Blood Vessel Types and Tunics

Blood Vessel Types

• Arteries transport blood away from the heart, typically oxygen-rich blood, towards capillaries.
• Veins transport blood from capillaries back to the heart, usually oxygen-poor blood.
• Capillaries are microscopic vessels with thin walls that enable substance exchange between blood and tissues, connecting the smallest arteries and veins.

Tunics (Layers) of Arterial and Venous Walls

• Arteries and veins have three layers called tunics, while capillaries do not have these layers.
• The tunica interna (intima) is the innermost layer in direct contact with blood, consisting of endothelium (simple squamous epithelium) and areolar connective tissue.
• The tunica interna acts as a permeable membrane and secretes chemicals for vessel diameter changes, and platelets adhere to it to form blood clots when damaged.
• The tunica media is the middle layer, usually the thickest, made of smooth muscle, elastic tissue, and collagen fibers, which contracts to regulate vessel diameter.
• The tunica externa (adventitia) is the outermost layer, made of connective tissue that anchors the vessel and provides a pathway for lymphatics, nerves, and small blood vessels.
• The inner half of vessel walls is nourished by diffusion from blood in the lumen.
• The outer half of vessel walls are supplied by a network of small blood vessels, called the vasa vasorum.

Arteries

• Arteries are resistance vessels that withstand high pressure from the heart's ejection of blood.
• Blood pressure in arteries fluctuates and pulsates.
• There are different types of arteries.

Elastic Arteries

• Elastic arteries, also known as conducting or large arteries, are the largest arteries (2.5-1.0 cm in diameter).
• The tunica media in these arteries comprises 40-70 layers of elastic tissue alternating with smooth muscle and collagen fibers.
• These vessels stretch when blood is ejected from the left ventricle and then recoil to their original diameter.
• Examples of elastic arteries include the aorta, pulmonary trunk, common iliac arteries, and subclavian arteries.

Distributing Arteries

• Distributing arteries, also known as muscular or medium arteries, are smaller branches (1.0 cm – 0.3 mm) that deliver blood to specific organs or body regions.
• Examples of distributing arteries include the femoral, renal, splenic, and brachial arteries.
• These arteries have up to 40 layers of smooth muscle in the tunica media, which is proportionally thicker than in elastic arteries.
• The extra muscle enables vasodilation and vasoconstriction to regulate blood distribution.

Resistance Arteries

• Resistance arteries, or small arteries, vary in location and number and do not have individual names (300-10 μm).
• Arterioles are the smallest resistance arteries.
• These vessels deliver blood to capillaries where gas and nutrient exchange occurs.
• Arterioles control blood flow to organs and tissues.
• Metarterioles directly connect arterioles to venules in some areas, bypassing capillaries.

Clinical Correlation: Aneurysm

• An aneurysm is a weak point in a vessel wall, usually an artery, forming a bulging sac.
• Aneurysms can put pressure on nerves, airways, and adjacent tissues.
• The primary risk is rupture, leading to severe internal bleeding.
• Aneurysms commonly occur in the abdominal aorta, renal arteries, and the Circle of Willis at the base of the brain.
• Genetic factors can cause a congenital weakness of the vessel wall.
• Arteriosclerosis (thickening and hardening of arterial walls) can result from fatty deposits, genetics, aging, and hypertension.
• Hypertension strains arterial walls, increasing aneurysm risk.
• Trauma can lead to aneurysm formation.
• Treatment includes monitoring, lifestyle changes, surgical repair, and endovascular repair with stents or grafts.

Arterial Sense Organs

• Arterial sense organs within artery walls monitor blood composition and pressure, relaying information to the brainstem.
• This information is used to regulate blood vessel diameter, heart rate, and respiration.
• Carotid sinuses are baroreceptors in the internal carotid arteries that monitor blood pressure changes
• Carotid bodies are chemoreceptors in the common carotid arteries to monitor blood composition (pH, O2, CO2).
• Aortic bodies are chemoreceptors in the aortic arch that function similarly to carotid bodies.

Capillaries

• This is where the exchange of nutrients, gases, and wastes occurs between blood and tissues.
• They connect the smallest arteries to the smallest veins.
• Types include continuous, fenestrated, and sinusoidal.

Continuous Capillaries

• Continuous capillaries are the most common type, found in skin, muscles, and the CNS.
• Endothelial cells are held together by tight junctions, creating a continuous lining.
• Small gaps allow the exchange of substances such as gases via simple diffusion.

Fenestrated Capillaries

• Fenestrated capillaries are found in tissues with high rates of filtration and absorption, such as the kidneys and small intestines.
• These vessels contain fenestrations, or pores, that allow for increased exchange of fluids and solutes.

Sinusoidal Capillaries

• Sinusoidal capillaries are characterized by irregularly shaped endothelial cells with large gaps.
• The large openings allow the passage of larger molecules such as proteins and formed elements.
• They are found in organs with high blood supply demands and where exchange of larger molecules is essential, such as the liver, spleen, and bone marrow.

Capillary Beds

• Capillary beds consist of a web of 10 to 100 capillaries.
• Capillary beds receive blood from a single arteriole and drain into a venule.
• Blood flow is regulated by arteriole constriction or dilation.

Veins

• Veins return blood to the heart.
• They are thin-walled and can easily expand, acting as capacitance vessels.
• Most circulating blood (~64%) is in veins, compared to arteries (13%).
• Veins have low, steady blood pressure and flow.

Postcapillary Venules

• These are the smallest veins (10-20 μm diameter) and collect blood from the capillaries.
• They are more porous than capillaries and participate in fluid exchange with tissues.

Muscular Venules

• Muscular venules receive blood from the postcapillary venules.
• Consist of only one or two layers of smooth muscle.

Medium Veins

• Medium veins range up to 10mm in diameter and are often named by the region they drain blood from.
• Many have venous valves to prevent backflow of blood.

Venous Sinuses

• These veins have very large walls and no smooth muscle and are not capable of vasoconstriction.
• They collect blood from various veins.

Large Veins

• These are the greatest in size (>10mm) and have smooth muscle in all three layers.

Clinical Correlation: Varicose Veins

• Varicose veins are enlarged, twisted veins that are often bulged and blue or dark purple.
• Blood tends to pool in the legs and stretch the veins as a result of standing for long periods of time.
• The stretching affects the venous valves’ ability to prevent backflow of blood, and the vessel walls weaken and develop into irregular dilations.
• Causes include valve malfunction, family history, age, gender, prolonged sitting or standing, obesity, and pregnancy.
• Hemorrhoids are varicose veins of the anal canal.
• Treatments include exercise, leg elevation, compression stockings, laser therapy, or ablation.

Routes of Blood

• The most common route of blood flow: heart → artery → arteriole → capillary → postcapillary venule → vein → heart.

Anastomoses

• Anastomoses are the merging of vessels other than capillaries.
• Arterial anastomoses converge two or more arteries to supply blood to the same area, such as the cerebral arterial circle (Circle of Willis).
• Venous anastomoses consist of one vein emptying directly into another vein.
• Arteriovenous anastomoses transport blood from an artery to a vein, bypassing capillary beds in areas like toes and ears.

Systemic Blood Vessels, The Aorta & Its Branches

• Blood leaves the heart and is distributed through all systemic arteries that arise from the aorta.
• The principal regions of the aorta are, the ascending aorta, aortic arch, and descending aorta.

Ascending Aorta

• This initial segment of the aorta gives rise to the left and right coronary arteries that supply the myocardium of the heart.

Aortic Arch

• Gives off three major arteries.
• The brachiocephalic trunk is the first branch.

  • It delivers blood towards the upper right limb.
    

  • The trunk bifurcates into the right common carotid artery, which supplies blood to the right neck and head, and the right subclavian artery, which supplies blood to the right arm and chest.
    

• The left common carotid artery supplies blood to the left side of the neck and head.
• The left subclavian artery supplies blood to the left arm and chest.

Descending Aorta

• The aortic arch turns inferiorly, becoming the descending aorta that passes through the thoracic and abdominal cavities.
• It is called the thoracic aorta when it is above the diaphragm and the abdominal aorta when it is below.

Arteries of the Head and Neck

• The common carotid arteries are major vessels that supply blood to the head and neck.
• Each common carotid artery branches into the internal carotid artery and the external carotid artery.
• The external carotid artery delivers blood to external structures of the face and cranium.

  • Some branches include the maxillary, facial, occipital, and temporal arteries.
    

• The internal carotid artery passes through the temporal bone into the cranial cavity.

  • This vessel supplies 80% of the cerebrum as well as the orbits.
    

• The internal carotid artery contributes to the circle of Willis (cerebral arterial circle) at the base of the brain.

  • This anastomosis connects major arteries, ensuring a constant blood supply to the brain.
    

• The vertebral arteries also supply structures in the head and neck and are part of the circle of Willis.

  • These arise from the subclavian arteries and travel up the neck through the cervical vertebrae.
    

Veins of the Head and Neck

• The head and neck are mainly drained by three pairs of veins, the internal and external jugular veins, and the vertebral veins.
• The internal jugular vein drains blood from most of the brain.

  • These vessels also pick up blood from other veins along the way such as the facial, superior thyroid, and superficial temporal veins.
    

• The external jugular vein drains blood from superficial facial and neck structures.
• The vertebral vein drains blood from the spinal cord, some deep muscles of the neck, and the cervical vertebrae.
• These drain into the subclavian veins, which then empty into the brachiocephalic veins.
• The brachiocephalic veins empty into the superior vena cava.
• The superior vena cava empties into the right atrium of the heart.

Arteries of the Upper Limb

• The upper limbs are supplied by one prominent artery.
• The artery changes its name along its course, starting with the subclavian artery.
• This blood flow is the same on the left and right upper limbs.
• The right subclavian artery arises from the brachiocephalic trunk, and the left subclavian artery arises directly from the aortic arch.
• These vessels pass under the clavicle and over the first rib, becoming the axillary arteries.
• The axillary artery continues through the axillary region (armpit) and continues as the brachial artery that continues down the middle of the upper arm and ends just distal to the elbow.

  • This artery is commonly used to measure blood pressure.
    

• The brachial artery branches at the elbow to form the radial and ulnar arteries.
• The radial artery descends along the forearm laterally while the ulnar branch descends the forearm medially.

  • These vessels will give rise to branches that will supply the hand.
    

• The radial artery is used to measure heart rate.

Veins of the Upper Limb

• The veins follow either superficial or deep paths.
• The blood flow is the same on both the left and right sides and is essentially the arterial blood flow in reverse.

Deep Veins

• These receive blood from the palmar region and fingers.
• The palmar region and fingers converge to form radial and ulnar veins that run alongside the radius and ulna, respectively.
• These vessels unit near the elbow to form the brachial vein.
• As this vessel continues up the brachium, it becomes the axillary vein just before the axillary region.
• This vessel continues toward the first rib and changes name to the subclavian vein.
• The subclavian vein continues posterior to the clavicle and meets the internal jugular vein of the neck.
• Here, the brachiocephalic vein is formed, and this pair of veins converge to form the superior vena cava.
• Blood directly empties into the right atrium.

Superficial Veins

• These travel through the subcutaneous tissue, and you may be able to see several of them through the skin of your forearm, hand, and arm.
• The cephalic vein travels up the lateral side of your forearm and is often where intravenous fluids are administered.
• The basilic vein travels along the back side of the forearm and runs deeper.

Arteries and Veins of the Thoracic Cavity

• The thoracic artery has several branches that provide blood to various structures throughout the thoracic cage.
• These arteries can be subdivided into the visceral branches or the parietal branches.
• The visceral branches consist of arteries that deliver blood toward the tissues around organs.

  • Examples include the pericardial, bronchial, esophageal, and mediastinal arteries.
    

• The parietal branches consist of arteries that deliver blood toward the more superficial tissues.

  • Examples include the posterior intercostal, subcostal, and superior phrenic arteries.
    

• Each artery is paired with a vein of the same name.
• Most of these veins drain into an unpaired vessel known as the azygos vein that drains into the inferior vena cava.

Arteries of the Abdominopelvic Region

• As the thoracic aorta descends through an opening in the diaphragm, its name becomes the abdominal aorta.
• This large artery has main branches that can be paired or unpaired.
• The inferior phrenic arteries are the first pair of arteries that branch from the abdominal aorta.

  • These supply the inferior surface of the diaphragm.
    

• The celiac trunk gives rise to arteries that will supply the upper abdominal organs.

  • The left gastric artery supplies the stomach and lower esophagus.
    

  • The splenic artery that supplies blood to the spleen.
    

  • The common hepatic artery that gives rise to branches that supply the liver, pancreas, stomach, and gall bladder.
    

• The superior mesenteric artery arises from the abdominal aorta and supplies the intestines.
• The renal arteries supply blood to the kidneys.
• The inferior mesenteric artery supplies the distal end of the intestines.
• The abdominal aorta forks at its lower end and gives rise to the common iliac arteries that supply blood to the lower limbs.

Abdominopelvic Veins: The Hepatic Portal System

• The most significant route of venous drainage from all structures below the diaphragm is the inferior vena cava (IVC).
• The body’s largest vein, this vessel collects deoxygenated blood from most abdominal viscera and lower limbs and drains into the heart’s right atrium.
• The kidneys and liver are the only organs that directly empty into the IVC.
• The hepatic portal system is a specialized group of veins that drain blood from digestive organs and transport it to the liver before emptying directly into the IVC.

  • This blood is oxygen-poor but nutrient-rich.
    

• After the digestive system has physically and chemically broken down ingested food, nutrients, and other substances are absorbed into the bloodstream through the small intestine and stomach walls.

  • This blood drains into a single vessel called the hepatic portal vein.
    

  • This vessel delivers the blood to the liver for processing, detoxification, and storing nutrients.
    

  • Once the blood has been processed, it leaves the liver through the hepatic vein, which drains into the IVC.
    

Arteries of the Lower Limbs

• The abdominal aorta forks at its lower end and gives rise to the common iliac arteries that supply blood to the lower limbs.
• The common iliac arteries branch into the external and internal iliac arteries.

  • The internal iliac branch supplies blood to the pelvic region.
    

  • The external iliac artery branches to form the femoral and deep femoral arteries.
    

• The femoral artery forms the popliteal artery at the knee and then branches to form anterior and posterior tibial arteries, and fibular arteries that supply blood to the foot.

Veins of the Lower Limbs

• The veins are subdivided into superficial and deep veins.
• The superficial dorsal venous arch drains into the great saphenous vein along the medial aspect of the legs.

  • This large vessel drains into the external iliac vein.
    

• Deoxygenated blood from the foot drains into vessels that lead to the fibular and tibial veins.

  • These vessels converge to form the popliteal vein.
    

• The popliteal vein drains into the femoral vein that merges with the deep femoral vein and great saphenous veins to form the external iliac vein.
• Blood from the external iliac vein drains into the common iliac vein that directly empties into the inferior vena cava.

Flow and Perfusion

• The circulatory system must deliver oxygen and nutrients to tissues at a rate that keeps up with metabolic needs to sustain life.
• Flow is the amount of blood flowing through a blood vessel, organ, or tissue within a given time.
• Perfusion is the flow per a given mass or volume of tissue, referring to the delivery of nutrients and oxygen to body tissues

Blood Pressure

• Blood pressure (BP) is the force exerted by blood against a vessel’s wall as it flows through.
• Blood pressure is often measured at the brachial artery with a device called a sphygmomanometer.
• It is usually expressed as a ratio of systolic pressure to diastolic pressure.
• Systolic blood pressure refers to the pressure in the arteries when the heart contracts and pumps blood into the circulation.
• Diastolic blood pressure represents the pressure in the arteries when the heart is at rest between beats.
• The normal blood pressure of a healthy adult is about 120/80 mm Hg (systolic is the top number, diastolic is the bottom number).
• The difference between the systolic and diastolic pressure is called pulse pressure.
• Mean arterial pressure (MAP) represents the average pressure in a patient’s arteries during one cardiac cycle.

Blood Pressure Cont.

• Chronic resting blood pressure higher than 140/90 is known as hypertension.
• Chronic low resting blood pressure is known as hypotension.
• Blood flows through capillaries and veins at a steady rate with little, if any, pulsation.
• Arteries become less distensible and absorb less force upon ventricular contraction as we age resulting in stiffening of arteries called arteriosclerosis.
  • The elastic tissues of the arterial wall deteriorate.
• Lipid deposits can also grow in arterial walls with age, giving the arteries a bonelike consistency as these deposits become calcified, a process known as atherosclerosis.
• Three factors physiologically determine blood pressure: resistance to flow, blood volume, and cardiac output.

Resistance to Flow

• Peripheral resistance is the opposition that blood encounters as it flows through smaller arteries and arterioles.
• It is a key determinant of blood pressure.
• Blood viscosity refers to the thickness of blood and is mainly determined by the concentration of albumin plasma proteins and erythrocytes.

  • When blood viscosity increases, the blood becomes thicker and more resistant to flow (moves more slowly).
    

• Longer vessels create more resistance than shorter ones.
• Blood vessel diameter can have a significant impact on blood flow.

Cardiac Output

• Cardiac output (CO) is the amount of blood pumped by the heart’s ventricles in one minute.
• Increased cardiac output raises blood pressure as more blood is pumped into the arteries.

Blood Volume

• Blood volume is the total volume of blood circulating in the body.
• With an increase in blood volume, the heart will pump a larger amount of blood into the arteries.
• This leads to increased pressure against the arterial walls.
• A decrease in blood volume tends to lower blood pressure.
• The kidneys play a big role in regulating blood volume, eliminating body fluids through urine or reabsorbing it into blood.
• The kidneys are part of a mechanism called the Renin-Angiotensin-Aldosterone System (RAAS) that regulates blood volume and blood pressure.

  • The kidneys release an enzyme called renin that triggers the production of angiotensin II.
    

Regulation of Blood Flow

• Regulatory mechanisms include local autoregulatory mechanisms, neuronal mechanisms, and hormonal mechanisms.

Local Control

• Autoregulation is the ability of tissues to monitor and regulate their own blood supply.
• Vasoactive chemicals such as histamine and prostaglandins stimulate vasodilation during trauma, inflammation, or exercise.
• Some tissues, especially hypoxic tissue, can increase their own perfusion by angiogenesis, or growth of new blood vessels.

Neuronal Control

• The central and autonomic nervous systems can exert control over blood vessels throughout the body.
• Most blood vessels are stimulated by sympathetic nerve fibers to constrict or dilate.
• A critical component of the autonomic nervous system called the vasomotor center plays a role in autonomic reflexes – baroreflexes, chemoreflexes, and medullary ischemic reflexes.
• The medulla oblongata contains a cardiovascular center, which is a group of neurons that regulate heart rate, contractility, and blood vessel diameter.
• Sympathetic innervation from this center by the cardiac nerves increases heart rate and contractility.
• Parasympathetic innervation by the vagus nerves decreases the heart rate.

Hormonal Control

• Different hormones can influence blood pressure and perfusion through vasoactive effects or regulation of water balance.
• Angiotensin II acts as a potent vasoconstrictor and increases blood volume by promoting water retention and stimulating the thirst center.

  • ACE-inhibitor medications can reduce this hormone, decreasing blood pressure and volume.
    

• Aldosterone (“salt-retaining hormone”) promotes the retention of sodium by the kidneys, which in turn promotes water retention.

  • This increases blood volume, leading to increased blood pressure.
    

• Natriuretic peptides are hormones released by the heart that increase sodium and water excretion, reducing blood volume and pressure.
• Antidiuretic hormone (ADH or Vasopressin) stimulates the kidneys to reabsorb water back into the blood, increasing blood volume.
• Catecholamines (epinephrine and norepinephrine) increase blood pressure by stimulating vasoconstriction.

Capillaries and Fluid Exchange

• Capillaries are the only vessels that can exchange substances between tissues and blood.
• The movement of materials through capillary walls involves three main mechanisms:

  • Diffusion: Small substances cross capillary walls via simple diffusion.
    

  • Transcytosis: Large molecules are transported in vesicles across the plasma membrane.
    

  • Filtration and Reabsorption: Fluid filters out from the arterial end of a capillary and osmotically reenters the capillary at the venous end.
    

• Capillary hydrostatic pressure facilitates the filtration of fluid and small molecules out of the capillaries and into tissues.
• Capillaries also experience colloid osmotic pressure due to the presence of proteins in the blood drawing fluid back into the capillaries.

Mechanisms of Venous Return

• The pressure within veins is so minimal that the heart relies on different mechanisms to maintain adequate venous return.
• Venous valves prevent blood from flowing backward.
• Skeletal muscle pump: Muscles contract, blood is moved towards the heart and not back into the capillaries through valves.
• Respiratory pump: During inhalation, pressure changes in the thoracic and abdominal cavities squeeze blood upward toward the heart.
• Gravity helps blood from the neck and head returns to the heart.

Clinical Correlations

• A thrombus is a blood clot that forms within a blood vessel and remains attached to the vessel wall.
• An embolus occurs when the clot or pieces of it breaks off and travels through the bloodstream and blocks smaller blood vessels.
• A transient ischemic attack (TIA) is a temporary disruption of blood flow to a part of the brain, resulting in temporary neurological symptoms.
• A cerebrovascular accident (CVA), or stroke, occurs when there is a sudden disruption of blood flow to the brain, leading to damage or death of brain cells.

Circulatory Shock

• Circulatory shock refers to any state in which cardiac output is insufficient to meet the body’s metabolic needs.
• Cardiogenic shock is caused by inadequate pumping by the heart, usually due to myocardial infarction.
• Low venous return (LVR) shock is caused by low cardiac output as too little blood is returned to the heart.
• Hypovolemic shock is produced by a loss of blood volume.
• Obstructed venous return shock occurs when any object compresses a vein and impedes blood flow.
• Venous pooling shock occurs when the body has a normal blood volume, but excess accumulates in the lower limbs.
• Neurogenic shock is a form of venous pooling due to a loss of vasomotor tone, resulting in vasodilation.

Circulatory Shock Cont.

• Septic shock occurs when bacterial toxins induce vasodilation and heightened capillary permeability.
• Anaphylactic shock arises from exposure to an allergenic antigen.

Edema

• Edema occurs when fluid accumulates in the interstitial space.
• Common causes include:
  • Increased capillary filtration via increased hydrostatic pressure or increased capillary permeability.
  • Kidney failure leads to water retention and hypertension.
  • Aging leads to greater permeability of capillaries.
  • Poor venous return prevents fluids from flowing back to the heart.
  • Reduced capillary reabsorption: A lack of albumin in blood plasma reduces the reabsorption of tissue fluid.
  • Obstructed lymphatic drainage blocks lymph return to the bloodstream and accumulates in tissue fluids.