Physiology & Anatomy: Homeostasis

Introduction to Physiology and Anatomy

• Physiology is the study of how the normal body functions.
• Physiology describes how the various systems of the human body work together to maintain life and health.
• Anatomy is the study of the structure of the body.
• Anatomy studies the physical relationships between body parts.

Homeostasis and the Internal Environment

• Homeostasis is the maintenance of constant conditions in the internal environment.
• The internal environment is also known as the extracellular fluid (ECF).
• The ECF composition is consistent throughout the body due to the rapid exchange of solutes and water between blood and interstitial fluid.
• Rapid blood circulation and capillaries being located very near to the cells also help maintain consistent ECF.

Factors Regulated by Homeostasis

• Nutrient molecule concentrations are homeostatically regulated.
• Oxygen (O₂) and carbon dioxide (CO₂) concentrations are homeostatically regulated.
• Waste product concentrations are homeostatically regulated.
• pH is homeostatically regulated.
• Water, salt, and electrolyte concentrations are homeostatically regulated.
• Volume and pressure are homeostatically regulated.
• Temperature is homeostatically regulated.

Components of Homeostatic Regulation

• Homeostatic regulation involves a receptor, a control center, and an effector.
• The receptor receives information about changes in the environment.
• The control center processes information it receives from the receptor.
• The effector responds to commands from the control center by opposing or enhancing the stimulus.

Interdependence of Cells, Body Systems, and Homeostasis

• Homeostasis is essential for the survival of cells.
• Body systems maintain homeostasis.
• Cells make up body systems.
• The relationship between cells, body systems and homeostasis underlies the foundation of physiology.

Contributions to Homeostasis by Body Systems

• The circulatory system transports materials throughout the body.
• The digestive system breaks down dietary food into smaller molecules for distribution to body cells.
• The digestive system transfers water and electrolytes from the external to the internal environment.
• The digestive system eliminates undigested food residues via feces.
• The respiratory system gets O₂ from and eliminates CO₂ to the external environment.
• The respiratory system is important for maintenance of proper pH.
• The urinary system removes excess water, salt, acid, and other electrolytes from plasma, eliminating them in urine.
• The skeletal system provides support and protection for soft tissues and organs.
• The skeletal system serves as a storage reservoir for calcium.
• The skeletal system along with the muscular system enables movement of the body and its parts.
• Bone marrow, part of the skeletal system, is the source of all blood cells.
• The muscular system facilitates movement.
• The integumentary system serves as an outer protective barrier.
• The immune system defends against foreign invaders and cancerous body cells.
• The immune system paves the way for repairing or replacing injured or worn-out cells.
• The nervous system is responsible for the rapid operation and coordination of bodily activities.
• The nervous system detects and initiates reactions to changes in the external environment.
• Secreting glands of the endocrine system regulate activities that require duration rather than speed.
• The endocrine system controls the concentration of nutrients and controls the internal environment volume and electrolyte composition by adjusting kidney function.
• The reproductive system is not essential for homeostasis, or the survival of individual cells.
• The reproductive system is essential for the survival of the human species.

Intrinsic and Extrinsic Controls on Homeostasis

• Intrinsic controls are local controls inherent in an organ.
• Intrinsic controls operate in all cells to help control intracellular as well as extracellular function.
• Extrinsic controls are regulatory mechanisms initiated outside an organ.
• Extrinsic controls operate throughout the entire body to control the interrelations between different organ systems.

Nervous and Hormonal Mechanisms in Homeostasis

• Nervous mechanisms are a rapid regulatory mechanism that regulates body functions using nerve impulses carried by nerve fibers.
• Arterial blood pressure (ABP) regulation via the vasomotor center (VMC) is an example of a nervous mechanism.
• Hormonal mechanisms are a slow regulatory mechanism that regulates body functions using hormones released from endocrine glands.
• Maintaining a constant glucose level is an example of a hormonal mechanism.

Negative Feedback Control

• Negative feedback control results in a function inhibiting itself, opposing an initial change.
• CO₂ regulation in the blood is achieved by the lungs; when CO₂ increases in arterial blood, pulmonary ventilation increases.
• ABP regulation by the VMC, when increased above normal, inhibits the VMC, which inhibits the heart + vasodilatation of blood vessels.
• Blood glucose level regulation, if blood glucose increases above normal, insulin hormone is secreted to maintain normal levels.

Positive Feedback Control

• Positive feedback control results in a function stimulating itself, amplifying an initial change.
• Parturition where the baby descends, stretches the uterine cervix, which causes reflex uterine contraction and more descent is an example of positive feedback.

Oxytocin and Positive Feedback

• Uterine contractions force the baby's head into the cervix.
• Stretch receptors in the cervix (detectors) detect the stretching.
• Nerve Impulses are triggered.
• The hypothalamus and posterior pituitary (control center) are stimulated.
• Oxytocin is released.
• Uterine smooth muscle contraction (effector) occurs.
• Stronger contractions force the baby's head further into the cervix.
• Inhibition occurs after delivery when there are no uterine contractions.

Disruptions in Homeostasis

• Disruptions in homeostasis can lead to illness and death.
• Pathophysiology refers to abnormal functioning of the body associated with disease.

Levels of Organization in the Body

• The basic living unit of the body is the cell.
• Each type of cell is specially adapted to perform one or a few particular functions.

Components of the Human Body

• The human body is composed of 60% water, 18% protein, 15% fat, and 7% minerals.
• Total body water is 60% of total body weight.
• Intracellular fluid accounts for 40% of total body water.
• Extracellular fluid accounts for 20% of total body water.
• Interstitial fluid accounts for 15% of total body water.
• Plasma accounts for 5% of total body water.
• Total body water is usually 75% of total body weight in young infants.
• Total body water is less in females because of higher fat content.
• Total body water is less in obese individuals.
• Total body water is less in old age.

Daily Water Balance

• Daily water input includes approximately 250 mL from metabolism.
• Daily water input includes approximately 750 mL from moist foods.
• Daily water input includes approximately 1500 mL from drinking.
• The total daily water input is approximately 2500 mL.
• Daily water output includes approximately 200 mL from sweat.
• Daily water output includes approximately 100 mL from feces.
• Daily water output includes approximately 700 mL from skin and lungs.
• Daily water output about 1500 mL from urine.
• The total daily water output is appproximately 2500 mL.

Effects of Water Loss

• A loss of 1% of total body water triggers thirst sensation.
• A loss of 5% of total body water leads to dehydration.
• A loss of more than 20% of total body water can result in death.

Importance of Fluids

• The fluids in the body are made up of water and dissolved substances, including electrolytes.
• Body fluids are crucial for body function.
• Temperature regulation is a function of body fluid.
• Body fluids provide a protective cushion.
• Body fluids act as a lubricant.
• Body fluids are reactants in chemical reactions.
• Body fluids act as a solvent.
• Body fluids help with transport.

Body Fluid Compartments

• Intracellular fluid (ICF) is the fluid enclosed within cells and also known as cytosol.
• Extracellular fluid (ECF) is the fluid found outside of cells.
• Interstitial fluid is the fluid surrounding the cells.
• Plasma is the fluid component of blood.

Electrolytes

• Electrolytes are ionic compounds that conduct electricity when dissolved in water.
• Electrolytes conduct electricity and are therefore essential for muscle and nerve function.
• Electrolytes exert osmotic pressure, keeping body fluids in their own compartments.
• Electrolytes function in acid-base balance and as buffers resisting pH changes in body fluids.
• Electrolytes act as buffers to resist pH changes in body fluids.
• Compounds that are not ionic and therefore have no electrical properties when dissolved in water include carbohydrates.

Distribution of Electrolytes

• The major cations of intracellular fluid (ICF) are potassium (K+) and magnesium (Mg2+).
• The major anions of intracellular fluid (ICF) are protein and organic phosphates (ATP, ADP, AMP).
• The major cation of extracellular fluid (ECF) is sodium (Na+).
• The major anions of extracellular fluid (ECF) are chloride (Cl-) and bicarbonate (HCO3-).
• The cardiovascular, pulmonary, renal, gastrointestinal, endocrine, and nervous systems maintain the composition of the ECF in a coordinated fashion.
• The cell membrane maintains the composition of the ICF, mediating transport of materials between the ICF and ECF through diffusion, osmosis, active transport, and vesicular transport.