Body Fluids & Homeostasis PDF

Body Fluids Introduction Organization of the human body - The human body is made up of different systems e.g. digestive system, whose functions are coordinated together. - Each system consists of many organs that are made up of many tissues with complementary f...

Body Fluids Introduction Organization of the human body - The human body is made up of different systems e.g. digestive system, whose functions are coordinated together. - Each system consists of many organs that are made up of many tissues with complementary functions. - Each tissue consists of millions of cells. The cell is the basic unit of structure and function in the body. - The cells of the human body, as well as those of other multi-cellular animals and plants, are eukaryotic (true nucleus) cells. - These cells contain a nuclear membrane surrounding the cell nucleus and numerous other membrane-bound structures. - These membrane-bound compartments, along with some particles and filaments, are known as cell organelles (little organs) e.g. mitochondria and ribosomes. Prokaryotic cells e.g. bacteria, lack these membranous structures. Fig. (1): Organization of human body -The eukaryotic cells are surrounded by a limiting barrier, the plasma membrane, which covers the cell surface. The plasma (cell) membrane allows selective communication between the intracellular and extracellular compartments and aids cellular movement. - On the other hand, each organelle performs specific function (ribosomes help in protein synthesis, and mitochondria help in production of energy) that contribute to the cell’s survival. Body Fluids - The body fluids are solutions of water, into which many organic molecules are dissolved (carbon-containing molecules such as carbohydrates, lipids, proteins, and nucleic acids), as well as inorganic molecules and ions (atoms with a net charge). - The total body fluids constitute about 65% i.e. 40-42 liters in an adult weighing 70 Kg. Compartments of body fluids: Body fluids are divided into 2 main compartments; A) Intracellular fluid (ICF): It is the fluid inside the cell. It constitutes two thirds (about 25-28 liters) of total body fluids. B) Extracellular fluid (ECF): It is the fluid outside the cell. It constitute one third (about 14-15 liters) of total body fluids. It is separated from the intracellular fluid by the cell membrane. Fig. (2): Body fluid compartments  Extracellular fluid (ECF) is known as the internal environment that supplies the cells with nutrients and other substances needed for cellular function. It consists of; a. Plasma (3-3.5 liters) inside the blood vessels. b. Interstitial fluid in spaces between cells (10-12 liters) and in lymph vessels. Plasma is separated from the interstitial fluid by the wall of the capillary. c. Transcellular fluid (1 liter) is present in body cavities e.g. digestive juices, and cerebrospinal fluid. Composition of body fluids: The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions, while ICF contains large amounts of potassium, magnesium, and phosphate ions (table 1). These differences are extremely important to the life of the cell. Concentration in mmol/L Intracellular fluid Interstitial fluid Plasma Na+ 10 145 140 Cations K+ 160 4 4 Ca+2 10-4 2 2 Cl- 3 100 115 Anions HCO3- 10 30 28 Proteins 55 10 16 Table (1): Composition of Body fluid Water Balance The total body water constitutes about 60% of the total body weight in adult men, while in women it constitutes 50% because their bodies contain much fat. Increase the percentage of fat in the body decreases the percentage of water. Children contain more water (70%) than adults, so water loss leads to rapid dehydration. In old age the water content is decreased (figure 3). Functions of body water: 1. It is required for all chemical reactions inside the body. 2. It acts as powerful solvent that dissolve various substances to make them accessible to body. So, it acts as a vehicle that carries nutrients and gases to the body cells and removes wastes from them. 3. It is important in regulation of body temperature as water has a high latent heat of evaporation (each gram H2O needs 0.58 ◦C to evaporate from the body). 4. It moistens tissues such as mouth, eyes and nose, and lubricates joints. 5. Water can pass easily through the cell membrane, so it helps in osmotic equilibrium between various body compartments. 6. Digestive function: water shares in the formation of digestive secretions and action of enzymes 7. Absorption: at the venous end of capillaries, GIT and kidney. 8. Filtration: at arterial end of capillaries and glomeruli. 9. Excretion: at GIT, kidney and skin. 10. Exchange; at capillary membrane, alveolar membrane and cell membrane. 11. Refractive medium in the eye Fig. (3): Percentages of water content in the body in different ages. Water balance: - Body water is kept constant by adjusting water input and water output. A) Sources of water input and output: 1) Water input or intake: (about 2400 ml/day) - Water input comes from 2 sources; a. Exogenous water: ingested in the form of water or liquid (about 2200 ml/day). b. Endogenous water: synthesized as a result of metabolism (about 200 ml/day). 2) Water output or loss: (about 2400 ml/day) - Under normal conditions, water loss occurs through the following routes; a. Insensible water loss (about 700ml) from lungs (water vapor) and skin (insensible perspiration) b. Sweating: only about 100 ml are lost under normal conditions, but in exercise sweating is increased. c. Feces (about 100ml). Water loss in feces increases in case of diarrhea. d. Urine (about 1500 ml) B) Control of Water Balance (figure 4): 1) Control of water input: It is controlled by thirst sensation. This is caused by stimulation of thirst center in anterior hypothalamus) by increased plasma osmolarity (hypertonicity), decreased blood volume (hypovolemia), and angiotensin II. It allows the person to drink much water. 2) Control of water loss It is controlled mainly by adjusting the urine volume. Volume of urine is controlled mainly by antidiuretic hormone (ADH) which is secreted the posterior pituitary gland. Its secretion is also stimulated by hypertonicity and hypovolemia.  Fig. (4): Regulation of water balance in case of decreased body fluid volume. Homeostasis  Definition: they are the mechanisms that keep the internal environment constant.  This is very important as the cells lie in the internal environment and this keeps the normal cellular functions as: - Body water. - Ions - Body temperature. - Blood glucose - pH. - Arterial blood pressure  There is a narrow range of change in homeostasis; more than this range the cell function is greatly affected.  All body systems work to maintain homeostasis as: - Nervous system (rapid). - Endocrine system (slow).  Feedback mechanisms:  Definition: These are the mechanisms that keep the system constant.  Types: 1. Negative feedback mechanisms: these mechanisms keep the system constant and the response inhibits the stimulus. e.g.  a) Increased CO2  hyperventilation  washout of excess CO2  decreased CO2 to normal. b) Increased blood glucose  increased insulin secretion  increased glucose utilization  decreased glucose level to normal. c) Increased arterial blood pressure (ABP)  reflex vasodilatation and decreased heart rate  decreased arterial blood pressure back to normal. 2. Positive feedback mechanisms: the response increases the stimulus. e.g. a) death cycles: * Heart failure decreased cardiac output Decreased coronary Decreased ABP flow *Heat stroke increased metabolic rate Increased heat production b) Some positive feedback cycles are useful. e.g. cervical dilatation increases the uterine contractions descent of baby more cervical dilatation  more uterine contractions  more descent of baby, till complete labor. References: 1. Costanzo, Linda S. "BRS Physiology (Board Review Series)." (2018). 2. Ganong, William F. "Review of medical physiology." (2020).