Blood Vessels & Blood: Study Guide for Anatomy & Physiology II

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Blood Vessels

Vessel Structure & Types

Blood vessels are classified based on their structure and function, which are adapted to their roles in the circulatory system.

Arteries: Carry blood away from the heart; have thick, elastic walls to withstand high pressure.
Veins: Return blood to the heart; have thinner walls, larger lumens, and valves to prevent backflow.
Capillaries: Microscopic vessels where exchange of gases, nutrients, and wastes occurs; walls are one cell thick.
Three Tunics (Layers) of Vessel Walls (Superficial to Deep):
1. Tunica externa (adventitia): Connective tissue, provides support and protection.
2. Tunica media: Smooth muscle and elastic fibers; controls vessel diameter; sympathetically innervated.
3. Tunica intima: Endothelial lining, minimizes friction.
Elastic arteries (e.g., aorta): Largest, most elastic; dampen pressure fluctuations.
Muscular arteries: Distribute blood to organs; more smooth muscle.
Arterioles: Smallest arteries; regulate flow into capillaries.
Venules: Collect blood from capillaries.
Veins: Serve as blood reservoirs; handle lowest pressure.
Highest pressure: Found in arteries, especially the aorta.

Blood Flow, Pressure & Resistance

Blood flow is determined by pressure gradients and resistance within the vessels.

Blood Pressure (BP): The force exerted by blood on vessel walls.
Peripheral Resistance: Opposition to flow; determined by vessel radius (most important), viscosity, and length.
Relationship: As resistance increases, blood pressure increases if flow is constant.
Pressure Gradient: Set by cardiac output and peripheral resistance.
BP is highest: In the aorta; drops most sharply in arterioles; lowest in veins.

Key Equation:

Where = blood flow, = pressure gradient, = resistance.

Mean Arterial Pressure (MAP)

MAP is the average pressure in the arteries during one cardiac cycle, crucial for organ perfusion.

Formula:
Normal MAP: 70–100 mmHg.
Measurement: Typically measured in the brachial artery using a sphygmomanometer.

Venous Return

Venous return is the movement of blood back to the heart, especially against gravity.

Skeletal Muscle Pump: Muscle contractions squeeze veins, pushing blood toward the heart.
Respiratory Pump: Breathing changes thoracic pressure, aiding venous return.
Venous Valves: Prevent backflow in veins.
Sympathetic Venoconstriction: Reduces venous volume, increasing return.

Regulation of Blood Pressure

Blood pressure is regulated by neural, hormonal, and renal mechanisms.

Short-term (Neural): Cardiovascular center in the medulla oblongata; baroreceptors (detect pressure changes); chemoreceptors (sense CO2, O2, pH).
Sympathetic stimulation: Increases heart rate and vasoconstriction.
Parasympathetic stimulation: Slows heart rate.
Hormonal: ANP lowers BP; others (e.g., ADH, angiotensin II) raise BP.
Long-term (Renal): Kidneys regulate blood volume via water/salt balance.
Hypertension: Chronic high BP; treated with lifestyle changes and medications.

Capillary Exchange

Capillaries allow exchange of substances between blood and tissues via several mechanisms.

Types of Capillaries:
- Continuous: Least permeable; found in skin, muscle, brain.
- Fenestrated: More permeable; found in kidneys, intestines.
- Sinusoidal: Most permeable; found in liver, bone marrow.
Exchange Routes: Diffusion (lipid-soluble), vesicular transport, bulk flow.
Bulk Flow: Movement of fluid driven by hydrostatic and colloid osmotic pressures.
Hydrostatic Pressure: Pushes water out; highest at arteriolar end.
Colloid Osmotic Pressure: Pulls water in; due to plasma proteins (albumin).
Net Filtration Pressure (NFP): Determines direction of fluid movement.

Key Equation:

Where = capillary hydrostatic pressure, = interstitial fluid hydrostatic pressure, = capillary osmotic pressure, = interstitial fluid osmotic pressure.

Edema: Excess fluid in tissues; caused by increased hydrostatic pressure, decreased osmotic pressure, or lymphatic blockage.

Systemic Vascular Anatomy

The systemic circuit includes all major arteries and veins supplying the body.

Largest artery: Aorta (ascending, arch, thoracic, abdominal regions).
Common iliac arteries: Branch from the abdominal aorta.
Major named vessels: Include carotid, subclavian, femoral arteries; jugular, subclavian, femoral veins.

Blood

Composition & Functions of Blood

Blood is a connective tissue with multiple functions, including transport, regulation, and protection.

Total blood volume: ~5 liters in adults.
After centrifugation:
- Plasma: ~55%; water, proteins, nutrients, hormones.
- Buffy coat: <1%; leukocytes and platelets.
- Erythrocytes: ~45% (hematocrit); red blood cells.
Major plasma proteins:
- Albumin: Maintains colloid osmotic pressure.
- Globulins: Include antibodies.
- Fibrinogen: Involved in clotting.

Erythrocytes (Red Blood Cells)

RBCs are specialized for oxygen transport and have a unique structure and life cycle.

Biconcave shape: Increases surface area for gas exchange; lacks nucleus for flexibility.
Hemoglobin: Binds oxygen; can also bind carbon monoxide (forms carboxyhemoglobin).
Lifespan: ~120 days; produced in red bone marrow.
Erythropoiesis: Formation of RBCs; regulated by erythropoietin (EPO) from kidneys in response to hypoxia.
Iron transport: By transferrin.
RBC breakdown: In spleen; heme converted to bilirubin (gives bruise color).
Common anemias:
- Aplastic: Marrow damage.
- Pernicious: B12 deficiency.

Leukocytes (White Blood Cells)

Leukocytes defend the body against infection and are classified by the presence of granules.

Granulocytes:
- Neutrophils: Most abundant; phagocytize bacteria.
- Eosinophils: Combat parasites.
- Basophils: Release histamine.
Agranulocytes:
- Monocytes: Large, U-shaped nucleus; become macrophages.
- Lymphocytes: B cells (produce antibodies), T cells (mature in thymus).
Leukopoiesis: Formation of WBCs from hematopoietic stem cells; myeloid and lymphoid lines.

Platelets & Hemostasis

Platelets are cell fragments essential for blood clotting (hemostasis), which occurs in three main steps.

Platelets: Fragments of megakaryocytes.
Hemostasis steps:
1. Vascular spasm (vasoconstriction)
2. Platelet plug formation
3. Coagulation (clotting cascade)
Coagulation pathways: Intrinsic, extrinsic, and common.
Key steps: Prothrombin activator converts prothrombin to thrombin; thrombin converts fibrinogen to fibrin.
Calcium ions and vitamin K: Essential for clotting; anticoagulants (e.g., warfarin/rat poison) interfere with these steps.

Clot Breakdown & Blood Typing

Clots are dissolved after healing, and blood typing is crucial for safe transfusions.

Clot retraction: Platelets contract to shrink the clot.
Fibrinolysis: Plasmin digests fibrin, dissolving the clot.
Thrombus: Stationary clot; Embolus: Traveling clot.
ABO and Rh systems: Determine blood type compatibility.
Antigens: On RBC surface; Antibodies: In plasma.
Agglutination: Clumping due to incompatible transfusion.
Transfusion compatibility:
- Type O: Universal donor (no A/B antigens).
- Type AB: Universal recipient (no anti-A/B antibodies).
- Rh+: Can receive Rh+ or Rh-; Rh- must not receive Rh+.

Key Tables

Vessel Type	Wall Thickness	Pressure	Valves
Artery	Thick	High	No
Vein	Thin	Low	Yes
Capillary	One cell layer	Very low	No

Leukocyte	Relative Abundance	Main Function
Neutrophil	Most	Phagocytosis of bacteria
Lymphocyte	2nd most	Immunity (B/T cells)
Monocyte	3rd	Become macrophages
Eosinophil	4th	Fight parasites
Basophil	Least	Release histamine

Blood Type	Antigens on RBC	Antibodies in Plasma	Can Donate To	Can Receive From
A	A	Anti-B	A, AB	A, O
B	B	Anti-A	B, AB	B, O
AB	A, B	None	AB	A, B, AB, O
O	None	Anti-A, Anti-B	A, B, AB, O	O

Additional info:

Practice calculations for MAP and NFP as these are common exam questions.
Memorize the order of vessel tunics and hemostasis steps for sequence-based questions.
Be able to explain regulatory mechanisms and the development of blood cells from stem cells.