FACTORS MAINTAINING CARDIAC OUTPUT

1. VENOUS RETURN

• When venous return increases, ventricular filling increases, so cardiac output also increases.
• Cardiac output is directly proportional to venous return.
• This holds true when the other three factors (force of contraction, heart rate, and peripheral resistance) remain constant.

I. Respiratory Pump

• The respiratory pump is the breathing activity that helps blood return to the heart during inspiration.
• It is also called the abdominothoracic pump.
• During inspiration, the thoracic cavity expands. This makes intrapleural pressure more negative.
• As a result, the diameter of the inferior vena cava increases, which increases venous return.
• At the same time, the diaphragm descends. This increases intra-abdominal pressure.
• The abdominal veins get compressed and blood is pushed upward toward the heart - increasing venous return (Fig. 95.1).
• The respiratory pump is much stronger during forced respiration and severe muscular exercise.

Inspiration → Expansion of thoracic cage → More negativity in intrapleural pressure → Increase in diameter of inferior vena cava → Increase in venous return

Inspiration → Descent of diaphragm → Increase in intra-abdominal pressure → Compression of abdominal veins → Increase in venous return

II. Muscle Pump

• The muscle pump is the muscular activity that helps blood return to the heart.
• When skeletal muscles contract, the vein between the muscles gets compressed.
• The valve of the vein proximal (closer) to the contracting muscles opens, and blood is pushed toward the heart (Fig. 95.2A).
• The valve of the vein distal (farther) to the muscles closes, preventing backflow.
• So, when muscular activity increases, venous return increases.

• The proximal valve closes - prevents backflow.
• The distal valve opens - allows blood to flow toward the muscle.

III. Gravity

• Gravitational force reduces venous return.
• When a person stands for a long time, gravity causes blood to pool in the legs. This is called venous pooling.
• Because of venous pooling, less blood returns to the heart.

IV. Venous Pressure

• Venous pressure affects venous return.
• Pressure in venules: 12 to 18 mm Hg
• Pressure gradually decreases in smaller and larger veins.
• In the inferior vena cava and superior vena cava: pressure falls to about 5.5 mm Hg
• At the junction of the venae cavae and right atrium: about 4.6 mm Hg
• Pressure in the right atrium is still lower and changes during cardiac action - it falls to zero during atrial diastole.
• This pressure gradient along the venous tree acts as a driving force for venous return.

V. Sympathetic Tone

• Venous return is aided by sympathetic (vasomotor) tone (see Chapter 100).
• Sympathetic stimulation causes constriction of venules (venoconstriction).
• Venoconstriction pushes blood toward the heart.

2. FORCE OF CONTRACTION

• Cardiac output is directly proportional to the force of contraction, when the other three factors remain constant.
• According to the Frank-Starling law, the force of contraction is directly proportional to the initial length of muscle fibers before contraction begins.
• Force of contraction depends on two things: preload and afterload.

Preload

• Preload is the stretching of cardiac muscle fibers at the end of diastole, just before contraction.
• It is caused by the filling of blood during diastole, which increases ventricular pressure.
• Stretching the muscle fibers increases their length, which increases the force of contraction.
• Therefore, force of contraction and cardiac output are directly proportional to preload.

Afterload

• Afterload is the force against which the ventricles must contract and eject blood.
• It is determined by arterial pressure.
• At the end of isometric contraction, the semilunar valves open and blood is ejected into the aorta and pulmonary artery - raising pressure in both vessels.
• The ventricles must now work against this increased pressure to continue ejection.
• Afterload for the left ventricle = aortic pressure
• Afterload for the right ventricle = pulmonary artery pressure
• Force of contraction and cardiac output are inversely proportional to afterload.

3. HEART RATE

• Cardiac output is directly proportional to heart rate, when the other three factors remain constant.
• A moderate change in heart rate does not significantly alter cardiac output.
• A marked increase in heart rate → cardiac output increases.
• A marked decrease in heart rate → cardiac output decreases.

4. PERIPHERAL RESISTANCE

• Peripheral resistance is the resistance offered to blood flow at the peripheral blood vessels.
• It is the load against which the heart pumps blood.
• Cardiac output is inversely proportional to peripheral resistance.
• Resistance is mainly offered at the arterioles, so they are called resistant vessels.
• Maximum peripheral resistance in the body is at the splanchnic region (see Chapter 99).

MEASUREMENT OF CARDIAC OUTPUT

• Direct methods are used only in animals.
• Indirect methods are used in both animals and humans.

Measurement by Direct Methods

1. Mechanical Flowmeter

• Used to measure cardiac output or blood flow to any organ.
• It has a measuring device in the middle with an inlet and outlet.
• The artery or vessel is cut, and the inlet and outlet of the flowmeter are inserted into the cut ends.
• As blood passes through, the measuring device calculates the amount of blood flow (Fig. 96.1).

2. Electromagnetic Flowmeter

• The flowmeter has two coils of wire.
• When placed on either side of a blood vessel, an electromagnetic field is produced around the vessel.
• When blood flows through the vessel, the magnetic field changes.
• Using electrodes, these changes are detected. This gives the velocity of blood flow.
• From velocity, the volume of blood flow is calculated.

• An electromagnetic probe is made with magnetic coils and electrodes.
• The probe has a cleft. The intact blood vessel is placed through this cleft (no cutting needed).
• The probe is connected to an electronic device to measure blood flow volume.
• Advantage: The blood vessel does not need to be cut.

3. Ultrasonic Doppler Flowmeter

• Ultrasound has a very high frequency - beyond the audible range of human ears.
• Ultrasound waves are transmitted through a blood vessel (transmitted waves).
• These waves hit blood cells (especially red blood cells) and bounce back (reflected waves).
• This effect is called the Doppler effect (named after its discoverer).
• The change in frequency between transmitted and reflected waves depends on the velocity of blood flow.
• By detecting this change, blood flow velocity and volume can be determined.

• The ultrasonic device produces ultrasound waves and receives the reflected waves.
• Electronic equipment measures the difference in frequencies to determine the volume of blood flow.

The Autonomic Nervous System (ANS)

What is it?

1. Sympathetic Nervous System - "Fight or Flight"

• Heart beats faster and stronger
• Blood pressure goes up
• Pupils dilate (eyes open wide to see more)
• Breathing becomes faster
• Blood flows to muscles (ready to run or fight)
• Digestion slows down (not important right now)
• Sweating increases
• Mouth becomes dry

Simple example: You suddenly see a dog charging at you. Your heart races, you breathe fast, your muscles tense up. That is your sympathetic system working.

2. Parasympathetic Nervous System - "Rest and Digest"

• Heart rate slows down
• Blood pressure decreases
• Digestion speeds up (stomach churns, intestines move)
• Saliva increases (mouth waters when you see food)
• Pupils constrict (get smaller)
• Bladder contracts (you feel the urge to urinate)
• Body conserves energy

Simple example: After eating a big meal, you feel sleepy and relaxed. Your digestion is working, heart is calm. That is your parasympathetic system working.

3. Key Differences - Side by Side

4. The Chemical Messengers (Neurotransmitters)

• Acetylcholine (ACh) - used by parasympathetic system (and in the first relay station of both systems)
• Norepinephrine - used by sympathetic system at the final target organs

Simple Memory Trick

Where Does "Sensory" Fit In?

• Efferent (outgoing) signals - brain sends commands to organs (sympathetic & parasympathetic)
• Afferent (incoming) signals - organs send sensory feedback back to the brain (e.g., pain, pressure, stretch from internal organs)

• Neuroanatomy through Clinical Cases, 3rd Ed.
• Harrison's Principles of Internal Medicine, 22E
• Guyton and Hall Textbook of Medical Physiology

Section 8: Cardiovascular System — Page 534

v. Sympathetic Tone

• Venous return is aided by sympathetic (vasomotor) tone. (See Chapter 100)
• Sympathetic tone causes constriction of venules.
• This venoconstriction pushes blood toward the heart.

2. FORCE OF CONTRACTION

• Cardiac output is directly proportional to force of contraction.
• This is true when the other three factors remain constant.
• According to the Frank-Starling law, force of contraction is directly proportional to the initial length of muscle fibers, before contraction begins.
• Force of contraction depends on two things: preload and afterload.

Preload

• Preload is the stretching of cardiac muscle fibers at the end of diastole, just before contraction.
• It is caused by filling of blood during diastole, which increases ventricular pressure.
• This stretching increases the length of muscle fibers.
• Longer fibers produce a stronger contraction and higher cardiac output.
• So, force of contraction and cardiac output are directly proportional to preload.

Afterload

• Afterload is the force against which the ventricles must contract and eject blood.
• It is determined by arterial pressure.
• At the end of the isometric contraction period, the semilunar valves open and blood is ejected into the aorta and pulmonary artery.
• Pressure rises in these two vessels.
• The ventricles must now work against this pressure to eject more blood.
• Afterload for the left ventricle = determined by aortic pressure.
• Afterload for the right ventricle = determined by pulmonary artery pressure.
• Force of contraction and cardiac output are inversely proportional to afterload.

3. HEART RATE

• Cardiac output is directly proportional to heart rate.
• This holds true when the other three factors remain constant.
• A moderate change in heart rate does not significantly change cardiac output.
• A marked increase in heart rate → cardiac output increases.
• A marked decrease in heart rate → cardiac output decreases.

4. PERIPHERAL RESISTANCE

• Peripheral resistance is the resistance offered to blood flow at peripheral blood vessels.
• It is the load against which the heart pumps blood.
• Cardiac output is inversely proportional to peripheral resistance.
• Resistance is mainly offered at the arterioles.
• So, arterioles are called resistant vessels.

MEASUREMENT OF CARDIAC OUTPUT

• In the body, maximum peripheral resistance is at the splanchnic region. (Details in Chapter 99)
• Cardiac output is measured by direct and indirect methods.
• Direct methods are used only in animals.
• Indirect methods are used in both animals and humans.

Measurement by Direct Methods

• In direct methods, cardiac output is measured using a cardiometer or flowmeter.
• Cardiometer is described in Chapter 88.
• Flowmeter is of different types.

1. Mechanical Flowmeter

• Used to measure cardiac output or blood flow to any organ.
• It has a measuring device in the middle, with an inlet and outlet.
• The artery entering the organ is cut.
• The inlet and outlet of the flowmeter are inserted into the cut ends of the vessel.
• As blood passes through, the measuring device determines the amount of blood flow. (See Fig. 96.1)

2. Electromagnetic Flowmeter

• This flowmeter has two coils of wire.
• When placed on either side of a blood vessel, an electromagnetic field is produced around the vessel.
• When blood flows through, the electromagnetic field changes.
• Using appropriate electrodes, these changes are detected.
• This gives the velocity of blood flow.
• From velocity, the volume of blood flow is calculated.

• An electromagnetic probe is made with magnetic coils and electrodes.
• The probe has a cleft.
• The intact blood vessel is placed through this cleft - no cutting needed.
• The probe is connected to an electronic device to measure blood flow volume.
• Advantage: The blood vessel does not need to be cut.

3. Ultrasonic Doppler Flowmeter

• Ultrasound has a very high frequency - beyond the audible range of human ears.
• Ultrasound waves are transmitted through a blood vessel. These are called transmitted waves.
• As these waves pass through the vessel, they hit blood cells (especially red blood cells).
• The waves bounce back - these are called reflected waves.
• The change between the transmitted and reflected waves depends on the velocity of blood flow.
• This effect is called the Doppler effect (named after its discoverer).
• By detecting the alteration in frequency, blood flow velocity and volume can be determined.

• The ultrasonic device produces ultrasound and receives reflected waves.
• Electronic equipment measures the frequency difference.
• From this, the volume of blood flow is determined.

• i. Can be used only on certain vessels.
• ii. Blood vessel has a limited life.
• iii. Size of cardiometer is large.
• iv. Diameter of the instrument must be equivalent to the vessel size.

Invasive and Non-Invasive Methods

• Invasive methods involve invasion or penetration of the body by injection or catheter.
• Non-invasive methods do not involve invasion - they work from outside parts of the body.
• Indirect methods are given below.

Indirect Methods

• Adolph Fick developed this principle.
• (Details follow in the next section.)

Peripheral Resistance

What is it?

Where does it mainly occur?

• That is why arterioles are called "resistant vessels".
• The maximum peripheral resistance in the body is at the splanchnic (gut/liver) region.

The Formula (Poiseuille-Hagen Law)

R = 8ηL / πr⁴

• R = Resistance
• η (eta) = Viscosity of blood
• L = Length of the vessel
• r = Radius of the vessel

• If radius doubles → resistance drops to just 6% of original (huge drop)
• If radius increases by just 19% → blood flow doubles

What Factors Affect Peripheral Resistance?

1. Radius of the Blood Vessel (Most Important)

• Smaller radius → More resistance → Less blood flow
• Larger radius → Less resistance → More blood flow
• Arterioles control their radius by contracting (vasoconstriction) or relaxing (vasodilation)

2. Viscosity of Blood

• Viscosity = how thick or sticky the blood is
• Whole blood is 3-4 times more viscous than water
• Plasma alone is about 1.8 times more viscous than water
• Viscosity mainly depends on hematocrit (the percentage of red blood cells in blood)
• Higher hematocrit → thicker blood → more resistance
• In very small vessels (arterioles, capillaries) - viscosity effect is less due to the Fahraeus-Lindqvist effect (red blood cells move to the centre, leaving thin plasma near the walls)

3. Length of the Vessel

• Longer vessel → more resistance
• This is relatively constant in the body and not a major regulating factor

Relationship with Cardiac Output and Blood Pressure

• Cardiac output is inversely proportional to peripheral resistance
• Blood pressure is directly proportional to peripheral resistance

Simple Real-Life Example

• Squeezing (narrowing) = vasoconstriction = more resistance = less flow, more pressure builds up
• Releasing (widening) = vasodilation = less resistance = more flow, pressure drops

Summary in Simple Points

1. Peripheral resistance = friction blood faces in blood vessels
2. Mainly occurs at arterioles
3. Controlled mainly by vessel radius (4th power relationship)
4. Also affected by blood viscosity and vessel length
5. More resistance → higher blood pressure, lower cardiac output
6. Less resistance → lower blood pressure, higher cardiac output
7. Arterioles are called resistant vessels because they are the main control point

• Ganong's Review of Medical Physiology, 26th Ed.
• Costanzo Physiology, 7th Ed.