Functions and Composition of Blood

Functions of Blood

The heart pumps 75 ml of blood with each contraction.
On average, the heart beats 70 times per minute.
Therefore, the heart pumps roughly 5,250 ml per minute, calculating as follows:
- $ext{75 ml/beat} imes ext{70 beats/minute} = 5,250 ext{ ml/min}$
The entire blood supply is circulated each minute.

Functions of Blood - Continued

Major Functions

Transportation: Blood is responsible for carrying various substances, which include:
  - Oxygen from the lungs to the tissues.
  - Nutrients from the digestive tract to the cells.
  - Wastes from the tissues to the excretory organs.
  - Carbon dioxide from the tissues back to the lungs.
  - Hormones from the endocrine glands to target organs.
Defense Mechanism: Blood provides protection against pathogens via:
- Phagocytosis by some white blood cells (WBCs) that engulf and digest microbes.
- Production of antibodies by some WBCs, which are proteins that disable pathogens.
Clotting: Prevents excessive fluid loss due to injury.

Functions of Blood - Concluded

Regulatory Functions

Body Temperature: Blood helps maintain a stable body temperature through its transportation capabilities.
Osmotic Pressure: Proteins dissolved in the plasma, such as albumins, assist in maintaining osmotic pressure, which is critical for regulating the body’s water-salt balance.
pH Regulation: Buffers present in blood maintain a constant pH level around 7.4.

Composition of Blood

Blood is classified as a liquid connective tissue composed of formed elements suspended in plasma.
Formed Elements:
  - Red Blood Cells (RBCs or Erythrocytes)
  - White Blood Cells (WBCs or Leukocytes)
  - Platelets (Thrombocytes)
Produced primarily in the red bone marrow.

Plasma Composition

Plasma is approximately 91% water and 9% salts and organic molecules.
Function of Solutes:
  - Help maintain osmotic pressure of blood.
  - Salts function as buffers stabilizing pH.
  - Other solutes include nutrients, wastes, and hormones.
Plasma Proteins:
- Most abundant organic molecules produced by the liver.
- Functions: Maintain osmotic pressure and transport molecules through the bloodstream.

Types of Plasma Proteins

Albumins:
  - Most abundant plasma proteins.
  - Contributes to osmotic pressure more than others.
  - Functions as transport molecules within blood.
Globulins:
- Some transport substances, while gamma globulins fight pathogens.
Fibrinogen:
- An inactive plasma protein that, when activated, forms blood clots.

Red Blood Cells (RBCs)

Structure:
- Biconcave shape increases surface area for gas exchange.
- Contain hemoglobin (Hb), a protein crucial for oxygen binding.
Functions of Hemoglobin:
  - Binds to oxygen, giving blood its red color.
  - Each heme group of Hb can bind up to four oxygen molecules.
  - Can also bind carbon monoxide, forming carboxyhemoglobin.
  - Oxygen-bound hemoglobin is referred to as oxyhemoglobin, while released oxygen is termed deoxyhemoglobin.

Transport of Carbon Dioxide in Blood

Forms of CO2 Transport:
  - 7% is transported dissolved in plasma.
  - 23% binds to the globin portion of hemoglobin, termed carbaminohemoglobin.
  - 70% is transported as bicarbonate ion ( $ext{HCO}_3^-$ ) in plasma.

Production of Red Blood Cells

Occurs in the red bone marrow.
As RBCs develop, they lose their nucleus and most organelles, resulting in limited lifespan (approx. 120 days).
Old RBCs are removed by macrophages, primarily in the liver and spleen.
The unique disc shape allows RBCs to navigate small capillaries, maximizing gas diffusion.

Erythropoietin (EPO)

A hormone produced by the kidneys in response to low oxygen levels in the blood, stimulating RBC production in the bone marrow.
Blood Doping: Refers to artificially increasing RBC count (often via EPO injections) to enhance athletic performance, which can lead to dangerous blood thickening and heart failure.

Kidney Response to Decreased Blood Oxygen

Process:
- Low blood oxygen levels trigger increased production of EPO from the kidneys.
- EPO promotes the production of RBCs from stem cells, raising oxygen levels back to normal.

Disorders Involving Red Blood Cells

Jaundice: Caused by the accumulation of heme in blood when the liver fails to excrete it, leading to a yellowish tint in skin and eyes.
Anemia: Characterized by a deficiency of RBCs or hemoglobin.
  - Iron-Deficiency Anemia: The most common type, resulting from inadequate dietary iron.
  - Pernicious Anemia: Due to lack of vitamin B12 necessary for RBC production.
  - Folic Acid Deficiency Anemia: Resulting from insufficient folic acid for RBC synthesis.
Hemolytic Anemia: Excessive rupture of RBCs.
Sickle-Cell Disease: A genetic disorder causing RBCs to adopt a sickle shape, making them prone to rupture due to abnormal hemoglobin structure.

White Blood Cells (Leukocytes)

Larger cells equipped with a nucleus, translucent unless stained.
Produced in the red bone marrow, regulated by colony-stimulating factor (CSF).
Play crucial roles in the immune system, with varying lifespans (some only days, others months or years).

Examples and Types of White Blood Cells

Granular Leukocytes:
  - Neutrophils: Most abundant WBCs (50-70%), termed polymorphonuclear leukocytes, first responders to infections, engulf pathogens via phagocytosis.
  - Eosinophils: Involved in combating parasitic infections and related to allergic responses, characterized by bilobed nuclei and large granules.
  - Basophils: Rarest WBCs, involved in inflammatory responses by releasing histamine, which dilates blood vessels but constricts air passages during asthma attacks.
Agranular Leukocytes:
- Lymphocytes: Constituting 25-35% of WBCs, include B cells that produce antibodies and T cells that destroy infected or cancerous cells.
- Monocytes: Largest type, differentiate into macrophages that phagocytize pathogens and cellular debris.

Disorders Involving White Blood Cells

Severe Combined Immunodeficiency Disease (SCID): Inherited condition preventing lymphocyte development.
Leukemia: A cancer leading to uncontrolled proliferation of abnormal WBCs.
Infectious Mononucleosis: Caused by the Epstein-Barr virus, affecting lymphocytes, resulting in fatigue and swollen lymph nodes.

Platelets and Blood Clotting

Platelets (Thrombocytes):
  - Result from fragmentation of megakaryocytes in red bone marrow; not considered true cells.
  - Approximately 200 billion platelets produced daily.
  - Function: Critical role in blood clotting (coagulation).
Plasma Proteins Involved in Clotting:
- Prothrombin and fibrinogen; vitamin K is essential for prothrombin formation.

Blood Clotting Process

Essential to prevent blood loss during vessel injury.
Involves 13 different clotting factors, calcium ions, and enzymes.
Platelets aggregate to form a plug at injury sites, releasing prothrombin activator to convert prothrombin to thrombin, necessitating calcium ions.

Steps in Blood Clot Formation

Blood vessel is punctured.
Platelets congregate to form a plug.
Platelets and damaged tissue cells release prothrombin activator, initiating enzymatic reactions.
Fibrin threads form, trapping RBCs, creating a clot.
The clot is temporary; plasmin enzyme degrades the fibrin network, thereby allowing tissue repair.
Serum escapes from the clot, containing all plasma components except fibrinogen and prothrombin.

Disorders Relating to Blood Clotting

Thrombocytopenia: Insufficient platelet count, potentially due to decreased production or increased breakdown, leading to excess bleeding.
Thromboembolism: Occurs when a thrombus (stationary clot) detaches and forms an embolism that can block vessels.
Hemophilia: Genetic condition characterized by a deficiency in a clotting factor, hindering normal clot formation.

Blood Types and Compatibility

Blood types determined by proteins (antigens) on RBC surfaces, where transfer of blood requires compatibility to prevent agglutination (clumping of RBCs).

ABO Blood Groups

Type A: Presence of A antigen, produces anti-B antibodies.
Type B: Presence of B antigen, produces anti-A antibodies.
Type AB: Both A and B antigens present, produces neither type of antibody.
Type O: Neither antigen present, produces both anti-A and anti-B antibodies.

Compatibility and Transfusions

Blood transfusion requires careful compatibility checks, noting that type O is a universal donor, whereas type AB is a universal recipient.
Crossmatching is essential to prevent adverse reactions during transfusions.

Rh Blood Groups

Rh Factor: An additional blood type antigen; absence denotes Rh negative (-), whereas presence indicates Rh positive (+).
Unlike anti-A and anti-B antibodies, anti-Rh antibodies develop only after exposure to the Rh factor.

Homeostasis and Body System Cooperation

Cardiovascular System: Pumps blood to deliver oxygen and nutrients; facilitates waste removal and clotting.
Digestive System: Nutrients delivered from the digestive tract; molecules for plasma protein and blood cell formation are provided here, along with necessary water for blood volume and clotting.
Urinary System: Blood vessels carry waste for excretion; kidney regulation of water-salt balance is critical for maintaining blood pressure.
Muscular System: Muscle contraction essential for blood movement through the heart and vessels, especially veins.
Nervous System: Controls heart contractions and blood vessel constriction or dilation.
Endocrine System: Blood carries hormones; for instance, epinephrine elevates blood pressure, while other hormones regulate blood volume and blood cell production.
Respiratory System: Facilitates gas transport; breathing aids venous return and acid-base balance stabilization.
Lymphatic System: Collects excess interstitial fluid, ensuring blood volume stabilization and drainage back into the cardiovascular system.
Skeletal System: Protects the heart, with red bone marrow producing blood cells and bone tissue storing calcium needed for coagulation.