Blood Vessels and Circulation

Structure of Vessels

• There are 5 general classes of vessels:
  1. Arteries: carry blood away from the heart.
  2. Arterioles: smaller branches of arteries.
  3. Capillaries: smallest vessels where gas, nutrient and waste exchange occur.
  4. Venules: small vessels that receive the blood from capillaries and begin to transport it back to the heart.
  5. Veins: large vessels that return blood to the heart.

Structure of Vessel Walls

• Both the walls of the arteries and veins have 3 distinct layers:
  1. Tunica intima: innermost layer that includes the endothelial lining and an underlying layer of connective tissue with elastic fibers. In arteries, there is an outer layer that contains a thick layer of elastic tissue called the internal elastic membrane.
  2. Tunica media: middle layer containing smooth muscle and loose CT that allows vasoconstriction and vasodilation. Separated by the surrounding tunica externa by a thin band of elastic fibers called the external elastic membrane.
  3. Tunica externa: outermost layer that is a CT sheath of elastic and collagen fibers. Thickest layer in veins.
• These layers contribute to the strength of the vessels and due to the thickness, they contain their own vessels to keep the cells alive called vasa vasorum.

Arteries vs. Veins

• Arteries have thicker walls.
• The tunica media of an artery contains more smooth muscle and elastic fibers.
• Arteries, due to the thicker walls, do not collapse like veins do when there is no pressure within the vessel.
• The endothelial lining of arteries cannot contract, so if an artery constricts, the endothelium is thrown into folds, giving a pleated appearance, unlike veins.
• Veins contain valves that prevent backflow of blood.

Arteries

• The thick walls of the arteries allow them to be resilient and accommodate the surges of pressure of the blood when pumped by the heart.
• The smooth muscle in the walls are controlled the sympathetic division of the ANS. Constriction of the smooth muscle (vasocontriction) decreases the lumen and blood flow (increased afterload). Relaxation of the smooth muscle (vasodilation) increases the lumen and blood flow (decreased afterload).

Types of Arteries

• Elastic arteries are large arteries (up to 1 inch in diameter), that transports large volumes of blood away from the heart (e.g.: aorta, pulmonary trunk).
• Very resilient due to high elastic fiber concentration in tunica media and low concentration of smooth muscle.
• Allows for “elastic rebounding” which allows it to contribute and perpetuate the pumping action of blood from the heart.
• Muscular arteries are medium sized arteries that distribute blood to the body’s skeletal muscles and internal organs.
• Have thick tunica media with high smooth muscle concentration.
• Examples external carotid, femoral, brachial arteries.
• Arterioles are small with poorly defined tunica externa and have more muscle than elastic arteries.
• Vasodilate when O_2 levels are low and vasoconstrict under sympathetic stimulation.
• Change in the diameter of the lumen affects the force required to push blood around the cardiovascular system. The more constricted the vessel the more pressure needed.
• The force opposing blood flow is called resistance (R), so arterioles are also called resistance vessels.
• Occasionally, local arterial pressure exceeds the capacity of the elastic components of the tunics producing and aneurysm, which is a bulge in the weakened wall of the artery.
• These can be very dangerous if they become large due to the possibility of rupture, internal bleeding, shock and death.
• Most serious locations are in the brain and the abdominal aorta.

Arteriosclerosis

• This is the thickening and hardening of the arterial walls.
• Two major forms of this disease:
  1. Focal calcification: deposition of Ca^{2+} salts following the degeneration of smooth muscle in the tunica media. Associated with diabetes and aging.
  2. Atherosclerosis: deposition of lipid in the tunica media (plaque) associated with damage to the endothelium. Increased with hypercholesterolemia, hypertension, smoking, diabetes, obesity, stress, Chlamydia, and birth control pills (estrogens slow plaque formation).
• Both forms decrease the lumen and may lead to coronary artery dz, MIs, and strokes.

Capillaries

• Microscopic vessels that weave through tissues forming extensive networks.
• The only blood vessel whose walls are permeable and allow exchange between the blood and the surrounding interstitial fluid of the tissues via passive or active transport.
• Consists of an endothelial tube inside a basal lamina. It lacks a tunica media and externa.
• The diameter is the size of a RBC.

Types of Capillaries

• Two types:
  1. Continuous capillaries: the endothelium is a complete lining. Located in all tissue except epithelium and cartilage. They permit diffusion of water, small solutes, and lipid-soluble materials (not RBC or plasma proteins).
  2. Fenestrated capillaries: contains pores in the endothelium that permit rapid exchange of water and large solutes. Found in the choroid plexus and endocrine organs.

Capillary Beds

• The network of capillaries is called a capillary bed, where a single arteriole gives rise to dozens of capillaries that empty into several venules.
• It is here where the arterial system and venous system meet forming an anastomosis.
• If one of the capillaries in the capillary bed is blocked, there are multiple other routes the blood can take. However, if a tissue becomes hypoxic, angiogenesis (formation of new blood vessels) can occur.

Veins

• Venules are the smallest type of veins and begin at the capillary beds. They may lack a tunica media and contain valves.
• Medium-sized veins are comparable in size to muscular arteries. The tunica media is thin but the tunica externa is very thick containing much elastic and collagen fibers. Contains valves.
• Large veins include the superior and inferior venae cavae and have all three layers.

Blood Flow

• Capillary blood flow is determined by the interplay between pressure (P) and resistance (R) in the cardiovascular system.
• To keep blood flowing, the heart must overcome the resistance to blood flow in all of the vessels.
• Blood flow is directly proportional to blood pressure and inversely proportional to resistance (based on friction between the endothelium and blood, vessel length, diameter, blood viscosity and turbulence).
• An addition of one pound of fat to the body mass increases the length of one’s blood vessels by 200 miles.

Arterial Blood Pressure

• The arterial pressure is much larger than that of the venous because in maintains blood flow through the capillary beds.
• It decreases as vessels extend further from the heart.
• Atrial blood pressure is highest during ventricular systole (systolic pressure) and least during ventricular diastole (diastolic pressure).
• When recording a blood pressure, the systolic pressure is placed over the diastolic pressure. Normal range is 90/60 to 140/90 mmHg.
• A pulse is a rhythmic pressure oscillation that accompanies each heartbeat. Normal pulse rate is 60-90 beats/minute.
• Hypertension is pressure over 140/90. Hypotension is abnormally low blood pressure.
• Tachycardia is a pulse over 90/min and bradycardia is a pulse under 60/min.

Venous Pressure and Return

• Venous pressure is very low compared to arteries.
• However, because there is very little resistance in this system, due to convergence of the veins, as well as the presence of valves, blood can return to the right atrium (venous return).
• Two factors also contribute to venous return:
  1. Muscular compression: contractions of the skeletal muscles surrounding the veins help push blood toward the heart.
  2. Respiratory pump: as we inhale, the thoracic cavity expands pulling air into the lungs and blood from smaller veins into the inferior vena cava. When we exhale, as air is forced out of the lungs, blood is pushed from the inferior vena cava into the right atrium.
• Both are important especially during exercise.

Capillary Exchange

• Necessary to maintain homeostasis and is responsible for providing the body’s tissues with the materials they need to live and removing their waste.
• Three steps:
  1. Diffusion: can occur through adjacent endothelial cells or pores of the fenestrated capillaries (water, ions, glucose, amino acids, urea, lipids, O2, CO2), or through ion channels in the plasma membrane of the endothelial cells (Na^+, K^+, Ca^{2+}). Large substances such as RBCs or plasma proteins are normally unable to diffuse out.
  2. Filtration: this is the retention of solutes as a solution flows across a porous membrane. This occurs with large solutes that cannot cross the endothelium. The driving force is the blood pressure within the capillary beds called hydrostatic pressure.
  3. Reabsorption: occurs as osmosis, where a solvent (water) diffuses to an area of higher solute concentration.
• All three are important for the bidirectional flow of materials across the capillary walls. If there is a disruption in homeostasis, edema occurs (abnormal accumulation of interstitial fluid).

Cardiovascular Regulatory Mechanisms

• Depending on the situations or activity, certain tissues of the body may need and increase of blood flow in order to deliver the necessary nutrients and O_2 it demands (tissue perfusion).
• In order to regulate which parts of the body receives more blood and which parts less, the body depends on three regulatory mechanisms to control the cardiac output (CO) and blood pressure (BP).

Regulatory Mechanisms

• Autoregulation: local factors or chemical changes in the interstitial fluid may have an effect on sphincters found within the capillary beds. These factors may be vasodilators (decreased tissue O_2, lactic acid formation, histamine, increased temperature), or vasoconstrictors (prostaglandins, thromboxanes).
• Neural mechanisms: adjusts cardiac output and peripheral resistance by controlling the force and rate of the heart contraction, vasoconstriction and vasodilation. Located in the medulla oblongata and influenced by the ANS. Also acts in response to baroreceptors stimulation in the walls of the carotid arteries, aortic arch and right atrium. If BP climbs, the baroreceptors stretch, triggering the CV centers in the medulla to decrease heart contraction force and rate and cause widespread peripheral vasodilation. If BP falls, the opposite occurs.
• Neural mechanisms also include chemoreceptor reflexes. Chemoreceptors located in the walls of the carotid arteries and aorta, detect levels of CO2, O2 or pH levels in the blood. If there is a rise in CO2, drop in O2 or fall in pH, the CV centers increase force and rate of heart contraction and peripheral vasoconstriction. The respiratory center, also in the medulla will be triggered to increase the rate and depth of respiration.
• Endocrine mechanism: CO and BP can also be influenced by certain hormones. If BP is too high, atrial natriuretic peptide is released by the fibers in the right atrial wall and increases urine production by the kidneys. If the BP is too low, ADH (from neurohypophysis), angiotensin II and erythropoetin (from kidneys) are produced.

CV Response to Exercise

• Exercise results in extensive vasodilation in order to deliver needed O_2 and nutrients to skeletal muscle. Venous return increases due to the skeletal muscle contractions and increased respiratory rate, and CO increases due to the rise in the venous return and stretch of the atria (Frank-Starling principle).
• Benefits of CV exercise decreases cholesterol levels (CAD, atherosclerosis, stroke), reduces stress, lowers BP and plaque formation.

Pulmonary Circuit

• As the right ventricle receives deoxygenated blood from the right atrium, it then pumps this blood through the pulmonary semilunar valve into the pulmonary trunk, which curves over the superior border of the heart. The pulmonary trunk diverges into the left and right pulmonary arteries, which still is carrying deoxygenated blood. These arteries then continue to branch into smaller and smaller arteries that become pulmonary arterioles and capillaries that surround each microscopic alveolus, where gas exchange occurs and the blood becomes oxygenated.
• The capillaries then become venules that converge to form larger vessels that ultimately become the two right pulmonary veins and two left pulmonary veins. These four veins are delivering oxygenated blood to the left atrium, which will be pumped into the body via the systemic circuit by the left ventricle.

Systemic Circuit

• The left ventricle pumps the oxygenated blood through the aortic semilunar valve into the ascending aorta. The aortic arch curves over the superior surface of the heart connecting the ascending aorta to the descending aorta.
• 3 elastic arteries originate from the aortic arch and deliver blood to the head, neck, shoulders and upper limbs: brachiocephalic trunk, left common carotid a., left subclavian a. The brachiocephalic trunk then divides to form the right subclavian a., and right common carotid a.

Subclavian Arteries

• From these arteries, the right and left vertebral aa. branch superiorly and supply blood to the brain and spinal cord. At the medulla the two vertebral aa., fuse to form the basilar a.
• As the r/l subclavian aa. travel laterally, they exit the thoracic cavity and pass across the superior border of the 1st ribs and become the right and left axillary aa. These arteries then crosses the axillae and become the right and left brachial aa., which supplies the upper limbs. As these arteries approach the coronoid fossae of the humerus, each artery divides into the radial a., which follow the radius, and the ulnar a., which follows the ulna to the wrist.

Radial and Ulnar Arteries

• These arteries supply the forearm.
• At the wrist, these arteries fuse to form the superficial and deep palmar arches, which supply blood to the hand and the digits.

Common Carotid Arteries

• These arteries ascend the neck and each divides into an external and internal carotid artery.
• The external carotid aa., supply blood to the neck, esophagus, pharynx, larynx, lower jaw and face.
• The internal carotid aa., ascend to the brain and supply the anterior half of the cerebrum, while the basilar a, supplies the posterior half of the cerebrum. Branches of the internal carotid aa., and branches of the basilar a., interconnect in a ring-shaped anastomosis called the circle of Willis, which encircles the pituitary gland and the infundibulum. This ensures blood supply to the brain, even if one of the arteries is compromised (CVA).

Descending Aorta

• As the descending aorta passes through the thoracic cavity, it is called the thoracic aorta. As it crosses the diaphragm, it enters the abdominal cavity. Here it is called the abdominal aorta.
• The abdominal aorta gives rise to three unpaired arteries listed from most superior to inferior:
  1. Celiac trunk: that delivers blood to the liver, stomach, and spleen. It then divides into three branches:
     • Left gastric a., that supplies the stomach and inferior esophagus.
     • Splenic a., that supplies the spleen, stomach and pancreas.
     • Common hepatic a., that supplies the liver, gall bladder, stomach, and duodenum.
  2. Superior mesenteric a., supplies the pancreas, duodenum, small intestine, and most of the large intestine.
  3. Inferior mesenteric a., supplies the distal colon and rectum.

Abdominal Aorta

• Inferior to the unpaired aa., the right and left renal aa., arise and travel laterally to the adrenal glands and the kidneys.
• At the L4 level, the descending aorta splits to form the right and left common illiac aa., which carry blood to the pelvis, lower limbs and portions of the intestines.
• At the lumbosacral joint, the common iliac aa., divide to form an internal and external iliac a. The internal iliac a., supply the urinary bladder, pelvis, genitalia, medial thigh and uterus.

External Iliac Arteries

• These arteries exit the abdominopelvic cavity and enter the anteromedial thigh, becoming the femoral a.
• The femoral a., continues inferiorly and posteriorly to the knee. At the popliteal fossa the femoral a., becomes the popliteal a., which then branches into the posterior tibial a., that supplies the posterior lower leg, and anterior tibial a., that supplies the anterior lower leg.
• When it reaches the ankle, the anterior tibial a., becomes the dorsalis pedis a., which supplies the foot and ankle.

Systemic Veins

• Another significant difference between arteries and veins is that arteries in the neck and limbs are located deep beneath the skin, protected by bones and soft tissues.
• Veins located in these areas generally have two sets: one superficial and one deep. This dual venous drainage is important for controlling body temperature: in hot weather, venous blood flows through the superficial veins where heat loss can occur; in cold weather blood is routed to the deep veins to minimize heat loss.

Superior Vena Cava

• This is a great vein that drains into the right atrium and receives blood from the tissues and organs of the head, neck, chest, shoulders and upper limbs.
• In the cranium, veins drain deoxygenated blood from the brain into dural sinuses, spaces between the two layers of dura mater.

Cranial Sinuses

• The largest sinus is the superior sagittal sinus found in the falx cerebri in the longitudinal fissure. This sinus receives blood from the majority of the cerebrum.
• Posteriorly, this sinus is continuous with the straight sinus located between the cerebrum and cerebellum and receives blood from the inferior cerebrum.
• The superior sagittal and straight sinuses empty into the right and left transverse sinuses which drains into a sigmoid sinus that leaves the skull as the right and left internal jugular vv., which are deep veins that drain into the right and left subclavian vv.

Superficial Veins of the Head and Neck

• The right and left external jugular veins receive blood from the superficial veins of the face (temporal, facial and maxillary vv.).
• These veins descend down either side of the neck, superficial to the SCM, and empties into the subclavian vv.
• The jugular venous pulse is sometimes palpable at the base of the neck.

Upper Limb Veins

• The superficial and deep palmer vv., of the hand interconnect to form the superficial and deep palmer venous arches respectively.
• The superficial arch empties into the cephalic v, which ascends along the radial forearm, and the basilic v., which ascends up the ulnar side.
• The median cubital v., found in the anterior elbow, passes from the cephalic v., medially and at an oblique angle to connect to the basilic v. It is at this vein that blood samples are usually taken. The basilic v., passes superiorly along the medial biceps brachii.
• The deep palmer vv drain into the radial v and the ulnar v.
• After crossing the elbow, the radial and ulnar vv fuse to form the brachial v.
• As the brachial v. ascends the upper arm, it fuses with the basilic v and becomes the axillary v., which enters the axilla.

Formation of the Superior Vena Cava

• The cephalic v joins the axillary v on the lateral surface of the 1st rib forming the subclavian v., which continues into the chest.
• The subclavian v merges with the external and internal jugular vv on that side and forms the brachiocephalic v., which enters the thoracic cavity.
• This vein then receives blood from the vertebral v on that side. The right and left brachiocephalic vv then fuse to form the superior vena cava.

Inferior Vena Cava

• This great vein also enters the right atrium.
• It collects blood from the tissues and organs inferior to the diaphragm.
• Deoxygenated blood in the sole of the foot is collected by the deep plantar venous arch, while blood from the dorsum and toes is collected by the dorsal venous arch.

Venous Draining in the Lower Limbs

• The deep plantar venous arch provides blood to the anterior tibial v. and posterior tibial v.
• The dorsal venous arch is drained by two superficial vv.: the great saphenous v. in the medial leg and thigh, and the small saphanous v. in the posteriolateral calf.
• The small saphanous v meets the popliteal v., formed by the union of both tibial vv and the fibular v.
• The popliteal v becomes the femoral v., at the femur and joins with the great saphanous v. The femoral v then penetrates the pelvic cavity and becomes the external iliac v.
• The external iliac v., also receives blood from the lower abdomen and pelvis. This vein is then joined by the internal iliac v, which drain the pelvic organs. Together, the internal and external iliac vv., form the common iliac v.
• Anterior to L5, the right and left common iliac vv fuse to form the inferior vena cava.

Abdominal Veins

1. Lumbar vv: drain the lumbar portion of the abdomen.
2. Gonadal vv: drain ovaries and testes.
3. Hepatic vv: originate in the liver.
4. Renal vv: largest tributaries of the IVC, drain the kidneys.
5. Suprarenal vv: drain the adrenal glands.
6. Phrenic vv: drain the diaphragm.

• All join directly into the IVC.

Hepatic Portal System

• Begins in the capillaries of the GI tract and ends in the liver.
• This system is different than other venous systems, because it contains high concentrations of dietary substances absorbed from the stomach and intestines (glucose, amino acids).
• At the liver, the macromolecules can be stored, metabolically converted or excreted.
• The largest vessel in this system is the hepatic portal v., which delivers blood to the liver. Receives blood from:
  1. Inferior mesenteric v: receives blood from the inferior 1/3 of the large intestine.
  2. Splenic v: receives blood from the spleen, stomach and pancreas.
  3. Superior mesenteric v: receives blood from the stomach, small intestine and proximal 2/3 of the large intestine.

Fetal Circulation

• Throughout fetal life the lungs are collapsed and non-functional. The fetus receives its O_2 from the mothers blood.
• Therefore, the right ventricle does not pump the majority of the blood to the lungs. Instead the blood from the right atrium enters directly into the left atrium through a hole in the interatrial septum called the foramen ovale.
• There is also a duct that connects the pulmonary and aortic trunks called the ductus arteriosus. Both structures allow blood to bypass the pulmonary circuit and supply the systemic circuit instead, where it is needed.
• After birth, the lungs are functional and the newborn can no longer rely on O_2 from maternal blood.
• Respiration therefore causes the foramen ovale to close forming the fossa ovalis, and the ductus arteriosus to constrict forming the ligamentum arteriosum.