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Blood is a specialized body fluid that circulates through the heart, arteries, veins, and capillaries of all vertebrates.
Functions of blood include:
- Transporting oxygen, nutrients, hormones, and waste products to and from the body’s tissues.
- Helping maintain homeostasis and support life.
Composition of blood:
- Plasma: Liquid portion (approximately 55%), carries dissolved substances such as proteins, glucose, hormones, and waste products.
- Formed elements: Cellular components (approximately 45%), which include:
- Red blood cells (RBCs): Carry oxygen using hemoglobin.
- White blood cells (WBCs): Defend against infection.
- Platelets: Help in blood clotting.

Blood is classified as a specialized connective tissue because:
- It originates from mesenchymal tissue (embryonic connective tissue).
- Performs conveying and transporting functions throughout the body.
Unlike other connective tissues, the extracellular matrix of blood is fluid (called plasma), in which cells (RBCs, WBCs, and platelets) are suspended.
Blood connects different organs and tissues by:
- Transporting gases, nutrients, hormones, and waste products.
- Maintaining homeostasis.

Approximately 8% of an adult's body weight is comprised of blood.
Volume of blood:
- Females: Approximately 4-5 liters
- Males: Approximately 5-6 liters
Blood's mean temperature: 38 °C
Blood pH: 7.35-7.45, making it slightly basic.
Color distinction:
- Blood in the arteries is brighter red due to higher levels of oxygen compared to blood in veins.

Transport
- Transports:
  - Substances:
  - Gases: Oxygen (O2) and Carbon Dioxide (CO2) between lungs and rest of the body.
  - Nutrients: From the digestive tract and storage sites to the rest of the body.
  - Waste products: To be detoxified or removed by the liver and kidneys.
  - Hormones: From the glands in which they are produced to their target cells.
  - Heat: To the skin to help regulate body temperature.
Protection
- Involves roles in inflammation:
  - Leukocytes: White blood cells that destroy invading microorganisms and cancer cells.
  - Antibodies and proteins: Destroy pathogenic substances.
  - Platelet factors: Initiate blood clotting and help minimize blood loss.
Regulation
- Helps regulate:
  - pH: By interacting with acids and bases.
  - Water balance: By transferring water to and from tissue
- Reference: Gyton and Hall, Text Book of Medical Physiology (11th edition).

Blood consists of components that also include:
- Tunica adventitia: The outer layer of blood vessels.
- Tunica media: The middle layer of blood vessels.
- Tunica intima: The inner layer of blood vessels.

Characteristics of RBCs:
- Highly specialized, biconcave, and anucleate cells designed for:
- Transporting oxygen from the lungs to tissues and carrying carbon dioxide back for exhalation.
- Biconcave shape increases surface area, facilitating efficient gas exchange.
- Mature RBCs lack nuclei and organelles to allow more space for hemoglobin, the key oxygen-carrying molecule.

RBCs are terminally differentiated, meaning they cannot divide.
Typical lifespan: Approximately 120 days.
Removal process:
- Removed by phagocytic cells in the liver and spleen after their lifespan.
Iron from hemoglobin is mostly recycled, and the heme portion is broken down into bile pigments and excreted by the liver.
Production rate: Around 3 million RBCs are cleared every second, with new ones produced in the bone marrow to maintain balance.

RBCs are denser than plasma and settle at the bottom after centrifugation, making up roughly 45% of total blood volume (termed hematocrit).
Above this layer lies a thin, cream-colored layer called the buffy coat, consisting of white blood cells and platelets.
The top layer consists of plasma, which is pale yellow in color and accounts for just under 55% of the total volume.
Hematocrit significance:
- Low hematocrit indicates anemia.
- High hematocrit may suggest polycythemia or dehydration.

A patient’s hematocrit is measured at 41%. Approximate percentage of the patient’s blood that is plasma:
- Solution: Plasma % = 100% − Hematocrit % − Buffy coat % ≈ 100 − 41 − 1 = 58%.
- The patient’s blood is approximately 58% plasma.

In adult humans, the hemoglobin (Hb) molecule consists of:
- Four polypeptides:
- Two alpha (α) chains of 141 amino acids each.
- Two beta (β) chains of 146 amino acids each.
Attached to each polypeptide is:
- A prosthetic group called heme.
Features of heme:
- Contains one atom of iron at the center.
- Allows each heme to bind one molecule of oxygen.

What would happen to red blood cells if the haem group were removed from haemoglobin?
a) Red blood cells would not be able to bind oxygen.
b) Red blood cells would not be able to reproduce.
c) White blood cells would not be able to reproduce.
d) Blood clot formation would be inhibited.
Which of the following statements is correct about erythrocytes?
a) They fight infection.
b) They clot blood.
c) They do not have a nucleus.
d) They are formed in the spleen.
The percentage of total volume used to determine the number of erythrocytes in a blood sample is referred to as:
a) Red blood cells count.
b) Reticulocyte count.
c) Haematocrit.
d) ESR.

At high altitudes, oxygen levels are low; consequently, the body of residents synthesizes more RBCs to obtain sufficient oxygen.