The Circulatory System Blood Vessel and Circulation1 Lecture

The Circulatory System: Blood Vessel and Circulation-1 Lecture # 5 (Chapter 20) � General Anatomy of Blood Vessels � Blood Pressure, Resistance, and Flow � Capillary Exchange � Venous Return and Circulatory Shock � Special Circulatory Routes � Anatomy of: ◦ pulmonary circuit ◦ systemic vessels of the axial region ◦ systemic vessels of the appendicular region

Blood Vessels Pulmonary arteries Pulmonary veins Aorta Vena cava Arteries They carry blood away from the heart (they are efferent vessels). Veins They carry blood back to (toward) the heart (they are afferent vessels. Capillaries They connect the arteries with the veins.

Conducting (large) artery Large vein Distributing (medium) artery Medium vein Arteriole Venule Capillary Metarteriole

The Vessel Wall Lumen Tunica interna It lines the blood vessel and is exposed to blood. Endothelium Tunica media It is simple squamous epithelium. Functions: It consists of smooth muscle, collagen, and elastic tissue. 1 - Acts as a selectively permeable barrier. 2 - Secretes chemicals that stimulate dilation or constriction of the vessel. 3 - Normally repels blood cells and platelets that may adhere to it and form a clot. 4 - When tissue around vessel is inflamed, the endothelial cells produce cell-adhesion molecules that induce leukocytes to adhere to the surface and causes leukocytes to congregate in tissues where their defensive actions are needed. Functions: 1 - Strengthens vessel and prevents blood pressure from rupturing them. 2 - Vasomotion – changes in diameter of the blood vessel brought about by smooth muscle. Tunica externa It consists of loose connective tissue that often merges with that of neighboring blood vessels, nerves, or other organs. Functions: 1 - Anchors the vessel and provides passage for small nerves, lymphatic vessels. Basement membrane Areolar connective tissue. Nerve Vasa vasorum They are small vessels that supply blood to at least the outer half of the larger vessels.

Arteries 1 - Conducting (elastic or large) arteries The biggest arteries: Aorta, common carotid, subclavian, pulmonary trunk, and common iliac arteries. They have a layer of elastic tissue, internal elastic lamina, at the border between interna and media. They have also an external elastic lamina at the border between media and externa. They expand during systole, recoil during diastole which lessens fluctuations in blood pressure. 2 - Distributing (muscular or medium) arteries They distribute blood to specific organs: brachial, femoral, renal, and splenic arteries. The smooth muscle layers constitute three-fourths of wall thickness. 3 - Resistance (small) arteries They control the amount of blood to various organs. They exhibit up to 25 layers of smooth muscle tissue (a thick tunica media) and relatively little elastic tissue. The smallest resistance arteries are the arterioles. Metarteriole 4 - Metarterioles They are short vessels that link arterioles to capillaries. Muscle cells form a precapillary sphincter about entrance to capillary. Constriction of these sphincters reduces or shuts off blood flow through their respective capillaries and diverts blood to other tissues. Arteriole Venule

Arterial Sense Organs External carotid artery Internal carotid artery Carotid body Carotid sinus It contains chemoreceptors They monitor blood chemistry and transmit signals to the brainstem respiratory centers to adjust respiratory rate to stabilize p. H, CO 2, and O 2 It contains baroreceptors They monitor blood pressure and send signals to the brainstem, which decreases the heart rate and dilates blood in response to high blood pressure. Common carotid artery Aortic bodies It contains chemoreceptors They monitor blood chemistry and transmit signals to the brainstem respiratory centers to adjust respiratory rate to stabilize p. H, CO 2, and O 2 Aorta Baroreceptors They monitor blood pressure and send signals to the brainstem, which decreases the heart rate and dilates blood in response to high blood pressure.

Capillaries Tissues: Oxygen, nutrients and hormones pass from the blood to the tissue fluid and the waste passes from the tissue fluid to the blood through the capillaries. Lungs: Oxygen passes from the air to the blood and carbon dioxide passes from the blood to the air through the capillaries. Nutrients, hormones, Wastes O 2 CO 2 Venule Capillary Arteriole Tissue Metarteriole Arteriole O 2 CO 2 Venule Capillary Lung Metarteriole Capillaries are composed of an endothelium and a basal lamina. They are absent or scarce in tendons, ligaments, epithelia, cornea and lens of the eye.

Types of Capillaries Continuous Capillaries They occur in most tissues. Endothelial cells have tight junctions forming a continuous tube with intercellular clefts allow passage of solutes such as glucose. Pericytes wrap around the capillaries and contain the same contractile protein as muscle. They contract and regulate blood flow. Fenestrated capillaries Sinusoids (discontinuous capillaries) They occur in organs that require rapid absorption or filtration (kidneys, small intestine). The sinusoids are discontinuous capillaries that occur in liver, bone marrow, and spleen. The endothelial cells riddled with holes called filtration pores (fenestrations), which allow passage of only small molecules. The endothelial cells are separated by wide gaps with no basal membrane. They allow proteins (albumin), clotting factors, and new blood cells to enter the circulation.

Capillary Beds Capillaries are organized into networks called capillary beds, usually supplied by a single metarteriole. When the sphincters are open, the capillaries are well perfused and engaged in exchanges with the tissue fluid. When the sphincters are closed, little to no blood flow occurs (skeletal muscles at rest).

Veins They have a greater capacity for blood containment than arteries. In large arteries, blood pressure averages 90 to 100 mm Hg, whereas in veins it averages about 10 mm Hg. Veins, therefore, do not require thick, pressure resistant walls They have thinner and flaccid walls and contain less muscular and elastic tissue than the arteries, that is why they can expand easily.

Small veins merge to form larger and larger ones as they approach to the heart. Postcapillary venules Muscular venules They are even more porous than capillaries so also exchange fluid with surrounding tissues. They are up to 1 mm in diameter. Tunica interna with a few fibroblasts around it. No tunica media. No tunica externa. They have a thin tunica externa. Most leukocytes emigrate from the bloodstream through venule walls. They have 1 or 2 layers of smooth muscle in tunica media. Medium veins They are up to 10 mm in diameter (radial vein, ulnar vein, saphenous veins). Large veins They are larger than 10 mm (venae cavae, pulmonary veins, internal jugular veins, and renal veins) They have a thin tunica media and thick tunica externa. They have some smooth muscle in all three tunics The tunica interna forms venous valves. The tunica externa is the thickest layer contains longitudinal bundles of smooth muscle The varicose veins result in part from the failure of these valves. Skeletal muscle pump propels venous blood back toward the heart.

Venous sinuses: They are veins with especially thin walls, large lumens, and no smooth muscle (dural venous sinus and coronary sinus of the heart). Unlike other veins, sinuses are not capable of vasomotion. Vasomotion: It is the capacity to adjust the radius of the blood vessels. This includes vasoconstriction, the narrowing of a vessel, vasodilation, the widening of a blood vessel.

Vasomotion has two purposes A generalized raising or lowering of blood pressure. A selectively modification of the perfusion of particular organs and rerouting blood from one region of the body to another Redirection of Blood Flow in Response to Metabolic Needs

Portal Systems Simplest and Most Common Route of Blood Flow heart arteries arterioles capillaries venules veins Blood passes through only one network of capillaries from the time it leaves the heart until the time it returns.

The Portal Systems Blood flows through two consecutive capillary networks before returning to heart. 1 - Between hypothalamus and anterior pituitary. 2 - In kidneys. 3 - Between intestines to liver. To the heart

Anastomosis: It is the point where two blood vessels merge. 1 - Arteriovenous anastomosis (shunt) The artery flows directly into the vein bypassing capillaries. 2 - Venous anastomosis One vein empties directly into another They are the most common anastomosis. They provide several alternative routes of drainage. For that reason vein blockage less serious than an arterial blockage. 3 - Arterial anastomosis Two arteries merge and provide collateral (alternative) routes of blood supply to a tissue. Coronary circulation and around joints. Circle of Willis (cerebral circulation) (a) Simplest pathway (1 capillary bed) (b) Portal system (2 capillary beds) (c) Arteriovenous anastomosis (shunt) (d) Venous anastomoses (e) Arterial anastomoses

Blood Pressure Blood pressure: It is the force that blood exerts against a vessel wall. Two pressures are recorded: Systolic pressure: It is the peak arterial BP taken during ventricular contraction (ventricular systole). Diastolic pressure: It is the minimum arterial BP taken during ventricular relaxation (diastole) between heart beats. Systolic blood pressure Ventricular systole Ventricular diastole Diastolic blood pressure

Blood pressure: It is the force that blood exerts against a vessel wall. Two pressures are recorded Systolic pressure: It is the peak arterial BP taken during ventricular contraction (ventricular systole). Diastolic pressure: It is the minimum arterial BP taken during ventricular relaxation (diastole) between heart beats. The Blood Pressure is measured at brachial artery of arm using sphygmomanometer. Normal value, young adult: 120/75 mm Hg Hypertension (high blood pressure): It is a chronic resting blood pressure higher than 140/90. Hypotension: It is chronic low resting BP Hypotension is caused by blood loss, dehydration, anemia

Blood pressure is physiologically determined by 3 principal variables: 1 - Cardiac output 2 - Blood volume It is regulated mainly by the kidneys, which have a greater influence than any other organ on blood pressure. 3 - Peripheral resistance to flow Blood viscosity Peripheral resistance depends on 3 factors Vessel length Vessel radius Blood viscosity: RBC count and albumin concentration elevate viscosity the most. Decreased viscosity with anemia and hypoproteinemia speed flow. Increased viscosity with polycythemia and dehydration slow flow. Vessel length: The farther liquid travels through a tube, the more cumulative friction it encounters. Pressure and flow decline with distance.

Systemic blood pressure (mm Hg) 120 100 Systolic pressure 80 60 40 Diastolic pressure 20 0 e na e Ve va ca e rg s La in ve l al s Sm in ve es l nu Ve ries a ill ap s C le io er rt A l al ies Sm ter ar e rg ies La rter a ta or A Increasing distance from left ventricle Because of arterial elasticity and the effect of friction against the vessel walls, all measures of blood pressure declines with distance.

Vessel Radius In a healthy individual, blood viscosity is quite stable, and vessel lengths do not change in the short term. Therefore, the only significant way of controlling peripheral resistance from moment to moment is by vasomotion. Vasomotion is a change in vessel radius produced by: 1 - Vasoconstriction by smooth muscle contraction. 2 - Vasodilation by relaxation of the smooth muscle. The arterioles are the most significant point of control over peripheral resistance and blood pressure. They produce half of the total peripheral resistance. A single drop of epinephrine here has caused the arteriole to constrict about one-third of its dilated diameter.

Vasomotion is a quick and powerful way of altering blood pressure and flow. There are three ways of controlling vasomotion: 1 - Local control 2 - Neural control 3 - Hormonal control 1 - Local control Tissues have the ability to regulate their own blood supply (autoregulation) a) Metabolic Theory of Autoregulation Perfusion It is the blood flow per given volume or mass of tissue (m. L/min/g) Wastes ( CO 2, H+, K+, lactic acid, adenosine) Wastes Perfusion H+, K+, ( CO 2, lactic acid, adenosine) Vasodilation Vasoconstriction Perfusion

Other mechanisms of local control b) Vasoactive chemicals: They are substances secreted by platelets, endothelial cells, and perivascular tissue that stimulate vasomotion. Histamine, bradykinin, prostaglandins, prostacyclin and nitric oxide stimulate vasodilation. Endothelins stimulate vasoconstrictor. c) Reactive hyperemia: If blood supply is cut off and then restored, flow increases above normal. d) Angiogenesis: It is the growth of new blood vessels Angiogenesis occurs in regrowth of uterine lining, around coronary artery obstructions, in exercised muscle, and malignant tumors controlled by growth factors

2 - Neural control The blood vessels are under remote control by the central and autonomic nervous systems Baroreceptors: They monitor blood pressure and send signals to the brainstem. BP 1 - Heart rate decreases 1 - Baroreflex 2 - Blood vessels dilate BP Sympathetic tone Chemoreceptors: They monitor blood chemistry, mainly transmit signals to the brainstem respiratory centers to adjust respiratory rate to stabilize p. H, CO 2, and O 2 When O 2 , CO 2 , and p. H , 2 - Chemoreflex they also induce a widespread vasoconstriction in the vasomotor center. BP Perfusion of lungs and gas exchange increases. 3 - Medullary ischemic reflex A drop in the perfusion of the brain produces an increase in the sympathetic tone with: - An increase the heart rate - Widespread vasoconstriction BP Sympathetic tone Perfusion of brain increases. Sympathetic tone Vasomotor center

3 - Hormonal control Hormones influence blood pressure by two mechanisms: Vasoactive effects Regulating water balance 1 - Angiotensin II It is a potent vasoconstrictor that raises the blood pressure. Angiotensin II ACE inhibitors are drugs used to treat hypertension 2 - Epinephrine and Norepinephrine They are adrenal and sympathetic catecholamines that stimulate smooth muscle to contract They produce vasoconstriction and rise the blood pressure (a adrenergic receptors), except in the coronary blood vessels and skeletal muscles where they produce vasodilation (b adrenergic receptors). 1 - Aldosterone It is produced by the adrenal cortex. It promotes retention of sodium and water in the kidneys, which increase blood volume and blood pressure. 2 - Atrial natriuretic peptide (ANP) It is secreted by the heart and antagonizes aldosterone. It promotes excretion of sodium and water in the kidneys, which lower blood volume and blood pressure 3 - Antidiuretic hormone It is produced by the hypothalamus and released by the posterior pituitary gland. It promotes water retention in the kidneys, which increases blood volume and blood pressure.

Capillary Exchange The most important blood in the body is in the capillaries because only through capillary walls are exchanges made between the blood and surrounding tissues. Capillary exchange: It is the two way movement of fluid across capillary walls. Chemicals given off by the capillaries Chemicals taken up by the capillaries Water, oxygen, glucose, amino acids, lipids, minerals, antibodies, hormones. ROUTES Water, carbon dioxide, ammonia and other wastes. 1 - Filtration pores (fenestrations) of the fenestrated capillaries MECHANISMS 1 - Diffusion 2 - Transcytosis 3 - Filtration 2 - Endothelial cell cytoplasm 3 - Intercellular clefts between endothelial cells 4 - Reabsorption

1 - Diffusion It is the most important form of capillary exchange. Glucose and oxygen being more concentrated in blood diffuse out of the blood. Carbon dioxide and other waste being more concentrated in tissue fluid diffuse into the blood. Capillary diffusion can only occur if the solute can permeate the plasma membranes of the endothelial cell, or find passages large enough to pass through (filtration pores and intracellular clefts). Lipid soluble substance (steroid hormones, O 2 and CO 2) diffuse easily through plasma membranes. Water soluble substances (glucose and electrolytes) must pass through filtration pores and intercellular clefts.

2 - Transcytosis Endothelial cells pick up material on one side of the plasma membrane by pinocytosis or receptor-mediated endocytosis, transport vesicles across cell, and discharge material on other side by exocytosis. It is important for fatty acids, albumin and some hormones (insulin).

3 - Filtration and Reabsorption

�Anatomy of: ◦ pulmonary circuit ◦ systemic vessels of the axial region ◦ systemic vessels of the appendicular region

Aorta and Major Branches L. common carotid a. R. common carotid a. R. subclavian a. L. subclavian a. Brachiocephalic trunk Aortic arch Superior vena cava Ascending aorta Descending aorta, thoracic (posterior to heart) Inferior vena cava Diaphragm Aortic hiatus Descending aorta, abdominal Anterior view

Anatomy of the Pulmonary Circuit Right pulmonary artery Superior lobar artery Middle lobar artery Inferior lobar artery Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Superior lobar arteries Left pulmonary artery Inferior lobar artery Pulmonary trunk

Superficial Veins of the Head and Neck Arteries of the Head and Neck Vertebral v. External carotid a. Internal carotid a. They supply blood to the brain. Vertebral a. External jugular v. Carotid sinus Thyroid gland Internal jugular v. Thyroid gland Axillary v. Brachiocephalic v. Subclavian v. Common carotid a. Subclavian a. Axillary a. Brachiocephalic trunk Lateral view (c) Superficial veins of the head and neck

Anterior communicating artery R. and L. posterior communicating arteries Cerebral Arterial Circle or Circle of Willis

Cerebral Arterial Circle or Circle of Willis Anterior communicating a. (1) Anterior cerebral a. (2) Internal carotid a. Posterior communicating a. (2) Middle cerebral a. Posterior cerebral a. (2) Basilar a. Vertebral a. Inferior view

Arteries of the Upper Limb Right subclavian artery Right axillary artery Right brachial artery Right radial artery Superficial palmar arch Right ulnar artery Deep palmar arch

Veins of the Upper Limb Left subclavian vein Left Brachiocephalic vein Left axillary vein Cephalic Brachial Basilic

Basilic Brachial Median cubital Cephalic Radial Ulnar Superficial and deep palmar plexuses

Major Branches of Abdominal Aorta Celiac trunk Liver Superior mesenteric a. Spleen Pancreas Pancreatic aa. Renal a. L. gastric a. Gonadal a. Common hepatic a. Splenic a. Inferior mesenteric a. Common iliac a. Internal iliac a. External iliac a. Celiac trunk: 1 - Common hepatic artery 2 - Splenic artery 3 - Left gastric artery

Arteries of the Lower Limb External iliac Internal iliac Femoral artery Popliteal artery Posterior tibial artery Anterior view Anterior tibial artery Fibular or peroneal artery Posterior view

Veins of the Lower Limb Great saphenous Femoral vein Popliteal Posterior tibial vein Anterior tibial vein Peroneal or fibular vein Lesser saphenous

The Hepatic Portal System Hepatic veins Hepatic portal v. Superior mesenteric v. Gastric vv. Splenic v. Inferior mesenteric v. The hepatic portal system all the blood draining from the abdominal digestive tract, as well as from the pancreas, gallbladder and spleen. The hepatic portal system gives the liver first claim to the nutrients before the blood is distributed to the rest of the body. It also allows the blood to be cleansed of bacteria and toxins picked up from the intestine.