Explain pulmonary and coronary circulation in detail

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Pulmonary and Coronary Circulation

PART 1: PULMONARY CIRCULATION

Overview

The pulmonary circulation is the circuit that carries deoxygenated blood from the right ventricle through the lungs for gas exchange, then returns oxygenated blood to the left atrium. It is entirely a low-pressure system - unlike the systemic circulation.

Vascular Structure

The pulmonary vascular bed resembles the systemic one with an important difference: the walls of the pulmonary artery and its large branches are only about 30% as thick as the wall of the aorta. The small arterial vessels contain relatively little smooth muscle compared to systemic arterioles. Pulmonary capillaries are large with multiple anastomoses, so each alveolus sits in a capillary basket.

Ganong's Review of Medical Physiology, 26th Ed.

Pressures and Flow

Parameter	Pulmonary	Systemic
Mean arterial pressure	~15 mm Hg	~95 mm Hg
Pressure gradient	~7 mm Hg	~90 mm Hg
Capillary pressure	~10 mm Hg	~25 mm Hg

The total blood volume in the pulmonary vessels at any time is about 1 L, of which less than 100 mL is in the capillaries. The mean velocity in the root of the pulmonary artery is the same as the aorta (~40 cm/s), falling off rapidly in smaller vessels. A red cell takes about 0.75 seconds to traverse pulmonary capillaries at rest and 0.3 seconds during exercise - just enough time for complete gas exchange at rest, though exercise leaves less margin.

Because pulmonary capillary pressure (~10 mm Hg) is far below plasma oncotic pressure (~25 mm Hg), an inward-directed gradient of ~15 mm Hg normally keeps the alveoli free of fluid. When pulmonary capillary pressure exceeds 25 mm Hg, pulmonary edema results.

Ganong's Review of Medical Physiology, 26th Ed.

Physiological Shunts

Two small exceptions prevent perfect oxygen saturation in the systemic arteries:

Some bronchial blood (which drains into pulmonary capillaries/veins) bypasses the right ventricle
A small amount of blood from coronary arteries flows into the left ventricle chambers

These create a physiological shunt - arterial PO₂ is about 2 mm Hg lower and hemoglobin saturation is 0.5% less than perfectly equilibrated alveolar blood.

Effect of Gravity - The Three Zones

Gravity significantly affects pulmonary blood distribution in the upright position:

Pulmonary lung zones showing ventilation and perfusion differences from apex to base

Zone 1 (Apex): Alveolar pressure (PA) > pulmonary arterial pressure (Pa) > pulmonary venous pressure (Pv). Capillaries collapse - no gas exchange. This is the physiological dead space zone. Under normal conditions Zone 1 is minimal or absent, but enlarges with reduced Pa or increased PA (e.g. positive pressure ventilation).

Zone 2 (Middle): Pa > PA > Pv. Blood flow is determined by the Pa - PA gradient (the "waterfall effect"). Venules downstream of the constriction collapse and fill with whatever blood passes through.

Zone 3 (Base): Pa > Pv > PA. Normal arteriovenous flow throughout. Blood flow is greatest here.

Perfusion increases progressively from apex to base. Ventilation also increases base-to-apex, but less steeply, so the V/Q ratio is high at the apex and low at the base.

Ganong's Review of Medical Physiology, 26th Ed.

Ventilation/Perfusion (V/Q) Ratio

The overall V/Q for the entire lung at rest is about 0.8 (4.2 L/min ventilation / 5.5 L/min blood flow).

If ventilation falls relative to perfusion → alveolar PO₂ falls, PCO₂ rises → venous admixture ("shunt-like" effect)
If perfusion falls relative to ventilation → alveolar PO₂ rises, PCO₂ falls → physiological dead space

Hypoxic Pulmonary Vasoconstriction (HPV)

This is the most distinctive feature of the pulmonary circulation. In all other vascular beds, hypoxia causes vasodilation. In the lungs, hypoxia causes vasoconstriction. Regional alveolar hypoxia triggers constriction of local arterioles, redirecting blood away from poorly ventilated alveoli (where gas exchange would be wasted) toward well-ventilated alveoli. This optimizes V/Q matching across the lung.

Costanzo Physiology, 7th Ed.

Autonomic and Humoral Control

Unlike the systemic circulation where arterioles are the primary resistance vessels, the pulmonary circulation has relatively sparse smooth muscle. Nonetheless, a range of receptors on pulmonary vessels modulate tone:

Substance	Receptor	Effect
Norepinephrine (α₁)	α₁	Constriction
Norepinephrine/Epi (β₂)	β₂	Relaxation
Acetylcholine (M₃)	M₃	Relaxation (endothelium-dependent)
Adenosine (A₂)	A₂	Relaxation
Endothelin (ETA)	ETA	Constriction
Prostacyclin (PGI₂)	IP	Relaxation
Bradykinin (B₂)	B₂	Relaxation (endothelium-dependent)

Ganong's Review of Medical Physiology, 26th Ed.

PART 2: CORONARY CIRCULATION

Clinical Importance

About one-third of all deaths in Western industrialized countries result from coronary artery disease. Understanding coronary physiology is one of the most important areas in medicine.

Guyton and Hall Textbook of Medical Physiology

Anatomic Considerations

The two coronary arteries arise from sinuses behind two of the cusps of the aortic valve at the root of the aorta.

Coronary arteries showing left and right coronary arteries, circumflex branch, anterior descending branch, septal and marginal branches (Ganong's)

Coronary arteries: right coronary artery, left coronary artery, left circumflex branch and left anterior descending branch (Guyton & Hall)

Left coronary artery → supplies the anterior and left lateral portions of the left ventricle, via:

Left anterior descending (LAD) branch - runs in the anterior interventricular groove
Left circumflex branch - runs in the left atrioventricular groove

Right coronary artery (RCA) → supplies most of the right ventricle, plus the posterior left ventricle in 80-90% of people ("right dominant" circulation)

The main coronary arteries lie on the surface of the heart (epicardial). Smaller arteries then penetrate into the muscle mass. Only the innermost ~0.1 mm of endocardium can receive nutrition directly from intracavitary blood - the rest depends entirely on coronary flow.

Guyton and Hall Textbook of Medical Physiology

Venous Drainage

~75% of coronary venous blood from the left ventricle returns via the coronary sinus → right atrium
Most right ventricular venous blood returns via anterior cardiac veins directly into the right atrium
A small amount drains via Thebesian veins directly into all four heart chambers (arteriosinusoidal vessels, sinusoidal capillaries)
A few anastomoses exist between coronary arterioles and extracardiac arterioles near the great vein orifices
Ganong's Review of Medical Physiology, 26th Ed.

Normal Coronary Blood Flow

Resting flow: approximately 70 mL/min/100 g heart weight = ~225 mL/min total = 4-5% of cardiac output
During strenuous exercise, cardiac work increases 6-9 fold; coronary flow increases 3-4 fold
The heart extracts about 70% of the oxygen from coronary arterial blood at rest - leaving very little reserve. Unlike skeletal muscle, the heart cannot significantly increase oxygen extraction; it must increase blood flow to meet greater oxygen demands.
Guyton and Hall Textbook of Medical Physiology

Phasic Flow - Systole vs. Diastole

This is a unique feature of coronary (especially left ventricular) circulation:

Phase	Left Ventricular Flow	Right Ventricular Flow
Systole	Near zero (intramyocardial pressure > aortic pressure in subendocardium)	Reduced but present (wall tension lower)
Diastole	Maximum (muscle relaxes, pressure gradient restored)	Continues throughout

During systole, left ventricular intramyocardial pressure actually slightly exceeds aortic pressure, completely stopping flow in the subendocardial vessels. The more superficial (epicardial) vessels receive some flow throughout the cycle. This is why:

Coronary flow is heavily diastole-dependent in the left ventricle
Tachycardia reduces left ventricular flow (shorter diastole)
Subendocardial ischemia is the most common site of myocardial infarction - these vessels are at greatest risk since they receive no systolic flow

The pressure differentials (from Ganong's):

Phase	Aorta (mmHg)	Left Vent (mmHg)	Right Vent (mmHg)
Systole	120	121	25
Diastole	80	0	0

Ganong's Review of Medical Physiology, 26th Ed.; Guyton and Hall Textbook of Medical Physiology

Epicardial vs. Subendocardial Supply

Epicardial coronary arteries on the outer surface supply most of the muscle. Smaller intramuscular arteries penetrate inward. A subendocardial arterial plexus lies immediately beneath the endocardium - this plexus is the most vulnerable area because intramuscular pressure is highest there during systole.

Guyton and Hall Textbook of Medical Physiology

Control of Coronary Blood Flow

1. Local Metabolic Control (Primary)

Coronary blood flow is regulated almost entirely by local arteriolar vasodilation in response to myocardial metabolic needs. The key mediators:

Adenosine - the most important vasodilator:

When O₂ demand exceeds supply → ATP degrades → AMP → adenosine released into tissue fluid
Adenosine causes profound arteriolar dilation → increased flow
After vasodilation, adenosine is reabsorbed into cardiac cells for ATP resynthesis

Other local vasodilators: adenosine phosphate compounds, K⁺, H⁺, CO₂, prostaglandins, nitric oxide

Hypoxia itself is also a direct stimulus: local low O₂ causes vasodilation to increase delivery.

Guyton and Hall Textbook of Medical Physiology; Costanzo Physiology, 7th Ed.

2. Nervous Control (Secondary)

Indirect (dominant) effects:

Sympathetic stimulation → increases HR and contractility → increased O₂ consumption → local metabolic vasodilation (dominant effect)
Vagal stimulation → slows heart → decreases O₂ demand → indirect coronary constriction

Direct effects:

Sympathetic α₁ receptors: vasoconstriction (coronary arteries)
Sympathetic β₂ receptors: vasodilation (coronary arteries)
Parasympathetic M₃ receptors: mild direct vasodilation
In practice, the metabolic (indirect) effects override the direct neural effects during exercise
Guyton and Hall Textbook of Medical Physiology

3. Autoregulation

Coronary blood flow is autoregulated between mean arterial pressures of about 60-140 mmHg. Flow remains relatively constant despite changes in perfusion pressure within this range, protecting the heart from both ischemia and overperfusion.

Summary Comparison

Feature	Pulmonary Circulation	Coronary Circulation
Driving pressure	~7 mm Hg gradient	~80 mm Hg (diastolic)
Vessel wall thickness	Thin (30% of aorta)	Thick epicardial arteries
Response to hypoxia	Vasoconstriction (unique!)	Vasodilation
Primary flow regulator	O₂/V-Q matching (HPV)	Adenosine/local metabolites
Flow timing	Continuous	Mostly diastolic (LV)
O₂ extraction at rest	~25%	~70%
Reserve mechanism	HPV redirects flow	Must increase flow (can't extract more)
Sympathetic effect	Mild (sparse muscle)	Indirect >> direct

Sources:

Ganong's Review of Medical Physiology, 26th Edition - Pulmonary and Coronary Circulation (Ch. 33-34)
Guyton and Hall Textbook of Medical Physiology - Coronary Circulation (Ch. 21)
Costanzo Physiology, 7th Edition - Special Circulations
Medical Physiology (Boron & Boulpaep) - Pulmonary pressure profiles

For long question about them how will I write a rout this topics?

Here's exactly how to structure (route) a long exam answer on Pulmonary and Coronary Circulation. This is a writing framework - what to write, in what order, and how much to cover at each step.

Exam Answer Route: Pulmonary Circulation

Opening Line (1-2 sentences)

Define it: "Pulmonary circulation is the circuit through which deoxygenated blood travels from the right ventricle to the lungs for gas exchange and returns oxygenated blood to the left atrium via the pulmonary veins."

1. Anatomy of the Vessels

Right ventricle → pulmonary trunk → left & right pulmonary arteries → arterioles → capillaries → venules → 4 pulmonary veins → left atrium
Wall structure: pulmonary artery walls are only 30% as thick as the aorta
Small arterioles have very little smooth muscle (unlike systemic)
Capillaries are large with multiple anastomoses - each alveolus sits in a capillary basket

2. Pressures and Flow (give numbers)

Pulmonary arterial pressure: 25/8 mmHg (systolic/diastolic), mean ~15 mmHg
Driving gradient: only ~7 mmHg (vs ~90 mmHg systemic)
Capillary pressure: ~10 mmHg
Oncotic pressure: ~25 mmHg → keeps alveoli dry
When capillary pressure > 25 mmHg → pulmonary edema
Total pulmonary blood volume: ~1 L (capillaries hold <100 mL)
RBC transit time: 0.75 s at rest, 0.3 s during exercise

3. Physiological Shunts

Bronchial blood drains into pulmonary veins (bypasses right heart)
Coronary blood drains into left ventricle chambers
Result: systemic arterial PO₂ is ~2 mmHg lower than alveolar air

4. Effect of Gravity - The 3 Zones (West's Zones)

This is a high-yield section - draw a lung diagram if allowed.

Zone	Location	Pressure relationship	Flow
Zone 1	Apex	PA > Pa > Pv	None (dead space)
Zone 2	Middle	Pa > PA > Pv	Determined by Pa - PA (waterfall effect)
Zone 3	Base	Pa > Pv > PA	Maximal, normal

(PA = alveolar pressure, Pa = arterial, Pv = venous)

5. V/Q Ratio

Overall V/Q at rest = 0.8
High at apex, low at base (due to gravity effects on both)
Low V/Q → venous admixture (shunt effect) → hypoxemia
High V/Q → physiological dead space → hypocapnia

6. Hypoxic Pulmonary Vasoconstriction (HPV) ← Most unique feature

Opposite to all other vascular beds
Hypoxia → vasoconstriction (not vasodilation)
Mechanism: local alveolar hypoxia → constricts local arterioles → diverts blood to well-ventilated alveoli
Purpose: optimize V/Q matching
Mediators: inhibition of K⁺ channels in smooth muscle, endothelin-1, reduced NO

7. Autonomic/Humoral Control

Sparse innervation overall
Vasodilators: NO, prostacyclin (PGI₂), bradykinin, β₂ stimulation
Vasoconstrictors: endothelin-1, thromboxane A₂, α₁ stimulation, serotonin

8. Clinical Correlates (end with these)

Pulmonary hypertension: mean PAP > 20 mmHg at rest
Pulmonary edema: capillary pressure > oncotic pressure
Pulmonary embolism: obstruction → dead space → hypoxia

Exam Answer Route: Coronary Circulation

Opening Line

"Coronary circulation is the system that supplies oxygenated blood to the myocardium via the coronary arteries arising from the root of the aorta, and drains deoxygenated blood back to the right atrium."

1. Anatomy - Arterial Supply

Both coronary arteries arise from sinuses behind aortic valve cusps at root of aorta
Left coronary artery (LCA) → splits into:
- Left Anterior Descending (LAD): anterior LV, septum
- Left Circumflex (LCx): lateral and posterior LV
Right coronary artery (RCA) → right ventricle + posterior LV (in 80-90% = right dominant)
Epicardial arteries → penetrating intramuscular arteries → subendocardial plexus

2. Venous Drainage

~75% left ventricular venous blood → coronary sinus → right atrium
Right ventricular blood → anterior cardiac veins → right atrium directly
Small amount → Thebesian veins → all four chambers

3. Normal Blood Flow Values (numbers examiners love)

Resting flow: ~70 mL/min/100g or ~225 mL/min total
= 4-5% of cardiac output
O₂ extraction at rest: ~70% (very high - barely any reserve left)
During exercise: flow increases 3-4 fold (work increases 6-9 fold)

4. Phasic Nature of Coronary Flow ← Key unique feature

Left ventricle: flow occurs mainly in diastole
- During systole: intramyocardial pressure slightly exceeds aortic pressure → vessels compressed → flow stops in subendocardium
- Pressure table: Systole - Aorta 120, LV 121 mmHg (net = -1, no flow)
- Diastole - Aorta 80, LV 0 mmHg (net = +80, maximum flow)
Right ventricle: mild phasic reduction in systole (lower wall tension)
Clinical implication: tachycardia → shorter diastole → reduced LV perfusion time → ischemia risk

5. Epicardial vs. Subendocardial Vulnerability

Epicardial arteries: on surface, protected, receive flow in both systole and diastole
Subendocardial plexus: deepest layer, highest compression during systole, zero flow during systole
Subendocardium is most vulnerable to ischemia → most common site of MI

6. Control of Coronary Blood Flow

A. Local Metabolic (Primary Controller)

Increased cardiac work → increased O₂ demand → local hypoxia → adenosine release (from AMP breakdown) → arteriolar dilation → increased flow
Other vasodilators: K⁺, H⁺, CO₂, NO, prostaglandins
Flow is regulated in direct proportion to metabolic needs

B. Autoregulation

Flow remains constant between mean arterial pressures of ~60-140 mmHg
Protects against hypoperfusion and overperfusion

C. Nervous Control (Secondary)

Indirect effects dominate: sympathetic → ↑HR + ↑contractility → ↑O₂ demand → metabolic vasodilation (net result = increased flow)
Direct sympathetic: α₁ → constriction | β₂ → dilation
Direct parasympathetic: mild direct dilation (ACh on M₃)
The indirect metabolic effect always overrides direct neural effects

7. Clinical Correlates

Angina pectoris: O₂ demand > supply, especially on exertion
Myocardial infarction: subendocardium most vulnerable
Tachycardia danger: shortens diastole → reduces LV perfusion → ischemia
Aortic stenosis/regurgitation: alters diastolic perfusion pressure → ischemia

General Exam Writing Tips for These Topics

Always start with a definition - examiners mark from the first line
Give numbers - pressures, flow rates, O₂ extraction - these show precision
Highlight unique features - HPV in pulmonary; diastolic-dominant flow in coronary
Draw simple diagrams if allowed - the 3-zone lung diagram and the coronary anatomy diagram each earn marks
End with clinical applications - shows you understand why the physiology matters
Use headings - makes the answer easy to follow and ensures you cover all points systematically

What do you mean by high pressure pulmonary vasodilatation in case of pulmonary circulation?

Great question - and I want to be upfront with you first.

Clarification

I did not use the phrase "high pressure pulmonary vasodilation" anywhere in my previous answers. You may be thinking of one of these related concepts I did cover:

What I actually said	What it means
Hypoxic Pulmonary Vasoconstriction (HPV)	Low O₂ → vessels constrict (unique to lungs)
Increased flow at the lung base	Higher arterial pressure at base due to gravity → more blood flow (Zone 3)
Capillary pressure > 25 mmHg → pulmonary edema	High capillary pressure overwhelms oncotic pressure

However - Here Is What "High Pressure Causing Vasodilation" Could Refer To

There is a real phenomenon in pulmonary physiology called recruitment and distension, which is the closest thing to "high pressure → vasodilation" in the pulmonary circuit:

Recruitment

When cardiac output increases (e.g. exercise), pulmonary arterial pressure rises slightly. This opens up previously collapsed capillaries (especially in Zone 1 and Zone 2 of the upright lung) - effectively "recruiting" new vessels. More vessels open → total resistance falls → pressure stays low despite increased flow.

Distension

Higher pressure also widens already-open capillaries (they are very compliant). Wider vessels → lower resistance → acts like vasodilation without any active smooth muscle change.

Combined effect: The pulmonary circulation can accommodate a 4-5 fold increase in cardiac output with only a small rise in pulmonary arterial pressure - because of recruitment + distension.

Why This Matters

This is the opposite of what happens in the systemic circulation:

Systemic: higher pressure → vessels constrict (autoregulation)
Pulmonary: higher pressure → passive vasodilation (recruitment + distension)

This keeps the pulmonary circuit as a low-resistance, high-flow system even during exercise.

Could you share where you read "high pressure pulmonary vasodilation"? It's possible it was phrased differently in your notes or textbook, and I can explain that exact concept precisely.

Correction in the above context that what is hypoxic pulmonary vasoconstriction

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Excellent - the textbooks give a very complete picture. Here is the full corrected explanation:

Hypoxic Pulmonary Vasoconstriction (HPV)

Definition

HPV is the response of pulmonary vascular smooth muscle to low alveolar PO₂ (hypoxia) by constricting - thereby increasing local vascular resistance and reducing blood flow to that hypoxic region.

This was first demonstrated by von Euler and Liljestrand and the functional logic they proposed still holds today.

Why It Is Unique

In systemic tissues, local PO₂ is determined by:

PO₂ = Q / VO₂ (flow delivered ÷ consumption)

So when O₂ falls → vessels dilate → more blood flows in → restores O₂ supply. This is the normal response everywhere else in the body.

In the lungs, local alveolar PO₂ is determined by:

PO₂ = VA / Q (ventilation ÷ perfusion)

So when O₂ falls in an alveolus, it means ventilation is poor there. Sending more blood to a poorly ventilated alveolus would waste flow - so the lung does the opposite: it constricts to redirect blood away.

Fishman's Pulmonary Diseases and Disorders

The Mechanism (Cellular Level)

Alveolar PO₂ falls in a region
Voltage-gated K⁺ channels in pulmonary vascular smooth muscle cells are inhibited
K⁺ efflux decreases → membrane depolarizes
Voltage-gated Ca²⁺ channels open
Intracellular Ca²⁺ rises → smooth muscle contracts → vasoconstriction

Key point: this mechanism is entirely local - it does not need:

The central nervous system
Autonomic innervation (HPV still occurs in transplanted, denervated lungs)
High PO₂ in the blood - it is the alveolar gas PO₂, not arterial blood PO₂, that drives the response

Proof: if you perfuse a lung with high-PO₂ blood while keeping alveolar PO₂ low - vasoconstriction still occurs.

Murray & Nadel's Textbook of Respiratory Medicine

Site of Action

Primarily the small precapillary arterioles
Small pulmonary veins also constrict slightly (~20-30% contribution to total resistance change)

Factors That Enhance HPV

Acidosis
Low mixed venous PO₂
Repeated hypoxic exposure
Perinatal hypoxia
Cyclo-oxygenase inhibition
Nitric oxide inhibition

Factors That Inhibit HPV

Alkalosis
Hypercapnia
Increased pulmonary vascular or alveolar pressure
Vasodilating prostaglandins
Nitric oxide (NO) - a key endothelium-derived relaxing factor
Calcium channel blockers
β₂ agonists (clinically important: bronchodilators like salbutamol can inhibit HPV and worsen hypoxemia in asthma)

The Biphasic Response (Important Detail)

HPV is not linear. As PO₂ falls progressively:

PO₂ down to ~35-40 mmHg → progressive vasoconstriction (beneficial)
PO₂ below ~35-40 mmHg → paradoxical vasodilation ("hypoxic vasodilation")

This very deep hypoxia causes such severe cellular dysfunction that smooth muscle can no longer maintain contraction.

Fishman's Pulmonary Diseases and Disorders

Physiological Purpose (V/Q Matching)

Scenario	What happens
Alveolus poorly ventilated (e.g. mucus plugging)	Local PO₂ falls → HPV constricts arteriole → blood diverted away
Blood goes to well-ventilated alveoli	Gas exchange occurs normally there
Net effect	V/Q mismatch is corrected → arterial PO₂ maintained

Clinical example: Bronchodilators (salbutamol) in asthma can inhibit HPV → blood flows back to poorly ventilated alveoli → arterial PO₂ actually drops despite airways opening up. This is a known paradox in asthma management.

Most Important Physiological Role - The Fetus

The single most important role of HPV is in fetal life:

Fetal lungs do not perform gas exchange → alveolar PO₂ is very low
HPV keeps pulmonary vascular resistance very high → only ~15% of fetal cardiac output goes to lungs
The rest is diverted through the foramen ovale and ductus arteriosus to the systemic circulation
At birth, lungs inflate → alveolar PO₂ rises → HPV releases → pulmonary resistance falls dramatically → normal neonatal circulation established
Murray & Nadel's Textbook of Respiratory Medicine

When HPV Becomes Harmful - Chronic Hypoxia

If hypoxia is global and prolonged (e.g. living at high altitude, COPD, chronic lung disease):

HPV is generalized across the whole lung → pulmonary arterial pressure rises throughout
Over time → pulmonary arterial hypertension (PAH)
Eventually → right ventricular hypertrophy → cor pulmonale → right heart failure

The reversibility of HPV is largely lost after 24-48 hours of sustained hypoxic exposure due to vascular remodeling.

Fishman's Pulmonary Diseases and Disorders

Summary in One Sentence

HPV is a local, autonomic-independent reflex where low alveolar PO₂ causes pulmonary arteriolar constriction via K⁺ channel inhibition and Ca²⁺ influx, redirecting blood to better-ventilated alveoli to optimize V/Q matching - but becomes pathological when hypoxia is global and sustained.

It's the oxygen level in the alveolus where the ventilation is poor. False. Then why does it... Constricts the vessels to redirect the blood flow away. Just why

This is a great "but WHY?" question. Let me break the logic down step by step.

The Core Logic - Think About It From the Blood's Perspective

Imagine you are a red blood cell. You enter a pulmonary capillary next to an alveolus. Your job is to pick up oxygen.

Now ask: what happens in two different scenarios?

Scenario A - Normal Alveolus (Good Ventilation)

Fresh air is flowing in regularly
Alveolar PO₂ = ~100 mmHg
You arrive at the capillary, pick up plenty of O₂, leave fully saturated
This is useful blood flow ✅

Scenario B - Blocked/Poorly Ventilated Alveolus

Air is not entering (mucus plug, collapsed alveolus, bronchospasm)
The alveolus just sits there - no fresh O₂ coming in
Whatever O₂ was there gets consumed quickly → alveolar PO₂ drops to very low levels
You arrive at the capillary, but there is no O₂ to pick up
You leave the capillary still deoxygenated
This blood flow was completely wasted ❌

So Why Does the Body Constrict?

The body detects the low alveolar PO₂ and essentially says:

"There is no point sending blood here. This alveolus has nothing to offer. Shut the vessels down and redirect blood somewhere it can actually pick up oxygen."

Constricting the vessel stops wasting cardiac output on an alveolus that cannot oxygenate blood.

The Redirect Effect

When blood is blocked from going to the bad alveolus, it has to go somewhere. It gets pushed toward neighbouring alveoli that ARE well-ventilated and have plenty of O₂.

Poor alveolus (low PO₂)          Good alveolus (normal PO₂)
      |                                    |
  [CONSTRICT]                         [OPEN]
      |                                    |
  No blood here              ← Blood diverted here
                                  Picks up O₂ normally ✅

This is the entire purpose - match blood flow to where ventilation is actually happening.

The Key Insight That Makes It Click

In the rest of the body (muscles, brain, kidney):

Low O₂ means the tissue is consuming too much - so send MORE blood to rescue it
Low O₂ → vasodilation → more flow → more O₂ delivery

In the lung, the logic is completely different:

Low alveolar O₂ does NOT mean the tissue is consuming it
It means no fresh air is arriving - the alveolus is simply not working
Sending more blood there does NOT fix the problem - it makes it worse
Because now even more deoxygenated blood enters the systemic circulation

So the lung evolved the opposite reflex - constrict and abandon a non-working alveolus rather than flood it with blood that returns unoxygenated.

One Analogy

Think of it like a factory assembly line:

Well-ventilated alveolus = a working station that has raw materials (O₂)
Poorly ventilated alveolus = a station where supplies have run out
Blood = workers being sent to stations

If Station B has no supplies, you stop sending workers there and redirect them to Station A where they can actually do something productive. Sending workers to an empty station is just wasted labour.

Bottom Line

Vasoconstriction in response to low alveolar PO₂ is not about rescuing the alveolus - it is about not wasting blood flow on an alveolus that cannot oxygenate it. The goal is always to maximise the efficiency of the entire lung, not to save one bad alveolus.

Explain cardiac cycle as if it is asked in my exam as long question

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I now have everything needed. Here is the complete exam-style long answer.

THE CARDIAC CYCLE

Definition

The cardiac cycle is defined as the sequence of mechanical and electrical events that repeats with every heartbeat - from the beginning of one heartbeat to the beginning of the next.

Duration = 60 / heart rate (beats/min)
At a normal HR of 75 beats/min → duration = 0.8 seconds
Of this: systole = ~0.3 s, diastole = ~0.5 s

Each cycle is initiated by spontaneous depolarization of the sinoatrial (SA) node in the superior lateral wall of the right atrium, which then conducts through both atria and down to the ventricles via the AV node with a delay of >0.1 s - allowing atria to contract and fill the ventricles before ventricular contraction begins.

Guyton and Hall Textbook of Medical Physiology

The Wiggers Diagram (draw this in the exam)

This diagram shows all cardiac events simultaneously:

Wiggers diagram from Ganong's - showing all 5 phases of cardiac cycle with pressures, volumes, ECG, heart sounds, venous pulse

Cardiac cycle diagram from Costanzo - 7 phases A through G with LV pressure, aortic pressure, atrial pressure, ventricular volume, venous pulse and ECG

Phases of the Cardiac Cycle

The cardiac cycle has 7 phases (Costanzo classification), which group into Systole and Diastole.

SYSTOLE

Phase A - Atrial Systole

Parameter	Events
ECG	P wave → PR interval
Trigger	Atrial depolarization (P wave)
Atria	Contract → left atrial pressure rises
Ventricle	Still relaxed; mitral valve open
Ventricular volume	Increases slightly (atrium actively pumps remaining blood into ventricle)
Heart sound	S4 (not audible normally; heard in stiff/hypertrophied ventricle)
Venous pulse	'a' wave - atrial pressure rise reflected back to jugular veins

The atria contribute only ~20-30% of total ventricular filling (70% is passive). At rest they act mainly as primer pumps; their contribution becomes more important at high heart rates when passive filling time is reduced.

Phase B - Isovolumetric Ventricular Contraction (IVC)

Parameter	Events
ECG	QRS complex (ventricular depolarization)
Trigger	Ventricles begin to contract
Pressure	LV pressure rises sharply
Valve event	As soon as LV pressure > LA pressure → Mitral valve closes
Volume	Constant - all valves are closed (mitral closed; aortic still closed)
Heart sound	S1 ("lub") - closure of mitral valve (and tricuspid slightly after)

This is called "isovolumetric" because volume does not change - all valves are shut - but pressure is building rapidly. The ventricle contracts with no outflow yet.

S1 may be split because mitral closes slightly before tricuspid (M1-T1 split).

Phase C - Rapid Ventricular Ejection

Parameter	Events
ECG	ST segment
Trigger	LV pressure exceeds aortic pressure
Valve event	Aortic valve opens
Pressure	LV pressure reaches its maximum (~120 mmHg)
Volume	LV volume falls rapidly (most of stroke volume ejected here)
Aortic pressure	Rises sharply as blood rushes in
Atrial filling	Begins - LA pressure slowly rises as blood returns from pulmonary veins
Heart sound	None

Phase D - Reduced Ventricular Ejection

Parameter	Events
ECG	T wave (ventricular repolarization begins)
Ventricle	Begins to repolarize → contracting less forcefully
Aortic valve	Still open
Volume	LV volume continues to fall, but more slowly
Aortic pressure	Starts to fall - blood running off into arterial tree faster than it is being added
Heart sound	None

At the end of phase D, ventricular volume is at its minimum = End-Systolic Volume (ESV) = ~70 mL

Stroke Volume = EDV - ESV = 130 - 70 = 60 mL (at rest) Ejection Fraction = SV/EDV = 60/130 = ~55-60% (normal ≥ 55%)

DIASTOLE

Phase E - Isovolumetric Ventricular Relaxation (IVR)

Parameter	Events
ECG	End of T wave
Trigger	Ventricular repolarization complete → ventricle relaxes
Pressure	LV pressure drops sharply
Valve event	When LV pressure < Aortic pressure → Aortic valve closes
Volume	Constant - all valves are again closed (aortic just closed; mitral still closed)
Heart sound	S2 ("dub") - closure of aortic valve (A2) then pulmonic valve (P2)
Aortic pressure	Shows dicrotic notch (incisura) - brief pressure blip at moment of aortic valve closure

Splitting of S2: Inspiration → increases venous return to right heart → prolongs right ventricular ejection → P2 closes later than A2 → physiological splitting. Heard best on inspiration.

Phase F - Rapid Ventricular Filling

Parameter	Events
ECG	Isoelectric (between T and P)
Trigger	LV pressure falls below LA pressure
Valve event	Mitral valve opens
Volume	LV volume rises rapidly (passive flow from atrium into ventricle)
Mechanism	Blood that accumulated in LA during systole now rushes in (suction + pressure gradient)
Heart sound	S3 - produced by sudden deceleration of blood hitting ventricular wall

S3 is normal in children and young adults. In adults over 40 it suggests ventricular failure (dilated, poor compliance ventricle).

Phase G - Reduced Ventricular Filling (Diastasis)

Parameter	Events
ECG	Isoelectric
Flow	Blood continues to flow slowly from atrium to ventricle
Volume	LV volume increases slowly
Pressures	Atrial and ventricular pressures nearly equalize
Heart sound	None

At the end of diastasis, LV volume = End-Diastolic Volume (EDV) = ~130 mL. Then the next P wave fires → atrial systole → cycle repeats.

Summary Table of All 7 Phases

Phase	Name	Valves	LV Volume	LV Pressure	ECG	Heart Sound
A	Atrial systole	Mitral open	Increases	Low, slight rise	P wave	S4 (if heard)
B	Isovolumetric contraction	ALL CLOSED	Constant	Rises sharply	QRS	S1
C	Rapid ejection	Aortic open	Falls rapidly	Maximum (~120)	ST segment	-
D	Reduced ejection	Aortic open	Falls slowly	Falling	T wave	-
E	Isovolumetric relaxation	ALL CLOSED	Constant	Falls sharply	After T	S2 + dicrotic notch
F	Rapid filling	Mitral open	Rises rapidly	Low	Isoelectric	S3 (if heard)
G	Diastasis	Mitral open	Rises slowly	Very low	Isoelectric	-

Jugular Venous Pulse (JVP) Waves

The JVP has three positive waves reflecting atrial pressure changes:

Wave	Cause	Timing
'a' wave	Atrial contraction	Before S1 (Phase A)
'c' wave	Tricuspid valve bulging into atrium during IVC	After S1 (Phase B)
'v' wave	Venous filling of atrium while tricuspid is closed	Late systole / early diastole

Between waves are descents: x descent (atrial relaxation) and y descent (tricuspid valve opening → rapid atrial emptying).

Heart Sounds - Summary

Sound	Cause	Normal/Abnormal
S1	Mitral + tricuspid closure	Normal (beginning of systole)
S2	Aortic + pulmonic closure	Normal (beginning of diastole)
S3	Rapid ventricular filling	Normal in young; heart failure in adults
S4	Atrial contraction against stiff ventricle	Always abnormal; LV hypertrophy, ischemia

Effect of Heart Rate on Cardiac Cycle

At 72 beats/min: systole ~0.4 s, diastole ~0.4 s
As HR increases: diastole shortens far more than systole
- At 200 beats/min: diastole = only 0.14 s
Consequences of very high HR:
1. Inadequate ventricular filling → reduced cardiac output
2. Reduced coronary perfusion time (LV fills coronaries only in diastole)
3. Risk of subendocardial ischemia

The maximum effective ventricular rate is ~230/min - beyond this, the AV node's refractory period prevents conduction.

Ganong's Review of Medical Physiology, 26th Ed.

Key Numbers to Quote in the Exam

Parameter	Value
Duration (HR 75)	0.8 s
Systole duration	~0.3 s
Diastole duration	~0.5 s
End-diastolic volume (EDV)	~130 mL
End-systolic volume (ESV)	~70 mL
Stroke volume	~60 mL
Ejection fraction	~55-65%
Peak LV systolic pressure	~120 mmHg
LV diastolic pressure	~0 mmHg
Aortic systolic/diastolic	120/80 mmHg

Clinical Correlates (End your answer with these)

S3 gallop → ventricular failure, dilated cardiomyopathy
S4 gallop → LV hypertrophy, hypertensive heart disease, aortic stenosis
Prolonged IVC (long Phase B) → aortic stenosis (hard to open aortic valve)
Prolonged IVR (long Phase E) → impaired relaxation, diastolic dysfunction
Reduced EF (<40%) → systolic heart failure
Dicrotic notch absence on arterial pulse → aortic regurgitation (valve leaks back)
Tachycardia → mainly shortens diastole → compromises filling and coronary perfusion

Sources: Costanzo Physiology 7th Ed.; Ganong's Review of Medical Physiology 26th Ed.; Guyton and Hall Textbook of Medical Physiology; Medical Physiology (Boron & Boulpaep)

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