blood vessels

Overview of Blood Vessels

  • Definition: The cardiovascular system's delivery system consisting of dynamic structures that begin and end at the heart.
  • Arteries: Carry blood away from the heart. They are typically oxygenated, with the exception of the pulmonary circulation and the umbilical vessels of a fetus.
  • Capillaries: These vessels make direct contact with tissue cells to serve cellular needs.
  • Veins: Carry blood toward the heart.

Structure of Vessel Walls (Tunics)

  • Tunica Intima:
    • Endothelium: Simple squamous epithelium that lines the lumen of all vessels. It is continuous with the endocardium. Its slick surface reduces friction.
    • Subendothelial Layer: Found in vessels larger than 1 mm1\,mm; consists of a connective tissue basement membrane.
  • Tunica Media:
    • Composition: Smooth muscle and sheets of elastin.
    • Control: Sympathetic vasomotor nerve fibers control vasoconstriction and vasodilation.
    • Function: Influences blood flow and blood pressure.
  • Tunica Externa (Tunica Adventitia):
    • Composition: Collagen fibers that protect and reinforce the vessel while anchoring it to surrounding structures.
    • Contents: Contains nerve fibers and lymphatic vessels.
    • Vasa Vasorum: Found in larger vessels to nourish the external layer.
  • Capillary Structure: Consists only of the endothelium with a sparse basal lamina.

The Arterial System

  • Elastic Arteries:
    • Description: Large thick-walled arteries with elastin present in all three tunics.
    • Function: Inactive in vasoconstriction. They act as pressure reservoirs, expanding and recoiling as blood is ejected from the heart.
  • Muscular Arteries:
    • Structure: Possess a thick tunica media with a higher proportion of smooth muscle.
    • Function: Active in vasoconstriction to distribute blood.
  • Arterioles:
    • Description: The smallest arteries.
    • Function: Lead to capillary beds and control flow via vasodilation and vasoconstriction.

Capillaries

  • General Characteristics:
    • Microscopic vessels with walls consisting only of a thin tunica intima.
    • Diameter: In the smallest vessels, one cell forms the entire circumference; the diameter allows only a single red blood cell (RBCRBC) to pass at a time.
    • Distribution: Found in all tissues except cartilage, epithelia, the cornea, and the lens of the eye.
    • Functions: Exchange of gases, nutrients, wastes, and hormones between blood and interstitial fluid.
    • Precapillary Sphincters: Regulate blood flow into true capillaries.
  • Capillary Types:
    • Continuous Capillaries: Abundant in skin and muscles. Intercellular clefts allow passage of fluids and small solutes. In the brain, tight junctions are complete, forming the blood-brain barrier.
    • Fenestrated Capillaries: More permeable than continuous capillaries. Function in absorption or filtrate formation (found in small intestines, endocrine glands, and kidneys).
    • Sinusoid Capillaries: Characterized by sluggish blood flow allowing for modification. Large molecules and blood cells pass between blood and tissues. Found in the liver, bone marrow, spleen, and adrenal medulla.

The Venous System

  • Venules: Formed when capillary beds unite. The smallest (postcapillary venules) are very porous, allowing fluids and white blood cells (WBCsWBCs) into tissues.
  • Veins:
    • Characteristics: No compliance and low pressure. They feature large lumens and thin walls.
    • Wall Layers: Thin tunica media and a thick tunica externa of collagen fibers and elastic networks.
    • Capacitance: Contain up to 65%65\% of the blood supply.
    • Resistance: Large-diameter lumens offer little resistance to flow.
    • Venous Valves: Prevent backflow; most abundant in the limbs.
    • Venous Sinuses: Flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain).

Physiology of Circulation: Definitions

  • Blood Flow (FF): Volume of blood flowing through a vessel, organ, or entire circulation in a given period (ml/minml/min).
  • Blood Pressure (BPBP): Force per unit area exerted on the wall of a blood vessel by blood, expressed in mm Hgmm\,Hg. The pressure gradient provides the driving force for movement.
  • Resistance (Peripheral Resistance): Opposition to flow; a measure of friction encountered in the systemic circulation.
    • Sources of Resistance:
      1. Blood Viscosity: The "stickiness" of blood due to formed elements and plasma proteins. Increased viscosity results in increased resistance.
      2. Total Blood Vessel Length: Longer vessels result in greater resistance.
      3. Blood Vessel Diameter: The greatest influence on resistance.

Mathematical Relationships in Circulation

  • Flow and Pressure: Blood flow (FF) is directly proportional to the blood pressure gradient (ΔP\Delta P).
    • If ΔP\Delta P increases, blood flow speeds up.
  • Flow and Resistance: Blood flow is inversely proportional to peripheral resistance (RR).
    • If RR increases, blood flow decreases.
  • The Formula:F=ΔPRF = \frac{\Delta P}{R}
  • Resistance Importance: Resistance is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter.

Systemic Blood Pressure

  • Generation: Pumping action of the heart generates flow; pressure results when flow is opposed by resistance.
  • Pressure Profile:
    • Highest in the aorta.
    • $0\,mm\,Hg in the right atrium.\n * The steepest drop occurs in the arterioles.\n* **Arterial Blood Pressure Terms:**\n * **Systolic Pressure:** Pressure in the aorta during ventricular contraction (average 120\,mm\,Hg).\n * **Diastolic Pressure:** Lowest level of aortic pressure during heart relaxation.\n * **Pulse Pressure:** The difference between systolic and diastolic pressure (the "throbbing" pulse).\n * **Mean Arterial Pressure (MAP):** The pressure that propels blood to tissues.\n * **Example Calculations:** If BP = 120/80\,mm\,Hg,then, thenMAP = 93\,mm\,Hg.\n\n# Venous Return and BP Maintenance\n\n* **Factors Aiding Venous Return:**\n 1. **Muscular Pump:** Skeletal muscle contraction "milks" blood toward the heart; valves prevent backflow.\n 2. **Respiratory Pump:** Pressure changes during breathing move blood toward the heart (squeezing abdominal veins as thoracic veins expand).\n 3. **Venoconstriction:** Under sympathetic control.\n* **Maintaining Blood Pressure:** Requires cooperation of the heart, blood vessels, and kidneys, supervised by the brain.\n * **Main Factors:** Cardiac Output (CO),PeripheralResistance(), Peripheral Resistance (PR), and Blood Volume.\n * **Correlated Formulas:**\n        CO = \frac{\Delta P}{R}\n        \Delta P = CO \times R\n        BP = CO \times PR\n\n# Blood Pressure Regulation Mechanisms\n\n* **Short-Term Neural Controls:** Operate via reflex arcs involving:\n * **Baroreceptors:** Located in carotid sinuses, aortic arch, and walls of large arteries of the neck and thorax.\n * Factors causing a decrease in BP: Arteriolar vasodilation, venodilation, and decreased cardiac output.\n* **Chemoreceptor Reflexes:** \n * Located in the aortic arch and large neck arteries.\n * Detect increased CO_2,dropped, droppedpH,ordropped, or droppedO_2.\n * Action: Signaling cardioacceleratory center to increase CO and vasomotor center to increase vasoconstriction.\n* **Hormonal Controls (Short-Term):**\n * Sympathetic activation: Adrenal glands release epinephrine and norepinephrine (increases CO and vasoconstriction).\n * Decreased pressure/volume causes kidneys to produce Renin, activating Angiotensin II (vasoconstrictor).\n * Antidiuretic Hormone (ADH) and Aldosterone are released for fluid retention.\n* **Hormonal Controls (Long-Term):**\n * Thirst stimulation.\n * Erythropoietin release for red blood cell formation.\n * Atrial Natriuretic Hormone (ANH): A powerful vasodilator that promotes fluid loss from kidneys to reduce blood volume and pressure.\n\n# Comparison of Neural and Endocrine Factors\n\n| Factor | Vasoconstriction | Vasodilation |\n| :--- | :--- | :--- |\n| **Sympathetic** | Arterioles in skin, viscera; skeletal muscle (high levels) | Arterioles in heart; skeletal muscle (low levels) |\n| **Parasympathetic** | No known innervation for most | Arterioles in external genitalia |\n| **Epinephrine** | Similar to sympathetic (alpha receptors) | Beta (\beta) receptors at low/moderate levels |\n| **Angiotensin II** | Powerful generalized vasoconstrictor | n/a |\n| **ADH** | Moderately strong generalized | n/a |\n| **ANH** | n/a | Powerful generalized vasodilator |\n\n# Measuring and Altering Blood Pressure\n\n* **Normal Ranges:**\n * Systolic: < 120\,mm\,Hg (sounds first occur as blood spurts).\n * Diastolic: < 80\,mm\,Hg (sounds disappear as blood flows freely).\n* **Variations:** Affected by age, sex, weight, race, mood, posture, and physical exertion.\n* **Hypertension:** Sustained pressure of 140/90\,mm\,Hg or higher.\n * Prehypertension: Elevated but not yet hypertensive.\n * Risks: Heart failure, vascular disease, renal failure, and stroke.\n* **Hypotension:** Pressure below 90/60\,mm\,Hg.\n * Usually not a concern unless it leads to inadequate tissue perfusion.\n\n# Blood Flow Dynamics\n\n* **Velocity:** Inversely related to total cross-sectional area.\n * Fastest in the aorta.\n * Slowest in capillaries (allows exchange time).\n * Increases again in the veins.\n* **Tissue-Specific Flow:**\n * **Skeletal Muscle:** REST: 1\,L/min.EXERCISE:Canincrease. EXERCISE: Can increase10\times((12,500\,ml/min).\n * **Brain:** Constant flow at 750\,ml/min. \n * Syncopy (fainting) occurs if MAP < 60\,mm\,Hg.\n * Cerebral edema occurs if MAP > 160\,mm\,Hg.\n * **Skin:** Functions in nutrient supply and temperature regulation.\n * **Lungs:** Low O_2causesvasoconstriction;highcauses vasoconstriction; highO_2 causes vasodilation (opposite of systemic) to direct blood to oxygen-rich areas.\n * **Heart:** Flow ceases during ventricular systole (compression) and occurs during diastole.\n\n# Blood Flow Totals (Rest vs. Strenuous Exercise)\n\n| Organ | Rest (ml/min)∣StrenuousExercise() | Strenuous Exercise (ml/min) |\n| :--- | :--- | :--- |\n| Brain | 750∣|750 |\n| Heart | 250∣|750 |\n| Skeletal Muscles | 1200∣|12,500 |\n| Skin | 500∣|1900 |\n| Kidneys | 1100∣|600 |\n| Abdomen | 1400∣|600 |\n| Other | 600∣|400 |\n| **Total** | **5800∗∗∣∗∗** | **17,500** |\n\n# Circulatory Shock\n\n* **Definition:** Condition where blood vessels are inadequately filled and blood cannot circulate normally to meet tissue needs.\n* **Hypovolemic Shock:** Results from large-scale blood loss.\n* **Vascular Shock:** Result of extreme vasodilation and decreased peripheral resistance.\n* **Cardiogenic Shock:** Result of an inefficient heart that cannot sustain circulation.\n\n# Anatomy of the Pulmonary and Systemic Circuits\n\n* **Pulmonary Circuit:**\n * Pulmonary Trunk: Exits right ventricle; divides into right and left pulmonary arteries.\n * Pulmonary Arteries: Lead to pulmonary capillaries.\n * Pulmonary Veins: Two pairs lead from capillaries to the left atrium.\n* **Systemic Aorta:**\n * **Ascending Aorta:** Rising from left ventricle (\sim 5\,cm$$).
    • Aortic Arch: Connects ascending to descending; ends at T4/T5 disk.
    • Descending Aorta: Subdivided into Thoracic (above hiatus) and Abdominal (below hiatus).
  • Major Arterial Branches:
    • Celiac Trunk: Supplies stomach (Left gastric), spleen (Splenic), and liver (Common hepatic).
    • Superior Mesenteric: Small intestine and first two-thirds of large intestine.
    • Inferior Mesenteric: Last third of large intestine.
    • Renal Arteries: Supply the kidneys.
    • Gonadal Arteries: Testes or ovaries.
    • Circle of Willis: Anastomosis at base of brain ensuring continual blood supply.
  • Major Venous Branches:
    • Superior Vena Cava: Drains areas superior to the diaphragm.
    • Inferior Vena Cava: Drains areas inferior to the diaphragm.
    • Hepatic Vein: Drains blood from the liver into the IVC.
    • Renal Vein: Largest vein entering the IVC, draining kidneys.
    • Internal Jugular Vein: Drains the brain; parallel to common carotid.