A&P II - lesson 6

What are tunics?

Tissue layers of blood vessels that surround a central space called the lumen.

What is the lumen?

The central space of a blood vessel where blood flows.

Name the three tunics of blood vessels (deep → superficial).

• Tunica intima

• Tunica media

• Tunica externa

What is the tunica intima composed of?

Endothelium (simple squamous epithelium) continuous with the endocardium of the heart.

Functions of the tunica intima

• Regulates vessel diameter with vasomotor nerves

• Releases tissue factor when injured

• Allows exchange of nutrients and gases in smaller vessels

What is the tunica media composed of?

• Circular smooth muscle

• Elastic fibers

• Innervated by vasomotor nerves

Function of the tunica media

Controls vessel diameter via vasoconstriction and vasodilation.

What is the tunica externa made of?

Dense irregular collagenous connective tissue.

Function of the tunica externa

Provides structural support and protection for the blood vessel.

What sensory receptors are found in some arteries?

• Baroreceptors (pressure receptors)

• Chemoreceptors (detect O₂, CO₂, and H⁺)

What are venous valves and where are they found?

Extensions of the tunica intima in veins that prevent backflow of blood.

Define hemodynamics

The dynamics of blood flow.

What three factors determine blood flow and blood pressure?

• Resistance

• Cardiac output

• Blood volume

Define blood pressure

The outward force blood exerts on the inner walls of blood vessels, measured in mmHg.

Define blood flow

The volume of blood that moves through blood vessels per minute; matches cardiac output.

Where is blood pressure highest?

Closest to the heart.

How does blood pressure change as blood moves away from the heart?

It decreases.

Relationship between blood flow and pressure

• Directly proportional to pressure gradients

• Indirectly proportional to peripheral resistance

What does systolic/diastolic blood pressure represent?

• Systolic: ventricular contraction

• Diastolic: ventricular relaxation

What circuit is measured when reporting blood pressure?

Systemic arterial circuit.

What three variables influence blood pressure?

• Peripheral resistance (PR)

• Cardiac output (CO)

• Blood volume

Define peripheral resistance (PR)

Hindrance to blood flow through peripheral vasculature.

Relationship between peripheral resistance (PR) and blood pressure

When peripheral resistance (PR) increases, blood pressure increases.

Equation relating CO and PR

CO × PR = ΔP
SV × HR × PR = ΔP

What increases cardiac output?

• Increased heart rate

• Sympathetic nervous system stimulation

• Drugs like caffeine

Relationship between CO and BP

Increased CO → increased BP.

How does blood volume affect blood pressure?

Increased blood volume → increased blood pressure.

Why does excess blood volume increase BP?

Veins can only stretch so much; excess blood shifts to arteries, raising pressure.

Short-term regulation affects which variables?

Peripheral resistance and cardiac output.

Long-term regulation affects which variable?

Blood volume.

Role of the sympathetic nervous system in BP regulation

• Releases epinephrine and norepinephrine

• Increases HR → increases CO

• Causes vasoconstriction → increases PR

• Result: increased BP

Role of the parasympathetic nervous system

• Releases acetylcholine

• Decreases HR → decreases CO

• Allows vasodilation by inhibiting sympathetic activity

• Result: decreased BP

What is the baroreceptor reflex?

Autonomic nervous system (ANS) monitoring of blood pressure via pressure receptors.

Where are baroreceptors located?

• Carotid sinus (common carotid artery)

• Aortic sinus (aortic arch)

What do chemoreceptors near baroreceptors monitor?

Blood O₂, CO₂, and H⁺ concentrations.

What neurotransmitter does the vagus nerve release to the SA node?

Acetylcholine.

Hormones that increase cardiac output

• Epinephrine

• Norepinephrine

• Thyroid hormones (increase receptor sensitivity)

Hormones affecting peripheral resistance

• Epinephrine (vasoconstriction)

• Angiotensin II (powerful vasoconstrictor)

• Atrial natriuretic peptide (ANP) → vasodilation

When BP is high, what hormone is released?

ANP from the heart.

Effect of ANP

• Decreases water retention by kidneys

• Decreases blood volume

• Lowers blood pressure

When BP is low, what system is activated?

Renin–angiotensin–aldosterone system (RAAS).

Hormones released when BP is low

• Renin → angiotensin II → aldosterone

• Antidiuretic hormone (ADH)

Effect of RAAS and ADH

• Increase water retention

• Increase blood volume

• Increase blood pressure

What do capillaries connect?

Arterioles and venules.

What are pericytes?

Undifferentiated connective tissue cells that help regulate blood flow through capillaries.

Define tissue perfusion

Blood flow into tissues allowing capillary exchange.

Define capillary exchange

Movement of nutrients, gases, ions, and wastes between blood and tissue cells.

Mechanisms of capillary exchange

• Diffusion

• Osmosis

• Transcytosis

What is transcytosis?

Transport of substances via endocytosis on one side of the cell and exocytosis on the other.

Continuous capillaries

Endothelial cells joined by tight junctions.

Fenestrated capillaries

Contain pores (fenestrations) allowing greater permeability.

Sinusoidal capillaries

Discontinuous endothelium allowing transfer of large substances.

What drives water movement across capillaries?

Filtration.

Define hydrostatic pressure (HP)

Force exerted by a fluid on the wall of its container.

Two hydrostatic pressures considered in capillaries

• HP of vessel

• HP of interstitial fluid

HP equation

HP of vessel – HP of interstitial fluid (mmHg)

Effect of HP

Pushes water out of capillaries.

Define osmotic pressure (OP)

Force needed to prevent water from leaving a container by osmosis.

What determines osmotic pressure?

Solutes that cannot pass through membranes (e.g., albumin).

Define colloid osmotic pressure (COP)

Difference in osmotic pressure between capillary and interstitial fluid due to plasma proteins.

COP equation

Capillary OP – Interstitial fluid OP (mmHg)

Effect of COP

Pulls water into capillaries.

Define net filtration pressure (NFP)

Overall movement of water considering both HP and COP.

NFP interpretation

• Positive NFP → filtration (water out)

• Negative NFP → absorption (water in)

Arteriole NFP equation

= arteriole HP – COP

Venule NFP equations

= venule HP – COP

What does overall NFP determine?

Whether a capillary has a net gain or loss of fluid.