Physiology of the Cardiovascular System

Beyond simply moving blood, the cardiovascular system maintains blood pressure, regulates gas and solute exchange, enables clot formation, and contributes to thermoregulation. This page focuses on blood pressure: what it is, how it's measured, how the body regulates it, and how it changes across the circulatory system.

Key Takeaways

• Blood pressure is the force blood exerts on vessel walls; it's reported as systolic (contraction) over diastolic (relaxation) pressure, e.g., 120/80 mmHg.

• Arterioles act as resistance vessels, constricting or dilating to raise or lower blood pressure and direct blood flow where it's needed.

• Hypertension can damage the heart, kidneys, brain, and eyes over time; pressure that's too low starves tissues of oxygen.

• ΔP = CO × TPR: blood pressure equals cardiac output multiplied by total peripheral resistance.

• ADH and aldosterone both raise blood pressure by increasing blood volume (water and sodium reabsorption, respectively); ANP lowers it by promoting salt/water loss and vasodilation.

• Pressure is highest in the aorta and large arteries, drops sharply at the arterioles, and is lowest in the capillaries and veins.

What Is Blood Pressure?

Blood pressure is the force that blood exerts on the walls of blood vessels. This pressure is what keeps blood flowing forward through the circulatory system and what allows oxygen, nutrients, and hormones to reach tissues throughout the body. Without it, nothing would move — but if pressure gets too high or too low, it can cause serious problems.

Measuring and Reporting Blood Pressure

Blood pressure is measured with a blood pressure cuff and reported as two numbers:

• Systolic pressure — the pressure in the arteries when the heart contracts.

• Diastolic pressure — the pressure when the heart relaxes between beats.

A blood pressure reading of 120 over 80 means 120 millimeters of mercury (mmHg) during systole and 80 mmHg during diastole.

How Blood Pressure Is Regulated

Arterioles as Resistance Vessels

Blood pressure is tightly regulated by the body, and one of the most important regulators is the smooth muscle in the walls of arterioles — the small branches of arteries. Arterioles can constrict to increase resistance and raise pressure, or dilate to decrease resistance and lower pressure, which is why they're often called resistance vessels. This control is especially important for directing blood to where it's needed most, like the muscles during exercise or the digestive organs after a meal.

Hypertension and Hypotension

If blood pressure is too high — hypertension — sustained high pressure can damage blood vessels and organs over time, especially the heart, kidneys, brain, and eyes. If pressure is too low, tissues won't receive enough oxygen. The goal is to keep blood pressure in a healthy range so blood moves forward effectively without causing harm.

The Blood Pressure Equation

Blood pressure is controlled according to a key equation:

ΔP = CO × TPR

The pressure difference across the circulation (ΔP) equals cardiac output (CO) multiplied by total peripheral resistance (TPR). If either cardiac output or resistance increases, pressure goes up; if either decreases, pressure goes down.

Cardiac output depends on heart rate and stroke volume. Total peripheral resistance depends primarily on how constricted or relaxed the blood vessels are — mainly the arterioles.

Hormonal Regulation of Blood Pressure

Hormones also play a major role in regulating blood pressure:

ADH increases water reabsorption in the kidneys, which increases blood volume and pressure. Aldosterone increases sodium reabsorption, pulling water along with it and raising pressure the same way. ANP works in the opposite direction, lowering blood pressure by promoting salt and water loss and causing vasodilation.

The Blood Pressure Gradient Across the Circulatory System

Blood pressure changes predictably as blood moves through the circulatory system. The highest pressure is found in the aorta and large arteries. As blood moves into the arterioles, there's a dramatic drop in pressure — this is where most of the resistance in the system occurs. By the time blood reaches the capillaries and then the veins, pressure is much lower.

This pressure gradient is what drives blood flow, and it's why blood can move all the way from the heart to the toes and back again.

Common MCAT Mistakes

• Confusing systolic and diastolic pressure. Systolic is the higher number, measured when the heart contracts; diastolic is the lower number, measured when the heart relaxes between beats. In 120/80, systolic is 120, diastolic is 80.

• Thinking arterioles are just passive pipes. Arterioles are active resistance vessels — smooth muscle in their walls constricts or dilates to control resistance and redirect blood flow, not just carry it. This is the primary mechanism behind TPR.

• Getting ΔP = CO × TPR backwards. Cardiac output and total peripheral resistance both raise pressure when they increase and lower it when they decrease — they don't move in opposite directions. Raising either factor raises ΔP.

• Mixing up which hormones raise vs. lower blood pressure. ADH and aldosterone both raise blood pressure by increasing blood volume (water and sodium reabsorption, respectively). ANP is the outlier — it lowers pressure via salt/water loss and vasodilation.

MCAT-Style Concept Check

Question: A patient's total peripheral resistance decreases while cardiac output stays constant. According to ΔP = CO × TPR, what happens to blood pressure, and which vessels are most directly responsible for this change?

• A) Blood pressure increases; the change occurs primarily in the capillaries.

• B) Blood pressure decreases; the change occurs primarily in the arterioles.

• C) Blood pressure stays the same; the change occurs primarily in the veins.

• D) Blood pressure decreases; the change occurs primarily in the aorta.

Answer: B

Explanation: ΔP = CO × TPR means blood pressure is directly proportional to total peripheral resistance when cardiac output is held constant — so a decrease in TPR produces a decrease in blood pressure. TPR is governed mainly by arterioles, the resistance vessels whose smooth muscle constricts or dilates to raise or lower resistance. The capillaries and veins are downstream of where most resistance is generated, and the aorta is a large conducting vessel, not a resistance vessel, so neither drives changes in TPR.

FAQ

What's the difference between systolic and diastolic blood pressure?

Systolic pressure is the force in the arteries when the heart contracts; diastolic pressure is the force when the heart relaxes between beats. A reading of 120/80 mmHg means 120 mmHg systolic and 80 mmHg diastolic.

Why are arterioles called resistance vessels?

Arterioles have smooth muscle in their walls that can constrict (raising resistance and pressure) or dilate (lowering resistance and pressure), giving the body fine control over where blood flows and how much overall resistance the circulatory system generates.

What does the equation ΔP = CO × TPR mean?

It means the pressure difference across the circulation (ΔP) equals cardiac output (CO) multiplied by total peripheral resistance (TPR). Increasing either cardiac output or resistance raises blood pressure; decreasing either lowers it.

How do ADH, aldosterone, and ANP affect blood pressure differently?

ADH and aldosterone both raise blood pressure by increasing blood volume — ADH through water reabsorption in the kidneys, aldosterone through sodium reabsorption that pulls water with it. ANP lowers blood pressure by promoting salt and water loss and causing vasodilation.