Chapter 18: The Cardiovascular System II – Blood Vessels

Overview of the Cardiovascular System: Blood Vessels

Introduction

The cardiovascular system is composed of the heart and an extensive network of blood vessels that transport blood throughout the body. Blood vessels are essential for delivering oxygen, nutrients, and removing waste products from tissues. The vasculature collectively measures over 60,000 miles in length, highlighting its vastness and importance.

Types of Blood Vessels

Arteries, Capillaries, and Veins

Blood vessels are classified into three main types based on their function and structure:

• Arteries: Distribution system; carry blood away from the heart under high pressure.

• Capillaries: Exchange system; facilitate the exchange of gases, nutrients, and wastes between blood and tissues.

• Veins: Collection system; return blood to the heart, often under lower pressure.

Structure of Arteries and Veins

Arteries and veins have three layers:

• Tunica intima: Innermost layer, composed of endothelium.

• Tunica media: Middle layer, contains smooth muscle and elastic fibers.

• Tunica externa: Outermost layer, provides structural support.

Blood Distribution in the Cardiovascular System

Blood is distributed unevenly among different vessel types:

• Systemic veins: 55%

• Pulmonary veins: 15%

• Systemic arteries: 10%

• Capillaries: 5%

• Pulmonary arteries: 3%

• Right and left heart: 6% each

Structural Differences Among Vessel Types

Type	Structure
Elastic arteries	Extensive elastic laminae in tunica intima and media
Muscular arteries	Thick tunica media with smooth muscle
Arterioles	Thin walls, all three tunics, dispersed smooth muscle
Venules	Thin walls, little smooth muscle
Veins	Thin smooth muscle, large lumen, valves present

Functions of Blood Vessels

Key Functions

• Conduct blood under high pressure to organs

• Control blood flow to organs and tissues

• Regulate blood pressure

• Feed and drain capillary beds

• Return blood to the heart

Atherosclerosis

Definition and Pathology

Atherosclerosis is the leading cause of death in the developed world. It affects large and medium-sized muscular arteries and is characterized by the formation of atherosclerotic plaques composed of lipids, cholesterol, calcium salts, and cellular debris within the tunica intima. Plaques often form at vessel branching points or curves.

Development and Consequences

• Endothelial injury (from high blood pressure, cholesterol, toxins, infections, high blood glucose) triggers inflammation.

• Phagocytes are drawn to the site, causing further vessel damage.

• Clots may form, leading to myocardial infarction or stroke.

Treatment of Atherosclerosis

• Dietary modifications

• Physical activity

• Medications to lower cholesterol, blood pressure, and manage glucose

• Smoking cessation

• Surgical interventions (e.g., stents, bypass grafts)

Hemodynamics: Physiology of Blood Flow

Blood Flow, Pressure, and Resistance

Hemodynamics refers to the study of blood flow. Blood flow is the volume of blood that flows per minute and is influenced by pressure and resistance.

• Blood Pressure: Outward force blood exerts on vessel walls.

• Resistance: Opposition to blood flow, affected by vessel radius, length, and blood viscosity.

Equation: $\text{Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

Factors Affecting Blood Pressure

• Vessel radius (greatest effect on resistance)

• Vessel length (increases with body size)

• Blood viscosity (increases with more RBCs or dehydration)

• Blood volume (increases with water intake)

• Sympathetic and parasympathetic nervous system activity

Blood Pressure in Different Portions of Circulation

• Systolic Pressure: Pressure during ventricular systole (~110–120 mm Hg)

• Diastolic Pressure: Pressure during ventricular diastole (~70–80 mm Hg)

• Pulse Pressure: Difference between systolic and diastolic (~40 mm Hg)

Mean Arterial Pressure (MAP)

MAP is the average pressure in systemic arteries during a cardiac cycle. It is estimated as:

$\text{MAP} = \text{DBP} + \frac{\text{SBP} - \text{DBP}}{3}$

Pressures in Pulmonary and Systemic Circuits

Circuit	Pressure
Pulmonary arteries	15 mm Hg
Pulmonary veins	5 mm Hg
Systemic arteries	120 mm Hg (systolic), 80 mm Hg (diastolic)
Arterioles	80–35 mm Hg
Capillaries	35–15 mm Hg
Venules	15–5 mm Hg
Veins	5–0 mm Hg

Venous Return Mechanisms

Mechanisms

• Valves prevent backflow

• Smooth muscle contraction

• Skeletal muscle pump

• Respiratory pump

Varicose Veins

Varicose veins are dilated, bulging veins caused by decreased venous return, often due to pregnancy, prolonged standing, obesity, or genetics. Blood pools in the lower limbs, stretching the veins.

Blood Pressure Regulation

Short-Term Maintenance

• Nervous system and endocrine system adjust resistance and cardiac output.

• Baroreceptor reflex and chemoreceptor stimulation play key roles.

• Hormones: Epinephrine, norepinephrine, thyroid hormone, angiotensin-II, ADH, ANP.

Long-Term Maintenance

• Endocrine and urinary systems adjust blood volume.

• Renin-angiotensin-aldosterone system regulates sodium and water retention.

• ADH increases water retention; ANP promotes water and sodium excretion.

Disorders of Blood Pressure

Hypertension

• May be acute or chronic

• Associated with coronary artery disease, stroke, heart failure, dementia, kidney disease, vascular disease

• Often called "the silent killer" due to lack of symptoms

• Treatment: Lifestyle modifications and medications

Hypotension

• May be chronic or acute

• Severe hypotension leads to circulatory shock, loss of consciousness, organ failure, death

• Causes: Blood loss, fluid loss, decreased cardiac output, excessive vasodilation (anaphylactic or septic shock)

Capillary Structure and Function

Types of Capillaries

Type	Structure	Location	Function
Continuous	Tight junctions	Skin, nervous tissue, muscle	Least leaky; narrow range of substances cross
Fenestrated	Small pores (fenestrations)	Kidneys, endocrine glands, small intestine	Moderately leaky; large volumes and larger substances cross
Sinusoidal	Large pores, irregular basal lamina	Liver, lymphoid organs, bone marrow, spleen	Leakiest; large substances and cells cross

Capillary Exchange Mechanisms

• Diffusion

• Transcytosis

• Bulk flow (filtration and reabsorption)

Pressures at Work in a Capillary

Hydrostatic and Osmotic Pressure

• Hydrostatic Pressure (HP): Drives water out of capillary

• Osmotic Pressure (OP): Draws water into capillary

Net Filtration Pressure (NFP):

$\text{NFP} = \text{Filtration} - \text{Reabsorption}$

Overall NFP drives water out by filtration, with 2–4 liters lost daily and returned by the lymphatic system.

Edema

Edema is the accumulation of excess water in interstitial fluid. Peripheral edema affects legs and feet; ascites affects the abdomen. Causes include increased hydrostatic pressure or decreased osmotic pressure.

Cerebrovascular Accident (CVA)

Types and Risk Factors

• Ischemic: Blockage of brain artery by clot; treated with clot-dissolving medications

• Hemorrhagic: Ruptured cerebral artery; treated with surgery

• Risk factors: Hypertension, atherosclerosis, diabetes, smoking, hypercholesterolemia, atrial fibrillation

• Symptoms: Sudden paralysis, loss of vision, difficulty speaking

The Big Picture of Blood Vessel Anatomy

Summary

Blood vessels are integral to the cardiovascular system, providing distribution, exchange, and collection functions. Their structure and function are closely tied to their role in maintaining blood flow, pressure, and tissue health.

*Additional info: Academic context and explanations were expanded for clarity and completeness. Tables were recreated and images included only when directly relevant to the paragraph content.*