Ch22 L10 Blood Vessels and Circulation

Introduction

  • Discussion on blood vessels and circulation

  • Overview of topics: blood flow from heart to lungs, heart to body, aging vessels

Histological Organization of Blood Vessels

  • Fundamental understanding required for all blood vessels (except capillaries)

  • Composition: three distinct layers, referred to as tunicas

Tunica Adventicia (or Tunica Externa)

  • Outermost layer of blood vessel

  • Composed of connective tissue sheath

  • Functions:

    • Anchors blood vessels to surrounding organs and structures

Tunica Media

  • Middle layer of blood vessel wall

  • Primarily composed of smooth muscle

  • Functions:

    • Regulates vessel diameter

    • Involvement in both vessel constriction and dilation

  • Vasodilation: Smooth muscle relaxation increases lumen diameter, allowing more blood flow

  • Vasoconstriction: Smooth muscle contraction decreases lumen diameter, restricting blood flow

Tunica Intima (or Tunica Interna)

  • Innermost layer of the blood vessel

  • Composed of:

    • Endothelium: a single layer of simple squamous epithelium lining the vessel

    • Provides a smooth surface for blood flow

    • Underlying areolar layer: a type of connective tissue

Distinction Between Arteries and Veins

  • General characteristics:

    • Arteries: Carry blood away from the heart

    • Veins: Carry blood towards the heart

    • Capillaries: Connect arteries and veins

Vessel Wall Differences

  • Arterial walls are thicker than venous walls

  • Reason: arteries contain significantly more smooth muscle in the tunica media

  • Arterial walls withstand higher blood pressures coming from the heart

Vessel Lumen Characteristics

  • Arteries maintain circular shape even when empty

  • Veins collapse and appear flatter when cut due to thinner walls

Endothelial Lining Distinctions

  • Arterial endothelial lining: features pleated folds from non-contractile epithelial cells, resulting from muscle contraction in the tunica media

  • Venous endothelial cells appear relatively flat, lacking these folds

Elastic Membranes

  • Arteries possess:

    • External elastic membrane between tunica media and tunica adventicia

    • Internal elastic membrane between tunica intima and tunica media

  • Functions of elastic components: allow arteries to stretch and recoil with each heartbeat

Presence of Valves

  • Many veins contain one-way valves to prevent backflow (especially in limbs)

  • Arteries do not possess valves due to higher blood pressure ensuring continuous forward flow

  • Muscle contractions surrounding veins help push blood past valves aiding its return to the heart

Types of Arteries

  • Blood flow from heart travels through progressively smaller arteries:

Elastic Arteries

  • Largest arteries (closest to the heart)

  • Examples: aorta, pulmonary trunk

  • Features: thick walls with high elastic fiber density

  • Function: tolerate pressure changes during cardiac cycles; expand and recoil

Muscular Arteries

  • Transition from elastic arteries

  • Examples: radial and ulnar arteries

  • Thicker tunica media with higher percentage of smooth muscle

  • Function: regulate blood flow to specific body regions through significant contraction and dilation

Arterioles

  • Much smaller than muscular arteries

  • Consist of thin or absent tunica adventitia

  • Tunica media: scattered smooth muscle cells

  • Role: feed into capillaries, crucial for regulating blood flow into capillary beds

Venous Circulation

  • Blood travels from tissues back to the heart via progressively larger veins:

Venules

  • Smallest veins collecting blood from capillaries

  • Resemble expanded capillaries, may lack a tunica media

Medium-sized Veins

  • Collect blood from venules

  • Examples: radial and ulnar veins

  • Have thin tunica media and larger adventitia

  • Presence of valves begins to show

Large Veins

  • Collect blood from medium-sized veins

  • Examples: superior and inferior vena cava

  • Very thick adventitia, thin tunica media and intima, often hard to distinguish

Capillaries

  • Smallest and most delicate blood vessels

  • Wall composition: 1-3 endothelial cells and underlying basal lamina

  • Diameter: small enough for red blood cells to pass in single file

  • Lack adventitia and media layers

  • Function: optimized for nutrient and gas exchange

  • Interconnected to form capillary beds for efficient exchange

Blood Volume Distribution

  • Total blood volume is distributed unevenly:

    • 65-70% within systemic venous system (veins)

    • 30-35% in arteries

  • Veins are more distensible, can expand more readily than arteries

  • Veins act as blood reservoirs, accommodating large blood volume changes

Functional and Structural Differences: Pulmonary vs. Systemic Circulation

  • Pulmonary circulation: Lower blood pressure than systemic

    • Shorter distance between heart and lungs, requires less pressure

    • Walls of pulmonary arteries thinner than systemic arteries

  • Systemic circulation: Higher pressure; arterial blood carries oxygenated blood away from the heart, while venous blood carries deoxygenated back to the heart

  • Pulmonary arteries: Carry deoxygenated blood to lungs

  • Pulmonary veins: Carry oxygenated blood back to heart

Specific Blood Vessel Patterns in the Body

  • Neck and limbs:

    • Presence of both deep and superficial veins

    • Superficial veins are easier to access for blood draws than deeper arteries

Specialized Systemic Circuits

Cerebral Arterial Circuit (Circle of Willis)

  • Represents critical arterial circle at base of brain

  • Designed for collateral circulation to minimize blockage impact

  • Components:

    • Anterior communicating artery

    • Anterior cerebral artery

    • Posterior communicating artery

    • Posterior cerebral artery

  • Function: provide vital connection between major arteries supplying the brain, ensuring alternative routes for blood flow

Hepatic Portal System

  • Begins in the capillaries of digestive organs (stomach, intestine, pancreas, spleen) and ends in the liver

  • Blood contains nutrients and toxins from digestion, requiring processing by the liver

    • Prevents nutrient flood into systemic circulation

    • Key veins: splenic vein, superior mesenteric vein, inferior mesenteric vein

Fetal Circulation

  • Distinct from adult circulation due to nonfunctional lungs/digestive system

  • Nutritional and respiratory needs met by placenta

  • Key vessels: umbilical arteries (carry mixed blood away from fetal heart) and umbilical veins (carry oxygenated blood into heart)

Bypasses in Fetal Circulation

  • Foreman ovale: connects right atrium to left atrium, bypassing pulmonary circuit

  • Ductus arteriosus: connects pulmonary trunk to aortic arch, allowing blood flow to systemic circulation

Changes in Cardiovascular System with Aging

  • General pattern: reduced efficiency and elasticity

  • Blood:

    • Decreased erythrocytes (red blood cells)

    • Blood pooling in veins, often due to less efficient valves

  • Heart:

    • Reduced efficiency translates to decreased cardiac output

    • Changes in pacemaker activity affecting heart rhythm

  • Blood Vessels:

    • Loss of elasticity

    • Can develop aneurysms: enlarged blood vessels due to weakened walls which may rupture

    • Aneurysm analogy: old hoses that burst under pressure

Conclusion

  • End of lecture for chapter 22 on vessels and circulation

  • Importance of noting names of vessels and circuits discussed

  • Reminder for students to reach out with questions

Varicose veins are caused by weakened valves and veins in the leg, leading to blood pooling. Factors contributing to this condition include:

  • Genetics: A family history of varicose veins increases risk.

  • Age: As people age, veins lose elasticity and valves may weaken.

  • Hormonal Changes: Hormonal fluctuations during pregnancy, menopause, or due to birth control pills can affect vein health.

  • Obesity: Excess weight puts additional pressure on veins.

  • Sedentary Lifestyle: Prolonged periods of standing or sitting can hinder blood circulation.

  • Injury: Damage to veins or valves from injuries may lead to varicosities.

Symptoms of varicose veins can include swelling, aching, and heaviness in the legs. Treatment options vary depending on the severity and may include lifestyle changes, compression stockings, and, in some cases, surgical procedures.

Case Study 1: Aging and Blood Vessel Elasticity

Patient Profile: An 82-year-old male with a history of high blood pressure and diabetes.
Clinical Observation: The patient experiences symptoms indicative of reduced blood flow, including fatigue and occasional dizziness when standing up.
Analysis:

  • Aging Effects: With aging, blood vessels lose elasticity due to structural changes in the tunica media, which is primarily composed of smooth muscle. This loss of elasticity leads to increased systolic blood pressure and possible diastolic dysfunction.

  • Diagnosis: Given the symptoms and medical history, the patient may be diagnosed with a form of vascular insufficiency potentially leading to orthostatic hypotension, where blood pressure drop occurs while standing, causing dizziness.

Case Study 2: Varicose Veins in Pregnancy

Patient Profile: A 34-year-old pregnant woman in her second trimester.
Clinical Observation: The patient reports swelling and visible bulging veins in her legs.
Analysis:

  • Hormonal Changes: Hormonal fluctuations during pregnancy, particularly increased levels of progesterone, weaken vein walls and valves, leading to conditions such as varicose veins.

  • Pressure Factors: The growing uterus exerts pressure on pelvic veins, further limiting venous return from the lower extremities.

    Varicose veins develop when small, one-way valves within leg veins weaken or fail, causing blood to flow backward and pool, which stretches and twists the veins. Common causes and risk factors include standing or sitting for long periods, pregnancy, obesity, aging, and family history.

  • Diagnosis: The patient may be diagnosed with pregnancy-related varicosities or chronic venous insufficiency, necessitating lifestyle alterations and possibly compression therapy.

Case Study 3: Sedentary Lifestyle and Venous Health

Patient Profile: A 45-year-old office worker with a sedentary job.
Clinical Observation: The patient complains of heaviness and swelling in the legs after long hours of sitting.
Analysis:

  • Impact of Sedentary Lifestyle: Prolonged periods of sitting can hinder venous return due to increased venous pressure in the legs, leading to symptoms of venous insufficiency.

  • Valvular Dysfunction: Inefficient function of venous valves contributes to symptoms of discomfort and swelling.

  • Diagnosis: The patient may be diagnosed with chronic venous insufficiency (CVI), which can lead to further complications such as varicose veins if lifestyle changes are not implemented.

Case Study 4: Cardiovascular Events Related to Obesity

Patient Profile: A 50-year-old female with a BMI of 32.
Clinical Observation: The patient experiences chest pain and shortness of breath during physical activity.
Analysis:

  • Obesity Impact: Excess weight impairs cardiac output and increases systemic blood pressure, raising the risk of cardiovascular disease.

  • Blood Vessel Changes: The arterial walls may show signs of atherosclerosis due to increased stress from elevated blood pressure and lipid levels.

  • Diagnosis: The patient's symptoms and profile may indicate the early onset of coronary artery disease (CAD), necessitating further cardiovascular evaluation and lifestyle modifications.

Case Study: Atherosclerosis in Middle Age

Patient Profile: A 55-year-old male with a history of high cholesterol and hypertension.

Clinical Observation: The patient reports chest pain during physical activity and has experienced episodes of shortness of breath. He has elevated LDL cholesterol levels and a family history of heart disease.

Analysis:

  • Pathophysiology of Atherosclerosis: The accumulation of lipids, inflammatory cells, and fibrous elements in the arterial wall leads to plaque formation in the arteries. This narrows the blood vessels and restricts blood flow, particularly during increased physical exertion.

  • Risk Factors: The patient’s high cholesterol, hypertension, and family history significantly contribute to the development of atherosclerosis.

  • Diagnosis: A non-invasive imaging test, such as an angiogram or ultrasound, may reveal narrowed coronary arteries, confirming a diagnosis of atherosclerosis, which puts the patient at greater risk for myocardial infarction (heart attack).

  • Management: Recommended interventions include lifestyle changes (diet and exercise), statin therapy to manage cholesterol levels, and monitoring for potential cardiovascular events.