA axis. **Overtraining syndrome** results from excessive exercise without adequate recovery, manifesting as fatigue, performance decline, mood disturbance, immunosuppression, and hormonal imbalance.

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