Physiology of the Cardiovascular System
High-Yield Summary
- Blood pressure = force blood exerts on vessel walls; reported systolic (contraction) over diastolic (relaxation), e.g. 120/80 mmHg.
- Arterioles are the key resistance vessels — constrict to raise pressure, dilate to lower it, and direct blood to where it's needed.
- ΔP = CO × TPR: blood pressure equals cardiac output × total peripheral resistance.
- ADH and aldosterone both raise blood pressure by increasing blood volume (water/sodium reabsorption); ANP lowers it via salt/water loss and vasodilation.
- Pressure is highest in the aorta/large arteries, drops sharply at the arterioles, and is lowest in capillaries and veins.
The Blood Pressure Equation
ΔP = CO × TPR
- ΔP = Pressure difference across the circulation
- CO = Cardiac output (= HR × SV)
- TPR = Total peripheral resistance — mainly set by arteriole constriction/dilation
- If CO or TPR increases, pressure goes up; if either decreases, pressure goes down.
Hormonal Regulation of Blood Pressure
- ADH (antidiuretic hormone)
- Increases renal water reabsorption → raises blood volume/pressure.
- Aldosterone
- Increases renal sodium reabsorption (water follows) → raises blood volume/pressure.
- ANP (atrial natriuretic peptide)
- Promotes salt/water loss + vasodilation → lowers blood pressure.
Blood Pressure Gradient Across the Circulatory System
- 1Aorta / large arteries — highest pressure.
- 2Arterioles — dramatic pressure drop; most system resistance occurs here.
- 3Capillaries — low pressure.
- 4Veins — lowest pressure.
Common MCAT Trap
- ADH and aldosterone RAISE pressure (both increase blood volume); ANP LOWERS it — don't group ANP with the other two.
- The biggest pressure drop in the whole circulatory system happens at the arterioles, not the capillaries — that's where most resistance is generated.
- Systolic = heart contracting; diastolic = heart relaxing — systolic is always the higher number in a normal reading.
Quick Recall
What is the blood pressure equation?
Which hormone lowers blood pressure, and how?
Where does blood pressure drop most sharply?
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