Blood vessels

BLOOD VESSELS
The role of blood vessels in the circulation of blood has been known since 1628, when William Harvey, an English anatomist, demonstrated that blood in veins always flowed toward the heart. Before that time, it was believed that blood was static or stationary, some of it within the vessels but the rest sort of in puddles throughout the body. Harvey showed that blood indeed does move, and only in the blood vessels (though he did not know of the existence of capillaries).In the centuries that followed, the active (rather than merely passive) roles of the vascular system were
discovered, and all contribute to homeostasis.The vascular system consists of the arteries, capillaries,
and veins through which the heart pumps blood throughout the body. As you will see, the major “business”of the vascular system, which is the exchange of materials between the blood and tissues, takes place in the capillaries. The arteries and veins, however, are just as important, transporting blood between the capillaries and the heart.Another important topic of this chapter will be
blood pressure (BP), which is the force the blood exerts against the walls of the vessels. Normal blood pressure is essential for circulation and for some of the material exchanges that take place in capillaries.
ARTERIES
Arteries carry blood from the heart to capillaries;smaller arteries are called arterioles. If we look at an
artery in cross-section, we find three layers (or tunics)of tissues, each with different functions
The innermost layer, the tunica intima, is the only part of a vessel that is in contact with blood. It is made of simple squamous epithelium called endothelium.This lining is the same type of tissue that forms the endocardium, the lining of the chambers of the heart.As you might guess, its function is also the same: Its extreme smoothness prevents abnormal blood clotting.The endothelium of vessels, however, also produces nitric oxide (NO), which is a vasodilator. The tunica media, or middle layer, is made of smooth muscle and elastic connective tissue. Both of these tissues are involved in the maintenance of normal blood pressure, especially diastolic blood pressure when the
heart is relaxed. The smooth muscle is the tissue affected by the vasodilator NO; relaxation of this muscle tissue brings about dilation of the vessel. Smooth muscle also has a nerve supply; sympathetic nerve impulses bring about vasoconstriction. Fibrous connective tissue forms the outer layer, the tunica externa. This tissue is very strong, which is important to prevent the rupture or bursting of the larger arteriesthat carry blood under high pressure The outer and middle layers of large arteries are
quite thick. In the smallest arterioles, only individual smooth muscle cells encircle the tunica intima. As mentioned, the smooth muscle layer enables arteries to constrict or dilate. Such changes in diameter are regulated by the medulla and autonomic nervous system, and will be discussed in a later section on bloodpressure.
VEINS
Veins carry blood from capillaries back to the heart;the smaller veins are called venules. The same three tissue layers are present in veins as in the walls of arteries, but there are some differences when compared to the arterial layers. The inner layer of veins is smooth endothelium, but at intervals this lining isfolded to form valves prevent backflow of blood and are most numerous in veins of
the legs, where blood must often return to the heart against the force of gravity.The middle layer of veins is a thin layer of smooth muscle. It is thin because veins do not regulate blood
pressure and blood flow into capillaries as arteries do.Veins can constrict extensively, however, and this function becomes very important in certain situations
such as severe hemorrhage. The outer layer of veins is also thin; not as much fibrous connective tissue is necessary because blood pressure in veins is very low.
Structures of Blood vessel walls
All blood vessels except capillaries have three layers: The tunica intima, The tunica Media and the Tunica adventitia. Capillary walls are composed of tunica intima only
Arterial System
Elastic (conducting) arteries: are the large arteries close to the heart that expand and recoil to accommodate changing blood volume.
Muscular (Distributing) Arteries carry blood to specific organs; They are less stretchy and more active in vasoconstriction.Arterioles regulate the blood flow into capillary beds
Arteriosclerosis is a degenerative vascular disease. Initiated by endothelial lesions, it progresses through fatty streaks, Atherosclerotic and arteriosclerotic stages

Capillaries
Capillaries are microscopic vessels with very thin walls. Most exhibit clefts, which aid in the exchange between the blood and interstitial fluid
Capillaries at sites of active absorption have fenestrations that enhance their permeability
Vascular shunts (metarterioles- through fare channels) connect the terminal arteriole and venule at opposite ends of a capillary bed
Most true capillaries arise from and rejoin the shunt channels
The amount of blood flowing into the true capillaries is regulated by precapillary sphincters

Venous System
Veins have comparatively larger lumens than arteries and a system of valves prevents backflow of blood. Respiratory and skeletal muscles pumps aid venous return of blood to the heart.
Most veins are normally only partially filled with blood; Thus, they can serve as blood reservoirs

Vascular Anastomosis
The joining together of arteries serving a common organ is called an arterial anastmosis
Such vascular patterns provide alternate channel for blood to reach the same organ.
Vascular anastomosis also form between veins and between arterioles and venules

PHYSIOLOGY OF CIRCULATION
Introduction to Blood flow, Blood Pressure and Resistance
Blood flow is the amount of blood flowing through a vessel, an organ, or the entire circulation in a given period of time
Blood pressure is the force per unit area exerted on a vessel wall by the contained blood
Resistance is opposition to blood flow; Blood viscosity and blood vessel length ad diameter contribute to resistance
Blood flow is directly proportional to blood pressure and inversely proportional to resistance
Systematic Blood Pressure
Systematic blood pressure is highest in the aorta and lowest in the venae cavae
The steepest drop in BP occurs in the arterioles where resistance is greatest
Arterial Bp Depends on compliance of the elastic arteries and on how much blood is forced into them
Arterial blood pressure is pulsatile
Arterial pressure peaks during systole (systolic pressure)
During Diastole, as blood is forced distally in the circulation by the recoiling elastic arteries, Arterial Bp drops to its lowest value, called the diastolic pressure
Pulse pressure is systolic pressure minus diastolic pressure
The mean arterial pressure (MAP)= diastolic pressure plus pressure/3, and is the pressure that keeps blood moving throughout the cardiac cycle.
Low capillary pressure ( 40 to 20 mmh Hg) Protects the delicate capillaries from rupture while still allowing adequate exchange across the capillary walls
Venous pressure is low( Declining valves and large lumens) and functional adaptations ( the muscular and respiratory pumps) promote venous return.
Maintaining Blood Pressure
Blood pressure varies directly with Co2 PR and Blood volume
Blood vessel diameter is the major factor determining PR and small changes in vessel (chiefly arteriolar) diameter significantly affect blood pressure.
Short term Mechanisms: Neural Control
BP is regulated by autonomic neural reflexes involving baroreceptors or chemoreceptor's, The vasomotor center (a sympathetic center that regulates blood vessel diameter), and vasomotor fibers which act on smooth muscle
Activation of the receptors by falling BP (and to a laswer extent by a rise in blood Co2 or falling blood pH or O2 levels) stimulates the vasomotor center to increase vasocnstriction and the cardioaccelatory center to increase heart rate contractility
Rising BP inhibits the vasomotor center (permitting vasodilation) and activates the cardio inhibitory center.
Higher brain centers ( cerebrum and hypothalamus) may modify neural controls of BP by influencing the meduallary control centers

Short Term Mechanisms Chemical Controls
Blood- borne chemicals that increase BP by promoting vasoconstriction include epinephrine and NE ( these also increase heart rate and contractility), ADH, angiotension II (generated in response to rennin release by kidney cells) and endothelin released by vascular endothelial cells
Chemicals that reduce BP by promoting vasodilation include atrial natriuretic peptide ( also causes a decline blood volume), nitric oxide released by the vascular endothelium, inflammatory chemicals and alcohol.

Long- Term Mechanisms: Renal regulation
The kidneys regulate blood pressure by regulating blood volume
In the direct mechanism, Rising Bp enhances filtrate formation and fluid losses in urine; Falling BP causes the kidneys to retain more water, increasing blood volume
Indirect renal regulation of blood volume involves the rennin-angiotensin mechanism, a hormonal mechanism
When BP falls the kidneys release rennin which triggers the formation of angiotensin II (a potent vasoconstrictor) and relapse of aldosterone,which causes salt and water to be retained
Monitoring Circulatory Efficiency
Pulse and blood pressure measurements are used to assess cardiovascular efficiency
The pulse is the alternating expansion and recoil of arterial walls with each heartbeat
Pulse points are also pressure points
Blood pressure is routinely measured by the auscultator method
Normal blood pressure in adult is 120/80 (systolic/diastolic)
Hypotension is rarely a problem
Hypertension is the major cause of myocardial infract, stroke and renal disease
Alternations in Blood Pressure
Hypotension or low blood pressure (systolic pressure below 100mmHg) is a sign of health in the well conditioned
In other individuals it warns of poor nutrition, disease or circulatory shock
Chronic hypertension (High blood pressure) is persistent BP readings of 140/90 Or higher.
It is dangerous because it indicates increased peripheral resistance which strains the heart and promotes vascular complications of other organs particularly the eyes and kidneys
Risk factors are high-fat diet, high-salt diet, obesity, advanced age, stress and being a member of the black race or a family with a history of hypertension
Blood Flow through Body Tissues
Blood flow is involved in delivering nutrients and wasters to and from cells, gas exchange absorbing nutrients and forming urine
Velocity of Blood flow
Blood flows fastest where the cross sectional area of the vascular bed is the least (aorta) and the slowest where the cross sectional is greatest (capillaries) the slow flow in capillaries allows time for nutrient waste exchanges
Auto regulation: Local regulation of blood flow
Auto regulation is the local adjustment of blood flow to individual organs based on their immediate requirements
It is largely controlled by local chemical factors that cause vasodilation of arterioles serving the area and open the precapillary sphincters Myogenic controls respond to changes in blood pressure
Blood Flow in Special Areas
In most instances. Auto regulation is controlled by oxygen deficits and accumulation of local metabolites However Auto regulation in the brain is controlled primarily by a drop in pH and by myogenic mechanisms and vasodilation of pulmonary circuit vessels occurs in response to high levels of oxygen
Blood flow through Capillaries and Capillary Dynamics
Nutrients gases and other solutes smaller than plasma proteins cross the capillary wall by diffusion.
Water soluble substance move through the clefts or fenestrations.
Fat soluble substances pass through the lipid portion of the endothelial cell membrane
Fluid flows occurring at capillary beds reflect the relative effect of outward (net hydrostatic pressure) forces minus the effect of inward (net osmotic pressure) forces
In general fluid flows out of the capillary bed at the arterial end and reenters the capillary blood at the venule endThe small net loss of fluid and protein into the interstial space is collected by lymphatic vessels and returned to the cardiovascular system
Circulatory shock occurs when blood perfusion of body tissues is inadequate
Most cases of shock reflect low blood volume (hypovolemic shock) abnormal vasodilation (vascular shock) or pump failure(cardiogenic shock)
Circulatory Pathways: Blood Vessels of the body
The pulmonary circulation transports O2 poor, Co2-aden blood to the lungs for oxygenation and carbon dioxide unloading
Blood returning to the right atrium of the heart is pumped via the pulmonary trunk, pulmonary arteries and lobar arteries to the pulmonary capillaries by the right ventricle
Blood issuing from the lungs is returned to the left atrium by the pulmonary veins
The systemic circulation transports oxygenated blood from the left ventricle to all body tissues via the aorta and it branches.Venous blood returning from the systemic circuit is delivered to the right atrium via the venae cavae.
Developmental Aspects of the Blood Vessels
The fetal vasculature develops from embryonic blood islands and mesenchyme and is functioning in blood delivery by the fourth week Fetal circulation differs from circulation after birth with respects to pulmonary and hepatic shunts and special umbilical vesselsThese are normally occluded shortly after birth Blood pressure is low in infants and rises to adult valves Age related vascular problems include varicose veins, hypertension and arteriosclerosis Hypertension is the most important cause of sudden cardiovascular death in middle aged men Arteriosclerosis is the most important cause of cardiovascular disease in the aged Aneurysm(an’u-rizm; aneurysm= a widening) A balloon like out pocketing of an artery wall that places the artery at risk for rupture ; may reflect a congenital weakness of the artery wall, but more often reflects gradual weakening of the artery by chronic hypertension or arteriosclerosis; the most common sites of aneurysm formation are abdominal aorta and arteries feeding the brain and kidneys
Angiogram ( an’je-o-gram”; angio= a vessel; gram= writing) diagnostic technique involving the infusion of a radiopaque substance into the circulation for x-ray examination of specific blood vessels.
Diuretic ( Diure= urintate) A chemical substance that promotes urine formation thus reducing blood volume; diuretic drugs are frequently prescribed to manage hypertension
Microangiopathic lesion (micro= small) An abnormal thickening of a capillary basement membrane due to the deposit of glycoprotein; The result is thickened- but leaky- capillary walls; a vascular hallmark of long standing diabetes mellitus
Phlebitis(fle-bot’ tis; phleb= vein;It is = Inflammation) Inflammation of a vein accompanied by painful throbbing and redness of the skin over the inflamed vessel; most often caused by bacterial infection or local physical trauma
Phlebotomy( fle-bot’ ome; tomy=cut) A venous incision made for the purpose of with drawing blood or blood letting
Thrombophelbitis: Condition of undesirable intravascular clotting initiated by a roughening of venous lining; often follows severe episodes of phlebitis; an ever-present danger is that the clot may become detached and form an embolus





References

•Hoar, W.S. (1983) General and Comparative Physiology, Prentice-Hall

•Snell, R.S.(2003) Clinical Neuroanatomy for Medical Students. Lippincott Williams &Wilkins

•Rosenzweig, M.R., S.M. Breedlove, and A.L. Leiman (2002) Biological Psychology: An Introduction to Behavioural, Cognitive and Clinical Neuroscience. Sinauer Associates