Endocrine System and Blood: Structure, Function, and Regulation

Endocrine System

Systemic Operation – Distant Control

The endocrine system regulates physiological processes through hormones, which are secreted into the extracellular fluid and transported by blood to distant target organs.

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

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

• 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 cells that express specific receptors for those hormones.

• Specificity: Target cells must express specific protein receptors for a hormone; may be on cell membrane or interior.

• Altering activity: Hormone binding alters cell activity, such as changing membrane permeability, protein synthesis, or enzyme activation.

• 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, binds to intracellular receptor, enters nucleus, binds to DNA to activate gene transcription, producing new proteins.
Second messenger system	Water-soluble hormones (e.g., nonsteroid hormones)	Hormone binds to membrane receptor on cell surface, activates intracellular signal (e.g., cAMP), causes changes in cellular function.

Mechanisms of Hormone Release

Hormone release is regulated by various stimuli:

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

• Humoral stimulus: Blood levels of certain ions or 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

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

Endocrine Disorders

Common disorders are associated with dysfunction of endocrine glands:

• Thyroid gland:

  • Goiter: Enlargement due to lack of iodine.

  • Graves disease: Overactivity, excessive hormone secretion.

  • Myxedema: Hypothyroidism, physical and mental sluggishness.

• Adrenal cortex:

  • Addison's disease: Underproduction of hormones; weakness, fatigue, bronze skin.

  • Cushing's syndrome: Overproduction of glucocorticoids; moon face, buffalo hump, high BP.

  • Hyperaldosteronism: Excess aldosterone; sodium retention, swelling.

• Pituitary gland:

  • Dwarfism: Too little GH during childhood.

  • Acromegaly: Hypersecretion of GH during adulthood; bones enlarge.

  • Diabetes insipidus: Too little ADH; excessive urination.

Blood

Location and Common Stem Cell

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

• Hematopoiesis: Occurs in red bone marrow (sternum, ribs, skull, vertebrae, pelvis, proximal ends of femur).

• Stem cell lineages:

  • Lymphoid stem cell: Produces lymphocytes.

  • Myeloid stem cell: Produces all other formed elements.

Hormone Regulation of Blood Cell Production

Blood cell production is controlled by hormones, mainly through negative feedback.

• Erythropoiesis: Kidneys produce erythropoietin in response to low oxygen; stimulates RBC production.

• Leukopoiesis: Colony stimulating factors (CSFs) and interleukins stimulate leukocyte production.

• Platelet production: Thrombopoietin stimulates production of platelets from megakaryocytes.

Physical Characteristics and Composition of Blood

Blood has unique physical and chemical properties essential for its function.

• pH: 7.35–7.45

• Temperature: ~100.4°F

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

• Viscosity: Excessive RBCs (polycythemia) increase viscosity, making it harder for the heart to pump.

• Volume: Male: 5–6L; Female: 4–5L

• Composition:

  • Plasma: 55%; water, proteins, other dissolved substances

  • Formed elements: 45%; RBCs, WBCs, platelets

Blood Types and Compatibility

Blood types are determined by the presence of specific antigens and antibodies, which affect compatibility for transfusions.

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

Blood is mixed with anti-A, anti-B, and anti-Rh serums to test for clumping, which indicates blood type. Compatibility is crucial to prevent immune reactions and hemolysis.

Hemostasis: Blood Clotting

Hemostasis is the process that stops bleeding after injury, involving three main steps:

• Vascular spasm: Immediate vasoconstriction reduces blood loss.

• Platelet plug formation: Platelets adhere to exposed collagen fibers and aggregate at the injury site.

• Coagulation: Clotting cascade converts prothrombin to thrombin, which then converts fibrinogen to fibrin, forming a stable clot.

Key conversion:

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

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

Rh Factor

The Rh factor is an antigen on RBCs that is important for blood compatibility, especially in pregnancy.

• Rh blood group determined by presence of 1 of 8 specific Rh antigens.

• Most significant is agglutination D.

• Rh+ has the antigen; Rh– does not.

• Rh– individuals can produce anti-Rh antibodies if exposed to Rh+ blood.

• Important in pregnancy: Rh– mother with Rh+ baby can develop antibodies that attack fetal RBCs in subsequent pregnancies.

Blood Cell Types: Characteristics and Functions

Cell type	Appearance	Function	Abundance	Lifespan
Erythrocyte	Amor. (disc); "bags of hemoglobin"	Transport oxygen; carry carbon dioxide away; do not consume any of the oxygen	5 million per mm3 blood; 45% blood volume (cell fraction)	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 response; phagocytosis of site of acute infection; ingest bacteria, fungi, and some viruses	40–75% WBCs	Increase during infection; die after 24 hours
Eosinophil (granulocyte)	Red granules; bilobed nucleus	Kill parasites; release enzymes attacking invaders; play a role in neutralizing histamine	1–6% WBCs	Highest in the tissues; circulate 3–8 hours before entering tissues

Additional info: Other blood cell types include basophils (release histamine), lymphocytes (B and T cells for immune response), and monocytes (become macrophages in tissues).