Human Physiology Blood Vessels and Circulation

Circulatory Physiology and Smooth Muscle

• This chapter covers the anatomy and physiology of blood vessels and smooth muscle, focusing on the circulatory system.
• Objectives include the anatomy of blood vessels, systemic circulatory pressures, and regulation of cardiovascular function.

Anatomy of Blood Vessels

• Blood vessels have a lumen surrounded by a wall.
• Major vessels (excluding capillaries) have three layers:
  • Tunica intima (innermost layer with endothelium).
  • Tunica media (middle layer composed mainly of smooth muscle and elastin).
  • Tunica externa (outermost layer with loose collagen fibers).
• Capillaries are composed of a single layer of endothelium.

Structure of Blood Vessel Walls

• Walls of vessels are composed of three tunics
• Tunica Intima:
  • Endothelium
  • Subendothelial layer
  • Internal Elastic lamina
• Tunica Media:
  • Smooth muscle
  • Elastic lamina
• Tunica Externa:
  • Loose connective tissue
• Capillary walls consist entirely of endothelium, resting on a basement membrane.

Smooth Muscle

• Smooth muscle fibers are spindle-shaped.
• Found in walls of internal organs, and blood vessels (except the heart).
• Fibers contract, causing constriction.
• Smooth muscle can contract in a 'corkscrew' manner.
• Smooth muscle cells lack T tubules and sarcomeres.
• Contain thin and thick filaments along with intermediate filaments.
• Dense bodies are analogous to Z-discs.

Microscopic Anatomy of Smooth Muscle

• Smooth muscle has a less developed sarcoplasmic reticulum (SR) compared to skeletal Muscle.
• T tubules are absent in smooth muscle
• Plasma membranes have pouch-like infoldings called caveoli, which are involved in Calcium (Ca2+) storage.

Organization of Myofilaments

• Thick filaments in smooth muscle have heads along their entire length.
• There is no troponin complex.
• Thick and thin filaments are arranged diagonally, creating a corkscrew-like contraction pattern.

Contraction of Smooth Muscle

• Whole sheets of smooth muscle contract in a synchronized, slow manner.
• Electrical coupling via gap junctions synchronizes contraction.
• Some smooth muscle cells act as pacemakers.
• Smooth muscle cells are self-excitatory and depolarize without external stimuli.
• Contraction relies on Ca2+ influx, Calmodulin, and Myosin Light Chain Kinase (MLCK)

Contraction Mechanism

• Calcium (Ca2+) entry in the cell triggers contraction.
• Ca2+ binds with calmodulin.
• This complex activates MLCK.
• MLCK phosphorylates myosin.
• Myosin-actin interaction leads to muscle contraction.

Special Features of Smooth Muscle Contraction

• Smooth muscle tone
• Slow, prolonged contractile activity
• Low energy requirements
• Response to stretch

Regulation of Smooth Muscle Contraction

• Neural Regulation: Sympathetic (norepinephrine) and parasympathetic (acetylcholine)
• Hormonal Regulation: Epinephrine, histamines, prostaglandins, ANG
• Stress: Stretch-activated channels
• Other: ATP, pH, CO2

Systemic Circulatory Pressures

• The main force for blood flow is ventricular contraction.
• Blood pressure results from elastic recoil of arteries after ventricular ejection.
• Blood pressure continuously decreases throughout the circulatory system.

Blood Pressure

• The main factor is ventricular contraction.
• Elastic recoil of arteries pushes blood forward continuously.
• Pressure is highest in arteries, gradually decreasing through arterioles, capillaries, venules, and veins.

Systemic Circulation Pressures

• Systolic pressure: Maximum pressure during ventricular contraction (typically 120 mmHg).
• Diastolic pressure: Minimum pressure during ventricular relaxation (typically 80 mmHg).
• Mean arterial pressure (MAP): Average pressure during one cardiac cycle (approximately [(2xDBP) + SBP]/3).
• Pressure decreases to zero in the venous system.
• Pressure waves disappear at capillaries due to the resistance they experience.
• Pressure in veins is steady, unlike that in arteries.

Measuring Blood Pressure

• A sphygmomanometer measures blood pressure.
• Cuff pressure is inflated to stop blood flow.
• Pressure is released and the first sound heard (Korotkoff sounds) indicates systolic pressure.
• The last sound heard indicates diastolic pressure.

Arterial Blood Pressure

• Arterial blood pressure reflects the elasticity of the arteries near the heart and the volume of blood forced into them.
• Two main factors are involved:
  • Compliance/distensibility (elasticity) of arteries.
  • Volume of blood forced into large arteries (from the heart).

Pulse Pressure

• Pulse pressure (PP) = Systolic Blood Pressure (SBP) - Diastolic Blood Pressure (DBP)

Capillary Blood Pressure

• Capillary blood pressure ranges from 20 to 40 mm Hg.
• Low capillary pressure is crucial to prevent damage.
• It allows filtration of nutrients, gases, and hormones out into tissues.

Venous Blood Pressure

• Venous blood pressure is relatively steady from cardiac cycles and only about 20 mm Hg.
• Blood flow in veins is continuous, unlike the pulsatile flow in arteries.

Factors Aiding Venous Return

• Respiratory pump, skeletal muscle pump, and valves
• Respiratory pump changes pressure to help blood flow.
• Skeletal muscle contraction compresses veins to aid blood flow against gravity.

Ohm's Law Applied to Blood Flow

• Flow (F) = Change in Pressure (ΔP) / Resistance (R)

Flow

• Actual volume of blood flowing through a vessel in a given period.
• Measured in ml/min and is equivalent to cardiac output.
• Relatively constant at rest, varies widely through organs based on immediate needs.

Pressure

• Force per unit area exerted by blood on the blood vessel wall.
• Expressed in millimeters of mercury (mmHg).
• Measured in reference to systemic arterial blood pressure in large arteries near the heart.

Maintaining Blood Pressure

• Maintained by the heart, blood vessels, and kidneys with brain supervision.
• Key factors: cardiac output (CO), total peripheral resistance (TPR), and blood volume.

Resistance

• Opposition to blood flow.
• Result of friction between blood and vessel walls.
• Influenced by blood viscosity, vessel length, and vessel diameter.

Resistance Factors

• Small-diameter arterioles significantly influence peripheral resistance.
• Atherosclerosis and fatty plaques cause turbulent blood flow leading to increased resistance.

Local Controls

• Automatic regulation of arteriolar radius.
• Myogenic responses regulate vessel diameter based on stretch.
• Paracrine substances (e.g., nitric oxide) alter vessel diameter in response to local factors.
• Metabolites (e.g., potassium ions) affect vessel diameter locally in response to changes in metabolism in affected tissues.

Extrinsic Controls

• Baroreceptors: Pressure sensors in the carotid sinus and aortic arch that trigger adjustments to blood pressure, based on stimuli (rising or falling BP).
• Chemoreceptors: Chemical sensors in the carotid body and aortic arch that respond to changes in blood chemistry (O2, CO2, and H+).
• Hormones: Epinephrine affects blood pressure, and hormones like angiotensin and aldosterone (Renal system) have long-term effects on blood pressure.

Regulation of Mean Arterial Pressure (MAP)

• Control systems (local and extrinsic) to maintain consistent MAP. Inputs influence responses throughout the vascular system.

Application Questions (provided in the original text)

• These are questions related to scenarios impacting blood pressure; answers require applying concepts learned from the study material.