Biology 211
Study Notes Exam 1
Endocrine System: a system of small glands scattered throughout the body that influences the metabolic activities of cells through hormones
- Hormones: chemical messengers released to the blood by the cells of endocrine glands that regulate the metabolic activities of other cells in the body
o Hormones signal target cells to perform specific chemical reactions
Endocrine Glands: pituitary, thyroid, parathyroid, adrenal, pineal and thymus glands.
- Organs with major functions outside the endocrine system containing endocrine tissue/cells: pancreas, gonads, hypothalamus (neuroendocrine organ)
- Tissues that produce hormones also found within: adipose cells, small intestine, stomach, kidneys, heart
Hormones:
Amino acid-based hormones: contain from a couple to many amino acidsŠ vary in size from simple amino acid derivatives (amines, thyroid hormone, peptides) to proteins (polypeptides)
Steroid hormones: synthesized from cholesterol (includes hormones from the gonads and adrenal cortex (outer region of the adrenal gland))
Eicosanoids: local hormones (paracrines); biologically active lipids released from nearly all cell membranes
- effects are highly localized, different from circulating hormones
- leukotrienes: chemicals that mediate inflammation & some allergic reactions
- prostaglandins: many targets/effectsŠ raise blood pressure, stimulate uterine contractions during birth, enhance blood clotting & inflammation
Eicosanoids are generally not considered part of the endocrine system, but will be discussed in later Chapters with the appropriate systems
Mechanisms of hormone action:
- alter plasma membrane permeability or electrical state
- stimulate synthesis of proteins within cells
- activate or deactivate enzymes
- induce secretory activity
- stimulate mitosis/cell division
- proteins and peptides cannot freely penetrate plasma membrane
- these hormones bind to a membrane receptor that starts a chain of reactions that activates an intracellular second messenger molecule
- cyclic AMP signaling:
o the hormone (first messenger) binds the membrane receptor; the receptor changes shape, which allows it to bind G protein
o G protein is activated; binds GTP & releases GDP
o activated G protein moves along membrane; binds to & activates (or inhibits) enzyme adenylate cyclase (GTP is hydrolyzed by GTPase activity of G protein)
o activated adenylate cyclase converts ATP to cyclic AMP (second messenger); if inhibited, adenylate cyclase will not catalyze its reaction
o cyclic AMP is free to circulate inside the cell; triggers activation of one to several protein kinase molecules; protein kinase phosphorylates (adds a phosphate group to) many proteins
- the phosphorylated proteins may be activated or inhibited by phosphorylation
- amplification effect: each activated adenylate cyclase can generate many cyclic AMP molecules; each protein kinase can catalyze hundreds of reactions
- the end effect depends on the target cell (in thyroid cells, binding of TSH to its receptor ends in the synthesis of thyroid hormone; in bone & muscle cells, growth hormone binding to its receptor ends in protein synthesis)
- cyclic AMP is rapidly degraded by the enzyme phosphodiesterase, but activation of protein kinase by cyclic AMP has generally already taken place
- PIP-calcium signaling mechanism:
o hormone (first messenger) binding to its receptor causes the receptor to bind inactive G protein
o G protein is activated; binds GTP & releases GDP
o activated G protein binds & activates a membrane-bound phospholipase enzyme; G protein becomes inactive
o phospholipase splits phosphatidyl inositol biphosphate (PIP2) to diacylglycerol (DAG) & inositol triphosphate (IP3); both DAG & IP3 are second messengers
o DAG activates protein kinases on the plasma membrane; IP3 triggers calcium ion release from the ER
o liberated calcium ions (also second messengers) alter activity of some specific enzymes and ion channels or bind to the regulatory protein calmodulin; calmodulin also activates specific enzymes to amplify the cellular response
- there also appear to be other second messenger systems used by some hormones that are less understood
Steroid hormones & direct gene activation:
- steroid hormones are lipid-based (synthesized from cholesterol) and can easily diffuse into target cells (no need for intracellular second messengers since the hormone can enter the cell)
- thyroid hormone also uses this mechanism
- hormone enters the nucleus; binds to & activates intracellular receptor
- the hormone-receptor complex binds a DNA-associated receptor protein, which turns on transcription & translation of the associated gene
- the protein synthesized in many cases is an enzyme that effects the metabolic activities of the cell to transmit the effects of the hormone
Hormones are specific for their target cells:
- hormones bind specific receptors; the receptor will only bind to one hormone
- the effects of the hormone depend on the blood levels of the hormone & the presence & number of receptors on the target cell
- up-regulation: increase in the # of receptors for a hormone on a target cell
- down-regulation: decrease in the # of receptors for a hormone on a target cell
The half-life of a hormone (persistence of a hormone in blood, a time indicating half its activity remaining) is brief (from a fraction of a minute to 30 minutes), but the effects of hormones can last for several minutes to hours
Control of hormone release:
Negative feedback: hormone secretion triggered by an external stimulus; as hormone levels rise, the hormones feed back to the metabolic pathway that produces them & inhibit their further release
Humoral stimuli: hormone release controlled by blood levels of specific ions and nutrients (e.g.: calcium or glucose)
Neural stimuli: nerve fibers stimulate hormone release (sympathetic neurons stimulate secretion of catecholamines (epinephrine & norepinephrine) fro the adrenal medulla))
Hormonal stimuli: other hormones regulate release of a hormone (e.g.: releasing & inhibiting hormones released by hypothalamus regulate release of hormones from pituitary)
Nervous system modulation: the nervous system can override normal homeostatic mechanisms for hormonal control (for example, to allow more glucose for fuel to be released during excitement (³fight or flight response²))
Major Endocrine Glands:
Pituitary Gland (Hypophysis): connected to hypothalamus by stalk called infundibulum
- Anterior Pituitary (Adenohypophysis):
o Growth Hormone (GH): stimulates cell division in most cells (major targets are bone & skeletal muscle)
§ IGFs (insulin-like growth factors or somatomedins) mediate most effects of GH
§ Hypersecretion: in children, can lead to gigantism; after long bones have developed, can lead to acromegaly
§ Hyposecretion: in children, can lead to pituitary dwarfism
o Prolactin (PRL): stimulates milk production by mammary glands of breasts
o Follicle-stimulating hormone (FSH): stimulates gamete production in gonads (ovaries & testes)
o Leutinizing hormone (LH): promotes production of gonadal hormones (testosterone, estrogen & progesterone)
o Thyroid-stimulating hormone (TSH): stimulates normal development of & secretion of hormones from thyroid gland
o Adrenocorticotropic hormone (ACTH or corticotropin): stimulates release of corticosteroid hormones from adrenal cortex
- Posterior pituitary (Neurohypophysis): receives & stores hormones from hypothalamus for later release
o Oxytocin: produced by paraventricular nucleus of hypothalamus; stimulates uterine contraction during childbirth & milk ejection during nursing
o Antidiuretic hormone (ADH): produced by supraoptic nucleus of hypothalamus; stimulates kidney tubules to retain water
§ deficiency of ADH secretion leads to diabetes insipidus
Hypothalamus: secretes releasing & inhibiting hormones that regulate release of hormones from anterior pituitary
- hypophyseal portal system: network of blood vessels that delivers hormones to anterior pituitary from hypothalamus
- Growth Hormone-Releasing Hormone (GHRH)
- Growth Hormone-Inhibiting Hormone (GHIH or somatostatin)
- Prolactin-Releasing Hormone (PRH)
- Prolactin-Inhibiting Hormone (PIH or dopamine)
- Gonadotropin-Releasing Hormone (GnRH)
- Thyrotropin-Releasing hormone (TRH)
- Corticotropin-Releasing hormone (CRH)
Thyroid Gland:
- Thyroid Hormone: amino acid hormones containing 2 tyrosine molecules each bound to iodine molecules; regulates metabolic activities of all cell types, especially glucose oxidation (energy & heat production)
o Thyroxine (T4): major hormone released from thyroid follicles (contains 4 iodine molecules)
o Triiodothyronine (T3): (contains 3 iodine molecules); generally formed from T4 by cleaving an iodine molecule
- Thyroid follicles are sacs lined with follicular cells and containing a substance called colloid; colloid contains thyroglobulin (tyrosine molecules linked to iodines)
- Thyroid hormone formed by joining 2 tyrosine-iodine complexes
- simple goiter: enlargement of thyroid gland due to lack of dietary iodine (thyroid hormone precursors accumulate in gland)
- hypothyroidism in infants may be associated with cretinism (underdeveloped thyroid gland); symptoms are short, stocky stature & may lead to mental retardation
- myxedema: hypothyroidism in adults (lethargy, weight gain, hair loss, slow pulse, etc,)
- treatment for hypothyroidism is generally thyroxine therapy
- Graves¹ disease: hyperthyroidism due to enlarged & overactive thyroid gland; produces exopthalmic goiter (swelling & protrusion of eyes)
- treatment of hyperthyroidism involves thyroid gland surgery &/or radioactive iodine
- Calcitonin: produced by parafollicular cells (C cells) of thyroid gland
o lowers blood calcium levels by inhibiting osteoclasts & stimulating calcium uptake by bones
Parathyroid Glands: paired glands on posterior aspect of thyroid gland
- Parathyroid hormone (Parathormone or PTH): raises blood calcium levels by stimulating osteoclasts, enhancing absorption of calcium by kidneys, & increasing absorption of calcium by cells of intestine
o PTH activates the inactive form of vitamin D in the kidneys; vitamin D enhances absorption of calcium by intestine
Adrenal Glands (Suprarenal Glands): pyramid-shaped glands above kidneys
- Adrenal Cortex: releases corticosteroid hormones
o Mineralocorticoids: released primarily by cells of zona glomerulosa; regulate salt concentrations in extracellular fluids
§ Aldosterone: primary mineralocorticoid: enhances sodium (& water) reabsorption from kidney tubules
· Sodium ion concentration in body fluids also regulated by rennin-angiotensin system, ACTH & atrial natriuretic peptide (ANP)
o Glucocorticoids: influence metabolism of body cells & help resist stressors
§ During times of stress (injury/blood loss), glucocorticoids stimulate gluconeogenesis (glucose synthesis) & mobilize fats & proteins to be used for energy to save glucose for the brain
§ Cortisol (hydrocortisone) is major glucocorticoid (also cortisone & corticosterone)
§ Glucocorticoids also prevent water loss from cells into tissue fluids; used as anti-inflammatory agents
- Gonadocorticoids: secondary source of sex hormones; primarily androgens (testosterone), but also estrogens
o may contribute to onset of puberty
- Addison¹s disease: low level of adrenal cortex hormones resulting in bronzing of skin, low blood sugar (low energy & weak immunity) & low blood sodium (low blood pressure)
- Cushing syndrome: high level of adrenal cortex hormones resulting in high blood sugar (& possibly diabetes mellitus), high blood sodium (hypertension), swelling & obesity & possible masculinization in women
- Adrenal medulla: releases catecholamines (norepinephrine & epinephrine)
o Release is stimulated by sympathetic nervous system (³fight or flight² response)
o Epinephrine: stimulates heart rate & metabolism
o Norepinephrine: influences peripheral vasoconstriction & blood pressure
Pancreas: releases insulin & glucagon from islets of Langerhans
- Insulin: released by beta cells of islets; lowers blood glucose levels by stimulating glucose storage & uptake of glucose by cells for energy
o Insulin deficiency may leads to diabetes mellitus
§ Insulin-dependent diabetes mellitus (IDDM): autoimmune disease where immune cells attack & destroy beta cells
§ Non-insulin-dependent diabetes mellitus (NIDDM): insulin receptors do not properly respond to insulin
- Glucagon: raises blood glucose levels by stimulating glucose removal from glycogen storage deposits in liver cells & gluconeogenesis
Gonads (ovaries & testes): produce steroidal sex hormones
- Ovaries: produce estrogens, progesterone, inhibin & relaxin
o estrogens (estrone & estradiol) & progesterone: produced by ovary cells are responsible for maturation of female reproductive organs & regulation of menstrual cycle
§ also, maintain pregnancy & prepare mammary glands for lactation
o inhibin inhibits FSH during ovarian cycle; relaxin released during pregnancy increases flexibility of pubic symphysis & helps dilate uterine cervix
- Testes: produce testosterone, an androgen (male sex hormone)
o testosterone: produced by cells of testes is responsible for maturation of male reproductive organs & sperm cell production
o inhibin inhibits FSH to regulate spermatogenesis
Pineal Gland: secretes melatonin
- pineal gland located in epithalamus of brain at roof of 3rd ventricle
- melatonin appears to be involved in maintenance or sleep/wake (day/night) cycles
o melatonin derived from the amino acid serotonin
o more melatonin released in darkness, less in light; norepinephrine from sympathetic fibers stimulate secretion of melatonin (may cause sleepiness)
o during sleep, plasma levels of melatonin increase & then decrease before awakening; therapeutic use to induce sleep still under investigation
Thymus: secretes thymopoietins & thymosins
- thymus located behind sternum superior to heart
- thymosin, thymic humoral factor (THF), thymic factor (TF) & thymopoietin involved with normal development of T cells (lymphocytes); may slow aging
Other Hormone-Producing Structures:
Heart: specialized cardiac muscle cells of atria secrete atrial natriuretic peptide (ANP), which reduces blood volume, blood pressure, & blood sodium levels
GI tract: enteroendocrine cells secrete hormones that aid in digestion
Placenta: secretes steroid hormones that help during pregnancy & human chorionic gonadotropin (hCG)
Kidney: secretes erythropoietin that stimulates red blood cell synthesis in bone marrow
Skin: secretes inactive vitamin D (cholecalciferol), which is activated by PTH in kidneys
Adipose Tissue: secretes leptin, which binds to neurons regulating appetite
control & leads to sensation of satiety
Chapter 17: Blood
Blood Functions:
- transport & distribution of oxygen & nutrients, carbon dioxide & metabolic waste, and hormones
- regulation of body temperature, normal pH and fluid volume in cells & tissues
- protection against blood loss (clotting) and infection (white blood cells)
Blood Characteristics:
- pH of blood is maintained between 7.35 and 7.45 by carbonic acid-bicarbonate ion buffer system
- blood accounts for ~ 8% body weight
- blood volume in adults is normally 5-6 L in males and 4-5 L in females
Blood Components:
- plasma & formed elements (erythrocytes, leukocytes & platelets)
- hematocrit: % of total blood volume occupied by erythrocytes (normally between 42% and 47% ± 5%)
Blood Plasma: fluid component of blood
- mostly (~ 90%) water
- contains over 100 different dissolved solutes, including:
o proteins: albumin, globulins, clotting proteins, etc.
§ albumin is majority of plasma protein; albumin is carrier molecule & contributes to plasma osmotic pressure
§ globulins include transport proteins & antibodies
o nutrients: sugars, amino acids, fatty acids, cholesterol, vitamins, etc.
o electrolytes: cations (positive ions) such as sodium, potassium, calcium & magnesium; anions (negative ions) such as chloride, phosphate & bicarbonate
o respiratory gases: oxygen & carbon dioxide
Formed Elements: erythrocytes, leukocytes & platelets
- Erythrocytes: red blood cells (RBCs)
o small cells; biconcave discs (flattened disc shape with thin, depressed centers – look like mini doughnuts)
o anucleate – RBCs have no nucleus
o function in gas transport
o most of contents of RBC (other than water) is the protein hemoglobin
§ hemoglobin is composed of 4 globin polypeptide chains each bound to a heme group
· heme is a ringlike compound with an iron atom at its center
· the iron atom in heme binds to oxygen
§ one hemoglobin molecule can bind to & transport up to 4 oxygen molecules
§ hemoglobin can also bind carbon dioxide; carbon dioxide binds to globin chain amino acids rather than heme
§ oxyhemoglobin: hemoglobin with bound oxygen
§ deoxyhemoglobin: hemoglobin with no bound oxygen
§ carbaminohemoglobin: hemoglobin with bound carbon dioxide
o hematopoiesis (hemopoiesis): blood cell formation; occurs in red bone marrow (in adults, in bones of girdles & proximal epiphyses of humerus & femur)
§ starts with stem cell called hemocytoblast (hematopoietic stem cell that is used to form all formed elements of blood)
§ erythropoiesis (erythrocyte production): hemocytoblast -> myeloid stem cell -> proerythroblast -> early erythroblast -> late erythroblast -> normoblast -> reticulocyte -> erythrocyte
· cell shrinks in size; cell accumulates hemoglobin protein during erythroblast stages; cell loses its nucleus in transition from normoblast to reticulocyte
· reticulocyte counts can be used as a rough indicator of the rate of RBC formation
· controlled hormonally by erythropoietin produced by the kidneys (responding to hypoxia (low oxygen levels))
· requires adequate supplies of iron & amino acids for hemoglobin and B vitamins (vitamin B12 and folic acid) for DNA synthesis
o since free iron is toxic, it is always transported in protein-iron complexes (such as ferritin)
o destruction of erythrocytes: RBCs last ~ 100-120 days in circulation
§ aged & damaged RBCs are broken down in small channels of the spleen, liver & marrow by macrophages
§ heme is broken from hemoglobin; iron is salvaged & stored and the remainder of the group is degraded to bilirubin (yellow pigment), which is picked up by the liver, converted into bile & excreted
§ globin chains are metabolized are broken down into amino acids for protein synthesis
o erythrocyte disorders:
§ anemias: conditions that involve blood with a very low oxygen-carrying capacity
· caused by an insufficient number of RBCs (hemorrhagic, hemolytic & aplastic anemias), decreased hemoglobin content (iron-deficiency & athlete¹s anemia) or abnormal hemoglobin (thalassemias & sickle cell anemia)
§ polycythemia: abnormal excess of RBCs; increases blood viscosity & can impair circulation
· can be treated by diluting blood with saline
· artificial polycythemia can be induced by infusing RBCs (blood doping used by some athletes to increase available oxygenŠ considered unfair by many games committees)
- Leukocytes: white blood cells (WBCs)
o only formed elements with nucleus & normal organelles
o involved in immune responses; protect the body from damage by bacteria, viruses, parasites, toxins & tumor cells
o diapedesis: white blood cells can move out of capillaries & into tissues
§ use amoeboid motion with flowing cytoplasmic extensions to move through tissue spaces
§ positive chemotaxis: follow chemical trail of other WBCs to sites of infection
o leukocytosis: condition of increased WBC count during infection (normal response)
o Granulocytes: WBCs with membrane-bound cytoplasmic granules
§ Neutrophils: most numerous WBCs (>50% of WBC volume)
· ~ 2x size of RBCs
· very fine, lightly staining granules containing enzymes or antibiotic-like proteins (defensins)
· nucleus has from 3-6 lobes (also known as PMNs (polymorphonuclear leukocytes))
· phagocytic cells (kill bacteria & fungi by oxidation), chemically attracted to sites of inflammation
§ Eosinophils: ~ 1-4% of WBC population; about size of neutrophils
· nucleus with 2 lobes (like telephone receiver)
· large, red-staining granules with enzymes
· digest invading parasitic flatworms & roundworms with digestive enzymes
· phagocytic; ingest immune complexes during allergic reactions
§ Basophils: ~ 0.5% of WBC population (rare); about size of neutrophils
· large purplish-black-staining granules containing histamine
· histamine: inflammatory chemical - vasodilator & chemoattractant – released by basophils
o Agranulocytes: WBCs without visible granules
§ Lymphocytes: small, medium & large sizes
· large spherical nucleus occupies most of cell volume
· most lymphocytes are in lymphatic organs
· T lymphocytes: fight virus-infected cells & tumor cells
· B lymphocytes: give rise to plasma cells that produce antibodies (immunoglobulins)
§ Monocytes: largest WBCs (2-3x size of RBCs)
· Large U or kidney-shaped nucleus
· differentiate into macrophages in tissues
· macrophages are phagocytic cells that destroy bacteria & help in immune response against viruses
o Leukopoiesis: WBC production
§ Stimulated by hormones (cytokines such as interleukins & colony-stimulating factors (CSFs) from macrophages & lymphocytes
§ Hemocytoblast differentiates into either myeloid stem cell or lymphoid stem cell
· Myeloid stem cell differentiates into myeloblast or monoblast
o Myeloblast will form granulocytes
o Monoblast will form monocytes
· Lymphoid stem cell differentiates into lymphoblast, which will form lymphocytes
o Leukocyte disorders:
§ Leukemias: cancer of myeloid or lymphoid cell lines
· Leukemias can be acute (rapidly advancing) or chronic (slowly advancing)
· Treated with radiation & chemotherapy & bone marrow transplant to replace cancerous cells
§ Infectious mononucleosis: highly contagious viral infection
· caused by Epstein-Barr virus (EBV)
· symptoms (fatigue, aches, fever) last a few weeks until virus is dealt with by immune system
- Platelets: cytoplasmic fragments of megakaryocytes with granules containing blood-clotting enzymes
o sometimes referred to as thrombocytes
o stick together to form a plug to prevent blood loss in torn vessels
o platelet formation regulated by hormone thrombopoietin
o hemocytoblast differentiates into megakaryoblast, which undergoes repeated mitoses without cell division; this results in the megakaryocyte (cell with large nucleus)
o extensions of megakaryocyte in bloodstream rupture to form platelets
o Hemostasis: stoppage of bleeding from a torn blood vessel
§ Vascular spasms: result in vasoconstriction; caused by damage to smooth muscle, chemicals & reflexes
§ Platelet plug formation: in response to blood vessel injury, platelets swell & form spiked processes
· this allows them to adhere to exposed collagen fibers surrounding vessel
· release of chemicals (serotonin, ADP) enhances vascular spasms & attracts more platelets to area (positive feedback)
· PGI2 released by endothelial cells limits platelet aggregation to area of injury
§ Coagulation (blood clotting): blood transformed from a liquid to a gel
· Prothrombin activator converts the plasma protein prothrombin to thrombin
· Thrombin catalyzes joining of fibrinogen molecules in plasma to form a fibrin mesh that seals vessel
· Clotting factors enhance clot formation (several require vitamin K for formation)
· Anticoagulants inhibit clotting
o Clot retraction & repair: within 30-60 minutes after injury, platelets contract, pulling on fibrin strands to pull ends of torn vessel closer together
§ Platelet-derived growth factor (PDGF) released by platelets to stimulate division of smooth muscle cells & fibroblasts to rebuild vessel wall
o Fibrinolysis: removes unneeded clots after healing has occurred
§ Tissue plasminogen activator (tPA) activates plasma plasminogen, which is converted to plasmin
§ Plasmin is an enzyme that digests fibrin, breaking down clot
o Factors limiting clot growth/formation:
§ Anticoagulants antithrombin III, protein C & heparin work to inhibit procoagulants & thrombin
o Disorders of Hemostasis:
§ Thromboembolytic disorders: a thrombus (clot) forms in an unbroken blood vessel; if it detaches from the vessel wall, the resulting embolus can travel through the blood & block blood vessels
· Free blood clots can be treated by anticoagulants aspirin, heparin & warfarin
o Bleeding disorders:
§ thrombocytopenia
§ impaired liver function
§ hemophilia
Transfusion & Blood Replacement:
- whole blood transfusions are used to treat conditions involving massive blood loss
- packed red cells can be used to treat anemias
- Human Blood Groups: red blood cells have many (perhaps > 100) cell surface antigens – glycoproteins known as agglutinogens
o antigens determining ABO and Rh blood groups cause transfusion reactions
o ABO blood groups:
§ type A blood individuals have surface antigen A
§ type B blood individuals have surface antigen B
§ type AB blood individuals have both A & B surface antigens
§ type O blood individuals have neither A nor B surface antigens
§ individuals make antibodies (agglutinins) against the antigen(s) not present on their red blood cells (e.g.: type A blood individuals will make anti-B agglutinins); this does not require previous exposure to the antigen(s)
o Rh blood groups:
§ Humans may also have one of several Rh factors present on the surface of their red blood cells
§ An individual without Rh factor will make antibodies against Rh factor, but only after exposure to the antigen
Transfusion reactions: agglutination & hemolysis
- following infusion of mismatched blood, agglutination occurs as antibodies complex with the foreign blood group antigens
- this blocks blood vessels & hinders blood flow; reduces oxygen availability to tissues, as the RBCs are lysed, hemoglobin escapes & may precipitate in kidney tubules leading to renal failure
- treatment involves diluting agents & diuretics
- type O- blood is the universal donor
- type AB+ blood is the universal recipient
Plasma & Blood Volume Expanders:
- plasma can be temporarily used to replace some blood volume when properly typed blood is not immediately available
- also, blood volume expanders such as albumin & dextran, which draw fluid into blood, can be used temporarily
Diagnostic Blood Tests:
- differential white blood cell count: to assess infection
- platelet count: used to assess thrombocytopenia
- complete blood count (CBC) used routinely to provide counts for all formed elements & tests for clotting factors
Chapter 20: The Lymphatic System
Functions of Lymphatic System:
- Draining excess interstitial fluid: lymphatic vessels drain excess fluid from tissue spaces & return it to the blood
- Transporting dietary lipids: lymphatic vessels transport lipids & lipid-soluble vitamins (A,D,E & K) absorbed by GI tract to the blood
- Carrying out immune responses: lymphatic tissue initiates specific immune responses to microbes or abnormal cells
Lymphatic Vessels (Lymphatics): system of drainage vessels that collects excess protein-containing interstitial fluid (fluid between cells) & returns it to blood
- used to return fluid escaped from blood into tissue spaces back to blood
- lymph is interstitial fluid that has entered lymphatic vessels
- form one-way system; blood flows toward heart
- Lymph capillaries: occur almost everywhere blood capillaries occur (except bones & teeth, bone marrow, & central nervous system (uses CSF to collect fluid))
o the edges of endothelial cells in walls of lymph capillaries loosely overlap forming minivalves to prevent backflow
o collagen filaments anchor the endothelial cells to connective tissue outside, allow the flaps to open when interstitial fluid volume increases such that fluid enters the lymphatic capillaries
o lymphatic capillaries (unlike blood capillaries) can easily take up proteins, foreign cells & debrisŠ fortunately lymph is circulated through lymphoid organs with immune cells to examine the fluid for undesirables
o lacteals are specialized lymphatic capillaries in the intestinal mucosa that carry a thick white fatty lymph (chyle) to the blood
o lymph flows from lymphatic capillaries to collecting vessels, trunks, and ducts
o lymphangitis: inflammation of lymphatics
o lymphatic ducts: right lymphatic duct drains lymph from right upper arm, right side of head & thorax; thoracic duct arising from the sac-like cisterna chyli drains the rest of the body
- Lymph transport: slow transport; lymph is not pumped, but flows by smooth muscle contraction in the walls of the vessels, pressure changes in the thorax during breathing & valves to prevent backflow
o Also, bundling with blood vessels helps along with movements in adjacent tissues
Lymphoid Cells:
- Lymphocytes: T cells & B cells
o T cells direct the immune response against virally-infected cells & cancer cells
o B cells produce plasma cells that synthesize antibodies
- Macrophages: phagocytize foreign substances & help activate T cells (along with dendritic cells)