Vascular System and Hepatic Portal Circulation

Administrative Information and Test Dates

  • Test Schedule:     * August 20: Test on Musculoskeletal Pharmacology.     * August 22: Test on Chapter 16 (Vascular System).
  • Class Materials: Review sheets and worksheets provided in class are described as "extremely, extremely helpful" for studying for the upcoming tests.
  • Attendance Note: The instructor will not be present on the day of the Chapter 16 test.

General Characteristics and Classification of Veins

  • Definition and Function: Veins are blood vessels that carry blood back toward the heart.
  • Principal Vein (Vena Cava): The main vein of the body is the vena cava, which is divided into two sections:     * Superior Vena Cava (SVC): This vessel receives deoxygenated blood from the head, shoulders, and arms.     * Inferior Vena Cava (IVC): This vessel receives deoxygenated blood from the lower part of the body.
  • Drainage Patterns:     * Some veins drain directly into the superior vena cava.     * Other veins drain into a second vein, which might then merge with another vein before finally emptying into the vena cava.     * Examples of major draining veins include the subclavian and jugular veins.
  • Structural Classifications:     * Superficial Veins: These reside near the surface of the body.     * Deep Veins: These are situated deeper within the body, are very well protected, and usually run alongside bones or corresponding arteries.

Specific Principal Veins and Clinical Significance

  • Hepatic Veins: These veins function specifically to drain the liver.
  • Internal Jugular Vein: This vein drains most of the blood from the brain.
  • Cephalic Vein: Identified as a frequent site for the administration of intravenous (IVIV) fluids.
  • Median Cubital Vein: This is the most common site used by healthcare professionals for drawing blood.
  • Popliteal Vein: This vein is located behind the knee.
  • Great Saphenous Vein: This is documented as the longest vein in the human body.     * Clinical Use: It is frequently harvested for use in surgical grafts.     * CABG: Specifically used in Coronary Artery Bypass Grafting (CABGCABG) procedures.

Procedures and Clinical Experiences (Catheterization and Stents)

  • Access Points: Surgeons often go through the groin (femoral artery) or the wrist (radial artery) to place stents or perform heart procedures.
  • Patient Experience during Stent Placement:     * Patients are typically kept awake during these procedures.     * Patients must lie completely still (prone or supine position).     * The patient may receive light sedation, but not full anesthesia, though this varies by surgeon and anesthesiologist.     * Movements, such as turning the head to watch the monitor, can be dangerous and are discouraged by the surgical team.
  • Risks: Moving during the procedure carries the risk of nicking a major vessel, such as the femoral artery, which could cause the patient to bleed out.
  • Ablation and Other Procedures: In some cases, such as cardiac ablation for Supraventricular Tachycardia (SVTSVT), patients may be put to sleep but could wake up if the heart rate is manipulated. Aneurysm clamps can also be placed via the groin while the patient is under anesthesia.

Veins of the Head and Neck

  • Internal Jugular Vein:     * Receives the majority of blood from the brain and the face.     * Flow Pathway: Internal Jugular $→$ Subclavian Vein $→$ Brachiocephalic Vein $→$ Superior Vena Cava.
  • External Jugular Vein:     * This is the more superficial of the jugular veins.     * It drains blood from the scalp, facial muscles, and other superficial structures.     * It drains directly into the subclavian vein.
  • Vertebral Vein: Drains the cervical vertebrae, the spinal cord, and several muscles of the neck.

The Hepatic Portal Circulation

  • Definition of a Portal System: A deviation from the standard circulatory path (Artery $→$ Capillary $→$ Vein). It occurs when one capillary bed drains into another capillary bed before the blood returns to the heart.
  • The Hepatic Portal Pathway:     1. Blood from digestive organs (stomach, pancreas, gallbladder, intestines) and the spleen flows into the Superior Mesenteric Vein and the Splenic Vein.     2. These form the Portal Vein (which also receives blood from the left and right gastric veins).     3. The Portal Vein channels blood into the liver.     4. Blood is distributed to microscopic sinusoids (the liver's capillaries).     5. Blood flows out of the sinusoids into the Hepatic Vein.     6. The Hepatic Vein drains into the Inferior Vena Cava to return to the heart.
  • Functions of the Liver as a Gatekeeper:     * Nutrient Regulation: The liver absorbs and processes nutrients from the GI tract. It converts excess glucose into glycogen for storage after meals and converts glycogen back to glucose during fasting to maintain blood sugar balance.     * Protein Synthesis: Free amino acids from protein digestion are synthesized into new proteins and proenzymes. Excess amino acids are converted to energy or broken down into urea for waste removal.     * Detoxification: The liver screens blood for toxins (like alcohol and drugs) and pathogens before they reach general circulation.     * Hormone Regulation and Blood Flow Control: Essential for maintaining overall metabolic stability.
  • Medical and Pharmacological Implications:     * Portal Hypertension: Increased pressure within the portal vein resulting from system disruptions.     * First Pass Effect: Many oral medications are metabolized and potentially deactivated by the liver before they can reach the general circulation.     * Alternative Routes: Some drugs must be administered via non-oral routes to bypass the liver.     * Prodrugs: These are medications specifically designed to be taken orally because they require liver conversion to become functional/active.
  • Other Portal Systems: The Hypophyseal Portal System, which connects the hypothalamus to the anterior pituitary gland to regulate hormone production.

Principles of Circulation and Blood Pressure

  • The Pressure Gradient: Blood flows due to differences in pressure between two points.     * Blood always moves from higher pressure to lower pressure.     * As blood moves further away from the heart, the blood pressure decreases.     * Flow Rule: Increased pressure difference = increased flow; increased resistance = decreased flow.
  • Determinants of Blood Pressure (BPBP):     1. Cardiac Output (COCO): The harder the heart beats (e.g., exercise), the higher the COCO and subsequent BPBP. A weak heart results in lower BPBP.     2. Blood Volume (BVBV): Declines in volume (dehydration or hemorrhage) cause BPBP to fall. The kidneys compensate by reducing urine output to preserve volume.     3. Resistance: Specifically Peripheral Vascular Resistance (PVRPVR), which is the friction of blood against vessel walls.

Resistance and Vessel Diameter

  • Viscosity: The thickness of blood, which typically remains stable.
  • Vessel Diameter: The body's primary method for controlling resistance.
  • Vasoconstriction:     * The narrowing of blood vessels.     * Effect: Increased resistance, higher BPBP, and decreased blood flow into tissues.     * Side Effects: Hypertension, pallor (loss of color) in extremities, tissue damage (ulcers or gangrene), headaches, dizziness, irregular heartbeats, and increased blood sugar.
  • Vasodilation:     * The increase in vessel diameter.     * Effect: Decreased resistance, lower BPBP, and increased blood flow into tissues.
  • Arterial Elasticity: Healthy arteries expand during systole to absorb force and recoil during diastole to maintain pressure and propel blood forward, protecting smaller vessels from pressure surges.

Blood Velocity and Diagnostics

  • Flow Rates by Location:     * Fastest: The Aorta.     * Slowest: The Capillaries. This is due to the distance from the left ventricle, path friction, and the high total cross-sectional area of the capillary beds. The slow rate is essential