Chapter 4: Homeostasis Notes

Anatomy and Physiology

Anatomy: Study of body structures and relationships.
Physiology: Study of how the body functions.

Organ Systems

Human body organized into eleven organ systems, each with related organs and common functions.
Some organs may be part of more than one system.
Organ systems include:
- Integumentary system: Skin, hair, nails, sweat and oil glands.
  - Functions: Protection, temperature regulation, waste elimination, sensation detection (touch, pain, warmth, cold), vitamin D production.
- Muscular system: Skeletal muscles.
  - Functions: Body movement, posture stabilization, heat generation.
- Skeletal system: Bones, joints, cartilages.
  - Functions: Support, protection, aids movement, houses blood-producing cells, stores minerals and lipids.
- Nervous system: Brain, spinal cord, nerves, special sense organs (eye, ear).
  - Functions: Regulates body activities via action potentials (nerve impulses), detects and interprets internal/external changes, responds with muscular contractions or glandular secretions.
- Endocrine system: Hormone-producing glands and cells.
  - Functions: Regulates body activities by releasing hormones transported in the blood to target cells.
- Cardiovascular system: Blood, heart, blood vessels.
  - Functions: Heart pumps blood, blood carries oxygen/nutrients to cells and CO2/wastes from cells, regulates pH/temperature/water content, defends against disease, and repairs damaged vessels.
- Lymphatic and immune system: Lymphatic fluid/vessels, spleen, thymus, lymph nodes, tonsils.
  - Functions: Returns proteins and fluid to blood, carries lipids from GI tract to blood, site of lymphocyte maturation and proliferation.
- Respiratory system: Lungs, air passages (pharynx, larynx, trachea, bronchial tubes).
  - Functions: Transfers oxygen from inhaled air to blood and carbon dioxide from blood to exhaled air. Regulates pH of body fluids, facilitates sound production via vocal cords.
- Digestive system: Organs of GI tract, accessory organs (salivary glands, liver, gallbladder, pancreas).
  - Functions: Physical and chemical digestion, nutrient absorption, elimination of undigested wastes.
- Excretory system: Kidneys, ureters, urinary bladder, urethra.
  - Functions: Produces, stores, and eliminates urine. Eliminates waste, regulates blood volume and composition, maintains pH and mineral balance, regulates red blood cell production.
- Reproductive system: Gonads (testes/ovaries) and associated organs (fallopian tubes, uterus, vagina, epididymis, vas deferens, penis).
  - Functions: Gonads produce gametes (sperm/oocytes) that unite to form a new organism. Associated organs transport and store gametes.

Homeostasis

Definition: Maintenance of near constant conditions in the internal environment.
Coined by Walter B. Cannon (1871-1945).
Dynamic condition where the body's equilibrium point can change over a narrow range.
Involves interplay between external forces and internal control mechanisms.
Pathological conditions may result when the body cannot maintain parameters within their normal range.

Important Constituents and Physical Characteristics of Extracellular Fluid

Oxygen: Normal value = 40 mmHg, Normal range = 35 – 45 mmHg, Approximate short-term non-lethal limit = 10 – 1000 mmHg
Carbon dioxide: Normal value = 40 mmHg, Normal range = 35 – 45 mmHg, Approximate short-term non-lethal limit = 5 – 80 mmHg
Sodium ion: Normal value = 142 mmol/L, Normal range = 138 – 146 mmol/L, Approximate short-term non-lethal limit = 115 – 175 mmol/L
Potassium ion: Normal value = 4.2 mmol/L, Normal range = 3.8 – 5.0 mmol/L, Approximate short-term non-lethal limit = 1.5 – 9.0 mmol/L
Calcium ion: Normal value = 1.2 mmol/L, Normal range = 1.0 – 1.4 mmol/L, Approximate short-term non-lethal limit = 0.5 – 2.0 mmol/L
Chloride ion: Normal value = 108 mmol/L, Normal range = 103 – 112 mmol/L, Approximate short-term non-lethal limit = 70 – 130 mmol/L
Bicarbonate ion: Normal value = 28 mmol/L, Normal range = 24 – 32 mmol/L, Approximate short-term non-lethal limit = 8 – 45 mmol/L
Glucose: Normal value = 85 mg/dl, Normal range = 75 – 95 mg/dl, Approximate short-term non-lethal limit = 20 – 1500 mg/dl
Body temperature: Normal value = 37.0 °C, Normal range = 36.0 – 38.0 °C, Approximate short-term non-lethal limit = 18.3 – 43.3 °C
pH: Normal value = 7.4 pH, Normal range = 7.3 – 7.5 pH, Approximate short-term non-lethal limit = 6.9 – 8.0 pH

Homeostatic Actions and Organ Systems Involved

Regulating respiratory gases: respiratory system, cardiovascular system
Coping with pathogens: cardiovascular system, integumentary system, immune system
Regulating blood glucose concentration: endocrine system, digestive system, cardiovascular system
Regulating body temperature: integumentary system, cardiovascular system, nervous system
Regulating blood osmolarity: renal system, cardiovascular system, endocrine system
Coordinating responses: nervous system, muscular system, skeletal system

Principles of Homeostasis

Internal environment disruptions originate from external and internal environments.
- External: Intense heat, lack of oxygen at high altitudes.
- Internal: Drop in glucose level after a night’s fast.
Body restores balance in the internal environment through:
- Genetic control systems in cells.
- Control systems within organs.
- Control systems throughout the body.
- Involvement of the nervous and endocrine systems.
Homeostasis involves the coordination of responses to a stimulus.
Generation of regulating response(s) or homeostatic action(s) for every stimulus detected.

Characteristics of Feedback Systems

Feedback system: Cycle of monitoring, evaluating, changing, re-monitoring, and re-evaluating a body condition.
Components: stimulus, receptor, control center, effector, and response.
Controlled condition: Monitored body condition (e.g., blood glucose concentration).
Stimulus: Any disruption to the controlled condition (e.g., body temperatures higher or lower than normal range, blood glucose concentration higher or lower than normal range of 70 – 110 mg/dl).
Receptor: Body structure that monitors changes, receives signals, and sends input to a control center.
- Detects stimulus.
- Examples: skin contains specialized nerve endings that detect temperature (cold/heat receptors); chemoreceptors detect partial pressure of carbon dioxide
Control center: Determines the range of values within which a controlled condition should be maintained. Evaluates input from receptors and generates output commands.
- Output commands: Nerve impulses or hormones.
Effector: Body structure that receives output commands from the control center and produces response(s).
- Responses lead to changes in the controlled condition.
Negative feedback mechanism: Output response reverses the stimulus and restores the parameter towards the set norm.
- Most body systems are controlled by negative feedback.
- Examples: blood pressure and blood glucose levels.
Positive feedback mechanism: Output response enhances or amplifies the stimulus.
- Examples: blood clotting and generation of nerve signals.

Thermoregulation

Process by which animals maintain an internal temperature within a tolerable range.
Core temperatures of humans are kept within a small range, between $36.0$ °C and $38.0$ °C.
Critical because biochemical and physiological processes are sensitive to temperature changes.
The skin is the major organ that controls heat and moisture flow.
Stimulus: Change in body temperature above or below the normal range of $36.0$ °C and $38.0$ °C.
Thermoreceptors in the skin detect changes to skin temperature or the external environment and send nerve impulses to the hypothalamus in the brain. The hypothalamus itself also contains thermoreceptors that can also detect changes to the temperature of the blood.
The hypothalamus functions as the control center.
The hypothalamus responds to body temperatures outside of the normal range by activating mechanisms that promote heat loss or gain.
The hypothalamus sends nerve impulses to effectors (relevant body parts) to produce responses.

Responses to High Body Temperature

Thermoreceptors in the skin detect the change and send nerve impulses to the hypothalamus in the brain.
The thermoreceptors in the hypothalamus are also stimulated as blood temperature rises.
Hypothalamus sends nerve impulses to effectors to bring about responses:
- Vasodilation of skin arterioles to allow more blood flow through the blood capillaries in the skin. This allows more heat to be lost through the skin by radiation, convection, and conduction.
- Sweat glands become more active, resulting in increased production of sweat. As more water from sweat evaporates from the surface of the skin, more heat is lost.
- Decrease in metabolic rate, to reduce the amount of heat produced within the body.

Responses to Low Body Temperature

Thermoreceptors in the skin detect the change and send nerve impulses to the hypothalamus in the brain.
The thermoreceptors in the hypothalamus are also stimulated as blood temperature drops.
Hypothalamus sends nerve impulses to effectors to bring about responses:
- Vasoconstriction of the skin arterioles so that less blood flows through blood capillaries in the skin. This reduces heat loss by radiation, convection, and conduction.
- Sweat glands become less active, resulting in decreased production of sweat. As less water from sweat evaporates from the surface of the skin, less heat is lost.
- Increased metabolic rate, to increase the amount of heat produced within the body.
- Shivering: Reflex contraction of body muscles that increases heat production when the above responses are insufficient.