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🫀 Guyton & Hall Physiology - Chapter 19

Role of the Kidneys in Long-Term Regulation of Arterial Pressure & Hypertension

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1. THE BIG CONCEPT: Why Kidneys Are the Ultimate BP Controller

• Short-term BP control = baroreceptors, vasomotor center, vasoconstriction (nervous system) - acts in seconds to minutes
• Long-term BP control = kidneys - acts over days, weeks, months
• The kidney is unique: it is the only mechanism with "infinite gain" for pressure control
  • "Infinite gain" = given enough time, the kidney can return BP to exactly normal, not just approximate it
  • All other controllers (baroreceptors, chemoreceptors) reset and lose effectiveness over time - the kidney never fully resets

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2. THE RENAL-BODY FLUID SYSTEM (Fundamental Concept)

1. ↑ Blood volume → ↑ Arterial pressure
2. ↑ Arterial pressure → ↑ Renal fluid/salt excretion (pressure diuresis + pressure natriuresis)
3. ↑ Excretion → ↓ Blood volume → pressure returns to normal

• A rise of just a few mm Hg can double urine water output (pressure diuresis) AND double salt output (pressure natriuresis)

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3. PRESSURE NATRIURESIS & PRESSURE DIURESIS (Mechanisms)

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4. EQUILIBRIUM POINT CONCEPT (The "Set Point" of BP)

• On a graph: Renal Output Curve vs Fluid Intake Line (horizontal)
• They intersect at ONE point = the equilibrium arterial pressure = the body's set point
• This is the only pressure at which output = intake - BP will always drift back here
• If intake ↑ → temporary fluid gain → BP rises → kidneys excrete more → new equilibrium at same pressure
• This is why the kidneys have "infinite gain" - the equilibrium point does not drift

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5. WHY INCREASED TOTAL PERIPHERAL RESISTANCE (TPR) ALONE CANNOT CAUSE CHRONIC HYPERTENSION

• If TPR ↑ but kidneys are normal → momentary BP rise → kidneys excrete more fluid → blood volume falls → BP returns to normal within days
• Chronic hypertension requires either:
  1. A shift in the renal pressure-natriuresis curve (kidney excretes LESS at any given pressure), OR
  2. ↑ Fluid/salt intake beyond what kidneys can handle

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6. RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM (RAAS) AND BP CONTROL

Components

Three stimuli for renin release:

1. Baroreceptors in JG cells - sense ↓ stretch (= ↓ pressure) in afferent arteriole
2. Macula densa - senses ↓ NaCl delivery to early distal tubule
3. Sympathetic NS - beta-adrenergic receptors on JG cells → ↑ renin

Actions of Angiotensin II:

• Fast (vasoconstriction): ↑ arteriolar resistance → ↑ BP within seconds
• Slow (volume retention):
  • ↑ Na⁺/water reabsorption directly in tubules
  • ↑ Aldosterone secretion (adrenal cortex) → ↑ Na⁺ reabsorption in collecting duct
  • ↓ GFR (constricts efferent > afferent arteriole)

Rapidity of RAAS response:

• Vasoconstrictor effect: peaks in ~20 minutes
• Volume retention effect: takes hours to days
• Together, prevents BP from dropping after hemorrhage, salt restriction, etc.

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7. ROLE OF RAAS IN MAINTAINING NORMAL ARTERIAL PRESSURE

• If salt intake drops → less Na⁺ delivered to macula densa → ↑ renin → ↑ Ang II → vasoconstriction + volume retention → BP stays normal
• RAAS prevents the BP from falling with:
  • Low salt diet
  • Hemorrhage
  • Upright posture (orthostatic stress)
• ACE inhibitors / ARBs block this system → ↓ BP (especially effective in high-renin states)

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8. TYPES OF RENAL HYPERTENSION

A. Volume-Loading Hypertension (reduced kidney mass + high salt)

• ↓ Kidney mass (e.g., one kidney removed) + ↑ salt intake → kidneys cannot excrete enough → fluid accumulates → ↑ BP
• Two phases:
  1. Early phase: ↑ cardiac output (CO) drives the ↑ BP; TPR normal or low
  2. Late phase (weeks-months): CO returns toward normal, but TPR rises (autoregulatory vasoconstriction) - this is called "autoregulatory hypertension"
  • Blood vessels constrict to protect tissues from excess flow → TPR ↑ → maintains high BP even as CO normalizes

B. Hypertension from Excess Aldosterone (Primary Aldosteronism / Conn's Syndrome)

• Adrenal tumor secretes excess aldosterone
• Aldosterone → ↑ Na⁺/water reabsorption → ↑ blood volume → ↑ BP
• Early: ↑ CO; Late: ↑ TPR (autoregulation)
• If sustained → structural kidney changes → worsens further
• High salt diet makes it worse

C. Hypertension from Renin-Angiotensin System (Renovascular Hypertension)

• Clip on renal artery of one kidney, other kidney removed
• Clipped kidney: ↓ pressure → ↑ renin → ↑ Ang II → vasoconstriction + Na⁺ retention
• Short term: ↑ Ang II causes vasoconstriction → ↑ BP
• Long term: ↑ BP pressure natriuresis in the clipped kidney is impaired (because of low perfusion pressure past the clip), so Na⁺ retained → ↑ blood volume maintains hypertension
• Renin levels normalize over time but hypertension persists (now volume-dependent)

• Clip on one renal artery; contralateral kidney is normal
• Clipped kidney: ↓ pressure → ↑ renin → ↑ Ang II → vasoconstriction → ↑ BP
• Normal kidney: faces elevated BP → pressure natriuresis → excretes excess Na⁺ and water
• Result: volume stays nearly normal, but ↑ renin/Ang II drives sustained hypertension
• Renin levels remain elevated (unlike one-kidney model)
• This model mimics renal artery stenosis in humans with normal contralateral kidney

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9. PRIMARY (ESSENTIAL) HYPERTENSION (~90-95% of all hypertension)

1. Obesity/excess adiposity - accounts for 65-75% of risk
   • ↑ CO (extra blood flow to adipose tissue)
   • ↑ SNS activity (kidneys) → impairs pressure natriuresis
   • Leptin from fat cells → stimulates hypothalamus → ↑ vasomotor center activity
   • ↓ Baroreceptor sensitivity
   • ↑ Ang II and aldosterone
2. Sympathetic activation impairs the kidney's ability to excrete Na⁺ at normal pressures → renal function curve shifts rightward → new equilibrium at higher BP
3. Genetic factors (monogenic hypertension):
   • All known mutations → impaired kidney function (either ↑ renal arterial resistance OR ↑ tubular Na⁺ reabsorption)
   • Monogenic hypertension is RARE (<1% of all hypertension)
   • Key point: all monogenic hypertension = final common pathway = impaired renal Na⁺/water excretion
4. Structural kidney changes:
   • Over time, hypertension damages kidneys → ↑ preglomerular resistance → ↓ glomerular membrane permeability → worsens hypertension (vicious cycle)

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10. INTEGRATED SUMMARY: "Guiding Principle"

The kidneys set the long-term arterial pressure. Every form of chronic hypertension ultimately requires either a shift in the renal pressure-natriuresis curve to the right, or increased fluid/salt intake. The nervous system, hormones (RAAS, aldosterone), and structural kidney changes all affect BP through the kidneys.

Three ways to shift the renal function curve rightward (causing hypertension):

1. ↑ Ang II / aldosterone → ↑ tubular reabsorption
2. ↑ Renal sympathetic nerve activity → ↑ renal vascular resistance + ↑ tubular reabsorption
3. Structural kidney disease → ↓ nephron mass, ↓ filtration surface

Treatment logic:

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11. HIGH-YIELD SUMMARY TABLE

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1. Definitions - CO vs VR, why they must be equal
2. Normal values - 5 L/min, cardiac index 3 L/min/m², age effects, obesity caveat
3. Frank-Starling - the heart is a slave to VR; peripheral circulation controls CO, not the heart
4. CO = sum of tissue flows - metabolism drives everything
5. Limits to CO - upper limits, when the heart actually becomes the bottleneck
6. Hypereffective vs Hypoeffective heart - what shifts the cardiac output curve up vs down
7. Pathological high-CO states (AV fistula, anemia, hyperthyroid, beriberi, sepsis) vs low-CO
8. Nervous system - SNS effects on heart AND Psf
9. Cardiac Output Curves - intrapleural pressure, tamponade shifts (with real diagram)
10. Venous Return Curves - full explanation of plateau, downslope, zero-point (with real diagram)
11. Mean Systemic Filling Pressure - definition, normal = 7 mmHg, the formula, what changes it
12. Resistance to Venous Return - why veins dominate (2/3), the formula
13. Combined analysis (equilibrium point) - the intersection diagram with worked scenarios (blood volume, SNS, AV fistula) - with real diagram
14. Measurement methods - Fick, dye dilution formula, echo, bioimpedance (with error rates)
15. Exercise - how Psf rises to 30 mmHg, RVR falls, and CO hits 25-35 L/min
16. Master summary table - every factor + mechanism
17. Key numbers - everything you need to memorize
18. Concept map - the full circular logic in one diagram

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🫀 GUYTON & HALL — CHAPTER 21

THE CORONARY CIRCULATION

Brutal fact upfront: ~1/3 of all deaths in industrialized countries = coronary artery disease. Most older adults have SOME coronary impairment. This is clinically the most important chapter in cardiovascular physiology.

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📌 SECTION 1: PHYSIOLOGICAL ANATOMY OF CORONARY BLOOD SUPPLY

🔴 Coronary Arteries - Layout

• Main coronary arteries lie on the surface (epicardial)
• Smaller arteries penetrate inward through the muscle mass
• The inner 0.1 mm of endocardium can get nutrition directly from ventricular blood - but this is negligible
• Virtually ALL cardiac nutrition depends on the epicardial coronary arteries

🔵 Coronary Venous Drainage

🟠 Epicardial vs. Subendocardial Vasculature

• Epicardial arteries = large surface vessels - supply most of the muscle
• Intramuscular arteries = penetrate from epicardial arteries
• Subendocardial arterial plexus = extra plexus right beneath endocardium
  • During systole, these are maximally compressed by contracting LV muscle
  • The extra subendocardial plexus vessels normally compensate for systolic compression
  • BUT: under ischemic conditions, subendocardium is the FIRST to infarct (explained later)

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📌 SECTION 2: NORMAL CORONARY BLOOD FLOW VALUES

Note: The heart's work increases 6-9× during strenuous exercise, but coronary flow only increases 3-4×. The heart compensates by increasing its energy efficiency - it extracts more O₂ and uses substrates more efficiently.

Sex difference: Coronary blood flow per gram of heart tissue is typically higher in women than men, though women's hearts are smaller.

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📌 SECTION 3: PHASIC CORONARY BLOOD FLOW (Systole vs. Diastole)

LEFT VENTRICLE:
During SYSTOLE  → coronary capillary flow FALLS TO LOW LEVELS
During DIASTOLE → coronary capillary flow is HIGH (up to 4-5× systolic)

Reason: LV muscle contraction compresses intramuscular vessels → 
        blocks flow during systole

• RV muscle contracts with far less force than LV
• So phasic changes in RV coronary flow are only partial (not as dramatic)

Clinical implication: The heart receives most of its blood during DIASTOLE. When heart rate rises, diastolic filling time is disproportionately shortened → heart can become ischemic even with normal coronary arteries at very high heart rates.

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📌 SECTION 4: CONTROL OF CORONARY BLOOD FLOW

🔴 PRIMARY Controller: Local Muscle Metabolism (MOST IMPORTANT)

The heart controls its own blood supply through metabolic vasodilation. When work ↑, O₂ demand ↑ → local vasodilators released → arterioles dilate → flow ↑ proportionally.

• Normal: 70% of O₂ is extracted from coronary arterial blood at rest
• This is enormously high compared to most tissues (which extract ~25%)
• Result: almost no O₂ reserve - the only way to supply more O₂ is to increase flow
• Blood delivered to resting LV: ~8 mL O₂/100g/min
• Minimum to keep muscle alive: ~1.3 mL O₂/100g/min

Warning: Pharmacological agents that block adenosine do NOT fully prevent coronary vasodilation during exercise - meaning NO single substance is fully responsible. It is a multi-factor system.

1. During ischemia: ATP → ADP → AMP → Adenosine
2. Adenosine dilates vessels (good - tries to restore flow)
3. BUT adenosine also diffuses OUT of the cell
4. After 30 min of severe ischemia → ~50% of the adenine base is LOST from cells
5. New adenine synthesis rate = only 2%/hour
6. Therefore: if ischemia persists >30 min → even restoring flow may be too late to save cells
7. This is a MAJOR cause of irreversible cardiac cell death in MI

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🔵 SECONDARY Controller: Nervous System

Indirect Nervous Effects (more important):

• Sympathetic stimulation → ↑ HR + ↑ contractility → ↑ O₂ demand → local metabolic vasodilation overrides → net effect = ↑ coronary flow
• Vagal stimulation → ↓ HR + ↓ contractility → ↓ O₂ demand → indirect coronary vasoconstriction

Direct Nervous Effects (less important):

Clinically dangerous: In some people, alpha vasoconstrictor effects are disproportionately severe → coronary vasospasm during excess sympathetic activation → ischemia → angina. This is Prinzmetal (variant) angina. Triggers: emotional stress, cocaine/amphetamines, cold exposure, tobacco.

Key rule: Metabolic control ALWAYS overrides direct nervous vasoconstriction within seconds. You cannot starve the heart of flow by nervous means if the heart is actively working.

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📌 SECTION 5: SPECIAL FEATURES OF CARDIAC MUSCLE METABOLISM

⚡ Energy Substrates

• At REST: ~70% from fatty acids, ~30% from glucose/lactate/other
• During ISCHEMIA: forced to use anaerobic glycolysis → glucose consumed rapidly + lactic acid accumulates → contributes to ischemic chest pain

⚡ ATP and the Adenosine Cycle

• 95% of energy → produced as ATP in mitochondria

• ATP → chemical energy for contraction + all cellular functions
• Severe ischemia: ATP → ADP → AMP → Adenosine
  • Adenosine diffuses out → attempts to vasodilate (good)
  • But intracellular adenine stores depleted → can't regenerate ATP (bad → cell death)

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📌 SECTION 6: ISCHEMIC HEART DISEASE

Who gets it?

• ~35% of Americans aged ≥65 years die from ischemic heart disease
• Obstructive coronary atherosclerosis = most common cause
• Women: lower prevalence of obstructive atherosclerosis than men (especially pre-menopause) BUT sex difference attenuates with age → remains the #1 killer in women too
• Coronary microvascular dysfunction and vasospasm cause ischemia even with NO obstructive plaques on angiogram

Atherosclerosis (Brief):

• Genetic predisposition + obesity + sedentary lifestyle + hypertension + endothelial damage
• Cholesterol deposits → fibrous invasion → calcification → plaque → partial or complete luminal obstruction
• Most common site: first few centimeters of major coronary arteries

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📌 SECTION 7: ACUTE CORONARY ARTERY OCCLUSION

Two Main Mechanisms:

• Plaque breaks through endothelium → contacts blood
• Platelets adhere → fibrin deposits → RBCs trapped → thrombus grows → occlusion
• If thrombus breaks free and flows distally → coronary embolus

• Direct smooth muscle irritation by atherosclerotic plaque edges
• Or local neural reflexes → excessive wall contraction
• Triggers: extreme emotional stress, cocaine, amphetamines, tobacco, cold exposure
• Can lead to secondary thrombosis
• Treatment: calcium channel blockers + nitrates (vasodilators)

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🔴 Collateral Circulation - The Life-Saving Variable

How much heart muscle dies after occlusion depends enormously on collateral circulation.

• Large coronary arteries: almost NO major anastomoses
• Small arteries (20-250 µm): many anastomoses exist

• Collaterals develop gradually alongside the narrowing
• Person may NEVER experience an acute episode
• But eventually: atherosclerosis outpaces collateral development → cardiac failure in old age
• Sometimes collaterals themselves develop atherosclerosis → makes things worse

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📌 SECTION 8: MYOCARDIAL INFARCTION

What happens immediately after occlusion:

1. Blood flow ceases distal to the blockage
2. Small collateral blood trickles in → area overfills with stagnant blood
3. Muscle uses last O₂ → hemoglobin becomes fully deoxygenated → tissue turns bluish-brown
4. Blood vessels become highly permeable → leak fluid → local edema
5. Cardiac cells swell (failed cellular metabolism)
6. Within a few hours → cell death

O₂ thresholds:

⚠️ Subendocardial Infarction - Why Subendocardium Dies First:

• Subendocardial muscle has:
  1. Higher O₂ consumption than epicardial muscle
  2. Blood supply maximally compressed during systole
• So ANY reduction in coronary flow hits subendocardium first
• Infarction starts subendocardially and then spreads outward toward epicardium over time
• This is why early treatment (within minutes) can limit infarct to the subendocardium (NSTEMI) instead of full-thickness (STEMI)

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📌 SECTION 9: CAUSES OF DEATH AFTER ACUTE CORONARY OCCLUSION

1. 💀 Decreased Cardiac Output (Cardiogenic Shock)

• Infarcted muscle cannot contract
• During systole, normal muscle contracts but dead/nonfunctional muscle bulges OUTWARD (paradoxical motion) due to high intraventricular pressure
• This wastes pumping force → CO drops more than expected from amount of muscle lost

Normal muscle contracts → ↑ intraventricular pressure → 
Dead muscle bulges OUT → pressure energy is wasted → 
Net pumping reduced disproportionately

• Cardiogenic shock = CO too low for peripheral tissue survival
• Occurs when >40% of LV is infarcted
• Mortality: 40-50% even with treatment

2. 💀 Pulmonary Edema (Blood Damming)

• Heart fails to pump forward → blood dams up in pulmonary veins
• Pulmonary capillary pressure rises above plasma colloid osmotic pressure
• Fluid transudes into lung interstitium and alveoli → pulmonary edema
• Impairs gas exchange → hypoxia → death

3. 💀 Ventricular Fibrillation (MOST COMMON cause of sudden death post-MI)

• First 10 minutes after infarction (primary fibrillation)
• Short relative safety period
• 1 hour later, lasting several hours (secondary fibrillation)
• Can also occur days later (less common)

4. 💀 Rupture of the Heart Wall

• NOT immediate - occurs 3-7 days after infarction
• Dead muscle fibers begin to degenerate → wall becomes thin and weak
• Dead muscle bulges outward more with each beat → progressive systolic stretch
• Eventually wall ruptures → blood fills pericardial space → cardiac tamponade → rapid death
• Monitored by: Echo / MRI / CT (looking for progressive systolic stretch)
• When RV ruptures → blood in pericardium → compresses heart → blocks right atrial filling → sudden death

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📌 SECTION 10: STAGES OF RECOVERY FROM ACUTE MI

Acute Zone Structure (Size Dependent):

• Little or no permanent muscle death
• Part of muscle temporarily nonfunctional (stunned) → recovers

[CENTER]: Complete blood flow cessation → dies within 1-3 hours

[MIDDLE]: Nonfunctional zone → no contraction, no impulse conduction
           (reversible if blood flow restored promptly)

[OUTER]: Still contracting but WEAKLY → mild ischemia, borderline viable

Scar Formation:

• Macrophages/phagocytes invade and remove dead tissue
• Replaced by fibrous scar tissue (not muscle)
• Scar contracts → no longer bulges outward passively → partially restores mechanical efficiency

⭐ Coronary Steal Syndrome:

• During recovery, if patient exercises → normal muscle vasodilates greatly
• Blood preferentially flows through normal muscle vessels (lower resistance)
• Flow through anastomotic channels to ischemic zone decreases (stolen)
• Ischemic zone gets even LESS blood during activity than at rest
• This is why: ABSOLUTE REST is mandatory in the acute phase of MI

Physical Activity Post-MI:

• Acute phase: Absolute rest - reduces workload, prevents coronary steal
• After clinical stabilization: Prescribed aerobic exercise is beneficial
• Benefits of exercise training post-MI:
  • ↑ Coronary endothelial function
  • ↓ Inflammation
  • ↑ Contractile function of surviving cardiomyocytes
  • Partial recovery of cardiac reserve

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📌 SECTION 11: HEART FUNCTION AFTER RECOVERY FROM MI

• Occasionally recovers almost fully (rare)
• More often: permanently reduced pumping capacity
• Normal cardiac reserve = 300-400% above resting needs
• Even when reserve is reduced to 100% → most daily activities still possible
• Inability to increase CO during strenuous exercise is the typical deficit
• Cardiac rehabilitation (exercise) can partially restore reserve

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📌 SECTION 12: CARDIAC PAIN IN CORONARY HEART DISEASE

Mechanism of Ischemic Cardiac Pain:

• Ischemia → muscle releases acidic metabolites (lactic acid) + pain-promoting substances: histamine, kinins, proteolytic enzymes
• Slowly moving/stagnant blood fails to clear these
• High concentrations → stimulate pain nerve endings in cardiac muscle
• Pain signals travel via afferent fibers to spinal cord

Angina Pectoris:

• Definition: chest pain due to transient cardiac ischemia without cell death
• Occurs when metabolic demand > available coronary supply
• Location: Beneath the upper sternum (precordial area)
• Referred pain locations (embryological basis):
  • Left arm and left shoulder (most classic)
  • Neck
  • Side of face
  • Why: During embryonic development, heart originates in the neck → heart + left arm share the same spinal cord pain segments

• Exercise (↑ heart metabolism)
• Emotional stress (sympathetic vasoconstriction + ↑ metabolism)
• Cold temperatures (↑ cardiac workload via vasoconstriction + shivering)
• Full stomach (↑ venous return → ↑ cardiac work)
• Pain character: hot, pressing, constricting; severe enough that patient stops all activity
• Duration: usually a few minutes; constant pain = severe ischemia

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📌 SECTION 13: TREATMENT OF CORONARY ARTERY DISEASE

💊 Drug Treatment for Angina

🔪 Aortic-Coronary Bypass Surgery (CABG)

• Indication: discrete atherosclerotic blockages at a few points; vessels otherwise normal
• Procedure: saphenous vein (leg) or internal mammary artery graft from aortic root to coronary artery distal to the blockage
• Usually 1-5 grafts per patient
• Results: anginal pain relieved in most patients; normal life expectancy if heart not severely pre-damaged

🎈 Coronary Angioplasty + Stenting (PCI)

• Indication: partial occlusion before total occlusion develops (also used for acute total occlusions)
• Procedure: 1mm balloon-tipped catheter inserted → guided to blockage → balloon inflated at high pressure → vessel lumen widened
• Results: flow increases 3-4× immediately; >75% relieved of ischemic symptoms for at least several years
• BUT: many still eventually need CABG
• Stent: Stainless steel mesh tube placed inside vessel after balloon dilation:
  • Keeps vessel open mechanically
  • Prevents vessel from re-collapsing (elastic recoil)
  • Reduces restenosis rates
  • Drug-eluting stents: coated with anti-proliferative drugs to reduce scar tissue re-stenosis

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📌 SECTION 14: MASTER SUMMARY TABLE

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📌 SECTION 15: CONCEPT DIAGRAM - CORONARY FLOW CONTROL LOGIC

↑ CARDIAC WORK (exercise, stress)
          ↓
   ↑ O₂ Demand → ↓ local O₂ tension
          ↓
  ATP → ADP → AMP → ADENOSINE + other vasodilators
  (+ K⁺, H⁺, CO₂, NO, prostaglandins)
          ↓
   Coronary arteriolar DILATION
          ↓
   ↑ Coronary blood flow (up to 3-4× resting)
          ↓
   O₂ supply matches demand → metabolism restored

CORONARY OCCLUSION TIMELINE:
0 sec   → Flow stops; collaterals start dilating
Minutes → <50% needed flow via collaterals
30 min  → Adenine base depleted → cells begin to die irreversibly
Hours   → Center of infarct dead; subendocardium affected first
8-24h   → Collateral diameters barely changed
Day 2-3 → Collateral flow doubles
~1 month → Near-normal collateral flow
3-7 days → Necrotic wall thin → RUPTURE RISK PEAK
Weeks   → Scar tissue replaces dead muscle

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