Blood Basics: quick revision, 12 bites

Blood: A Specialized Connective Tissue

Blood is more than a red liquid.

Blood is a **connective tissue** with a fluid matrix called **plasma** and formed elements: **RBCs**, **WBCs**, and **platelets**. Plasma carries nutrients, wastes, and hormones.

Takeaway. Blood = plasma + cells, a transport system for nutrients, wastes, and hormones.

Plasma Carries Respiratory Gases

Gases dissolve in plasma too.

Plasma carries small amounts of **oxygen** and **carbon dioxide** in dissolved form. Most oxygen is in RBCs, but about **1.5%** is dissolved in plasma. About **5-7%** of CO₂ is dissolved, mostly as **bicarbonate ions**.

Takeaway. Plasma carries ~1.5% O₂ and 5-7% CO₂, mostly as bicarbonate.

Red Blood Cells (Erythrocytes)

Biconcave oxygen carriers.

**RBCs** are **biconcave**, disc-shaped cells with a central depression. They contain **haemoglobin**, which binds oxygen and gives blood its red color. They transport oxygen and a little CO₂.

Takeaway. RBCs are biconcave cells packed with haemoglobin for gas transport.

RBC Count and Formation Sites

Millions of RBCs per drop.

Normal RBC count is **4 to 5.5 million per mm³** of blood. Before and immediately after birth, RBCs form in **liver and spleen**. In adults, they form in **red bone marrow** of short bones like sternum, ribs, and vertebrae.

Takeaway. RBCs are made in liver/spleen early, then red bone marrow in adults.

RBC Life Cycle and Removal

RBCs live about 120 days.

Takeaway. RBCs lose nucleus, live ~120 days, then are destroyed in spleen/liver.

RBC Turnover is Massive

2 million RBCs replaced every second.

The body has more than **30 trillion RBCs**. About **2 million RBCs die and are replaced every second** to maintain steady supply.

Takeaway. RBC production and destruction are continuous and rapid.

White Blood Cells (Leukocytes)

Colorless defenders of the body.

**WBCs** are **colorless** and have **irregular shapes**. They are formed in red bone marrow, and some mature in lymph nodes, tonsils, thymus, or spleen. They defend against diseases.

Takeaway. WBCs are irregular, colorless cells that protect against infections.

RBCs vs WBCs

Key differences at a glance.

Takeaway. RBCs are numerous, red, and carry gases; WBCs are fewer, colorless, and defend.

WBC Count and Lifespan

About 7000 WBCs per mm³.

Normal adult WBC count is about **7000 per mm³**. Their **lifespan depends on the body's needs**—some live hours, others days or longer. There are several types of WBCs.

Takeaway. WBC count is ~7000/mm³, and lifespan varies with immune demand.

Remember RBC vs WBC Numbers

Millions vs thousands.

Think: **RBC = Rich in count** (millions), **WBC = Warriors** (fewer, but fight). RBC count ~4-5.5 million, WBC ~7000.

Takeaway. RBCs are in millions, WBCs in thousands.

Exam Focus: RBC Formation Sites

Liver/spleen vs bone marrow.

Remember the shift: **Before/after birth → liver and spleen**; **adults → red bone marrow** of sternum, ribs, vertebrae. This is a common exam point.

Takeaway. RBC formation site changes from liver/spleen to bone marrow with age.

RBCs Lack Nucleus

No nucleus, more space for oxygen.

During formation, RBCs break down their **nucleus and organelles**. This leaves more room for **haemoglobin**, maximizing oxygen-carrying capacity.

Takeaway. RBCs sacrifice nucleus to pack more haemoglobin.