Cardiovascular System: Blood Vessels and Pressure (Chapter 19 Study Guide)

Cardiovascular System: Blood Vessels and Pressure

Learning Objectives

• Describe the overall contributors to blood pressure: cardiac output and total peripheral resistance.

• Compare and contrast the three types of arteries and three types of capillaries.

• Describe veins and list the features in veins that help promote blood flow back to the heart.

• Describe the relationships between pressure, flow, and resistance across arteries, capillaries, and veins.

• Compare and contrast the neural, hormonal, and long-term regulation of blood pressure via the RAAS system.

Contributors to Blood Pressure

Cardiac Output (CO) and Total Peripheral Resistance (TPR)

Blood pressure is determined by the interaction of cardiac output and total peripheral resistance. These factors are regulated by neural, hormonal, and renal mechanisms.

• Cardiac Output (CO): The volume of blood pumped by the heart per minute. Calculated as:

$CO = SV \times HR$

• Stroke Volume (SV): Amount of blood ejected per beat.

• Heart Rate (HR): Number of heartbeats per minute.

• Total Peripheral Resistance (TPR): The resistance to blood flow offered by all of the systemic vasculature, primarily determined by arteriolar diameter.

$MAP = CO \times TPR$

• Mean Arterial Pressure (MAP): The average pressure in the arteries during one cardiac cycle. It can be estimated as:

$MAP = \frac{\text{Systolic BP} + 2 \times \text{Diastolic BP}}{3}$

• Factors affecting TPR: blood viscosity, vessel length, and vessel radius (radius has the greatest effect).

Types of Blood Vessels

Arteries

Arteries carry blood away from the heart and are classified based on size and function:

Type	Structure	Function	Key Features
Elastic arteries	Largest diameter, thick-walled, abundant elastic tissue	Conduct blood from heart to medium-sized vessels	Pressure reservoir, not active in vasoconstriction
Muscular arteries	Thickest smooth muscle, less elastic tissue	Distribute blood to organs	Active in vasoconstriction
Arterioles	Smallest arteries, single layer of smooth muscle	Control flow into capillary beds	Major site of resistance, regulate blood flow and pressure

Capillaries

Capillaries are the smallest blood vessels, consisting of a single layer of endothelial cells. Their primary function is the exchange of materials between blood and interstitial fluid.

Type	Structure	Location	Function
Continuous	Least permeable, tight junctions	Skin, muscles, lungs, CNS	Permit limited exchange; form blood-brain barrier in CNS
Fenestrated	Endothelial cells with pores (fenestrations)	Kidneys, intestines, endocrine glands	Allow increased permeability for filtration and absorption
Sinusoid	Most permeable, large gaps, few tight junctions	Liver, bone marrow, spleen	Allow passage of large molecules and cells

Veins

Veins return blood to the heart and act as blood reservoirs. They have thinner walls and larger lumens compared to arteries.

• Venules: Collect blood from capillaries; post-capillary venules have porous walls for exchange and communication with arterioles.

• Veins: Contain little smooth muscle, cannot withstand high pressure, and contain the largest percentage of blood volume (about 60%).

• Adaptations for venous return:

  • Venous valves: Prevent backflow of blood.

  • Muscular pump: Skeletal muscle contractions "milk" blood toward the heart.

  • Respiratory pump: Pressure changes during breathing move blood toward the heart.

  • Sympathetic vasoconstriction: Smooth muscle contraction pushes blood toward the heart.

  • Large lumen: Reduces resistance to flow.

Relationships Between Pressure, Flow, and Resistance

Hemodynamic Principles

• Blood Flow (F): Volume of blood moving through a vessel per unit time. Equivalent to cardiac output for the entire systemic circulation.

• Blood Pressure (BP): Force exerted by blood on vessel walls.

• Resistance (R): Opposition to flow, mainly due to friction within blood vessels.

The relationship is described by:

$F = \frac{\Delta P}{R}$

• $\Delta P$ = Pressure difference between two points

• R = Resistance (mainly determined by arterioles)

Therefore, blood flow is directly proportional to the pressure gradient and inversely proportional to resistance.

Factors Affecting Resistance

• Blood viscosity: Increased viscosity increases resistance (e.g., polycythemia).

• Blood vessel length: Longer vessels increase resistance.

• Blood vessel radius: Most important factor; resistance varies inversely with the fourth power of the radius.

$R \propto \frac{1}{r^4}$

Capillary Exchange: Bulk Flow and Diffusion

Mechanisms of Exchange

• Diffusion: Movement of solutes (e.g., O2, CO2, glucose) down concentration gradients; slower than bulk flow.

• Bulk Flow: Movement of fluid and small solutes driven by pressure gradients; primary mechanism for exchange between plasma and interstitial fluid.

Direction and amount of fluid flow depend on two opposing forces:

• Hydrostatic pressure: Force exerted by fluid pressing against a wall (pushes fluid out of capillaries).

• Colloid osmotic pressure: Pressure exerted by plasma proteins (mainly albumin) pulling fluid into capillaries.

Force	Source	Effect
Capillary hydrostatic pressure	Blood pressure in capillary (35 mm Hg at arterial end, 17 mm Hg at venous end)	Pushes fluid out of capillary
Capillary colloid osmotic pressure	Plasma proteins (albumin), ~26 mm Hg	Pulls fluid into capillary
Interstitial fluid hydrostatic pressure	Fluid in interstitial space (usually ~0 mm Hg)	Pushes fluid into capillary (usually negligible)
Interstitial fluid colloid osmotic pressure	Proteins in interstitial fluid (usually very low, ~1 mm Hg)	Pulls fluid out of capillary (usually negligible)

• Net filtration occurs at the arterial end; net reabsorption at the venous end.

• The lymphatic system returns excess interstitial fluid to the blood.

Regulation of Blood Pressure

Short-Term Regulation

• Neural controls: Baroreceptors (pressure sensors) in carotid sinuses and aortic arch detect changes in BP and adjust heart rate, contractility, and vessel diameter via the autonomic nervous system.

• Hormonal controls: Epinephrine, norepinephrine, and atrial natriuretic peptide (ANP) can increase or decrease BP by affecting heart rate, contractility, and vessel tone.

Long-Term Regulation

• Renal (kidney) mechanisms: Control blood volume by adjusting water and salt excretion.

• Renin-Angiotensin-Aldosterone System (RAAS): Activated by low BP or low blood volume; increases BP by vasoconstriction and increasing blood volume via aldosterone-mediated sodium retention.

Summary Table: Comparison of Arteries, Capillaries, and Veins

Feature	Arteries	Capillaries	Veins
Direction of blood flow	Away from heart	Connect arteries and veins	Toward heart
Wall structure	Thick, muscular, elastic	Single endothelial layer	Thin, less muscular
Pressure	High	Low	Lowest
Valves	Absent	Absent	Present (in limbs)
Function	Transport blood under pressure	Exchange of materials	Blood reservoir, return blood to heart

Key Equations

• Mean Arterial Pressure (MAP):

$MAP = CO \times TPR$

• Cardiac Output (CO):

$CO = SV \times HR$

• Blood Flow (F):

$F = \frac{\Delta P}{R}$

• MAP estimation from BP:

$MAP = \frac{\text{Systolic BP} + 2 \times \text{Diastolic BP}}{3}$

Clinical Application: Edema

• Edema: Accumulation of excess interstitial fluid due to imbalance in filtration and reabsorption, or lymphatic obstruction.

• Can be caused by increased capillary hydrostatic pressure, decreased plasma protein concentration, or lymphatic blockage.

Summary Points

• Blood pressure is regulated by cardiac output and total peripheral resistance.

• Arteries, capillaries, and veins have distinct structures and functions.

• Capillary exchange is governed by hydrostatic and osmotic pressures.

• Short- and long-term mechanisms maintain blood pressure homeostasis.

Additional info: This guide integrates textbook-level explanations and fills in context for diagrams and tables referenced in the slides.