7.3 Cardiovascular regulation and adaptation

The cardiovascular system adapts to various stimuli to maintain homeostasis. Autonomic regulation, including baroreceptor and chemoreceptor reflexes, fine-tunes heart rate and blood pressure. Hormonal control, like the renin-angiotensin-aldosterone system, further regulates blood volume and pressure.

Exercise and temperature changes trigger specific cardiovascular adaptations. Regular exercise improves heart function and vascular health, while thermoregulation involves adjusting blood flow to maintain body temperature. These mechanisms showcase the system's remarkable flexibility and responsiveness.

Autonomic Regulation

Baroreceptor Reflex

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• Baroreceptors detect changes in blood pressure located in the walls of major arteries (aortic arch and carotid sinuses)
• Increased blood pressure stimulates baroreceptors sends signals to the cardiovascular center in the medulla oblongata
• Cardiovascular center responds by decreasing sympathetic activity and increasing parasympathetic activity to the heart and blood vessels
• Results in decreased heart rate, cardiac contractility, and peripheral resistance leading to a decrease in blood pressure back to normal levels
• Opposite occurs when blood pressure decreases baroreceptors are less stimulated, leading to increased sympathetic activity and decreased parasympathetic activity to restore blood pressure

Chemoreceptor Reflex

• Chemoreceptors detect changes in blood oxygen, carbon dioxide, and pH levels located in the carotid and aortic bodies
• Decreased blood oxygen or increased carbon dioxide and hydrogen ion concentrations stimulate chemoreceptors
• Signals are sent to the cardiovascular center in the medulla oblongata, which responds by increasing sympathetic activity
• Leads to increased heart rate, cardiac contractility, and peripheral resistance to improve blood flow and oxygen delivery to tissues
• Chemoreceptor reflex is important for maintaining adequate tissue oxygenation and acid-base balance

Autonomic Nervous System Control

• Sympathetic nervous system activation causes:
  • Increased heart rate and contractility via release of norepinephrine on beta-1 receptors in the heart
  • Vasoconstriction of arterioles via alpha-1 receptors in vascular smooth muscle increases peripheral resistance and blood pressure
  • Vasodilation in skeletal muscle and coronary arteries via beta-2 receptors improves blood flow during exercise
• Parasympathetic nervous system activation causes:
  • Decreased heart rate via release of acetylcholine on muscarinic receptors in the sinoatrial node
  • Vasodilation of arterioles in some vascular beds (gastrointestinal tract) via nitric oxide release from endothelial cells
• Balance between sympathetic and parasympathetic activity is crucial for maintaining cardiovascular homeostasis and responding to changing demands

Hormonal Control

Renin-Angiotensin-Aldosterone System (RAAS)

• RAAS is a hormonal cascade that regulates blood pressure and fluid balance
• Decreased renal perfusion pressure or sympathetic activation stimulates juxtaglomerular cells in the kidneys to release renin
• Renin converts angiotensinogen (produced by the liver) to angiotensin I
• Angiotensin-converting enzyme (ACE) in the lungs converts angiotensin I to angiotensin II
• Angiotensin II is a potent vasoconstrictor that increases peripheral resistance and blood pressure
• Angiotensin II also stimulates the adrenal cortex to release aldosterone, which promotes sodium and water retention by the kidneys, increasing blood volume and pressure
• ACE inhibitors and angiotensin receptor blockers (ARBs) are common medications used to treat hypertension by blocking the RAAS

Atrial Natriuretic Peptide (ANP)

• ANP is a hormone secreted by atrial myocytes in response to increased atrial stretch (due to increased blood volume)
• ANP acts on the kidneys to promote natriuresis (sodium excretion) and diuresis (water excretion), reducing blood volume and pressure
• ANP also causes vasodilation of arterioles and veins, decreasing peripheral resistance and venous return
• ANP counteracts the effects of the RAAS, helping to maintain fluid balance and prevent excessive increases in blood pressure
• Synthetic ANP (nesiritide) is sometimes used to treat acute decompensated heart failure by reducing preload and afterload

Cardiovascular Adaptations

Exercise Adaptation

• Regular aerobic exercise leads to beneficial cardiovascular adaptations that improve performance and reduce the risk of cardiovascular disease
• Cardiac adaptations include:
  • Increased stroke volume due to increased ventricular size and contractility (cardiac hypertrophy)
  • Decreased resting heart rate (bradycardia) due to increased parasympathetic tone
  • Increased cardiac output during exercise due to increased stroke volume and heart rate
• Vascular adaptations include:
  • Increased capillary density in skeletal muscle improves oxygen and nutrient delivery
  • Improved endothelial function and nitric oxide production leads to better vasodilation and blood flow regulation
  • Increased arterial compliance reduces afterload on the heart
• These adaptations allow for more efficient oxygen delivery and utilization during exercise, increasing aerobic capacity (VO2 max)

Thermoregulation

• The cardiovascular system plays a crucial role in maintaining body temperature within a normal range (thermoregulation)
• During heat stress (exposure to high temperatures or exercise):
  • Cutaneous vasodilation increases blood flow to the skin, facilitating heat loss through radiation, conduction, and convection
  • Sweating is initiated, and evaporation of sweat from the skin surface cools the body
  • Cardiac output increases to meet the demands of increased skin blood flow while maintaining adequate perfusion to other organs
• During cold stress:
  • Cutaneous vasoconstriction reduces blood flow to the skin, minimizing heat loss
  • Shivering generates heat through involuntary muscle contractions
  • Increased metabolic rate (thermogenesis) in brown adipose tissue generates heat
• Autonomic nervous system and hormones (thyroid, catecholamines) regulate these cardiovascular and metabolic responses to maintain thermal homeostasis