Textbook

Introduction

1. CARS

2. Psych/soc

3. Bio/biochem

4. Chem/phys

4.1 Translational motion, forces, work, energy, and equilibrium

4.2 Fluids in circulation of blood, gas movement, and gas exchange

4.2.1 Fluids and circulatory system fluids

4.2.2 Gas phase

4.3 Electrochemistry and electrical circuits and their elements

4.4 How light and sound interact with matter

4.5 Atoms, nuclear decay, electronic structure, and atomic chemical behavior

4.6 Unique nature of water and its solutions

4.7 Nature of molecules and intermolecular interaction

4.8 Separation and purification methods

4.9 Structure, function, and reactivity of bio-relevant molecules

4.10 Principles of chemical thermodynamics and kinetics, enzymes

Wrapping up

4.2.1 Fluids and circulatory system fluids

Achievable MCAT

4. Chem/phys

4.2. Fluids in circulation of blood, gas movement, and gas exchange

Fluids and circulatory system fluids

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Fluids

Liquids and gases are considered fluids because they can flow and take the shape of their containers.

A key property of fluids is density, defined as an object’s mass ( $m$ ) divided by its volume ( $V$ ):

$ρ = \frac{m}{V}$

For example, water typically has a density of 1 $\frac{g}{mL}$ (equivalent to 1 $\frac{g}{cm ^{3}}$ or 1 $\frac{kg}{L}$ or $\frac{1000 k g}{m ^{3}}$ ).

Specific gravity is the ratio of a substance’s density to the density of water. By definition, water’s specific gravity is 1.

Buoyancy and Archimedes’ principle

Buoyancy is described by Archimedes’ principle: the buoyant force on an object equals the weight of the fluid it displaces.

Mathematically:

$F_{B} = m_{displaced} g = ρ_{fluid} V_{submerged} g$

Here, $V_{submerged}$ is the volume of the object below the fluid surface.

An object:

Floats if the buoyant force equals its weight
Rises if the buoyant force exceeds its weight
Sinks if the buoyant force is less than its weight.

Hydrostatic pressure

Hydrostatic pressure in fluids follows Pascal’s law, which states that any pressure applied to a liquid is transmitted equally in all directions.

This idea is captured by:

$F_{1} / A_{1} = F_{2} / A_{2}$

This means the pressure (force per unit area) applied at one point is transmitted throughout the fluid. As a result, a small force applied over a small area can produce a larger output force over a larger area.

The relationship $A_{1} d_{1} = A_{2} d_{2}$ shows how the distances moved adjust so that the work done (force times distance) stays constant across the system.

In a fluid at rest, pressure increases with depth according to:

$P = ρ g h$

where:

$P$ is pressure
$ρ$ is the fluid density
$g$ is the gravitational constant
$h$ is the depth below the surface

At the surface, where $h = 0$ , the pressure contributed by the fluid is zero.

At any depth, the pressure given by $ρ g h$ is called the gauge pressure because it does not include atmospheric pressure.

To find the absolute pressure on a submerged object, add atmospheric pressure to $ρ g h$ .

Viscosity: poiseuille flow and continuity equation ( $A \cdot v$ = constant)

When a viscous fluid flows through a pipe, the velocity profile across the pipe is parabolic (fastest in the center and slowest near the walls).

This flow is described by the continuity equation, which states that the product of cross-sectional area ( $A$ ) and linear velocity ( $v$ ) remains constant:

$A \cdot v$ = constant

This is another way of saying the volume flow rate ( $d V / d t$ ) is constant along the pipe.

To see why, note that a small volume element can be written as $d V = A \cdot d L$ , where $d L$ is a small length along the pipe. Dividing by time gives:

$\frac{d V}{d t} = A \cdot \frac{d L}{d t} = A \cdot v$

So if $d V / d t$ is constant, then $A \cdot v$ must also be constant.

Concept of turbulence at high velocities

At low speeds, fluids tend to show laminar flow, where layers move smoothly and predictably.

As velocity increases, flow can become turbulent, meaning it becomes chaotic and forms swirling eddies.

Surface tension allows a liquid’s surface to support very light objects - such as insects walking on water - because of the cohesive forces between molecules of the solvent.

Bernoulli’s equation ( $P + \frac{1}{2} ρ v^{2} + ρ g h = constant$ ) states that in steady flow, the sum of pressure ( $P$ ), kinetic energy per unit volume ( $\frac{1}{2} ρ v^{2}$ ), and gravitational potential energy per unit volume ( $ρ g h$ ) remains constant.

Venturi effect, pitot tube

The Venturi effect describes how a fluid’s pressure decreases when it flows through a constricted section of a pipe or channel. Using Bernoulli’s principle, when the passage narrows, fluid velocity increases. That increase raises the kinetic energy per unit volume, so the pressure must drop. Devices such as Venturi meters use this effect to measure flow rate by comparing pressures in the wide and narrow sections.

A pitot tube measures the speed of a moving fluid (often air around an aircraft). It has an opening facing into the flow, where the fluid is brought to rest, producing stagnation pressure (static pressure plus the pressure associated with motion). Comparing stagnation pressure to the static pressure measured elsewhere allows you to determine the fluid’s velocity using Bernoulli’s principle.

Pitot tube showing stagnation and static pressure measurement

Circulatory system physics arterial and venous systems; pressure and flow characteristics

In the arterial system, the heart generates a high-pressure pulse that drives blood through relatively narrow vessels with strong, elastic walls. This structure helps maintain arterial pressure and shape blood flow so each heartbeat delivers blood at sufficient pressure and velocity.

According to the continuity equation, velocity is inversely related to cross-sectional area. So, compared with veins, arteries (with smaller cross-sectional area) tend to have higher pressure and relatively higher flow velocity.

As blood moves into smaller arterioles and then capillaries, the total cross-sectional area increases greatly. This causes flow velocity to slow, which supports nutrient exchange and gas exchange between blood and tissues.

By the time blood reaches the venous system, pressure has dropped substantially. Veins have thinner, more compliant walls, allowing them to stretch and hold varying volumes of blood, which contributes to lower venous pressure.

Because venous blood returns under low pressure, several mechanisms support venous return:.

Skeletal muscle contractions compress veins, pushing blood forward, while valves in the veins prevent backflow - together forming a “muscle pump.”
Additionally, changes in thoracic pressure during respiration help draw blood toward the heart, supporting venous flow against gravity.

Together, the arterial and venous systems regulate fluid pressure and flow, maintaining a closed-loop circulatory system that can adjust to changing demands.

Fluids: properties and definitions

Fluids: liquids and gases; flow and take container shape
Density: $ρ = \frac{m}{V}$
Specific gravity: density relative to water (water = 1)

Buoyancy and Archimedes’ principle

Buoyant force = weight of displaced fluid: $F_{B} = ρ_{fluid} V_{submerged} g$
Float: buoyant force = object weight; sink: buoyant force < object weight
Volume submerged determines buoyant force

Hydrostatic pressure and Pascal’s law

Pascal’s law: pressure applied to fluid transmitted equally, $F_{1} / A_{1} = F_{2} / A_{2}$
Pressure increases with depth: $P = ρ g h$
- Gauge pressure: excludes atmospheric pressure
- Absolute pressure: atmospheric + gauge pressure
Work conservation: $A_{1} d_{1} = A_{2} d_{2}$

Viscosity, Poiseuille flow, and continuity equation

Viscous flow: parabolic velocity profile in pipes
Continuity equation: $A \cdot v$ = constant (volume flow rate conserved)
$d V / d t = A \cdot v$

Turbulence and surface tension

Laminar flow: smooth, orderly at low velocities
Turbulent flow: chaotic, eddies at high velocities
Surface tension: cohesive forces at liquid surface support light objects

Bernoulli’s equation and applications

Bernoulli’s equation: $P + \frac{1}{2} ρ v^{2} + ρ g h = constant$
- Pressure, kinetic energy, and potential energy per unit volume conserved
Venturi effect: pressure drops as fluid velocity increases in constriction
Pitot tube: measures fluid velocity via stagnation and static pressure difference

Circulatory system physics: arterial and venous systems

Arterial system: high pressure, elastic walls, higher velocity (smaller area)
Capillaries: large total cross-sectional area, slow flow for exchange
Venous system: low pressure, compliant walls, volume reservoir
- Venous return aided by skeletal muscle pump, valves, and thoracic pressure changes
Pressure and flow regulated to maintain closed-loop circulation

Fluids and circulatory system fluids

Fluids

Liquids and gases are considered fluids because they can flow and take the shape of their containers.

A key property of fluids is density, defined as an object’s mass ( $m$ ) divided by its volume ( $V$ ):

$ρ = \frac{m}{V}$

For example, water typically has a density of 1 $\frac{g}{mL}$ (equivalent to 1 $\frac{g}{cm ^{3}}$ or 1 $\frac{kg}{L}$ or $\frac{1000 k g}{m ^{3}}$ ).

Specific gravity is the ratio of a substance’s density to the density of water. By definition, water’s specific gravity is 1.

Buoyancy and Archimedes’ principle

Buoyancy is described by Archimedes’ principle: the buoyant force on an object equals the weight of the fluid it displaces.

Mathematically:

$F_{B} = m_{displaced} g = ρ_{fluid} V_{submerged} g$

Here, $V_{submerged}$ is the volume of the object below the fluid surface.

An object:

Floats if the buoyant force equals its weight
Rises if the buoyant force exceeds its weight
Sinks if the buoyant force is less than its weight.

Hydrostatic pressure

Hydrostatic pressure in fluids follows Pascal’s law, which states that any pressure applied to a liquid is transmitted equally in all directions.

This idea is captured by:

$F_{1} / A_{1} = F_{2} / A_{2}$

The relationship $A_{1} d_{1} = A_{2} d_{2}$ shows how the distances moved adjust so that the work done (force times distance) stays constant across the system.

In a fluid at rest, pressure increases with depth according to:

$P = ρ g h$

where:

$P$ is pressure
$ρ$ is the fluid density
$g$ is the gravitational constant
$h$ is the depth below the surface

At the surface, where $h = 0$ , the pressure contributed by the fluid is zero.

At any depth, the pressure given by $ρ g h$ is called the gauge pressure because it does not include atmospheric pressure.

To find the absolute pressure on a submerged object, add atmospheric pressure to $ρ g h$ .

Viscosity: poiseuille flow and continuity equation ( $A \cdot v$ = constant)

When a viscous fluid flows through a pipe, the velocity profile across the pipe is parabolic (fastest in the center and slowest near the walls).

This flow is described by the continuity equation, which states that the product of cross-sectional area ( $A$ ) and linear velocity ( $v$ ) remains constant:

$A \cdot v$ = constant

This is another way of saying the volume flow rate ( $d V / d t$ ) is constant along the pipe.

To see why, note that a small volume element can be written as $d V = A \cdot d L$ , where $d L$ is a small length along the pipe. Dividing by time gives:

$\frac{d V}{d t} = A \cdot \frac{d L}{d t} = A \cdot v$

So if $d V / d t$ is constant, then $A \cdot v$ must also be constant.

Concept of turbulence at high velocities

At low speeds, fluids tend to show laminar flow, where layers move smoothly and predictably.

As velocity increases, flow can become turbulent, meaning it becomes chaotic and forms swirling eddies.

Surface tension allows a liquid’s surface to support very light objects - such as insects walking on water - because of the cohesive forces between molecules of the solvent.

Venturi effect, pitot tube

Circulatory system physics arterial and venous systems; pressure and flow characteristics

Because venous blood returns under low pressure, several mechanisms support venous return:.

Skeletal muscle contractions compress veins, pushing blood forward, while valves in the veins prevent backflow - together forming a “muscle pump.”
Additionally, changes in thoracic pressure during respiration help draw blood toward the heart, supporting venous flow against gravity.

Together, the arterial and venous systems regulate fluid pressure and flow, maintaining a closed-loop circulatory system that can adjust to changing demands.

Achievable MCAT

4. Chem/phys

4.2. Fluids in circulation of blood, gas movement, and gas exchange

Fluids and circulatory system fluids

6 min read

Font

Discuss

Feedback

Fluids

Liquids and gases are considered fluids because they can flow and take the shape of their containers.

A key property of fluids is density, defined as an object’s mass ( $m$ ) divided by its volume ( $V$ ):

$ρ = \frac{m}{V}$

For example, water typically has a density of 1 $\frac{g}{mL}$ (equivalent to 1 $\frac{g}{cm ^{3}}$ or 1 $\frac{kg}{L}$ or $\frac{1000 k g}{m ^{3}}$ ).

Specific gravity is the ratio of a substance’s density to the density of water. By definition, water’s specific gravity is 1.

Buoyancy and Archimedes’ principle

Buoyancy is described by Archimedes’ principle: the buoyant force on an object equals the weight of the fluid it displaces.

Mathematically:

$F_{B} = m_{displaced} g = ρ_{fluid} V_{submerged} g$

Here, $V_{submerged}$ is the volume of the object below the fluid surface.

An object:

Floats if the buoyant force equals its weight
Rises if the buoyant force exceeds its weight
Sinks if the buoyant force is less than its weight.

Hydrostatic pressure

Hydrostatic pressure in fluids follows Pascal’s law, which states that any pressure applied to a liquid is transmitted equally in all directions.

This idea is captured by:

$F_{1} / A_{1} = F_{2} / A_{2}$

The relationship $A_{1} d_{1} = A_{2} d_{2}$ shows how the distances moved adjust so that the work done (force times distance) stays constant across the system.

In a fluid at rest, pressure increases with depth according to:

$P = ρ g h$

where:

$P$ is pressure
$ρ$ is the fluid density
$g$ is the gravitational constant
$h$ is the depth below the surface

At the surface, where $h = 0$ , the pressure contributed by the fluid is zero.

At any depth, the pressure given by $ρ g h$ is called the gauge pressure because it does not include atmospheric pressure.

To find the absolute pressure on a submerged object, add atmospheric pressure to $ρ g h$ .

Viscosity: poiseuille flow and continuity equation ( $A \cdot v$ = constant)

When a viscous fluid flows through a pipe, the velocity profile across the pipe is parabolic (fastest in the center and slowest near the walls).

This flow is described by the continuity equation, which states that the product of cross-sectional area ( $A$ ) and linear velocity ( $v$ ) remains constant:

$A \cdot v$ = constant

This is another way of saying the volume flow rate ( $d V / d t$ ) is constant along the pipe.

To see why, note that a small volume element can be written as $d V = A \cdot d L$ , where $d L$ is a small length along the pipe. Dividing by time gives:

$\frac{d V}{d t} = A \cdot \frac{d L}{d t} = A \cdot v$

So if $d V / d t$ is constant, then $A \cdot v$ must also be constant.

Concept of turbulence at high velocities

At low speeds, fluids tend to show laminar flow, where layers move smoothly and predictably.

As velocity increases, flow can become turbulent, meaning it becomes chaotic and forms swirling eddies.

Surface tension allows a liquid’s surface to support very light objects - such as insects walking on water - because of the cohesive forces between molecules of the solvent.

Venturi effect, pitot tube

Circulatory system physics arterial and venous systems; pressure and flow characteristics

Because venous blood returns under low pressure, several mechanisms support venous return:.

Skeletal muscle contractions compress veins, pushing blood forward, while valves in the veins prevent backflow - together forming a “muscle pump.”
Additionally, changes in thoracic pressure during respiration help draw blood toward the heart, supporting venous flow against gravity.

Together, the arterial and venous systems regulate fluid pressure and flow, maintaining a closed-loop circulatory system that can adjust to changing demands.

Fluids: properties and definitions

Fluids: liquids and gases; flow and take container shape
Density: $ρ = \frac{m}{V}$
Specific gravity: density relative to water (water = 1)

Buoyancy and Archimedes’ principle

Buoyant force = weight of displaced fluid: $F_{B} = ρ_{fluid} V_{submerged} g$
Float: buoyant force = object weight; sink: buoyant force < object weight
Volume submerged determines buoyant force

Hydrostatic pressure and Pascal’s law

Pascal’s law: pressure applied to fluid transmitted equally, $F_{1} / A_{1} = F_{2} / A_{2}$
Pressure increases with depth: $P = ρ g h$
- Gauge pressure: excludes atmospheric pressure
- Absolute pressure: atmospheric + gauge pressure
Work conservation: $A_{1} d_{1} = A_{2} d_{2}$

Viscosity, Poiseuille flow, and continuity equation

Viscous flow: parabolic velocity profile in pipes
Continuity equation: $A \cdot v$ = constant (volume flow rate conserved)
$d V / d t = A \cdot v$

Turbulence and surface tension

Laminar flow: smooth, orderly at low velocities
Turbulent flow: chaotic, eddies at high velocities
Surface tension: cohesive forces at liquid surface support light objects

Bernoulli’s equation and applications

Bernoulli’s equation: $P + \frac{1}{2} ρ v^{2} + ρ g h = constant$
- Pressure, kinetic energy, and potential energy per unit volume conserved
Venturi effect: pressure drops as fluid velocity increases in constriction
Pitot tube: measures fluid velocity via stagnation and static pressure difference

Circulatory system physics: arterial and venous systems

Arterial system: high pressure, elastic walls, higher velocity (smaller area)
Capillaries: large total cross-sectional area, slow flow for exchange
Venous system: low pressure, compliant walls, volume reservoir
- Venous return aided by skeletal muscle pump, valves, and thoracic pressure changes
Pressure and flow regulated to maintain closed-loop circulation

Fluids and circulatory system fluids

Fluids

Liquids and gases are considered fluids because they can flow and take the shape of their containers.

A key property of fluids is density, defined as an object’s mass ( $m$ ) divided by its volume ( $V$ ):

$ρ = \frac{m}{V}$

For example, water typically has a density of 1 $\frac{g}{mL}$ (equivalent to 1 $\frac{g}{cm ^{3}}$ or 1 $\frac{kg}{L}$ or $\frac{1000 k g}{m ^{3}}$ ).

Specific gravity is the ratio of a substance’s density to the density of water. By definition, water’s specific gravity is 1.

Buoyancy and Archimedes’ principle

Buoyancy is described by Archimedes’ principle: the buoyant force on an object equals the weight of the fluid it displaces.

Mathematically:

$F_{B} = m_{displaced} g = ρ_{fluid} V_{submerged} g$

Here, $V_{submerged}$ is the volume of the object below the fluid surface.

An object:

Floats if the buoyant force equals its weight
Rises if the buoyant force exceeds its weight
Sinks if the buoyant force is less than its weight.

Hydrostatic pressure

Hydrostatic pressure in fluids follows Pascal’s law, which states that any pressure applied to a liquid is transmitted equally in all directions.

This idea is captured by:

$F_{1} / A_{1} = F_{2} / A_{2}$

The relationship $A_{1} d_{1} = A_{2} d_{2}$ shows how the distances moved adjust so that the work done (force times distance) stays constant across the system.

In a fluid at rest, pressure increases with depth according to:

$P = ρ g h$

where:

$P$ is pressure
$ρ$ is the fluid density
$g$ is the gravitational constant
$h$ is the depth below the surface

At the surface, where $h = 0$ , the pressure contributed by the fluid is zero.

At any depth, the pressure given by $ρ g h$ is called the gauge pressure because it does not include atmospheric pressure.

To find the absolute pressure on a submerged object, add atmospheric pressure to $ρ g h$ .

Viscosity: poiseuille flow and continuity equation ( $A \cdot v$ = constant)

When a viscous fluid flows through a pipe, the velocity profile across the pipe is parabolic (fastest in the center and slowest near the walls).

This flow is described by the continuity equation, which states that the product of cross-sectional area ( $A$ ) and linear velocity ( $v$ ) remains constant:

$A \cdot v$ = constant

This is another way of saying the volume flow rate ( $d V / d t$ ) is constant along the pipe.

To see why, note that a small volume element can be written as $d V = A \cdot d L$ , where $d L$ is a small length along the pipe. Dividing by time gives:

$\frac{d V}{d t} = A \cdot \frac{d L}{d t} = A \cdot v$

So if $d V / d t$ is constant, then $A \cdot v$ must also be constant.

Concept of turbulence at high velocities

At low speeds, fluids tend to show laminar flow, where layers move smoothly and predictably.

As velocity increases, flow can become turbulent, meaning it becomes chaotic and forms swirling eddies.

Surface tension allows a liquid’s surface to support very light objects - such as insects walking on water - because of the cohesive forces between molecules of the solvent.

Venturi effect, pitot tube

Circulatory system physics arterial and venous systems; pressure and flow characteristics

Because venous blood returns under low pressure, several mechanisms support venous return:.

Skeletal muscle contractions compress veins, pushing blood forward, while valves in the veins prevent backflow - together forming a “muscle pump.”
Additionally, changes in thoracic pressure during respiration help draw blood toward the heart, supporting venous flow against gravity.

Together, the arterial and venous systems regulate fluid pressure and flow, maintaining a closed-loop circulatory system that can adjust to changing demands.

Fluids and circulatory system fluids

Fluids

Buoyancy and Archimedes’ principle

Hydrostatic pressure

Viscosity: poiseuille flow and continuity equation (A⋅v = constant)

Concept of turbulence at high velocities

Venturi effect, pitot tube

Circulatory system physics arterial and venous systems; pressure and flow characteristics

Fluids and circulatory system fluids

Fluids

Buoyancy and Archimedes’ principle

Hydrostatic pressure

Viscosity: poiseuille flow and continuity equation (A⋅v = constant)

Concept of turbulence at high velocities

Venturi effect, pitot tube

Circulatory system physics arterial and venous systems; pressure and flow characteristics

Fluids and circulatory system fluids

Fluids

Buoyancy and Archimedes’ principle

Hydrostatic pressure

Viscosity: poiseuille flow and continuity equation (A⋅v = constant)

Concept of turbulence at high velocities

Venturi effect, pitot tube

Circulatory system physics arterial and venous systems; pressure and flow characteristics

Fluids and circulatory system fluids

Fluids

Buoyancy and Archimedes’ principle

Hydrostatic pressure

Viscosity: poiseuille flow and continuity equation (A⋅v = constant)

Concept of turbulence at high velocities

Venturi effect, pitot tube

Circulatory system physics arterial and venous systems; pressure and flow characteristics

Viscosity: poiseuille flow and continuity equation ( $A \cdot v$ = constant)

Viscosity: poiseuille flow and continuity equation ( $A \cdot v$ = constant)

Viscosity: poiseuille flow and continuity equation ( $A \cdot v$ = constant)

Viscosity: poiseuille flow and continuity equation ( $A \cdot v$ = constant)