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Blood is a fluid connective tissue. It transports oxygen, carbon dioxide, and glucose.
It helps in thermoregulation due to its contact with cells that generate heat through cellular respiration.
Cellular respiration: $Glucose + O<em>2 \rightarrow CO</em>2 + H_2O + Heat$
Shivering is an involuntary muscle contraction that increases cellular respiration to produce heat, raising blood temperature.
Men typically have 5-6 liters of blood, while women have 4-5 liters due to stature and menstruation.
Iron supplementation is more critical for women due to periodic blood loss during menstruation.
- Excess iron in men can lead to hemochromatosis, a condition where iron deposits damage organs.
- Treatment for hemochromatosis includes therapeutic bloodletting (donating blood).
Red blood cells circulate for approximately four months.
- They lack a nucleus, preventing mitosis and protein synthesis for repair.
- The bioconcave disc shape of red blood cells allows flexibility for navigating capillaries and maximizes hemoglobin content for transporting respiratory gases.
Capillaries are the smallest blood vessels, requiring cells to pass through in single file.
Red blood cells lack a nucleus and ribosomes; thus they cannot transcribe mRNA to synthesize new proteins.

All blood cells originate from a stem cell called a hemocytoblast.
- When more red blood cells are needed, hemocytoblasts differentiate into cells with a nucleus.
- These cells create hemoglobin and eventually dissolve their nucleus to become mature red blood cells (erythrocytes).

Red bone marrow, found in spongy bone of vertebrae, ilium, and sternum, is responsible for blood cell production.
- Compact bone is dense and doesn't allow for bone growth.
- Spongy bone contains open spaces to allow hematocytoblasts production
Hematopoiesis is the process of making more blood cells.
Bone marrow biopsies can be performed to examine blood cells, often taken from the ilium due to accessibility and reduced risk to organs.
Hematologists are blood doctors while nephrologists are Kidney doctors (related to the Nephron).

Low oxygen levels or decreased red blood cell numbers signal the body to produce more red blood cells.
Rapid blood volume decrease may indicate internal bleeding, prompting platelet production to fix leaks.

White blood cells protect against infections and have nuclei, enabling complex functions.
Mnemonic to remember the abundance of white blood cells: "Never Let Monkeys Eat Bananas" (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils).
- Neutrophils: Most abundant, respond to acute infections.
- Lymphocytes: Involved in acquired immunity, produce antibodies.
- Monocytes: Clean-up crew, elevated during chronic infections.
- Eosinophils: Elevated during parasitic infections and allergic reactions.
- Basophils: Least abundant, release chemicals (like histamine) to aid other blood cells.
Antihistamines counteract histamine's leakiness effect, reducing symptoms like runny nose and swelling.

Platelets are cytoplasmic fragments pinched off from megakaryocytes.
- They are essential for blood clotting due to their stickiness.
- Platelets have a lifespan of about ten days.
Aspirin thins blood by making platelets less sticky, reducing the risk of coronary artery blockage.
- It must be taken regularly to maintain the effect.
- Platelet comes from cell fragments.

The heart sends blood to the lungs, where oxygen is taken in and carbon dioxide is released (pulmonary ventilation and external respiration).
Boyle's Law.
Oxygen goes into the capillaries, while carbon dioxide goes into the lungs. Gas exchange occurs through simple squamous cells (Type I cells in the lungs).
- Type II are cuboidal producing surfactant to allow external respiration to work
Internal respiration occurs at the tissues.
The lymphatic system is a parallel system to the circulatory system, collecting excess fluids and immune cells.
- Lymph nodes filter lymph, trapping pathogens; doctors palpate lymph nodes to check for swelling, indicating infection.
Lymph nodes are strategically located (e.g., in joints) to act as checkpoints against infection spread.
Lymph node biopsies are used to check for disease spread, such as cancer metastasis.
- Cancer spreads via blood vessels or lymphatic vessels.
Lymphatic disruption can lead to lymphedema (swelling) due to fluid buildup.

Non-specific (innate) defenses provide broad protection.
- Skin with stratified squamous epithelium protects against external threats.
- Inflammation, characterized by redness, heat, swelling, and pain, is a non-specific response.
- Phagocytosis: Eating cells (endocytosis)
- Exocytosis: pooping it out
Acquired (adaptive) immunity develops after exposure to a specific pathogen, creating memory cells for targeted responses.
- Involves B cells and T cells (lymphocytes).
- Long