Blood: Structure, Function, and Cellular Components – Study Notes for Anatomy & Physiology

Blood and the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body and maintaining homeostasis. It consists of:

• The heart – a muscular pump

• Blood vessels – a series of conducting hoses

• Blood – a fluid connective tissue

Components and Functions of Blood

Characteristics and Major Functions

• Blood is a specialized connective tissue containing cells suspended in a fluid matrix (plasma).

• Functions include:

  • Transporting dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes

  • Regulating pH and ion composition of interstitial fluids

  • Restricting fluid losses at injury sites (clotting)

  • Defending against toxins and pathogens (immune response)

  • Stabilizing body temperature

• Physical characteristics:

  • Temperature: 38°C (100.4°F)

  • pH: Slightly alkaline (7.35–7.45)

  • High viscosity

  • Blood volume: ~7% of body weight (in kg); a 75-kg person has ~5.25 L

Composition of Whole Blood

• Whole blood consists of:

  • Plasma (fluid matrix)

  • Formed elements (cells and cell fragments)

• Fractionation is the process of separating whole blood into plasma and formed elements.

Plasma

• Makes up about 55% of blood volume

• Over 90% of plasma is water

• Contains dissolved plasma proteins and other solutes

• Similar in composition to interstitial fluid due to exchange across capillary walls

Plasma Proteins

Protein	Percentage	Function
Albumins	60%	Major contributors to plasma osmolarity; transport fatty acids, thyroid hormones, steroid hormones
Globulins	35%	Antibodies (immunoglobulins); transport globulins for hormones, lipids, and metals
Fibrinogen	4%	Soluble protein for clotting; converted to fibrin
Other proteins	1%	Enzymes, hormones, proenzymes

• Most plasma proteins are made in the liver; antibodies are made by plasma cells; peptide hormones by endocrine organs.

Formed Elements

• Include red blood cells (RBCs), white blood cells (WBCs), and platelets

• Produced by hematopoiesis in red bone marrow

Red Blood Cells (Erythrocytes)

Structure and Function

• Make up 99.9% of formed elements

• Contain hemoglobin (Hb), a red pigment that binds and transports O2 and CO2

• Small, biconcave discs with a thin central region and thicker outer margin

• Large surface-area-to-volume ratio for efficient gas exchange

• Flexible to pass through narrow capillaries (7–8 μm RBC through 4 μm capillary)

• Mature RBCs are anucleate (lack nuclei), lack mitochondria and ribosomes, and live about 120 days

Red Blood Cell Count and Hematocrit

Parameter	Adult Male	Adult Female
RBC count (million/μL)	4.5–6.3	4.2–5.5
Hematocrit (%)	46	42
Hemoglobin (g/dL)	14–18	12–16

Hemoglobin Structure and Function

• Complex quaternary structure: 4 globular protein subunits (2 alpha, 2 beta chains), each with a heme group

• Each heme contains one iron ion that binds O2 (forms oxyhemoglobin, HbO2)

• Deoxygenated form is deoxyhemoglobin

• Can also bind CO2 to form carbaminohemoglobin

• Fetal hemoglobin binds O2 more readily than adult hemoglobin

RBC Turnover and Erythropoiesis

• About 1% of RBCs are replaced daily (~3 million/sec)

• Formation of RBCs is called erythropoiesis

• Occurs in red bone marrow (myeloid tissue) in adults

• Requires amino acids, iron, folic acid, and vitamins B12 and B6

• Stimulated by erythropoietin (EPO), a hormone released by kidneys and liver in response to hypoxia

Stages of RBC Maturation

1. Hematopoietic stem cell (hemocytoblast)

2. Myeloid stem cell

3. Proerythroblast

4. Erythroblast stages

5. Reticulocyte

6. Mature RBC

Hemoglobin Recycling

• Macrophages engulf aged RBCs and break down hemoglobin

• Iron is removed from heme and stored or transported by transferrin

• Heme is converted to biliverdin (green), then to bilirubin (yellow)

• Bilirubin is excreted in bile; excess causes jaundice

• Converted by intestinal bacteria to urobilins and stercobilins

Disorders

• Anemia: Reduced hematocrit or hemoglobin, leading to decreased oxygen delivery

• Hemoglobinuria: Hb in urine due to excessive hemolysis

• Hematuria: Whole RBCs in urine, usually from kidney or vessel damage

• Pernicious anemia: Due to lack of vitamin B12

Blood Types

Surface Antigens and Blood Groups

• Surface antigens on RBCs determine blood type and are recognized by the immune system

• Main antigens: A, B, and Rh (D)

ABO Blood Group

Blood Type	Surface Antigens	Plasma Antibodies
Type A	A	Anti-B
Type B	B	Anti-A
Type AB	A and B	None
Type O	None	Anti-A and Anti-B

Rh Blood Group

• Rh positive (Rh+): Rh antigen present

• Rh negative (Rh−): Rh antigen absent

• Only sensitized Rh− individuals have anti-Rh antibodies

Transfusion Reactions and Compatibility

• Agglutinogens: Surface antigens on RBCs

• Agglutinins: Antibodies in plasma that attack foreign antigens

• Agglutination: Clumping of foreign cells

• Cross-reaction: Occurs if incompatible blood types are mixed, leading to agglutination and hemolysis

• Cross-match testing: Performed before transfusions to check compatibility

• Type O is the universal donor, but cross-reactions can still occur due to other antigens

White Blood Cells (Leukocytes)

Structure and Function

• Have nuclei and other organelles, but lack hemoglobin

• Defend against pathogens, remove toxins and wastes, attack abnormal or damaged cells

• Most are in connective tissue or lymphatic organs; only a small fraction circulates in blood (5,000–10,000/μL)

Characteristics of Circulating WBCs

• Can migrate out of bloodstream (diapedesis)

• Capable of amoeboid movement

• Attracted to specific chemical stimuli (positive chemotaxis)

• Some are phagocytic

*Additional info: Further classification of WBCs (granulocytes vs. agranulocytes), platelet structure/function, and blood clotting mechanisms are covered in subsequent sections of the source material.*