Blood Vessel Function and Structure

• Vasculature is the arrangement of blood vessels in the body.

Outline of Vasculature

• General anatomy of blood vessels
• Arteries
• Capillaries
• Veins
• Physiology of circulation
• Blood pressure
• Regulation of blood pressure
• Capillary exchange
• Venous return
• Circulatory shock

General Anatomy of Blood Vessels

• Three categories of blood vessels are arteries, veins, and capillaries.
• The categories differ by direction of blood flow, pressure they withstand, and histological structure of their walls.
• Arteries and veins have three tissue layers: tunica interna, tunica media, and tunica externa.
• Capillaries consist of only the tunica interna, the innermost layer.
• Tunica interna is the inner lining of the vessel and creates the borders of the lumen, being in direct contact with blood.
• It consists of simple squamous epithelium called the endothelium, which acts as a selectively permeable barrier.
• Tunica interna also secretes chemicals to prevent adhesion and platelet aggregation.
• Tunica interna also secretes chemicals that dilate or constrict blood vessels, regulating blood flow.
• Tunica media is the middle layer, usually the thickest which consists of smooth muscle, collagen, and elastic tissue.
• Smooth muscle carries out vasoconstriction and vasodilation.
• Tunica externa is the outermost layer which consists of loose connective tissue, anchoring the vessel to surrounding tissues.
• The tunica externa provides passage for small nerves, lymphatic vessels, and other smaller blood vessels.
• Arteries carry blood away from the heart.
• Veins carry blood back to the heart.
• Capillaries are microscopic vessels connecting the smallest arteries to the smallest veins.

Arteries: Qualities

• Arteries have a resilient structure, endure a surge in pressure with each heartbeat.
• The vessel retains a round shape when empty because arteries are more muscular than veins.
• Three types categorized by size exist: conducting (large), distributing (medium), and resistance (small).
• Large arteries branch into smaller arteries.

Arteries: Conducting (elastic or large)

• Includes the largest arteries like the aorta and pulmonary trunk.
• They can be up to 1 inch in diameter.
• The tunica media is dominated by layers of perforated elastic sheets of tissue.
• Smooth muscle and collagen are also present but less visible.
• Arteries expand during systole and recoil during diastole due to the abundance of elastic tissues.
• Elasticity protects smaller vessels downstream by reducing pressure surge during ejection from the heart as well as preventing blood pressure from dropping, which propels blood.

Arteries: Distributing (muscular or medium)

• Distributing arteries are smaller than elastic ones, distributing blood to specific organs.
• Think of conducting arteries as interstate highways and distributing arteries as exit ramps/state highways.
• The tunica media is dominated by smooth muscle with less elastic tissue.
• Examples include the femoral, brachial, or renal arteries.

Arteries: Resistance (small)

• These are too variable in number and location to have individual names.
• The tunica media is primarily smooth muscle with very little elastic tissue.
• Arterioles are the smallest of the resistance arteries.
• They are the initial point of control over how much blood an organ/tissue receives.

Capillaries

• Materials (wastes, hormones, nutrients) must pass through vessel walls to reach tissues.
• This process occurs primarily in capillaries, often called exchange vessels.
• There are an estimated 1 billion capillaries in the body, placing each cell within 4-6 cell widths of the nearest capillary.
• Capillaries are composed of an endothelium and a basement membrane, with walls as thin at 0.2-0.4 μm.
• They range in diameter from 5-9 μm, but cells can stretch to 7.5 μm
• Three types of capillaries are continuous, fenestrated, and sinusoids.

Capillaries: Continuous

• These are the most common type.
• The tubes of endothelial cells rolled up like a burrito and held together by tight junctions, with gaps between cells called intercellular clefts.
• Intercellular clefts allow small solutes to pass through.
• Pericytes can differentiate into new muscle and endothelial cells to help repair the capillaries, or other vessels.

Capillaries: Fenestrated

• Fenestrated capillaries have large filtration pores.
• Intercellular clefts are about 4 nm wide, with the filtration pores much larger, at 20-200 nm wide.
• They allow much larger molecules to pass through and are located where rapid filtration is needed like the kidneys.

Capillaries: Sinusoids

• These conform to the shape of the surrounding tissue.
• Their passages are 30-40 μm wide and the endothelial cells are separated by wide gaps, with large pores to pass large cells.
• Proteins and blood cells can pass through the pores of sinusoids.
• Sinusoids location include the liver, allowing albumin and clotting factors to enter, as well as bone narrow, allowing newly formed cells to enter circulation.

Capillary Beds

• The capillaries are organized in capillary beds, webs of 10-100 vessels per arteriole.
• Each capillary has a precapillary sphincter at the beginning which can dilate to let blood into a capillary, or constrict to prevent blood flow.
• Capillary beds blood flow is regulated to match the tissues needs.
• There is not enough blood to fill the vascular system at once, with ~75% shut down at any given time.

Veins

• Veins are regarded as capacitance vessels because they are relatively thin-walled, flaccid.
• Easy to accommodate increased blood volume, the have a greater capacity for blood than arteries.
• At rest, ~64% of blood is in systemic veins, and only 15% is in the arteries.
• Veins are distant from the ventricles of the heart, so are subjected to lower pressure, with steady blood flow.
• Veins do not require a muscular walls to withstand pressure surges.
• Small veins merge to form larger and larger veins.
• Three are types of vein, categorized on the basis of their size, including venules, medium sized veins and large veins.

Veins: Venules

• Venules receive blood from capillaries.
• Venules range up to 1mm in diameter.
• The smallest venules have no tunica media and are porous.
• White blood cells leave bloodstream to migrate into tissues via venules.

Veins: Medium Veins

• Medium veins range up to 10mm in diameter.
• They have a thicker tunica media and externa compared to venules.
• Many medium veins have individual names, such as the brachial vein.
• Many have venous valves, because under less pressure, blood movement requires skeletal muscle.
• Valves prevent backflow.

Veins: Large Veins

• Diameter is greater than 10mm.
• Includes veins that empty into the heart with two vena cava and four pulmonary veins.

Variations in Circulatory Routes

• Most common route is where oxygenated heart leaves via arteries through the capillary bed then deoxygenated blood returns via the veins.
• The variation to this blood pattern is portal system, where blood flows through two capillary system before returning to the heart.
• Portal systems are found where a substance is to be picked up by one capillary bed, then immediately given off by another.
• Anastomoses are another variation which involve routes that bypass the capillaries and goes directly from an artery to a vein or from one vein to another or from one artery to another.

Physiology of Circulation

• Blood flow refers to the amount of blood passing any given point.
• Perfusion is the flow relative to a given mass of tissue (ml/min per 100 grams of tissue).
• A small organ can therefore have greater relative perfusion but lower overall flow than a larger organ.
• Blood flow is governed by the fundamental principles that govern fluid flow: pressure and resistance.

Blood Pressure

• Blood pressure is the force exerted by blood on a vessel wall.
• Blood always flows down a pressure gradient (high to low).
• The greater the pressure gradient, the greater the flow.
• Arterial blood pressure is expressed as a ratio of systolic pressure to diastolic pressure.
• Systolic pressure is the pressure generated by left ventricle contraction, and diastolic pressure is the minimum blood pressure falls when ventricles relax.
• Blood pressure is measured with a sphygmomanometer, with readings expressed in millimeters of mercury (mmHg).
• A typical healthy adult is 120/80.
• Persistent high BP is associated with cardiovascular disease.
• A persistent resting blood pressure of 130/80 is considered hypertension.
• It does not include temporary elevations due to emotions or exercise.
• Chronic low blood pressure is called hypotension.
• Hypotension could result from blood loss, dehydration, or inability to regulate blood pressure fluctuation in old age.
• There is no defined numerical criteria for hypotension.

Regulation of Blood Pressure

• Determined by 3 principles:
  • Cardiac output (stroke volume x heart rate)
  • Blood volume
  • Regulated mainly by kidneys
  • Resistance to flow

Peripheral Resistance

• Peripheral resistance is the measure of difficulty of blood flowing through a vessel, caused by friction with vessel walls.
• It is the opposition to flow in vessels away from the heart.
• An higher resistance decreases flow, unless the heart pumps harder to compensate
• Resistance is determined by:
  • Viscosity
  • Length
  • Radius

Peripheral Resistance: Viscosity

• Viscosity is the "thickness" of the blood.
• Higher viscosity increases resistance, impeding flow.
• Blood viscosity is determined by concentration of erythrocytes and albumin.

Peripheral Resistance: Vessel Length

• The farther a liquid travels through a vessel, the more cumulative resistance encounters.
• Both pressure and flow accordingly decline distally.

Peripheral Resistance: Vessel Radius

• It is is most important variable in flow as vessel length and blood viscosity usually don't change rapidly.
• Regulation can be accomplished by adjusting radius of blood vessels.
  • Vasoconstriction- narrowing of vessel
  • Vasodilation- widening of vessel
• Achieved by contraction or relaxation of smooth muscle in the tunica media.

Regulation of Blood Pressure and Flow: Hormonal

• Perfusion adapts to ever-changing demand.
• Mechanisms controlling perfusion are local, neural, and hormonal.

Regulation of Blood Pressure and Flow: Local Control

• A tissue with a high metabolic rate has an increased need of oxygen and nutrients and produces more by-products, such as CO₂, lactic acid, etc.
• Vessels dilate and perfusion increases when this high metabolic rate exists.
• Long-term high metabolic demand causes angiogenesis, generating a denser capillary network.
• Long-term muscular conditions and cancerous tissues can cause local control.

Regulation of Blood Pressure and Flow: Neural Control

• Blood vessel diameter is also regulated by autonomic nervous system.
• Vessels have sensory nerve endings called baroreceptors, which monitor BP, sending signals to the medulla oblongata.
• When BP rises above normal, the vasomotor center of the medulla oblongata sends a signal to dilate blood vessels, decreasing resistance/BP falls.
• When BP falls below normal, the vasomotor center sends a signal to constrict blood vessels, increasing resistance raising the BP rise.
• These reactions are called baroreflexes.
• BP is constantly adjusted to respond to a variety of circumstances, such as gravity drawing pooling blood when standing up quickly.

Regulation of Blood Pressure and Flow: Hormonal Control

• Hormones can affect blood pressure by constricting or dilating blood vessels.
• Epinephrine and Norepinephrine are released from the adrenal medulla and sympathetic nerves, respectively.
• Epinephrine and Norepinephrine dilate some vessels and constrict other vessels.
• Angiotensin II is produced through collaboration of the liver, kidneys, and lungs.
• It is a potent which raises BP, for which angiotensin-converting enzyme (ACE) is necessary.
• Hypertension is often treated with ACE inhibitors, lowering Angiotensin levels.
• Natriuretic peptides are other hormonal mechanisms affecting blood pressure, by regulating urinary loss, and in turn affects blood volume.
  • They are secreted by the heart when BP is too high.
  • Stimulate kidneys to secrete more sodium; water then follows by osmosis, excreted by the body, lowering overall blood and BP.
• Aldosterone promotes Na and water retention in the body, raising blood volume and BP in turn.
• Antidiuretic hormone does the same, promoting retention and water.

Capillary Fluid Exchange

• Facilitated by diffusion, filtration, and osmosis.

Capillary Fluid Exchange: Diffusion

• If a substance is more concentrated in blood compared to surrounding tissue and is capable of crossing the wall, then it can cross the capillary via the substance
• Diffuses through the plasma membrane:
  • Oxygen
  • Carbon dioxide
  • Steroid hormones

Capillary Fluid Exchange: Filtration

• Blood pressure forces fluid through the capillary wall, carrying solutes from coffee.

Capillary Fluid Exchange: Osmosis

• Osmosis is a high concentration of Na, protein, and erythrocytes in the blood. Water moves from areas of low solute concentration to areas of high concentration.
• Capillaries tend to absorb water from surrounding tissues.
• Some solutes may be dissolved in the water and follow the blood into the capillaries known as solvent drag.

Exchange Summary

• Arterial end of the capillary primarily gives off fluid to tissues and delivers nutrients/oxygen.
• Venous end of capillary primarily absorbs fluid and picks up materials like metabolic waste and excess water.
• Blood entering the capillary from the arteriole is under higher pressure, so filtration override osmosis, so fluid leaves the capillary.
• As blood reaches venous end due to the low pressure level there, osmosis overrides pressure and absorbs more fluid than it gives off at venous end due to the low-pressure level.

Venous Return

• After the blood exchanges materials, it's time for it to return.
• Flow back to the heart is called venous return which can be aided by gravity but anything below the heart opposes it.
• Contributing factors are gravity aids and opposes.
• Blood pressure helps but much less than the arteries and still insufficient to move blood so skeletal and thoracic pumps assist in maintaining bloodflow.
• The skeletal muscle pump, involving the relaxation and flexing of muscle on limbs causes the squeezing of blood vessels through and maintenance of blood flow.

Venous Return: Skeletal Muscle Pump

• As lower and upper limbs are exercised, veins are squeezed in effect and blood flow is encouraged through relaxation.
• Medium veins on limbs have valves that enable the process and the regulation of blood, preventing return.
• Muscles contracting force blood up and the valves prevent backflow.

Venous Return: Thoracic Pump

• When you inhale, your chest expands, creating a squeezing effect for blood flow.
• Pressure in the thoracic cavity drops below the pressure in the abdominal cavity.
• This pressure difference squeezes the abdominal portion of the inferior vena cava, pushing blood up.
• This, and the skeletal pump, are partly why exercise and deep breathing are good for blood circulation.

Circulatory Shock

• Circulatory shock is when the heart fails to pump enough blood to meet the demands of organs.
• This can be a result of a major blood loss, compressing of vessels by tumors, and blood flow blocking of venous pooling.
• Venous pooling can result from allergic reactions.
• Standing can result from venous pooling via the skeletal muscle and if it occurs for too long, fainting occurs as a result.
• Most of the time the body can recover from circulatory shock, with severe cases leading to death.

Explore blood vessel layers, vasoconstriction/dilation, and endothelium. Understand arteries, capillaries, and veins' roles in circulation. Learn about blood pressure and precapillary sphincters.