Endocrine System and Blood: Structure, Function, and Regulation

Endocrine System

Systemic Operation – Distant Control

The endocrine system regulates physiological processes through hormones, which can act locally or be transported via the bloodstream to distant target organs.

• Transport via blood: Hormones are secreted into extracellular fluid and carried by blood to target sites.

• Scope of control: Includes reproduction, growth, development, immune system, homeostasis, and metabolic balance.

• Example: The hypothalamus stimulates the anterior pituitary to release hormones that travel to distant targets (e.g., thyroid, gonads).

Local Operation – Cellular Mechanisms

Hormones can also act locally, affecting only cells with specific receptors for that hormone.

• Specificity: Target cells must express specific protein receptors for hormone binding.

• Altering activity: Hormone binding alters cell activity, such as changing membrane permeability, promoting protein/enzyme synthesis, or stimulating mitosis.

• Mechanisms: Hormones act through two main mechanisms:

Mechanism	Hormone type	Key steps
Direct gene activation	Lipid-soluble hormones (e.g., steroid hormones)	Hormone diffuses through plasma membrane, enters nucleus, binds to DNA to activate gene transcription
Second messenger system	Water-soluble hormones (e.g., nonsteroid hormones)	Hormone binds to membrane receptor, generates intracellular signal (e.g., cAMP), causes change in cellular function

Mechanisms of Hormone Stimulus

• Hormonal stimulus: Endocrine glands activated by other hormones (e.g., hypothalamus stimulates pituitary).

• Humoral stimulus: Blood levels of certain ions/molecules stimulate hormone release (e.g., PTH and calcitonin for calcium, insulin and glucagon for glucose).

• Nerve stimulus: Nerve impulses stimulate hormone release, often in response to stress (e.g., sympathetic nerves trigger adrenal glands).

Major Endocrine Hormones and Their Functions

Hormone	Source	Function
Growth hormone (GH)	Anterior pituitary	Regulates growth of bones & muscles; stimulates fat breakdown & protein building; maintains blood sugar levels
Prolactin (PRL)	Anterior pituitary	Stimulates milk production after childbirth
Thyroid stimulating hormone (TSH)	Anterior pituitary	Stimulates growth and activity of the thyroid gland
Adrenocorticotropic hormone (ACTH)	Anterior pituitary	Regulates endocrine activity of the adrenal cortex
Follicle stimulating hormone (FSH)	Anterior pituitary	Stimulates follicle development in ovaries and sperm development in testes
Luteinizing hormone (LH)	Anterior pituitary	Triggers ovulation and stimulates testosterone production
Oxytocin	Posterior pituitary	Stimulates uterine contraction during labor; milk ejection during breastfeeding
Antidiuretic hormone (ADH)	Posterior pituitary	Inhibits urine production; causes vasoconstriction, increases BP
Thyroxine (T4) / Triiodothyronine (T3)	Thyroid	Major metabolic hormone; controls rate of energy production
Calcitonin	Thyroid	Decreases blood calcium levels by depositing calcium in bone
Parathyroid hormone (PTH)	Parathyroid	Increases blood calcium levels by stimulating osteoclasts

Endocrine Disorders

• Thyroid gland:

  • Goiter: Enlargement due to lack of iodine

  • Graves disease: Overactivity, excessive hormone, increased metabolism

  • Myxedema: Hypothyroidism, sluggishness, fatigue, cold, dry skin

• Adrenal cortex: Dysfunction causes Addison’s disease (deficiency) or Cushing’s syndrome (excess)

• Pituitary gland: Disorders include dwarfism, acromegaly, diabetes insipidus

Blood

Location and Common Stem Cell

Blood cells are formed in red bone marrow from hematopoietic stem cells, which differentiate into various cell types.

• Hematopoiesis: Formation of blood cells in bone marrow (sternum, ribs, vertebrae, pelvis, proximal ends of femur).

• Stem cell lineages:

  • Lymphoid stem cell: Produces lymphocytes (B and T cells)

  • Myeloid stem cell: Produces all other formed elements (erythrocytes, platelets, granulocytes, monocytes)

Hormone Regulation of Blood Cell Production

• Erythropoiesis: Controlled by erythropoietin (EPO) from kidneys in response to low oxygen

• Leukopoiesis: Controlled by colony stimulating factors (CSFs) and interleukins

• Platelet production: Controlled by thrombopoietin

Physical Characteristics and Composition of Blood

• pH: 7.35–7.45

• Temperature: ~100.4°F

• Oxygen-rich blood: Bright red; oxygen-poor: dull, dark red

• Viscosity: Increased RBCs (polycythemia) make blood more viscous

• Volume: Male: 5–6L; Female: 4–5L

• Composition: Plasma (55%), formed elements (45%: RBCs, WBCs, platelets)

Blood Types and Compatibility

Type	Antigens present	Antibodies produced	Donor	Recipient
A	A	Anti-B	A & AB	A & O
B	B	Anti-A	B & AB	B & O
AB	A & B	Neither Anti-A nor Anti-B	AB only	Universal recipient
O	None	Both Anti-A & Anti-B	Universal donor	O only

Rh Factor

• Rh blood group: Determined by presence of 1 of 8 specific Rh antigens on RBCs

• Most significant: Agglutination D

• Rh+: Has Rh antigen (most Americans)

• Rh-: Lacks Rh antigen; can produce Rh antibody if exposed to Rh+ blood

• Importance: In transfusion and pregnancy (e.g., Rh incompatibility in pregnancy)

Hemostasis: Blood Clotting

• Vascular spasm: Immediate vasoconstriction after vessel injury

• Platelet plug formation: Platelets adhere to exposed collagen, forming a plug

• Coagulation: Clotting cascade converts prothrombin to thrombin, which then converts fibrinogen to fibrin

Key conversion:

• $\text{prothrombin} \rightarrow \text{thrombin}$

• $\text{thrombin} \rightarrow \text{fibrinogen} \rightarrow \text{fibrin}$

Blood Cell Types: Characteristics and Functions

Cell type	Appearance	Function	Abundance	Lifespan
Erythrocyte	Amorphous, biconcave disc; "bags of hemoglobin"	Transport oxygen; carry carbon dioxide away; do not consume any of the oxygen	5 million per mm3 blood; 45% of blood volume	100–120 days
Leukocyte	Nuclei and organelles	Defense against pathogens	4,800–11,000 per mm3 blood	Varies
Neutrophil (granulocyte)	Fine granules; deep purple nucleus	First responder; phagocytosis of site of infection; release enzymes to degrade pathogens	40–75% WBCs	Increase during infection; 6 hours to a few days
Eosinophil (granulocyte)	Red granules; bilobed nucleus	Kill parasites; release enzymes in allergic reactions; neutralize histamine	1–6% WBCs	Highest in the blood during allergic reactions; 3–8 hours before entering tissues

Additional info: Academic context and explanations have been expanded for clarity and completeness. Tables have been recreated and formatted for study purposes.