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Basics of Hemodynamics

Haemodynamica

For medical students2 min readUpdated 2026-10-10

Hemodynamics describes the physical principles of fluid motion through the vascular system. The primary driver of blood flow is the pressure gradient, while the main retarding factor is hydrodynamic resistance, which depends on friction and vessel caliber.

Main factorPressure gradient between the beginning and end of the vascular bed

ResistanceArises from fluid friction against vessel walls and between adjacent fluid layers

ContinuityAn equal volume of blood flows through any consecutive cross-sectional segments per unit of time

Driving Force of Blood Flow: Pressure Gradient

For blood to move through a vascular tube, a pressure difference (denoted as ΔP) must exist. This parameter reflects the difference between the pressure at the beginning of a vascular segment and at its end.

The pressure gradient is the absolute and primary factor forcing blood forward. From a physical standpoint, this gradient can be established in three ways:

A significant increase in pressure at the very beginning of the vessel.
A marked decrease in pressure at the end of the vascular bed.
A combined effect of both factors, which provides the most effective potential difference for fluid movement.

Obstacles to Blood Flow: Hydrodynamic Resistance

While the pressure difference pushes blood forward, hydrodynamic resistance (denoted as R) constantly opposes this movement. The nature of this resistance lies in the frictional forces that inevitably arise during the flow of any real fluid.

Resistance in the vascular bed consists of two main types of friction:

Internal fluid friction: Occurs between individual layers of the flowing blood itself. In physiology and physics, this phenomenon is termed viscosity.
External friction: Occurs when the moving fluid comes into direct contact with the inner wall of the blood vessel. The more actively blood rubs against the wall, the more pressure energy is expended to overcome this obstacle.

Impact of Vessel Diameter

The third critical factor determining the pattern of blood movement is the vessel diameter. Although closely related to hydrodynamic resistance, it is distinguished as an independent hemodynamic parameter.

The diameter (lumen) of the vascular tube directly determines how strongly fluid-to-wall friction will manifest. Altering vessel diameter is the primary mechanism by which the body can rapidly regulate both resistance and flow velocity in a specific segment of the vascular bed.

General Principle of Continuity

One of the fundamental laws of hydrodynamics applicable to the vascular system is the principle of continuity.

The essence of this principle is as follows: if we take any consecutive segments of the vascular bed (for example, the total cross-sectional area of all arteries versus all veins), an absolutely identical volume of blood flows through them in a single unit of time.

It is important to understand that this principle refers specifically to the equality of blood volumes (quantity), rather than equality of linear blood flow velocity, pressure, or peripheral resistance across these segments.

Mnemonic

To remember the factors of hemodynamics, think of a garden hose: water pressure from the faucet to the exit (Pressure Gradient), the width of the hose (Diameter), and the roughness of its inner walls (Resistance and Friction).

Frequently asked questions

What formula is used to calculate the hydrodynamic resistance of blood vessels?

The hydrodynamic resistance of blood vessels is calculated using a physical formula that accounts for vessel geometry and blood properties. The formula for hydrodynamic resistance is: $R = \frac{8l\eta}{\pi r^4}$.

Parameter breakdown:

$R$ — hydrodynamic resistance;
$l$ — length of the blood vessel;
$r$ — vessel radius;
$\eta$ — blood viscosity;
$\pi$ — mathematical constant pi.

How does linear blood flow velocity depend on the total cross-sectional area of the vascular bed?

Linear blood flow velocity is inversely proportional to the cross-sectional area of vessels of a given caliber. According to the continuity equation, the larger the total cross-sectional area of the vascular bed, the lower the fluid velocity.

In the aorta, the total cross-sectional area is minimal, so linear velocity is maximal.
In the capillaries, there is a sharp peak and maximum total cross-sectional area for vessels of this caliber; consequently, linear blood velocity drops to minimal values.

What is the difference between volumetric flow rate and linear flow velocity?

Volumetric flow rate characterizes the volume of fluid passing through, whereas linear velocity reflects the distance traveled by a fluid particle.

Feature	Volumetric Flow Rate ($Q$)	Linear Velocity ($V$)
Definition	Amount of fluid (volume) passing through a vessel cross-section per unit time	Distance traveled by a blood particle per unit time
Variation across the bed	Remains constant throughout the vascular system	Varies depending on the total cross-sectional area of vessels of a given caliber
Calculation formula	Ratio of pressure gradient to resistance	Ratio of volumetric flow rate to cross-sectional area

What blood factors determine its internal friction (viscosity)?

Internal friction (viscosity) of blood is determined by hematocrit, plasma composition, and physicochemical properties of the medium. Key factors:

Hematocrit — the fraction of blood volume made up of cells (primarily erythrocytes). The higher it is, the greater the viscosity.
Plasma proteins — concentration and types of proteins.
Acid-base balance (ABR) — inversely related to viscosity.
Hypercapnia — increases viscosity (venous blood is more viscous than arterial).
Flow velocity in microvessels — in the microcirculation, viscosity is highly dependent on velocity (axial streaming decreases viscosity).

What is the blood pressure at the beginning of the systemic circulation (aorta) and at its end ( venae cavae)?

At the beginning of the systemic circulation, blood pressure is at its maximum, while at the end, it is at its minimum.

In the aorta, pressure is high with pronounced pulse pressure fluctuations; its drop in this segment is minor—averaging from 100 to 99 mm Hg. As blood moves forward, pressure gradually drops due to friction. At the end of the systemic circulation—in the venae cavae and right atrium—pressure is very low, dropping nearly to 0 and potentially becoming slightly negative (down to –2 mm Hg).

What is the main driving force of blood flow?

The main driving force is the pressure gradient—the difference between the pressure at the beginning and the end of the vascular bed.

Why does hydrodynamic resistance occur in blood vessels?

It arises due to friction: both between the layers of the moving fluid itself (which determines its viscosity) and as a result of fluid friction against the inner vessel wall.

What does the principle of continuity state in hemodynamics?

According to this principle, a strictly identical quantity (volume) of blood flows through any consecutive segments of the vascular bed per unit of time.

Go deeper

Poiseuille's law and mathematical calculation of hydrodynamic resistance
Blood viscosity: influencing factors and clinical significance
Types of blood flow: laminar and turbulent fluid flow
Mechanisms of pressure gradient generation in arterial and venous beds