Blood flow and circulation are vital for life. The heart pumps blood through arteries, capillaries, and veins, delivering oxygen and nutrients to tissues. This complex system ensures every cell gets what it needs to function.

Blood pressure regulation involves vessel size, cardiac output, and blood volume. Hormones and the nervous system play key roles. Understanding these mechanisms helps us grasp how the body maintains homeostasis and responds to different situations.

Blood Flow and Circulation

Blood flow through the body

• Blood pumped out of the left ventricle through the aorta, the largest artery in the body
• Aorta branches into smaller arteries carrying oxygenated blood to various organs and tissues (brain, heart, kidneys)
• Arteries branch into smaller arterioles controlling blood flow to specific capillary beds
• Arterioles branch into capillaries, the smallest blood vessels enabling gas and nutrient exchange between blood and tissues
• Deoxygenated blood flows from capillaries into venules, which merge to form veins
• Veins carry deoxygenated blood back to the heart, entering the right atrium (superior and inferior vena cava)
• Blood flows from the right atrium to the right ventricle, then pumped to the lungs for oxygenation (pulmonary arteries)
• Oxygenated blood returns to the left atrium (pulmonary veins), then to the left ventricle to begin the cycle again

Regulation of blood pressure

• Vessel size regulates blood pressure through vasoconstriction (narrowing) and vasodilation (widening)
  • Vasoconstriction increases blood pressure by reducing vessel diameter and increasing resistance to blood flow
  • Vasodilation decreases blood pressure by increasing vessel diameter and reducing resistance to blood flow
• Cardiac output, the volume of blood pumped by the heart per minute, influences blood pressure
  • Increased heart rate and stroke volume lead to higher cardiac output and blood pressure
  • Decreased heart rate and stroke volume result in lower cardiac output and blood pressure
• Blood volume directly affects blood pressure
  • Higher blood volume increases blood pressure by increasing the volume of blood in the vessels
  • Lower blood volume decreases blood pressure by reducing the volume of blood in the vessels
• Hormones play a crucial role in regulating blood pressure
  • Antidiuretic hormone (ADH) increases water retention, leading to increased blood volume and pressure
  • Aldosterone promotes sodium and water retention, increasing blood volume and pressure
  • Atrial natriuretic peptide (ANP) promotes sodium and water excretion, decreasing blood volume and pressure
• The nervous system regulates blood pressure through the autonomic nervous system
  • Sympathetic nervous system increases heart rate, contractility, and vasoconstriction, raising blood pressure (fight-or-flight response)
  • Parasympathetic nervous system decreases heart rate and contractility, lowering blood pressure (rest-and-digest response)
• Baroreceptors in blood vessels detect changes in blood pressure and trigger appropriate responses

Additional blood pressure regulation mechanisms

• Renin-angiotensin-aldosterone system (RAAS) regulates blood pressure and fluid balance
• Endothelium releases vasoactive substances to control blood vessel diameter
• Autoregulation maintains constant blood flow to organs despite changes in blood pressure
• Frank-Starling law describes how the heart adjusts its output based on venous return

Blood Pressure Measurement

Systolic vs diastolic pressure

• Blood pressure measured in millimeters of mercury (mmHg) consists of systolic and diastolic values
• Systolic blood pressure represents the maximum pressure in the arteries during ventricular contraction (systole)
  • Normal systolic blood pressure is less than 120 mmHg
  • High systolic pressure (>140 mmHg) indicates increased cardiac output or increased peripheral resistance
• Diastolic blood pressure represents the minimum pressure in the arteries during ventricular relaxation (diastole)
  • Normal diastolic blood pressure is less than 80 mmHg
  • High diastolic pressure (>90 mmHg) suggests increased peripheral resistance or decreased venous return
• The significance of blood pressure measurements lies in identifying abnormalities and associated health risks
  • Hypertension, or consistently high blood pressure, increases the risk of cardiovascular disease (heart attack, stroke) and kidney damage
  • Hypotension, or consistently low blood pressure, can cause dizziness, fainting, and reduced organ perfusion (shock)
• Pulse pressure, the difference between systolic and diastolic pressures, provides information about arterial stiffness and cardiac output
  • Normal pulse pressure ranges from 30 to 50 mmHg
  • Increased pulse pressure (>60 mmHg) suggests reduced arterial compliance or increased stroke volume
  • Decreased pulse pressure (<25 mmHg) may indicate heart failure or decreased stroke volume