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PY5.1-16 | Cardiovascular Physiology — Part 2

Arterial Blood Pressure (PY5.9, PY5.10)

Arterial blood pressure is the lateral force exerted by blood on the walls of the arteries. It has two components:

Figure: Arterial Blood Pressure (PY5.9, PY5.10)

• Systolic blood pressure (SBP) — the peak pressure during ventricular ejection. Normal: ~120 mmHg.
• Diastolic blood pressure (DBP) — the lowest pressure during ventricular diastole. Normal: ~80 mmHg.
• Pulse pressure = SBP - DBP = ~40 mmHg. It reflects stroke volume and arterial compliance.
• Mean arterial pressure (MAP) = DBP + 1/3(Pulse pressure) = 80 + 1/3(40) = ~93 mmHg. MAP is the average pressure driving blood through the systemic circulation. MAP below 60 mmHg = inadequate organ perfusion.

Factors affecting arterial BP:
1. Cardiac output (CO) — BP = CO x Total Peripheral Resistance (TPR)
2. Peripheral resistance — mainly determined by arteriolar smooth muscle tone
3. Blood volume — more volume = more pressure (think: overfilled balloon)
4. Elasticity of arterial walls — loss of elasticity (arteriosclerosis) increases systolic BP and pulse pressure
5. Viscosity of blood — increased viscosity (polycythaemia) increases resistance

The arterial pulse (PY5.10) — the pressure wave that travels along the arterial wall with each heartbeat. It travels much faster (4-12 m/s) than the blood itself (~0.5 m/s). The radial pulse at the wrist is where you routinely feel it.

Characteristics to assess: Rate, Rhythm, Volume (amplitude), Character (contour), Condition of the arterial wall, Symmetry (compare both sides).

Abnormal pulses: Pulsus paradoxus (exaggerated fall in BP during inspiration — cardiac tamponade), water-hammer pulse (wide pulse pressure — aortic regurgitation), pulsus alternans (alternating strong and weak beats — left ventricular failure).

Venous Pressure and JVP (PY5.11)

Venous blood pressure is much lower than arterial — about 10-15 mmHg in peripheral veins, falling to nearly 0 mmHg in the right atrium (central venous pressure, CVP). The veins are a capacitance system — they hold ~60-70% of total blood volume.

Figure: Venous Pressure and JVP (PY5.11)

Factors aiding venous return:
1. Skeletal muscle pump — contraction of leg muscles squeezes blood upward through one-way venous valves ('peripheral heart')
2. Respiratory pump — inspiration decreases intrathoracic pressure, creating a suction effect on the great veins
3. Venous valves — prevent backflow (especially important in the lower limbs)
4. Sympathetic venoconstriction — reduces venous capacitance, pushing blood toward the heart
5. Cardiac suction — ventricular relaxation creates a suction effect
6. Vis a tergo — the 'push from behind' — the residual pressure from arterial blood flowing through capillaries into veins

Jugular Venous Pressure (JVP) — examined in the internal jugular vein, it reflects right atrial pressure. The JVP waveform has 3 positive waves (a, c, v) and 2 descents (x, y):

• a wave — atrial contraction (right atrial systole)
• c wave — bulging of the tricuspid valve into the right atrium at the start of ventricular systole
• x descent — atrial relaxation + descent of the AV ring during ventricular systole
• v wave — passive filling of the right atrium while the tricuspid valve is closed
• y descent — opening of the tricuspid valve, blood flowing from atrium into ventricle

Clinical significance: Raised JVP = right heart failure, fluid overload, cardiac tamponade. Giant 'a' waves = tricuspid stenosis, pulmonary hypertension. Cannon 'a' waves = complete heart block (atrium contracts against a closed tricuspid valve). Absent 'a' waves = atrial fibrillation.

Blood Pressure Regulation — Baroreceptors and Beyond (PY5.8)

Blood pressure must be maintained within a narrow range — too low and organs don't get blood; too high and vessels are damaged. The body has short-term and long-term mechanisms.

Figure: Blood Pressure Regulation — Baroreceptors and Beyond (PY5.8)

SHORT-TERM REGULATION (seconds to minutes) — Neural:

1. Baroreceptor reflex — THE most important short-term mechanism. Baroreceptors are stretch receptors in the carotid sinus (CN IX — glossopharyngeal nerve) and aortic arch (CN X — vagus nerve). When BP rises: baroreceptors fire more -> nucleus tractus solitarius (NTS) in medulla -> increased parasympathetic output (vagus) + decreased sympathetic output -> decreased HR, decreased contractility, vasodilation -> BP falls. When BP falls: the opposite occurs.

The baroreceptor reflex operates continuously — it is the reason your BP doesn't crash every time you stand up.

1. Chemoreceptor reflex — peripheral chemoreceptors in the carotid body and aortic body respond to hypoxia, hypercapnia, and acidosis. They primarily regulate respiration but also cause vasoconstriction (increasing BP) when stimulated.

1. CNS ischaemic response (Cushing response) — when cerebral blood flow is severely reduced, the vasomotor centre fires massively, causing intense vasoconstriction and hypertension. This is a last-resort mechanism — it operates only at MAP < 60 mmHg. The Cushing triad (hypertension + bradycardia + irregular respiration) is a sign of raised intracranial pressure.

LONG-TERM REGULATION (hours to days) — Renal and Hormonal:

1. Renin-Angiotensin-Aldosterone System (RAAS) — when renal perfusion falls, the kidneys secrete renin -> angiotensinogen is converted to angiotensin I -> ACE (in lung capillaries) converts it to angiotensin II -> vasoconstriction + aldosterone release (Na+ and water retention) -> increased blood volume and BP.

1. ADH (vasopressin) — released from the posterior pituitary when osmolarity rises or blood volume falls. Causes water retention and vasoconstriction.

1. ANP (atrial natriuretic peptide) — released from atrial myocytes when the atria are stretched (volume overload). Causes Na+ and water excretion, vasodilation -> reduces BP. It opposes RAAS.

1. Pressure natriuresis — the ultimate long-term mechanism. When BP rises, the kidneys excrete more Na+ and water, reducing blood volume until BP returns to normal. Guyton called this the 'infinite gain' mechanism — given enough time, it can return BP to its set point regardless of what other mechanisms do.