Vascular System: Blood Vessels and Structure

Vascular System Overview

• The vascular system is comprised of two systems: the blood vascular system and the lymphatic vascular system.
• The blood vascular system provides a continuous flow transporting blood to and from the heart.
• The lymphatic vascular system aids with immunity and the regulation of interstitial fluid.
• The vascular system facilitates the exchange of gases, nutrients, and metabolic wastes across vessel walls.

Blood Vessel Types

• Arteries carry blood away from the heart.
• Veins carry blood towards the heart.
• Capillaries are the site of exchange between blood and tissues.

Structure of Blood Vessel Wall

• Blood vessels have three layers or tunics: tunica intima/interna, tunica media, and tunica adventitia/externa.
• The tunica intima is the smooth inner layer that repels blood cells and platelets, lined by endothelium.
• The tunica media is the middle layer, typically the thickest, with smooth muscle, collagen and some elastic fibers facilitating vasomotion.
• The tunica externa is the outermost layer of loose connective tissue.
• Smaller vessels may lack all three tunics.
• The smooth muscle of the tunica media is controlled by the autonomic nervous system (ANS).
• Stimulation of the sympathetic system releases norepinephrine, leading to vasoconstriction in vessels of the skin, GI, reproductive and urinary tracts, but vasodilation in coronary, brain and skeletal muscle vessels.
• Stimulation of the parasympathetic system releases acetylcholine in vessels of the skin, GI, reproductive and urinary tracts.

Arterial System and Types of Arteries

• There are four types of arteries: conduction or elastic, distributing or muscular, resistance or arterioles, and metarterioles.
• Conducting (elastic) arteries, such as the pulmonary artery, aorta and common carotid, are the largest and contain perforated sheets of elastic tissue in the tunica media.
• These arteries expand during systole and recoil during diastole, lessening fluctuations in blood pressure.
• Distributing (muscular) arteries distribute blood to specific organs, with smooth muscle layers constituting 3/4 of the wall thickness, such as the femoral and splenic arteries.
• Resistance (arterioles) arteries control the amount of blood within various organs.
• Metarterioles are short vessels connecting arterioles to capillaries.
• Smooth muscle cells form a precapillary sphincter at the entrance to the true capillary.

Types of Capillaries

• Continuous capillaries occur in most tissues, with endothelial cells having tight junctions and intercellular clefts for solute passage.
• Fenestrated capillaries are located in the kidneys and small intestine.
• They facilitate rapid absorption or filtration.
• They have filtration pores spanned by a thin glycoprotein layer that only allows passage of small molecules.
• Sinusoids are found in the liver, bone marrow, and spleen and have irregular blood-filled spaces with large fenestrations allowing proteins and blood cells to enter, and are dilated and continuously open to blood flow.

Capillary Beds and Precapillary Sphincters

• A capillary bed consists of a metarteriole, true capillaries with precapillary sphincters, thoroughfare channel, and post-capillary venules.
• Precapillary sphincters regulate blood flow into the true capillaries.
• Three-quarters of the capillaries are closed at any given time.
• Precapillary sphincters are regulated by vasomotor nerve fibers of the autonomic nervous system, heat, and local chemicals like O2, CO2, and pH.

Capillary Exchange

• Capillaries regulate fluid, electrolyte and nutrient exchange.
• The balance of forces on the arteriole and venous ends of the capillaries include hydrostatic and colloid osmotic pressures.
• Hydrostatic pressures include capillary blood pressure (outward) and interstitial hydrostatic pressure (inward).
• Colloid osmotic pressures include blood COP (inward, due to albumin) and interstitial COP (outward).
• Outward filtration force on the arterial side is 13 mmHg, while inward reabsorption force on the venous side is 7 mmHg.
• The excess filtration force is 6 mmHg.
• About 3.6L/day of volume must enter the lymph vessels to return to the circulatory system.

Venous System

• There are six types of veins: large, medium, small, venules, post-capillary venules, and venous sinuses.
• Veins, contrasted with arteries, have lower blood pressure at 10 mmHg.
• They possess thinner walls, less muscular and elastic tissue, but expand easily and have high capacitance.
• Valves aid skeletal muscles in upward blood flow.

Circulatory Routes

• The classic route is the simplest pathway: heart → arteries → arterioles → capillaries → venules → veins.
• In the portal system, blood flows through two consecutive capillary networks before returning to the heart like in the hypothalamus (anterior pituitary), kidneys or liver.
• The hepatic portal system picks up nutrients from the intestinal tract and carries them directly to the liver to be processed.

Anastomoses

• Anastomoses is the point where two blood vessels merge.
• Arteriovenous anastomosis is when an artery flows directly into a vein.
• Venous anastomosis is the most common and less serious form with alternate drainage of organs.
• Arterial anastomosis provides collateral circulation, such as in the coronary arteries.

Blood Pressure Basics

• Blood pressure is the force exerted by blood against the blood vessel wall, generated by heart contractions creating hydrostatic pressure.
• Blood is pushed down the vessel towards an area of lower pressure.
• Driving force for blood flow is a difference in blood pressure (ΔP) between different areas.
• In the systemic loop, ΔP is the pressure difference between the start of the aorta and the right atrium (95 - 5mmHg).
• The greater the ΔP, the greater the flow, which translates to Flow = ΔP/R.
• Blood pressure is mainly determined by the amount of blood flowing through the vessels and the resistance of the vessels to this flow.
• It is typically measured at the brachial artery of the arm and is pulsatile.
• Systolic pressure is the blood pressure during ventricular systole.
• Diastolic pressure is the blood pressure during ventricular diastole.
• Pulse pressure is the difference between systolic and diastolic pressure.
• A normal value for a young adult is 120/80 mm Hg.

Mean Arterial Pressure (MAP)

• Mean arterial pressure (MAP) is measured at given intervals in the cardiac cycle.
• The best estimate of MAP is Diastolic pressure + (1/3 of pulse pressure).
• Arterial elasticity is important for maintaining steady blood flow throughout the cardiac cycle and smoothing out pressure fluctuations and stress on small arteries.
• BP rises with age, as arteries become less distensible.
• Venus ΔP is 18 – 2 = 16.
• Venus pressure aids by: pressure gradient, skeletal muscle pump in limbs and thoracic pump.

Mechanisms Supporting Venous Return

• The pressure gradient from venules to right atrium.
• Inhalation reduces thoracic pressure and increases abdominal pressure (thoracic pump).
• This causes blood to be driven upwards
• Central venous pressure also fluctuates.
• 2 mmHg during inhalation, 6 mmHg during exhalation
• Blood flows faster with inhalation.
• Cardiac suction involves the expansion of the atrial space which increases volume and decreases pressure ultimately = ΔP = increased flow.
• Congestion is distention of veins due to elevated venous (and capillary) blood pressure, which increases blood pressure and edema due to increased filtration forces.
• Venous obstruction caused by compression, thrombosis, and heart failure, increases fluid filtration from the capillaries
• This excess fluid accumulates in tissue and may lead to edema.
• Ascites is defined as fluid accumulation in the abdominal cavity.
• Tissue necrosis occurs if oxygen delivery and waste removal are impaired.
• Pulmonary edema can cause suffocation.
• Cerebral edema is associated with headaches, nausea, seizures, and can lead to coma.
• Circulatory shock has reduced tissue fluid and impaired return of blood in veins

Lymphatic System

• The components of the lymphatic system include lymph, lymphatic vessels (lymphatics), lymphocytes/phagocytes/other immune cells, and lymphoid tissues and lymphoid organs.
• Lymph is a fluid similar to plasma but does not contain plasma proteins.
• Lymphatic vessels (lymphatics) are the network that carries lymph from peripheral tissues to the venous system.
• The function of the lymphatic system is to: Collect fluid and solutes lost by the capillaries, about 3.6 L/day, distribute hormones/nutrients/waste products and protection against disease from production/maintenance and distribution of lymphocytes.
• Lymphatic vessels return fluid lost from the capillaries to the venous system while they also transport hormones, nutrients and waste products.
• Lymphatic vessels are formed from buds coming off the veins, it begins with lymphatic capillaries including terminal lymphatics
• Distribution of Lymphatic vessels is originating from most capillary beds. Bone teeth, bone marrow, Nervous system, and cerebral spinal fluid lack lymphatic capillaries

Difference of Lymphatic Capillaries

• Lymphatic capillaries differ from blood capillaries since:
  • lymphatic capillaries start as pockets, and originate in the capillary beds
  • lymphatic capillaries have larger diameters, and thinner walls
  • Blood capillaries are flat or irregular in section
• overlapping endothelial cells of lymphatic capillaries is loosely bounded together serving as one way valves
  • the one-way valves prevent backflow into intercellular space

Lymphedema

• Lymphedema Results from blockage of lymph drainage from a limb
• Lymphedema Causes severe swelling, and Interferes with immune system function
• Results from tumors, excess lipid accumulation in fat cells in obese people, inflammatory destruction of connective tissue, surgical or radiation therapy damage to lymphatic system
• Elephantiasis is a term used to describe lymphedema caused by parasitic worm infections

Most Frequent Vascular System Problems

• Atherosclerosis presents as lipid, calcium, and fibrous deposits in the intima of large and medium sized arteries.
  • Most significant disease in the US in terms of death and morbidity
  • Blockage of the coronaries and vessels serving the brain are the most dangerous
    • Myocardial infarcts (heart attacks) leading cause of death
    • Cerebral infarcts (strokes) fifth most common cause of death
• Thrombi is a Blood clot formation that is be anchored to vessel wall, and a Deep vein thrombosis of legs is common particular in bedridden
• Emboli is defined as When pieces of a thrombus break off and and Can travel from the leg to the lungs, which is a pulmonary embolism.
  • May result in gradual development of pulmonary hypertension

Most Frequent Serious Vascular Problem with Varicosities

• Varicosities are Permanently dilated venous channels due to incompetent valves
  • Commonly caused by elevated venous pressure
• Common in leg veins = varicose veins
• Common in veins of anus =
• Problems:
  • Discomfort, Disfigurement, Edema, Ulceration, formation of thrombi
• Hypertension can be asymptomatic but present with headaches.
  • Most hypertension cases are idiopathic (of unknown cause)
  • If untreated it can lead to: Heart failure, Kidney disease, Myocardial infarcts, and Cerebral infarcts

General Symptoms, Signs, and Tests For Vascular Problems

• Occlusive disease of arteries, Include thrombi, emboli, atheromas (accumulation of degenerative material in the intima), and inflammatory occlusion, Is marked by Symptoms that may be sudden = infarction or gradual = ischemic atrophy where either results in loss of function of the organ affected
• Arteriography or angiography are both procedures that introduce radio-opaque substances into the vessels to observe it's distribution
• Sphygmomanometer is a Non-invasive monitor of blood pressure

Specific Diseases of the Vasculature

• Genetic/Developmental diseases include: Congenital variations in the vascular system and/or Angiomas
  • Angiomas are blood proliferation with the following properties
    • Hemangiomas: Local proliferations of capillaries that may be present at birth
    • Lymphangiomas: Small "birth mark" to large “port wine stains", Rare causes problems
• Inflammatory/Degenerative diseases are usually caused by arteriosclerosis, which is where there is Hardening of the arteries and loss of elasticity
  • generic term referring to thickening of the tunica intima and tunica media of small arterioles
  • Includes Atherosclerosis plaque formation.

Atherosclerosis

• A degenerative condition that gets worse with age
• the development of fibrofatty plaque in the intima of elastic and mostly muscular arteries
• Plaque is termed as atheromatous plaque
• Risk factors include genetics, toxins, cigarette smoke, diabetes, HTN, or autoimmune disorders.
• Damaged endothelium allows fats to be deposited underneath the intmia.

Hypertension

• High blood pressure presents with Upper limits of normal, being Systolic 120-140 mm Hg and normal Diastolic is 80-90 mm Hg
• Over 50 million Americans with hypertension
• Increase risks of developing of arteriolsclerosis
• Essential Hypertension are defined as Also termed primary or idiopathic, No known cause or Is multifactorial involving genetic and/or environmental factors and Higher rate in African Americans , 90% of all cases of hypertension
• Salt can influence hypertension.
• With these conditions hypertension occurs due to. Reduced blood flow at the Kidney triggering renin release and Reduced ability of water and salt regulation.
• Caused by kidney failure.
• Hormonal regulation of blood pressure is often Renin/Angiotensin II driven, Works to increase blood pressure
• Renin causes drop in renal blood pressure and increased sympathetic blood vessels
• Increases water retention.

Vasculitis

• General term for uncommon diseases and features inflammation of the blood vessels that Is caused by a certain patterns of distribution of blood vessel
• Do not know what causes most types of vasculitis and is thought to be autoimmune.
• Vasculitis Results in is a small to large infarcts of areas supplied by the inflamed vessel
  • This Can lead to renal failure, loss of vision

Thrombophlebitis

• phlebitis or vein inflammation related to a thrombus (blood clot)
• Migrans, occurs at different locations that Are caused increased the chance of blood clotting and long periods between movement.
• States such as pregnancy
  • Heparin
  • Warfarin
  • Surgery Complications include : pulmonary embolism

Functional vascular disease

• Includes Raynaud's, ray-NOHZ, is causes an autoimmune hyperactivation of in in your fingers and toes in response to cold temperatures
  • More common in women.
• Often secondary Raynauds is caused by Lupus, RA, Sclerderma.
  • Carpal Tunnel or smoking can worsen symptoms.

The effects and Symptoms of Aneurysm

• Organ Failure causes Shock a life threatening failure of blood pressure that causes the death of tissue; can be caused by the cardiovascular system, is marked by low blood pressure and rapid heart pressure.
• Ischemic Shock which affects the heart, cause and death.
  • Caused by reduced blood supply, or allergic reaction.
• To the lungs which damages endothelially cells and may lead to oedema or other damage to to the internal organs that leads to internal failure, two progressive and fatal conditions resulting from prolong and rapid.