Circulatory System and Blood

Blood and Blood Vessels

Learning Objectives

• State the different types of blood vessels and their functions.

• Describe the structural differences between blood vessels.

• List the main components of blood and its functions.

• Describe the role of white blood cells.

• Explain the structure and function of red blood cells.

Cardiovascular System Components

The cardiovascular system consists of:

• A pump: the heart.

• Conducting tubes: blood vessels.

• Fluid: blood.

Blood Composition

• Blood is a connective tissue with three main components:

  • Cells

  • Fibers

  • Fluid

Cells

• Red blood cells (erythrocytes)

• White blood cells (leukocytes)

• Platelets (cell fragments)

Fibers

• Fibrinogen: Converted to fibrin, important for blood clotting.

Fluids

• Plasma: Fluid and solutes (proteins and electrolytes).

Blood Types

• Classification based on the presence or absence of specific antigens on red blood cells.

• Four main types: A, B, AB, O.

  • Letter indicates the presence of a specific antigen on the surface of red blood cells.

• Positive or negative sign indicates the presence or absence of the rhesus antigen.

Blood Type Distribution and Antigens

• Group A: 40% of the population, A antigen on red blood cells, antibodies against group B.

• Group O: No antigens on their surface, the universal donor.

Rhesus Antigen

• Important for pregnant women due to potential incompatibility between mother and developing baby.

• Checked during pregnancy to manage potential issues, especially in second or subsequent pregnancies.

Detailed Composition of Blood

Plasma

• If a blood sample is spun down, it separates into cells and fluid.

  • Mostly water (over 90%)

    • Important for dissolving solutes and transport.

  • Ions (electrolytes)

    • Sodium, potassium, calcium, magnesium, chloride, bicarbonate.

    • Important for osmotic balance, pH maintenance, membrane permeability.

  • Plasma proteins

    • Albumin: Maintains osmotic balance and blood volume.

    • Fibrinogen: Important for blood clotting.

    • Immunoglobulins (antibodies): Immune defense mechanisms.

  • Nutrients

    • Glucose, fatty acids, vitamins.

  • Waste products

  • Respiratory gases

    • Oxygen and carbon dioxide.

  • Hormones

Cellular Elements

• Erythrocytes (red blood cells)

  • 5-6 million per microliter of blood.

  • Transport oxygen and remove carbon dioxide.

• Leukocytes (white blood cells)

  • Thousands in the bloodstream.

  • Defense and immune cells.

  • Five main types: basophils, eosinophils, lymphocytes, monocytes, neutrophils.

• Platelets

  • Cell fragments, abundant in the blood.

  • Stimulate blood clotting.

Visual Representation of Blood Composition

• Formed elements:

  • Mostly red blood cells, some white blood cells and platelets

• Fluid component:

  • Nearly all water, some solutes, and proteins

• Proteins:

  • Maintain blood volume, immune response, and blood clotting

• Solutes:

  • Provide nutrients, balance pH, and osmotic gradients

Red Blood Cells (Erythrocytes)

• Structure informs function.

• Cannot grow or divide, anucleate, lacking organelles.

• Main job is to transport gases.

  • Transport oxygen to tissues and remove carbon dioxide.

Hemoglobin Molecule

• Made of four protein components:

  • Two alpha globin chains (green)

  • Two beta globin chains (yellow)

  • Heme molecules (red), one per globin chain

    • Iron at the center of each heme molecule, binding site for oxygen or carbon dioxide

• Two types of hemoglobin:

  • Oxyhemoglobin: Oxygen bound, bright red blood (arterial blood).

  • Deoxyhemoglobin: Oxygen not bound, deep red-purple blood (venous blood).

White Blood Cells (Leukocytes)

• Divided into two main groups:

Granulocytes

• Contain granules within the cells:

  • Neutrophils

    • First responders, the paramedics of the leukocytes.

    • First line of defense against bacterial infection.

    • ~50% of cell population.

    • Engulf other cells, bacteria (phagocytosis).

  • Eosinophils

    • Dumbbell-shaped nucleus.

    • 1-4%.

    • Important for parasitic infections.

    • Release toxins to kill parasites.

  • Basophils

    • Involved in allergic reactions.

    • Release mediators, such as histamine.

Agranulocytes

• Lymphocytes and monocytes:

  • Monocytes

    • Precursor cell, matures into macrophages in tissues.

    • Phagocytic cells.

  • Lymphocytes

    • B lymphocytes: Produce antibodies against specific proteins.

    • T lymphocytes: Involved in cellular immunity (T helper cells and cytotoxic T cells).

    • Natural killer cells: Seek and destroy infected cells.

Hematopoiesis

• Process of new blood cell formation.

• Hematopoietic stem cell can differentiate into any blood cell type.

  • Differentiates into lymphoid stem cells (lymphocytes) or myeloid stem cells (red blood cells, megakaryocytes, granulocytes, monocytes).

• Terminology of lymphoid and myeloid is used in classifying different types of blood cancer, such as lymphoid and myeloid leukemia.

Platelets

• Cell fragments derived from megakaryocytes.

• Contain a membrane, mitochondria, but no nucleus.

• Contain enzymes important for hemostasis and secrete factors.

  • Vasoconstrictors

  • Clotting factors

  • Growth factors

  • Chemical attractants

• Activated when exposed to collagen (component of blood vessel walls).

• Stick together to form a platelet plug.

• Involved in blood clotting, work with fibrin and red blood cells to form a blood clot.

Blood Vessel Structure and Function

• Three main types: veins, capillaries, arteries.

Types of Blood Vessels

• Veins: Carry blood into the heart.

  • Give rise to smaller venules, then capillaries.

• Capillaries: Small blood vessels for diffusion (gas exchange).

• Arteries: Carry blood away from the heart.

  • Arterioles are strong and elastic leading into arteries.

• Capillaries:

  • Single cell thick for gas diffusion.

• Arteries:

  • Thick walls, muscular, smaller lumen.

• Veins:

  • Large lumen, thinner walls.

Blood Vessel Structure (Veins and Arteries)

• Continuous with the heart, three main layers:

  • Tunica externa (external)

    • Connective tissue, collagen and elastic fibers.

    • Contains and prevents over-stretching

  • Tunica media (middle)

    • Smooth muscle and elastic connective tissue.

    • Allows contraction and relaxation (vasoconstriction and vasodilation).

    • Changes blood flow and pressure and helps propel blood

  • Tunica intima (intima or inside)

    • Endothelium, basement membrane, connective tissue, internal elastic lamina.

    • Smooth endothelial surface facilitates seamless blood flow, preventing clotting.

Endothelium

• Squamous endothelium comprises the internal layer of blood vessels.

• Single layer of flattened endothelial cells (like a fried egg).

• Maintains a selectively permeable barrier.

• Non-thrombogenic (anti-clotting) under normal conditions.

• Modulates blood flow by contracting and relaxing smooth muscle.

• Releases factors to modulate the immune response (express markers, secrete hormones).

• Involved in metabolism and can modify lipoproteins.

  • Particularly relevant for hardening of the arteries (atherosclerosis).

Large Blood Vessels (Arteries and Veins)

• Structure is continuous with the heart and we've already touched on the three layers.

• Arterial wall:

  • Endothelial layer, thick smooth muscle.

• Venous blood vessel wall:

  • Endothelial layer, much less smooth muscle.

• Reflects function: arteries are under high pressure and regulate blood flow, while veins have low pressure and carry blood back to the heart with a large lumen to accommodate blood volume.

Arteries and Arterioles

• Arteries:

  • Biggest blood vessels, branching into arterioles.

  • Elastic and muscular, thick walls and small lumen.

  • Dynamic: vessel lumen can constrict or dilate to control blood pressure.

  • The seat of blood pressure control.

  • Regulate blood flow to capillaries.

Capillaries

• Microcirculation near every cell, abundant in tissues with high metabolic activity (muscles, liver, kidneys, brain).

• Site of nutrient, gas, and waste exchange between blood and tissue fluid.

• Single layer of squamous epithelium (endothelium) and basement membrane.

  • Thin structure facilitates gas diffusion.

Venules and Veins

• Drain the capillary network and carry blood back to the heart.

• Thinner walls, capacitance blood vessels (high capacity), blood reservoir.

• 70% of blood volume is in veins at any time.

• Low blood pressure (about 10 mm Hg) and veins have valves to prevent backflow.

• Skeletal muscle pump assists blood flow in veins:

  • Contraction of skeletal muscles squeezes veins and pushes blood towards the heart.

Arterial and Venous Systems Summary

Arterial System

• A for away from the heart: carries blood away.

• High pressure system: a pressure reservoir.

• No valves.

• Small lumen.

• Thick muscular walls.

Venous System

• Veins they in, carry blood into the heart.

• Low pressure system: reservoir for most blood volume.

• Valves prevent backflow of blood.

• Large lumen.

• Thin walls.

Functions of Blood

• Hematopoiesis:

  • Formation of blood cells from hematopoietic stem cells.

• Erythropoiesis:

  • Production of red blood cells.

Steps for Erythropoiesis

• Kidney:

  • Produces erythropoietin (EPO) to stimulate red blood cell production.

• Bone marrow:

  • EPO stimulates red blood cell production.

  • Continuous production because red blood cells live about 120 days

• Gas exchange:

  • Red blood cells circulate in the bloodstream, exchanging gases and delivering oxygen

• Macrophage:

  • Macrophages in the liver, spleen, and bone marrow recycle old or damages red blood cells.

  • Heme is broken down and secreted in bile.

Blood Functions Summary

• Transport:

  • Gases, nutrients, hormones, waste products.

• Regulation:

  • Ions, pH, body temperature.

• Protection:

  • Restricting fluid loss, defending against pathogens and toxins.

Transport Functions

• Carries oxygen from lungs.

• Picks up carbon dioxide from tissues.

• Transports nutrients absorbed from the digestive tract.

• Carries hormones produced in endocrine glands.

• Absorbs waste products from metabolically active cells.

Red Blood Cell Structure

• Bioconcave disc structure and is flexible.

• Allows bending and folding to pass through small capillaries.

• Can form stacks for orderly movement through blood vessels.

• Large surface area allows for efficient gas exchange.

Hemoglobin

• Structure is important for function.

• Oxygen doesn't dissolve easily in water, so it needs hemoglobin.

• Four oxygen molecules per hemoglobin molecule.

• Hemoglobin can also transport carbon dioxide.

• Breathe in:

  • Oxygen into the bloodstream carried by arteries

• Capillary beds

  • Oxygen offloaded and hemoglobin loads carbon dioxide

• Back to heart and exhale. Carbon dioxide:

  • Acidic so it needs to be removed.

• Hemoglobin can transport both oxygen and carbon dioxide. Note on carbon monoxide poisoning: has a 200x greater affinity for hemoglobin!

Carbon Monoxide Poisoning

• Will push oxygen off the hemoglobin and bind instead.

• So oxygen is not getting to the tissues!

• Leads to death if untreated.

• Treatment:

  • 100% oxygen therapy

The Regulation of pH and Ion Composition

• Ions diffuse between tissue and bloodstream.

  • Calcium is used as an example.

• Important for absorbing and neutralizing acids generated by metabolically active tissue.

  • Includes buffering system for high blood pH.

• The pH scale ranges:

  • Acidic in red.

  • Alkaline in blue.

• Normal is: 7.35-7.45.

Body Temperature Regulation

• Blood absorbs heat generated by metabolically active tissue and redistributes it around the body.

  • High body temperature: blood rushes to the skin to lose heat.

  • Low body temperature: blood is diverted away from the skin to retain heat at core organs.

Restricting Fluid Loss

• Platelets are involved in blood clotting, important for hemostasis.

• Break in the blood vessel wall activates the clotting cascade.

  • Platelets, red blood cells, and clotting factors come together to form a blood clot.

• Factors that activate clotting and factors that inhibit clotting maintain balance. So if bleeding too much then control the process.

• Five main stages of hemostasis when injured, and platelets activated from collagen by vessels, then forms plugs.
  Then fibrin in cascade.

Pathogen Defense

• White blood cells can migrate into the tissues to remove debris and attack pathogens.

• B cells produce antibodies that can neutralize or tag invading organisms.

Disruption to Blood Homeostasis

• There are a few examples of when it's not meeting requirements:

  • Hypoxemia: ventilation mismatch

    • Decreased respiratory issue.

      • This stems from issues to drugs, stroke.

  • Anemia: not enough hemoglobin.

    • Causes stems from missing items.

    • Consequences: heart failure.

  • Hemorrhage: loss of fluid and urgent to treat.