Cardiovascular System: Blood Vessels

Blood Vessels

• Blood is carried in a closed system of vessels that begins and ends at the heart.
• The intricate control of blood pressure is a key concept.
• The body must be flexible to deal with varying blood pressure conditions.

Controlling Blood Pressure

• The body regulates blood pressure effectively.
• Homeostasis disturbance can lead to various medical conditions.

Role of Blood Vessels in Medical Conditions:

• Cancer: Tumor angiogenesis (formation of new blood vessels) supplies malignant cells.
• Atherosclerosis: Lipid lumps (atheromas) in blood vessel walls cause cardiovascular disease.
• Inflammation: Increases blood vessel permeability.
• Trauma/Spontaneous Damage: May lead to hemorrhage.
• Occlusion: Ruptured plaque or emboli can cause ischemia (insufficient blood supply) and necrosis (tissue breakdown).
• Vasculitis: Inflammation of the vessel wall due to autoimmune disease or infection.

Main Pathologies Related to Blood Vessels:

• Aneurysm
• Atherosclerosis
• Cerebrovascular Accident (Stroke)
• Dilated cardiomyopathy
• Diastolic dysfunction
• High Blood Pressure (Hypertension)
• Venous Thromboembolism
• Preeclampsia
• Shock
• Vasculitis
• Raynaud's Disease

Types of Blood Vessels

• Arteries: Carry blood away from the heart.
• Capillaries: Contact tissue cells and serve cellular needs directly.
• Veins: Carry blood toward the heart.

Generalized Structure of Blood Vessels

• Arteries and veins have three tunics (coverings):
  • Tunica interna.
  • Tunica media.
  • Tunica externa.
• Lumen: Central blood-containing space.
• Common structures to mention
  • Smooth
  • Muscle
  • Structural support

Tunica Interna (Intima)

• Endothelial layer lining the lumen of all vessels.
• In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present.

Tunica Media

• Smooth muscle and elastic fiber layer, regulated by the sympathetic nervous system.
• Controls vasoconstriction/vasodilation of vessels.

Tunica Externa

• Collagen fibers that protect and reinforce vessels.

Vessel Wall Layers (Outside to Inside)

• Tunica externa, tunica media, tunica interna

Structure and Function Logic

• Tunica interna: Slick, same in veins and arteries.
• Arteries: Thicker tunica media to handle higher pressures.
• Muscle: Strong.
• Veins: Big lumen because they are low pressure.

Tunica Enlargement with High Blood Pressure

• Tunica media enlarges due to muscle responding to strength.

Artery, Vein, Capillary structure in depth:

• Detailed description on the structure and function of multiple types of blood vessels ranging from Large Vein, Elastic Artery, Muscular Artery, Medium-Sized Vein, Venule and Fenestrated capillaries.

Two Types of Arteries

• Conducting (Elastic): The highways
• Distributing (Muscular): City streets and neighborhood streets

Elastic (Conducting) Arteries

• Thick-walled arteries near the heart, such as the aorta and its major branches.
• Large lumen allows low-resistance conduction of blood.
• Contain elastin in all three tunics.
• Withstand and smooth out large blood pressure fluctuations.
• Allow blood to flow continuously through the body.

Muscular (Distributing) Arteries and Arterioles

• Muscular arteries: Distal to elastic arteries; deliver blood to body organs.
• Thick tunica media with more smooth muscle and less elastic tissue.
• Active in vasoconstriction.
• Arterioles: Smallest arteries, lead to capillary beds.
• Control flow into capillary beds via vasodilation and constriction.
• Main source of blood pressure.

Principle of Complementarity

• Difference between elastic and conducting arteries in their tunics.

Innervation of Blood Vessel Layers

• Sympathetic nervous system (SNS) innervates the tunica media because SNS cares about muscle.

Thickness of Tunics

• Elastic arteries have a thicker tunica externa; distributing arteries have a thicker tunica media.

Capillaries

• Smallest blood vessels.
• Walls consist of a thin tunica interna, one cell thick.
• Allow only a single RBC to pass at a time.
• Pericytes on the outer surface stabilize their walls.

Three Structural Types of Capillaries:

• Continuous
• Fenestrated
• Sinusoids

Continuous Capillaries

• No holes
• Brain, Skin, Muscles
• Control flow

Fenestrated Capillaries

• Holes.
• Small Intestine, Kidneys.
• Allow some leakage.

Sinusoids

• Large holes.
• Liver, Marrow, Lymph Nodes.
• Open to allow processing of blood.

Continuous Capillaries

• Abundant in the skin, muscles, and brain.
• Tightly packed cells, held together with tight junctions.
• Small intercellular clefts allow passage of some fluids.
• In the brain, they form the blood-brain barrier.

Fenestrated Capillaries

• Found in small intestines, endocrine glands, and kidneys.
• Endothelium riddled with pores (fenestrations).
• Greater permeability to solutes and fluids.

Sinusoids

• Highly modified, leaky, fenestrated capillaries with large lumens.
• Found in the liver, bone marrow, lymphoid tissue, and some endocrine organs.
• Blood flows sluggishly, allowing for modification in various ways.

Capillary Beds

• A net of capillaries composed of:
  • True capillaries
  • Vascular shunt

Blood Flow Through Capillary Beds

• Precapillary sphincter: Cuff of smooth muscle that surrounds each true capillary.
• Regulates blood flow into the capillary.
• Controlled by nerves, $O<em>2$ levels, $CO</em>2$ levels, lactic acid, and other metabolites.
• Allows local independent control of blood flow.

Metaphor for Blood Flow

• Space heater in every room versus heating the whole house with a furnace.
• More efficient to move a smaller volume of blood where needed.
• Capillaries have some control of flow via thermostats.

Capillary Beds - Shunt

• Capillary sphincters opening and closing depends on:
  • Nerves
  • $O_2$ levels
  • $CO_2$ levels
  • lactic acid
  • other metabolites

Vessels Providing Direct Access

• Capillaries are the only vessels that provide direct access to nearly every cell in the body.

Venous System

Venules

• Formed when capillary beds unite.
• Allow fluids and WBCs to pass from the bloodstream to tissues.

Veins

• Formed when venules converge.
• Capacitance vessels (blood reservoirs) that contain 65% of the blood supply.
• Lower blood pressure than arteries.
• Adaptations to return blood to the heart:
  • Large-diameter lumens, which offer little resistance to flow.
  • Valves (resembling semilunar heart valves), which prevent backflow of blood.

Venous Sinuses

• Specialized, flattened veins with extremely thin walls.
• Examples: coronary sinus of the heart and dural sinuses of the brain.

Vascular Anastomoses

• Merging blood vessels, more common in veins than arteries.
• Arterial anastomoses provide alternate pathways (collateral channels) for blood to reach a given body region.
• If one branch is blocked, the collateral channel can supply the area with adequate blood supply.

Blood Flow

• Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period.
• Measured in ml per min.
• Equivalent to cardiac output (CO) for the entire vascular system.
• Relatively constant when at rest.
• Varies widely through individual organs, according to immediate needs.

Location of Blood Supply

• 65% of blood supply in the venous system
• 13% in systemic arterial system
• 7% in the heart
• 15% in the pulmonary circuit

Blood Location Proportion Question

• Up to 65% of the body's blood supply is found in the vessels with large lumens (veins).

Blood Pressure (BP)

• Force per unit area exerted on the wall of a blood vessel by its contained blood.
• Expressed in millimeters of mercury (mm Hg).
• Measured in reference to systemic arterial BP in large arteries near the heart.

Resistance

• Opposition to flow.
• Causes blood pressure as blood pushes against vessel walls.
• Three types of resistance:
  • Blood viscosity
  • Total blood vessel length
  • Blood vessel diameter

Resistance Factors

• Blood Vessel Diameter:
  • Changes are frequent and significantly alter peripheral resistance.
  • Resistance varies inversely with the fourth power of vessel radius:
  • $R \propto \frac{1}{r^4}$
  • If radius is doubled, resistance is 1/16 as much.
• Only vessel diameter can be changed rapidly.
• Small-diameter arterioles are major determinants of peripheral resistance.

Sympathetic Tone

• Increased Tone = Vasoconstriction.
• Decreased Tone = Vasodilation.

Turbulence

• Fatty plaques from atherosclerosis cause turbulent blood flow and dramatically increase resistance.

Systemic Blood Pressure

• Highest in the aorta.
• Declines throughout the pathway.
• Is 0 mm Hg in the right atrium.
• Steepest change occurs in the arterioles.

Measuring Blood Pressure

• Arterial BP reflects:
  • Elasticity (compliance or distensibility) of arteries close to the heart.
  • Amount of blood forced into them at any given time.

Importance of Elasticity

• Indicates amount of plaque in vessels.
• Diminished control of BP if vessels are not elastic.

Pulsatile Blood Pressure

• Blood pressure in elastic arteries near the heart is pulsatile (BP rises and falls).
• Systolic pressure and diastolic pressure fluctuate.

Arterial Blood Pressure Terms

• Systolic pressure: Pressure exerted on arterial walls during ventricular contraction.
• Diastolic pressure: Lowest level of arterial pressure during a ventricular cycle.
• Pulse pressure: Difference between systolic and diastolic pressure.
  • $Pulse Pressure = Systolic Pressure - Diastolic Pressure$
• Mean arterial pressure (MAP): Pressure that propels the blood to the tissues.
  • $MAP = Diastolic Pressure + \frac{1}{3} Pulse Pressure$

Importance of MAP

• A MAP below 60mmHg indicates insufficient organ perfusion.
• A MAP above 160mmHg results in cerebral edema.

Capillary Blood Pressure

• Capillary BP ranges from 20 to 40 mm Hg.
• Low capillary pressure is desirable to avoid rupturing fragile capillaries.
• Sufficient to force filtrate out into interstitial space.

Venous Blood Pressure

• Venous BP is steady and changes little during the cardiac cycle.
• Pressure gradient in the venous system is only about 20 mm Hg.
• A cut vein has even blood flow; a lacerated artery flows in spurts.

Factors Aiding Venous Return

• Respiratory “pump”: Pressure changes during breathing suck blood toward the heart.
• Muscular “pump”: Contraction of skeletal muscles “milk” blood toward the heart.
• Valves prevent backflow during venous return.

Varicose Veins

• Conditions that increase chances:
  • An 8-hour flight
  • Pregnancy
  • Standing at attention for long periods of time

Maintaining Blood Pressure

• Requires:
  • Cooperation of the heart, blood vessels, and kidneys.
  • Supervision of the brain.

Main Factors Influencing Blood Pressure

• Cardiac output (CO)
• Peripheral resistance (PR)
• Blood volume

Controls of Blood Pressure

• Short-term controls alter cardiac output and vessel diameter.
• Long-term controls regulate blood volume.

Short-Term Mechanisms

• Vasomotor center: Changes vessel diameter.
• Cardiovascular center: Changes cardiac output.
• Baroreceptors.
• Chemoreceptors.
• Higher brain centers.
• Bloodborne chemicals and hormones.

Baroreceptor Reflexes

• Negative feedback loops that respond to changes in blood pressure.

Chemicals that Increase Blood Pressure

• Adrenal medulla hormones: norepinephrine and epinephrine.
• Antidiuretic hormone (ADH).
• Angiotensin II.
• Endothelium-derived factors: endothelin and prostaglandin-derived growth factor (PDGF).

Chemicals that Decrease Blood Pressure

• Atrial natriuretic peptide (ANP).
• Nitric oxide (NO).
• Inflammatory chemicals: histamine, prostacyclin, and kinins.
• Alcohol.

Long-Term Mechanisms: Renal Regulation

• Control BP by altering blood volume.
• Increased BP stimulates kidneys to eliminate water.
• Decreased BP stimulates kidneys to increase blood volume and BP.

Kidney Action and Blood Pressure

• Direct renal mechanism alters blood volume.
• Indirect renal mechanism involves the renin-angiotensin mechanism.

Blood Flow Through Tissues

• Involved in:
  • Delivery of oxygen and nutrients to tissue cells.
  • Removal of wastes.
  • Gas exchange in the lungs.
  • Absorption of nutrients from the digestive tract.
  • Urine formation by the kidneys.

Perfusion Triangle

• Heart (Pump Function):
• Blood Vessels (Container Function)
• Blood (Content Function)

Autoregulation of Blood Flow

• Short-term
  *Metabolic (cellular respiration ; $O<em>2$, $CO</em>2$
  *Myogenic Protective reactions
• Long-term
  • Angiogenesis

Velocity of Blood Flow

• Changes as it travels through the systemic circulation.
• Inversely proportional to the cross-sectional area.
• Slow capillary flow allows adequate time for exchange between blood and tissues.

Capillary Exchange of Respiratory Gases and Nutrients

• Oxygen, carbon dioxide, nutrients, and metabolic wastes diffuse between the blood and interstitial fluid along concentration gradients.
• Water-soluble solutes pass through clefts and fenestrations.
• Lipid-soluble molecules diffuse directly through endothelial membranes.

Forces Causing Fluid Flow

• Hydrostatic and osmotic pressures.

Capillary Exchange: Fluid Movements

• Direction and amount of fluid flow depends upon the difference between:
  • Capillary hydrostatic pressure (HPc).
  • Capillary colloid osmotic pressure (OPc).

Capillary Hydrostatic Pressure (HPc)

• Pressure of blood against the capillary walls.
• Tends to force fluids through the capillary walls.
• Greater at the arterial end of a bed than at the venule end.

Capillary Colloid Osmotic Pressure (OPc)

• Created by nondiffusible plasma proteins, which draw water toward themselves.

Net Filtration Pressure (NFP)

• Considers all the forces acting on a capillary bed.
• $NFP = (HPc - HPif) - (OPc - OPif)$
• $NFP = (HPc - HPif) - (OPc - OPif)$
• At the arterial end of a bed, hydrostatic forces dominate (fluids flow out).
• At the venous end of a bed, osmotic forces dominate (fluids flow in).

Edema

• Related to capillary exchange and other medical conditions such as congestive heart failure and liver disease.

Circulatory Shock

• Any condition in which blood vessels are inadequately filled, and blood cannot circulate normally.
• Results in inadequate blood flow to meet tissue needs.

Types of Circulatory Shock

• Hypovolemic shock: Large-scale blood loss.
• Vascular shock: Poor circulation resulting from extreme vasodilation.
• Cardiogenic shock: The heart cannot sustain adequate circulation.