Circulatory System: Capillary Beds & Blood Vessels

Circulatory System

Capillary Beds

• Capillary beds are an interwoven network of capillaries found between arterioles and venules.
• Microcirculation refers to the flow of blood through the capillary bed.
• Capillary beds are made up of two types of vessels: vascular shunts and true capillaries.
• Vascular shunt directly connects the arteriole to the venule.
• True capillaries are the vessels directly involved in exchanges.

Blood Vessel Structure and Function

• Blood vessels work with the lymphatic system to circulate fluids with the blood.
• Arteries carry blood away from the heart, which is oxygenated, apart from the pulmonary circulation and umbilical vessels of a fetus.
• Capillaries come into direct contact with tissue cells to fulfill cellular requirements
• Veins transport blood back to the heart, which is deoxygenated, except in the pulmonary circulation and umbilical vessels of a fetus.

Structure of Blood Vessel Wall

• Vessels consist of a lumen, which is a blood filled space
• All vessels, except capillaries, have three layers or tunics
• Tunica Intima is the innermost tunic.
• Tunica Media is the middle tunic.
• Tunica Externa is the outermost tunic.
• Capillaries consist of the endothelium and a spare basal lamina.

Tunica Intima

• A simple squamous epithelium lines the lumen of all vessels.
• The endothelium is continuous with the endocardium.
• A slick surface reduces friction.
• The subendothelial layer is a connective tissue basement membrane that is found in vessels larger than 1 mm.

Tunica Media

• The middle layer is mainly composed of smooth muscle and elastin sheets.
• Sympathetic vasomotor nerve fibers innervate this layer, allowing for vasoconstriction and vasodilation.

Tunica Externa

• This outermost layer is composed of collagen fibers that protect, reinforce, and anchor walls to surrounding structures.
• It contains nerve fibers, lymphatic vessels, and tiny blood vessels, called vasa vasorum.

Summary of Blood Vessel Anatomy

• Elastic arteries have an average lumen diameter of 1.5 cm and a wall thickness of 1.0 mm.
• Muscular arteries have an average lumen diameter of 0.6 cm and a wall thickness of 1.0 mm.
• Arterioles have a lumen diameter of 37.0 µm and a wall thickness of 6.0 µm.
• Elastic tissue contains elastin.
• Smooth muscle in muscular arteries are active in vasoconstriction.
• Arterioles have an average lumen diameter of 9.0 µm and a wall thickness of 0.5 µm.
• Capillaries have a lumen diameter of 9.0 µm and a wall thickness of 0.5 µm.
• Venules have a lumen diameter of 20.0 µm and a wall thickness of 1.0 µm.
• Veins have a lumen diameter of 5.0 mm and a wall thickness of 0.5 mm.
• Elastic arteries function as pressure reservoirs that expand and recoil as blood is ejected from the heart.
• Arterioles are resistance arteries that control flow into the capillary beds using vasodilation and vasoconstriction.

Capillaries

• Capillaries deliver gases, nutrients, wastes, and hormones between the blood and interstitial fluid in almost every cell except for cartilage, epithelia, the cornea, and the lens of the eye.
• Only a single red blood cells can pass through at a time.
• The walls are made of endothelial cells that are held by tight junctions with intercellular clefts.
• Spider shaped stem cells called pericytes stabilize the capillary walls, control permeability, and aid in vessel repair.

Veins

• Veins have larger-diameter lumens that offer less resistance, but have adaptations to return blood to the heart due to having less pressure than arteries.
• Adaptations include venous valves that prevent back flow of blood, and are commonly found in limbs.
• Venous sinuses are flattened veins with thin walls made only of endothelium, such as the coronary sinus of the heart and dural sinuses of the brain.

Anastomoses

• Anastomoses are interconnections of blood vessels
• Arterial anastomoses provide alternate pathways (collateral channels).
• They ensure constant flow, even if an artery is blocked, and are typical in joints, abdominal organs, the heart, and the brain.
• Arteriovenous anastomoses are shunts in capillaries.
• An example of an arteriovenous anastomoses is the metarteriole-thoroughfare channel.
• Venous anastomoses block blood flow.

Clinical Aspects of Venous Anatomy

• Varicose veins are dilated and painful due to leaky valves.
• Factors, such as heredity, and/or conditions that hinder venous return contribute to varicose veins.
• Prolonged standing or pregnancy can cause blood to pool, weakening valves and affecting over 15% of adults.
• Increased venous pressure can lead to hemorrhoids - strained varicosities in anal veins.

Definition of Terms

• Blood pressure (BP) is the force per unit area exerted on a blood vessel wall by blood.
• BP is expressed in mm Hg and measured as systemic arterial BP near large arteries.
• The pressure gradient is the force that drives blood from higher to lower pressure areas.
• Resistance is the opposition to flow, or the amount of friction encountered by blood vessel walls in systemic circulation.
• Important sources of resistance include blood viscosity (R ~ 1/V), total blood vessel length (RL) and blood vessel diameter (R1/radius4).

Blood Vessel Diameter

• Fluid closer to walls moves slower than in the middle of the tube (laminar flow).
• Major determinants of peripheral resistance are small-diameter arterioles whose radius changes frequently.
• Sudden changes in vessel diameter or plaques from atherosclerosis increase resistance, disrupting laminar flow and causing turbulence, increasing resistance.

Relationship Between Flow, Pressure and Resistance

• Blood flow (F) (equals CO) is directly proportional to the blood pressure gradient (ΔΡ) (mean arterial pressure, MAP).
• Blood flow is inversely proportional to peripheral resistance (R).
• F = ΔP/R.
• R is a huge influence on local blood flow, because it is easily changed by altering diameter.

Arterial Blood Pressure

• Blood pressure is determined by the Elasticity of arteries close to the heart.
• Blood pressure is determined by the volume of blood pushed into them at any time.
• Blood pressure is pulsatile near the heart.
• Systolic pressure is the exerted pressure in the aorta during ventricular contraction (~120 mm Hg)
• Diastolic pressure is the aortic pressure when the heart is at rest.
• Pulse pressure is the difference between systolic and diastolic pressure, which is felt under the skin as pulsing.
• Mean arterial pressure (MAP) is the pressure that propels blood to tissues
• As a result pulse pressure phases out near the arterial tree, then the heart spends more time in diastole, so it's not a simple average of the numbers.
• MAP = diastolic pressure + ⅓ pulse pressure, for example, if 120/80; PP = 120 - 80 = 40; so, MAP = 80 + (⅓) * 40 = 80 + ~13 = 93 mm Hg.
• Pulse pressure and MAP decline with increased distance from the heart.

Arterial Blood Pressure (Cont.)

• Clinical monitoring of circulation can assess how efficiently the circulatory maintains blood flow.
• Vital signs are pulse, blood pressure, respiratory rate, and body temperature.
• Taking a pulse on the radial pulse is most routinely used.
• Pressure points help reduce hemorrhaging and stop blood-flow.

Arterial Blood Pressure Measuring

• Systemic arterial BP is measured by auscultatory methods using a sphygmomanometer.
• To measure blood pressure wrap the cuff around the upper arm, and exceed systolic pressure to block the brachial artery.
• Pressure is released slowly, and listen with a stethoscope for Korotkoff sounds.
• Systolic pressure should normally be less than 120 mm Hg, it is the point the sounds start as blood begins to spurt through the artery.
• Normal Diastolic pressure is normally less than 80 mm Hg, and sounds will disappear entirely due to the artery no longer constricted.

Capillary Blood Pressure

• Starts at 35 mm Hg at the start of a capillary and lowers to 17 mm Hg by the end.
• Low capillary pressure is ideal for fragile capillaries, because high BP will rupture them, and low pressure can force filtrate into interstitial spaces.

Venous Blood Pressure

• Venous blood pressure changes little during cardiac cycle.
• The pressure gradient is small at 15 mm Hg.
• Venous return requires adaptations to help the low pressure.
• Low pressure comes from cumulative effects of peripheral resistance.
• The muscular pump uses skeletal muscles to push blood up, in tandem with valves stopping back flow.
• A respiratory pump moves blood toward heart by compressing abdominal veins as thoracic veins expand.
• Sympathetic venoconstriction causes smooth muscles to move blood toward the heart.

Regulation of Blood Pressure

• Maintains blood flow in the brain, heart, blood vessels, and kidneys.
• Three main factors are cardiac output (CO), peripheral resistance (PR), and blood volume.
• Remember that Flow (F) is mean arterial pressure (MAP) /resistance; F = ΔP/R.
• In other words cardiac output (CO) is arterial pressure/resistance; i.e. CO = ΔP/R. Therefore rearranging yields ΔΡ = CO × R = MAP..
• Blood pressure (MAP) is directly proportional to CO and PR
• Recall if CO = SV x HR, then MAP = SV x HR x R
• Any increases in SV, HR, or R will cause an increase in blood flow.
• SV depends on venous return (end diastolic volume) EDV
• Heart rate depends on the brain
• Resistance depends on vessel diameter.

Homeostatic Imbalances in Blood Pressure

• Transient elevations in BP occur during changes in physical position, and during bodily exertion, emotions, and fever.
• Age, sex, weight, race, mood and posture can affect blood pressure
• Hypertension is increased arterial pressure of 140/90 mm Hg
• prehypertension can result from transient adaptations such as fever, during bodily exertion, and emotional problems
• Prolonged hypertension can cause heart failure, organ damage, and stroke
• Prolonged hypertension can also stiffen the heart
• In Primary hypertension a cause isn't identified
• Risk factors of primary hypertension heredity, diet, obesity, age, diabetes mellitus, stress, and smoking
• Secondary hypertension is less common, with a clear source of the problem
• Commonly due to obstructed renal arteries, kidney disease, and endocrine disorders such as hyperthyroidism and Cushing's syndrome

Hypotension

• Hypotension is low blood pressure less than 90/60 mm Hg
• It isn't concerning unless tissues don't get enough blood
• It can also hint towards long life and less cardiovascular illness.
• Orthostatic hypotension is temporary low BP and being dizzy when standing up/reclining
• Chronic hypotension hints poor nutrition and could be Addison's disease.
• Acute hypotension is serious circulatory shock.
• In circulatory shock, hypovolemic shock is extensive blood loss.
• Vascular shock results from extreme vasodilation and low peripheral resistance.
• Cardiogenic shock happens when the heart is struggling to maintain the demands for blood.

Control of Blood Flow

• There are four control functions to blood flow, delivery of gasses, wastes etc, gas exchange, nutrient absorption, and for urine formation in the kidneys
• Flow has the right amount to perform the function that a tissue or organ is made for
• Flow is controlled on the nervous system, hormones, and arteriolar smooth muscle tissue
• Flow is controlled by sympathetic control
• Autoregulation and arterial resistance

Control Areas

• Neural connections control the circulatory system
• Hormones affect the system
• Resistance and tissue need drives intrinsic control of the body

Blood Flow in Special Areas (Brain)

• Blood flow to the brain must be constant, neurons have low tolerance for ischemia
• Flow averages 750 ml/min.
• The brain is in extreme systemic pressure ranges with 60 mm Hg can cause fainting and 160 mm Hg cerebral edema
• Control is 1) metabolic and 2) myogenic
• In high pH or CO₂ vessels will experience marked vasodilation
• Very high CO₂ levels can supress metabolic balance in cells

Blood Flow in Special Areas (Skin)

• Skin regulation happens on the surface. surface venous plexus adjusts and varies flow from 50to 2500 ml as needed using the nervoussystem..
• Temperature rises as blood passes.
• Hypothalamic signals reduce vasomation causing more dilation.
• Blood flows into capillary beds, warming it and transferring the heat.

Blood Flow in Special Areas (Heart)

• The heart has lots of aortic pressure.
• Ventricular systole contracts the coronary vessels.
• Stored mygloblin can support oxygen.
• Flow is controlled by myoglobin.
• Contraction uses up to 65% of delivered O₂
• 25% is used by rest of tissues.

Blood Flow in Special Areas (Lungs)

• There's little change in pulmonary veins and venules; they're nearly the same.
• Vessels average 24/10 versus 120/80 mm Hg.
• Autoregulatory mechanisms opposite of other tissues, low causes more constriction and high causes dilation.

Capillary Exchange Speed

• Velocity fluctuates when blood moves thru the system.
• Speed is fast in the aorta, capillaries are slow, then they increase again at the veins
• Speed inversely in relationship of vessel

Capillary Movement

• Diffusion thru the membrane moves lipid solubles
• Movement thru intercellular clefts transport water solubles
• Fenestrations is another route for water movement
• Transports move particles using vesicles.

Colliding Fluid: Blood Flow

• Hydrostatic pressures exist in blood
• They push back with hydrostatic force
• More hydrostatic means more pressure
• Capillary pressures are 35 on arterial vs 17 on venous
• Osmotic is the force to pull back thru capillarieds
• "Sucking is another word for that force.
• 26 is how hard those are sucked back in.

Fluid-Flow

• All forces act on the capillary bed
• Pressure drives the fluids towards arterial end, and then the absorption re-enters at venous.
• The remainder returns to lymphatics.

Clinical Aspects of the Capillary System

• Edema happens from fluid increases
• Edema occurs when fluids are pulled outward on capillaries
• Lymph problems may occur and cause edema.
• Low proteins may be to blame, a decrease to proteins pulls them into tissues
• The result is edema to lung tissues
• Too much causes pitting, excess has to be drained by renal help