Blood Vessel Structure and Function

Blood Vessel Structure and Function

• Blood vessels are a dynamic delivery system starting and ending at the heart, working with the lymphatic system to circulate fluids.
• Arteries carry blood away from the heart, typically oxygenated, except in pulmonary circulation and the umbilical vessels of a fetus.
• Capillaries facilitate direct contact with tissue cells, serving cellular needs.
• Veins carry blood toward the heart, usually deoxygenated, except in pulmonary circulation and the umbilical vessels of a fetus.

Structure of Blood Vessel Wall

• All vessels have a lumen, which is a central blood-containing space, surrounded by a wall.
• Vessel walls (excluding capillaries) consist of three layers, also known as tunics: tunica intima, tunica media, and tunica externa.
• Capillaries consist of endothelium with a sparse basal lamina.

Arteries

• Arteries are categorized into three groups based on size and function: elastic arteries, muscular arteries, and arterioles.

Elastic Arteries

• Elastic arteries have thick walls with large, low-resistance lumens, also called conducting arteries for conducting blood from the heart.
• Elastin is present in all three tunics, prevalent in the tunica media.
• Contain smooth muscle, but are relatively inactive in vasoconstriction.

Muscular Arteries

• Muscular arteries arise from elastic arteries that deliver blood to body organs.
• Diameters range from pinky-finger size to pencil-lead size.
• Comprise most of the body's named arteries.
• Have a thick tunica media with more smooth muscle and less elastic tissue, compared to elastic arteries.
• They are active in vasoconstriction.

Arterioles

• Arterioles represent ths smallest of all arteries.
• Larger arterioles have all three tunics, while smaller arterioles consist mainly of a single layer of smooth muscle around endothelial cells.
• Play a role in controlling flow into capillary beds through smooth muscle vasodilation and vasoconstriction.
• They are also known as resistance arteries because changes in diameter alter blood flow resistance.
• Arterioles lead to capillary beds.

Capillaries

• Capillaries are microscopic vessels with diameters so small that only a single red blood cell can pass at a time.
• Their walls consist of a thin tunica intima, and the smallest capillaries feature one cell forming the entire circumference.
• Capillaries are supplied to almost every cell, with the exception of cartilage, epithelia, the cornea, and the lens of the eye.
• They function in the exchange of gases, nutrients, wastes, and hormones between blood and interstitial fluid.

Types of Capillaries

• All capillary endothelial cells are joined by tight junctions with intercellular clefts that facilitate the passage of fluids and small solutes.
• Three types of capillaries exist: continuous, fenestrated, and sinusoidal.
• Continuous capillaries are abundant in the skin, muscles, lungs, and CNS.
  • Continuous capillaries in the brain are uniquely structured to form the blood-brain barrier.
  • Tight junctions enclose them, lacking intercellular clefts.
• Fenestrated capillaries are located in areas of active filtration, absorption, or hormone secretion such as kidneys, intestines, and endocrine glands.
  • They have Swiss cheese-like pores in their endothelial cells, called fenestrations, to increase permeability.
• Fewer tight junctions exist in sinusoidal capillaries, with larger intercellular clefts and incomplete basement membranes, allowing for increased permeability.
  • They're only found in the liver, bone marrow, spleen, and adrenal medulla.
  • Blood flow is sluggish, allowing time for blood modification.
  • Contain macrophages in the lining to capture and destroy foreign invaders.

Capillary Beds

• Capillary beds are interwoven networks of capillaries located between arterioles and venules.
• Two types of vessels exist in capillary beds: vascular shunts and true capillaries.
  • Vascular shunts directly connect arterioles to venules.
  • True capillaries are involved in exchange.

Veins

• Veins carry blood toward the heart.
• They form as capillary beds unite into postcapillary venules, which merge into larger veins.
• All tunics are present in veins, but the walls are thinner having large lumens compared to corresponding arteries.
• The tunica media is thin, but the tunica externa is thick containing collagen fibers and elastic networks.
• Large lumen and thin walls make veins good storage vessels, called capacitance vessels, because they store up to 65% of total blood supply.
• Due to less pressure in the veins adaptations ensure the return of blood to the heart
  • Large-diameter lumens offer little resistance.
  • Venous valves prevent backflow and are most abundant in veins.

Anastomoses

• Vascular anastomoses are interconnections of blood vessels.
• Arterial anastomoses create alternate pathways (collateral channels) to maintain continuous flow, even if an artery is blocked; abundant but, not found in the retina, kidneys, or spleen.
• Arteriovenous anastomoses are shunts in capillaries like the metarteriole-thoroughfare channel.
• The abundance of venous anastomoses ensures occluded veins rarely block blood flow.

Blood Flow

• Blood flow is the volume of blood flowing through a vessel, organ, or the entire circulation in a given time period.
• It's measured in ml/min and equals cardiac output (CO) for the whole vascular system.
• Blood flow is relatively constant at rest but varies at individual organ levels based on needs.

Blood pressure

• Blood pressure (BP) is the force per unit area exerted by blood on a vessel wall, expressed in mm Hg.
• Systemic arterial BP is measured in large arteries near the heart.
• Pressure gradient provides the force that keeps blood moving from higher- to lower-pressure areas.

Resistance

• Resistance (peripheral resistance) is the opposition to flow.
• It’s measured by the amount of friction blood encounters with vessel walls, mainly in peripheral (systemic) circulation.
• Important resistance sources: blood viscosity, total vessel length, and vessel diameter.

Blood Viscosity

• Blood viscosity is the thickness or "stickiness" of blood, due to formed elements and plasma proteins.
• Greater viscosity means molecules can't slide past each other easily.
• Increased viscosity leads to increased resistance.

Total Blood Vessel Length

• The longer the vessel, the greater the resistance encountered.

Blood Vessel Diameter

• Blood vessel diameter has the greatest effect on resistance.
• Frequent changes frequently alter peripheral resistance.
• Viscosity and blood vessel length are relatively constant.
• Fluid moves more slowly near walls than in the middle of a tube (laminar flow).
• If the radius increases, resistance decreases, and vice-versa.

Relationship Between Flow, Pressure, and Resistance

• Blood flow (F) is directly proportional to the blood pressure gradient (P). If P increases, blood flow speeds up.
• Blood flow is inversely proportional to peripheral resistance (R). If R increases, blood flow decreases, so F = P/R.
• The factor that most influences local blood flow is vessel diameter which is easily changed.

Systemic Blood Pressure

• The pumping action of the heart generates blood flow.
• Pressure results when flow is opposed by resistance.
• Systemic pressure is highest in the aorta and declines throughout the pathway.
• The steepest pressure drop occurs in arterioles.

Regulation of Blood Pressure

• Maintaining blood pressure (BP) requires cooperation from the heart, blood vessels, and kidneys, all overseen by the brain.
• The three primary factors heart rate, peripheral resistance, and blood volume.
• Factors can be affected by short-term neural and hormonal controls and long-term renal controls.

Short-Term Regulation: Neural Controls

• Two main neural mechanisms control peripheral resistance.
  • Mean arterial pressure (MAP) regulation through vessel diameter alteration, affecting resistance.
  • An example: If blood volume drops, all vessels constrict except those to the heart and brain.
• Blood distribution to organs can be altered in response to specific demands.
• Neural controls work via reflex arcs that include the cardiovascular center of the medulla, baroreceptors, chemoreceptors, and higher brain centers.

Role of the Cardiovascular Center

• The cardiovasular center is composed of sympathetic neurons in the medulla.
• Consists of cardiac enters (cardioinhibitory and cardioacceleratory centers) and vasomotor center.
• The vasomotor center transmits steady impulses via sympathetic efferents (vasomotor fibers) to vessels, causing moderate constriction (vasomotor tone).
• Receives inputs from baroreceptors, chemoreceptors, and cortex.

Baroreceptor Reflexes

• Baroreceptors are located in the carotid sinuses, aortic arch, and the walls of large neck and thorax arteries.
• If MAP is high: Increased blood pressure stimulates baroreceptors.
  • This increases input to the vasomotor center.
  • Motor and cardioacceleratory centers are stimulated by the the cardioinhibitory center.
  • This results in decreased blood pressure.

Chemoreceptor Reflexes

• Aortic arch and large neck arteries detect an increase in CO2, or a fall in pH or O2.
• Blood pressure increase by signaling cardioacceleratory center to increase CO
• They signal the vasomotor center to increase vasoconstriction.

Influence of Higher Brain Centers

• Blood reflexes that regulate bllod pressure are found in the medulla.
• Hypothalamus and cerebral cortex can modify arterial pressure by relays to medulla.
• The hypothalamus increases blood pressure during stress and mediates redistribution of blood flow during exercise and changes in body temperature.

Short-Term Mechanisms: Hormonal Controls

• Hormones regulate BP short-term, via changes in peripheral resistance, or long-term, via changes in blood volume.
• Adrenal medulla hormones such as epinephrine and norepinephrine increase CO and cause vasoconstriction.
• Angiotensin II stimulates vasoconstriction.
• High levels of ADH cause vasoconstriction.
• Atrial natriuretic peptide decreases BP by antagonizing aldosterone, which causes decreased blood volume.

Long-Term Mechanisms: Renal Regulation

• Long-term BP regulations is controlled through blood volume control via kidneys.
• Baroreceptors adapt quickly to chronic high or low BP so are ineffective long term.
• The kidneys regulate arterial blood pressure using a direct renal and an indirect renal (renin-angiotensin-aldosterone) mechanism.

Direct Renal Mechanism

• This alters blood volume independently of hormones.
• Increased BP or blood volume causes elimination of urine, reducing BP.
• Decreased BP or blood volume causes kidneys to conserve water conserving water, and BP rises.

Indirect Mechanism

• Decreases arterial pressures causes renin release from the kidneys initiating the renin-angiotensin-aldosterone mechanism
• Renin enters blood and catalyzes conversion of angiotensinogen from liver to angiotensin I
• Angiotensin-converting enzyme, especially from lungs, converts angiotensin I to angiotensin II.
• Angiotensin II then stabilizes BP and ECF in four ways it stimulates aldosterone secretion, stimulates the posterior pituitary to release ADH, triggers thirst, and acts as a vasoconstrictor.

Summary of Blood Pressure Regulation

• Blood pressure must be high enough to provide adequate tissue perfusion, but not too high to damage blood vessels.
• If BP to brain is too low, perfusion is inadequate, and person loses consciousness.
• If BP to brain is too high, person could have a stroke.

Homeostatic Imbalances in Blood Pressure

• Transient elevations in BP occur during changes in posture, physical exertion, emotional upset, or fever.
• Age, sex, weight, race, mood, and posture may also cause BP to vary.

Hypertension

• Hypertension involves sustained elevated arterial pressure of 140/90 mm Hg or higher.
• Prolonged hypertension is a heart and vascular disease, and also causes can renal failure and stroke.
• It results in the heart must work harder, and the myocardium enlarges, weakens, and becomes flabby
• It then accelerates atherosclerosis further weakening the cardiovascular system

Primary Hypertension

• 90% of hypertensive conditions have an unidentified underlying cause.
• Risk factors of high blood pressure include heredity, diet, obesity, age, diabetes mellitus, stress, and smoking.
• Although this form of hypertension as no cure, it can be controlled through a low stress high activity lifestyle and antihypertension drugs.

Secondary Hypertension

• Secondary hypertension has known identifiers and is less common than primary hypertension.
• Underlying causes include obstructed renal arteries, kidney disease, and endocrine disorders such as hyperthyroidism and Cushing's syndrome
• Treatments for secondary hypertension focuses on correcting the underlying cause.

Hypotension

• Hypotension involves is low low blood pressure below 90/60 mm Hg.
• It is generally not a concern however inadequate blood flow indicates a medical concern
• Often, it suggests long life and a lack of cardiovascular illness

Circulatory Shock

• Circulatory shock occurs due to blood vessels inadequately filling and therefore cannot circulate blood properly.
• Inadequate tissue needs cannot be circulated, and tissue needs cant be met
• A lack of Hypovolemic shock can results from large-scale blood loss,
• Extreme vasodilation and decreased peripheral resistance results in Vascular shock
• When an insufficient heart cannot sustain adequate circulation cardiogenic shock results.

Control of Blood Flow

• Tissue perfusion is blood flow through the bodies's tissues.
• Involves delivery of O₂ and removal of tissue cell wastes, gas exchange (lungs), absorption of nutrients (digestive tract), and urine formation (kidneys).
• Proper tissue and organ function is determined by rate of flow.

Extrinsic And Intrinsic Factors That Control Blood Flow

• Extrinsic controls are factors controlled through the sympathetic nervous system and hormones which control blood flow throughout the entire body.
• These extrinsic factors act on arteriolar smooth muscle to reduce blood flow to regions that need it the least.
• Intrinsic Controls: Autoregulation, where blood flow is adjusted locally to fulfill precise tissue necessities.
  • Local arterioles which feed capillaries can be altered in diameter.
  • By varying the resistance in their own arterioles, organs regulate their own supply of blood.
  • As an adaption skeletal tissue during exercise has a significantly higher blood flow than when at rest when blood flow is redirected.

Autoregulation: Intrinsic Regulation

• Regulation of blood flow to that specific area due to a set of local conditions.
• Metabolic controls that affect autoregulation include increase in levels of metabolic products (H+, K+, adenosine, and prostaglandins), and declining levels of O2.

Effects of Change in Levels of Local Chemicals

• Local chemicals cause direct relaxation of arterioles and relaxation of precapillary sphincters
• Can cause endothelial cells to release nitric oxide (NO).

Myogenic Controls

• Responses causes stimulation of the MAP due to vascular smooth muscle contraction, perfusion remains constant to prevent tissue injury or dysfunction.
• Vessel wall stretches which is referred as vascular smooth musle causes a reduction of bloodflow to the tissue
• Reduced arterial pressure results in less muscle stretch allowing an increase in blood flow to the tissue.

Velocity of Blood Flow

• Velocity of blood flow fluctuates during systemic circulation
• Typically with the fastest is aortic velocity, then slowest in capillaries, before increasing in veins again
• Total cross-sectional area is inversely related to speed
  • Capillaries have slow flow due to large surface area, allowing for adequate exchange between blood and tissues.

Capillary Exchange of Respiratory Gases and Nutrients

• Diffusion causes many molecules that pass.
• Concentration gradients regulate movement.
• The molecules have four unique pathways through the capillary.
  • Passing directly by endothelial membranes by lipids.
  • Passing through clefts or fenestrations to find water-soluble solutes.
  • Active transport by vesicles.
• Fluids are returned through both hydrostatic and colloid osmotic pressures.

Hydrostatic Pressures

• Fluid pressing against the wall and are composed of two types.
  • Blood capillary pressure with a higher fluid tension
  • Fluid pressure from the vessel where lymphatic arteries drain.

Colloid Osmotic Pressures

• Can be both capillary and intersitial in nature.
• Albumin is often the cause in intersitial fluids.
• Fluids are returned at an increased rate in intertitial nature.