Blood Vessels

Essentials of Human Anatomy & Physiology

General Overview of the Circulatory System

  • Components:

    • Lungs

    • Heart

    • Veins

    • Vena Cava

    • Aorta

Blood and Blood Vessels

Types of Blood Vessels

  • Arteries

  • Arterioles

  • Capillaries

  • Venules

Physical Characteristics of Blood

  • Color Range:

    • Oxygen-rich blood is scarlet red

    • Oxygen-poor blood is dull red

  • pH Level:

    • Must remain between 7.35 and 7.45

  • Temperature:

    • Blood temperature is slightly higher than body temperature

  • Volume:

    • Accounts for about 8% of body weight

    • Healthy adult males: 5-6 L

    • Healthy adult females: 4-5 L

Composition of Blood

Blood

  • The only fluid tissue in the human body

  • Classified as connective tissue

    • Living cells = formed elements

    • Non-living matrix = plasma

Blood Plasma

  • Composed of approximately 90% water

  • Includes many dissolved substances:

    • Nutrients (e.g., glucose, fatty acids, vitamins, amino acids)

    • Salts (metal ions)

    • Respiratory gases

    • Hormones

    • Proteins

    • Waste products

Formed Elements

  • Composition: 45% of blood

    • Erythrocytes (Red Blood Cells)

    • Functions: Transport oxygen and assist in carbon dioxide transport

    • Leukocytes (White Blood Cells):

    • Functions: Defense and immunity

    • Types include:

      • Basophil

      • Eosinophil

      • Neutrophil

      • Lymphocyte

      • Monocyte

    • Platelets:

    • Functions: Blood clotting

Functions of Blood

Distribution

  • Delivery of oxygen from lungs to body cells

  • Transport of nutrients from digestive tract to body cells

  • Removal of metabolic waste products to elimination sites:

    • CO₂ to lungs

    • Nitrogenous wastes to kidneys

  • Transport hormones from endocrine system to target organs

  • Maintenance of body temperature through heat absorption and distribution

Protection

  • Maintenance of normal pH levels

  • Adequate fluid volume maintenance:

    • Sodium chloride and other salts act with blood proteins to prevent excessive fluid loss

  • Prevention of blood loss:

    • Platelets and plasma proteins initiate clot formation

  • Prevention of infection:

    • Antibodies, complement proteins, white blood cells aid in defense against foreign invaders

Erythrocytes (Red Blood Cells)

Function and Anatomy

  • Main Function: Carry oxygen

  • Shape:

    • Biconcave disks

    • Essentially bags of hemoglobin

  • Key Features:

    • Anucleate (no nucleus)

    • Contains very few organelles

    • Contains spectrin (a protein that enables RBCs to change shape and aggregate)

Hemoglobin

  • Structure: Iron-containing protein

  • Function:

    • Binds strongly, but reversibly to oxygen

    • Each hemoglobin molecule has four oxygen-binding sites

Erythrocyte Disorders

Anemia

  • Definition: A condition of reduced oxygen-carrying ability of the blood

  • Symptoms: Individuals may feel perpetually fatigued, pale, short of breath, and chilly

Common Causes of Anemia

  1. Decrease in number of RBCs:

    • Hemorrhagic anemias: Result from blood loss

    • Hemolytic anemias: RBCs are prematurely lysed or ruptured

    • Pernicious anemia: Vitamin B12 deficiency affecting absorption

    • Aplastic anemias: Due to destruction or inhibition of red marrow

    • Bone marrow cancer: Replaces marrow with cancer cells

    • Toxins or radiation: May cause marrow replacement by connective tissue

  2. Decrease in hemoglobin content:

    • Iron deficiency anemia: Inadequate intake, absorption issues, or chronic blood loss

    • Sickle cell anemia: Abnormal hemoglobin causes RBCs to become crescent-shaped, leading to their rupture and blockage in vessels

Polycythemia

  • Condition characterized by an excessive number of RBCs

  • May arise from bone cancer or high altitudes

  • Increases blood viscosity, resulting in sluggish blood flow

Leukocytes (White Blood Cells)

Function

  • Essential for body defense against disease

    • Complete cells with a nucleus and organelles

    • Able to move into and out of blood vessels (diapedesis)

    • Can move via ameboid motion and respond to chemicals released by damaged tissues

  • Abnormal levels:

    • Leukocytosis: Abnormally high levels, generally indicating infection

    • Leukopenia: Abnormally low levels, often caused by certain drugs

Conditions Related to Leukocytes

  • Leukemia: Cancerous conditions of WBCs leading to the production of immature WBCs that cannot adequately fight disease

  • Infectious mononucleosis: Highly contagious viral disease caused by Epstein-Barr virus, characterized by tiredness, sore throat, and low-grade fever. Recovery usually occurs within a few weeks without a cure.

Platelets

Function and Formation

  • Derived from ruptured multinucleate cells (megakaryocytes)

  • Essential for clotting processes

Hematopoiesis

Definition

  • Blood cell formation occurring in red bone marrow

  • In adults, occurs in:

    • Flat bones of the skull and pelvis

    • Ribs

    • Sternum

    • Proximal epiphyses of humerus and femur

Fate of Erythrocytes

  • Erythrocytes wear out in 100 to 120 days

  • When worn out, they are eliminated by phagocytes in the spleen or liver

  • Lost cells are replaced by the division of hemocytoblasts

Regulation of Erythrocyte Production

  • Controlled by the hormone erythropoietin, primarily produced by the kidneys in response to reduced oxygen levels in the blood

  • Homeostasis maintained through negative feedback related to blood oxygen levels

  • Imbalances in red blood cell counts lead to increases in erythropoiesis

Hemostasis

Definition

  • The stoppage of blood flow resulting from a break in a blood vessel

  • Involves three phases:

    1. Platelet plug formation

    2. Vascular spasms

    3. Coagulation

Platelet Plug Formation

  • Activated by exposure of collagen fibers due to injuries; platelets become sticky and adhere to these fibers

  • Anchored platelets release chemicals, attracting more platelets, leading to plug formation

Vascular Spasms

  • Platelets release serotonin causing blood vessel muscles to spasm, narrowing the blood vessel and decreasing blood loss

Coagulation

  • Involves over 30 substances in the clotting process

  • Procoagulants and anticoagulants maintain the delicate balance determining whether blood clots

Blood Clotting

  • Usually occurs within 3 to 6 minutes

  • Clots remain until the endothelium regenerates, during which platelets contract, pulling debris closer and squeezing out serum

  • Clots are broken down after tissue repair via fibrinolysis

Undesirable Clotting

  • Thrombus: A clot in an unbroken blood vessel that can be deadly in critical areas, such as the heart

  • Embolus: A thrombus that dislodges and travels within the bloodstream, potentially clogging vessels in critical areas, like the brain

Blood Vessels: The Vascular System

  • Functions to take blood to the tissues and back

  • Components include:

    • Arteries

    • Arterioles

    • Capillaries

    • Venules

    • Veins

Structure of Blood Vessels

  1. Tunica Interna:

    • Composed of endothelium and loose connective tissue

    • Contains internal elastic lamina

  2. Tunica Media:

    • Contains smooth muscle and elastic fibers

    • Also features external elastic lamina

  3. Tunica Externa:

    • Composed primarily of connective tissue

Differences Between Blood Vessel Types

  • Arteries: Thickest walls

  • Skeletal Muscle Action: Helps move blood in veins toward the heart

Movement of Blood Through Vessels

  • Most arterial blood is pumped by the heart

  • Veins rely on the milking action of muscles to assist blood movement

Capillary Beds

  • Comprised of true capillaries, which are exchange vessels

  • Oxygen and nutrients cross to cells while carbon dioxide and metabolic waste products cross into blood

  • Capillary walls are only one cell layer thick, facilitating exchange between blood and tissues

Major Arteries and Veins of Systemic Circulation

  • Important arterial paths include major arteries enumerated in the text.

  • Significant venous paths follow major veins as well, facilitating blood return to the heart.

Essentials of Human Anatomy & Physiology

The Cardiovascular System

  • Definition and Structure

    • A closed system comprising the heart and blood vessels.

    • Primary functions include:

    • Pumping blood by the heart.

    • Circulating blood through blood vessels to all body parts.

    • Delivering oxygen and nutrients.

    • Removing carbon dioxide and other waste products.

The Heart

Location

  • Situated in the thorax between the lungs.

  • Positioned anterior to the vertebral column and posterior to the sternum.

  • The pointed apex of the heart is directed towards the left hip.

  • Size: Approximately that of a fist.

  • Weight: Less than one pound.

  • Pericardium: A double membrane encapsulating the heart.

    • Parietal Pericardium: The outer layer.

    • Visceral Pericardium: The layer closest to the heart.

    • Pericardial Fluid: Fills the space between the layers of the pericardium to prevent friction during heartbeats.

Heart Wall Structure

  • Composed of three layers:

    • Epicardium:

    • The outermost layer.

    • Composed of connective tissue.

    • Also known as the visceral layer of the pericardium.

    • Myocardium:

    • The middle layer.

    • Primarily consists of cardiac muscle and forms the bulk of the heart.

    • Endocardium:

    • The innermost layer.

    • Composed of endothelium lining the chambers and valves of the heart.

Chambers of the Heart

  • Contains four chambers that function as two separate pumps:

    • Right Atrium: Receiving chamber for deoxygenated blood.

    • Left Atrium: Receiving chamber for oxygenated blood.

    • Right Ventricle: Pumping chamber for deoxygenated blood to the lungs.

    • Left Ventricle: Pumping chamber for oxygenated blood to the body.

Heart Valves

  • The heart contains four key valves that ensure one-way blood flow:

    • Atrioventricular Valves: Situated between the atria and ventricles.

    • Tricuspid Valve: Located on the right side.

    • Bicuspid Valve (Mitral Valve): Located on the left side.

    • Semilunar Valves: Positioned between the ventricles and arteries.

    • Pulmonary Semilunar Valve: Located at the exit of the right ventricle.

    • Aortic Semilunar Valve: Located at the exit of the left ventricle.

Functioning of Heart Valves

  • Valves open and close in response to blood flow:

    • Atrioventricular Valves:

    • Blood fills the atria, increasing pressure and forcing the valves open.

    • Atria contract to push additional blood into the ventricles.

    • As ventricles contract, intraventricular pressure rises, pushing blood against the valve flaps.

    • Chordae Tendineae: Strands of connective tissue that keep the valves closed to prevent backflow.

    • Semilunar Valves:

    • Open when ventricles contract, allowing blood to flow into the aorta and pulmonary trunk.

    • Close when ventricles relax, preventing backflow from the arteries.

Heart Sounds

  • The heart emits two distinguishable sounds:

    • Lub: Associated with the closure of the AV valves; characterized as louder and longer with the bicuspid closing slightly before the tricuspid.

    • Dup: Results from the closure of the semilunar valves; the aortic valve closes just before the pulmonary valve.

    • The sounds originate from blood colliding with the closed valves.

  • Murmurs: Abnormal heart sounds indicating potential issues

    • Incompetent Valve: Does not close properly, causing a swishing noise as blood flows backward.

    • Stenotic Valve: A narrowed valve that produces a high-pitched sound as blood is forced through it.

Coronary Circulation

  • The myocardium of the heart is nourished by its own circulatory system, known as coronary circulation.

  • Coronary Arteries: Supply oxygenated blood to the myocardium.

  • Cardiac Veins: Drain deoxygenated blood from the myocardium into the right atrium.