Lecture 2: Fluid Composition and Homeostasis PDF (Human Physiology and Disease – 2025)

# Lecture 2: Fluid composition and homeostasis ## Human Physiology and Disease – 2025 ### Dr. Vadim V. Fedorov, Ph.D. Professor of Physiology and Cell Biology 5196 Graves Hall [email protected] ## Objectives: - **Body fluids are separated by membranes into fluid compartments** - **KNOW**:...

# Lecture 2: Fluid composition and homeostasis ## Human Physiology and Disease – 2025 ### Dr. Vadim V. Fedorov, Ph.D. Professor of Physiology and Cell Biology 5196 Graves Hall [email protected] ## Objectives: - **Body fluids are separated by membranes into fluid compartments** - **KNOW**: the terms and concepts - **DO**: Be able to calculate different volumes if given the weight of a person - **Homeostasis** - **KNOW**: Definitions and concept of relative constancy - **DO**: Be able to fill in a reflex arc diagram with terms - **Additional concepts** - **KNOW**: Definitions and concepts, (e.g. positive and negative feedbacks, feedforward mechanisms; messengers: hormones, neurotransmitters, or paracrine and autocrine agents) - **DO**: Be able to identify examples of each concept ## Body Fluid Compartments The cells that make up complex organisms exist in an "internal sea" of extracellular fluid. In mammals with a closed vascular system, the extracellular fluids are compartmentalized to enable exchange of nutrients and waste. A diagram of fluid compartments is shown, with the following labels: - Intracellular fluid - 28 L - Capillary - Interstitial fluid - 11 L - Red blood cell - Plasma - 3 L ## Internal Fluid Compartments If cells can perform their own fundamental activities, why are the functions of the organ systems essential for survival? - Most of the cells are isolated from the external environment - Existence of a stable internal environment made up of fluids in distinct compartments. - The constant balance between these internal fluid compartments is necessary for the normal functioning of the entire body. ## Volumes of Body Fluid Compartments | Compartment | Amount | | ----- | ----- | | Total Body Water (TBW) | 60% of body weight | | Intracellular Fluid | 2/3 TBW | | Extracellular Fluid (ECF) | 1/3 TBW | | Interstitial Fluid | 80% ECF | | Plasma | 20% ECF | A diagram of fluid compartments is shown, with the following labels: - Intracellular fluid - 28 L - Capillary - Interstitial fluid - 11 L - Red blood cell - Plasma - 3 L ## What is the approximate water content in a 70 kg person? - Percentage of the body that is water: 60% - How much water is that for a 70 kg person? 70 kg * 0.6 = 42 kg - How much is that in liters? 42 L (since 1 L of water weighs 1 kg) - Where do we find that water? 2/3 of it is found inside the cells, it is called the Intracellular Fluid Volume. - How much is that in our example? 42 L * 2/3 = 28 L ## Extracellular Fluid Volumes - Calculate the Extracellular Fluid Volume or ECF (1/3 TBW) 42 L * 1/3 = 14 L (or 42 L – 28 L = 14 L) - 20% of the extracellular fluid is inside the cardiovascular system and is constantly being pumped around to transport materials. This is: The Plasma Volume. - 80% of ECF is located between the cells; it is called The Interstitial Fluid Volume. - In our example, calculate the plasma volume and the interstitial fluid volume. Plasma volume = 14 L * 0.2 = 2.8 L Interstitial fluid volume = 14 L*0.8 = 11.2 L ## How much blood volume is that for a 70 kg person? - Does that mean that the blood volume is 2.8 L? Answer: No, in addition to the plasma, there are blood cells, RBC and WBC. The total blood volume is ~5 L. - **Diagram**: body fluid compartments (study and interpret) A diagram shows: - Total body water (TBW) - Volume = 42 L, 60% body weight - Extracellular fluid (ECF) (Internal environment) - Volume = 14 L, 1/3 TBW - Intracellular fluid - Volume = 28 L, 2/3 TBW - Interstitial fluid - Volume = 11 L, 80% of ECF - Plasma - Volume = 3 L, 20% of ECF ## Cell membrane separates ECF from the ICF - What separates the intracellular fluid (ICF) from the extracellular fluid (ECF) compartment? The cell membrane or plasma membrane - The intracellular fluid is very different from the extracellular fluid in composition. e.g. sodium, potassium, proteins concentrations. - The composition of the Extracellular Fluid is similar to seawater. A diagram is shown, with the following labels: - Capillary wall - Cell membrane - Blood cells - Blood vessel - Plasma - Interstitial fluid - Intracellular fluid - ECF - ICF - Cell membrane The text in the image is described in detail, covering the different compartments of the extracellular fluid. ## The Extracellular Fluid Compartment Compositions - What separates the interstitial fluid from the blood plasma? The capillary wall. - The compositions of the plasma and the interstitial fluid are very similar e.g. sodium, calcium, potassium concentrations. - **One big difference**: The plasma contains **proteins (plasma proteins)** while the interstitial fluid contains very few of these. - Plasma proteins are very important in keeping the fluid inside the cardiovascular system. ## Swelling and Water Accumulation in the Body - What is it called when there is too much interstitial fluid? edema ## Lecture 2 Homeostasis - What does this mean at the level of the CELL, TISSUE, and ORGAN? ## Homeostasis Definition - **Claude Bernard (1813-1878)** French Physiologist introduce the concept of a constant internal environment that is a prerequisite for good health - **Walter Cannon (1871-1945)** American scientist introduced the term: **Homeostasis**: the relative constancy of the internal environment. - This is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance, being kept within certain pre-set limits (homeostatic range). "Change is met with mechanisms that restore constancy." The Wisdom of the Body, 1932 ## Homeostasis is a Dynamic Constancy - Homeostasis is a dynamic process (e.g. blood glucose levels through the day) - It also means that any variable does not stay rigidly constant, but constantly fluctuates around a normal range. - Eventually, during healthy conditions, it gets restored to normal. - But in disease, this imbalance may be the cause or is difficult to reset. - Hence, Homeostasis is a state of dynamic constancy. A graph is shown, representing blood glucose concentration (mg/dL) over the course of a day. ## The Internal Environment and Homeostasis - Cells are very sensitive to the composition of the fluid that surrounds them. - **Great Example**: Every cell is like a battery. - at rest: the inside of the cell is slightly negative. - (separation of charge – like a battery) - activated cell: the battery reverses and for a very short time the inside of the cell becomes positive. - (This produces an action potential. We will discus it during Lecture 6). - this activity is very sensitive to and dependent upon the concentrations of sodium, potassium and other ions in the interstitial fluid. A diagram is shown, representing a cell with extracellular fluid and intracellular fluid, with charge notations. ## Homeostasis Minimizes Internal changes - It has been observed that whenever there is a change in the extracellular fluid composition, the body initiates **reactions** to **correct or to minimize that change.** - Much of physiology is a study of mechanisms that mediate response to change - using **homeostatic control systems.** A graph is shown, representing blood glucose concentration (mg/dL) over the course of a day. ## Homeostasis Example - Example from the whole body level: How can you increase your body temperature when it drops? 1. Need a sensor or sensors 2. Need an integrating center to compare against a set point 3. Need effectors ## Body Temperature Regulation by Homeostasis - Regulation of body temperature is a physiological example - e.g. In response to a fall in body temperature: - the core temperature of the body is continuously sensed by **thermoreceptors located in the body.** - that temperature information is sent to the **integrating center located in the hypothalamus (in the brain).** - in the integrating center, this body core temperature is continuously compared to **the set point (normally 37° or 98.6° F)** - If the body core temperature has fallen to 36C, the integrating center will produce changes in the effectors to bring the body temperature back up to 37C. ## Negative Feedback Regulation - In this case, the **stimulus** is **decreased body temperature.** - The **effectors** and their **responses** are: 1. Blood vessels in the skin → **constriction** 2. Voluntary muscles → **curling up to a ball, adding clothes** 3. General muscle activity → **shivering** - Together, these effector responses return the body temperature closer to its normal value. - This is an example of **Negative Feedback regulation.** - This is shown in the **homeostasis flow diagram on the next slide.** ## Homeostasis: Negative feedback Diagram A diagram is shown, representing a negative feedback mechanism for body temperature regulation. The following labels are shown: - Begin - Room temperature - Heat loss from body - Body temperature - Constriction of skin blood vessels - Curling up - Shivering - Heat loss from body - Heat production - Return of body temperature toward original value - Figure 1-5 - **Note**: Don't think that because of this negative feedback body temperature is controlled perfectly. Exercising will increase body temperature. Spending time in a cold environment will decrease body temperature. **Negative feedback will minimize these fluctuations.** ## Homeostatic Reflex Arc - Homeostatic control systems use **negative feedback** to keep variable within a certain ran - The general structure of a physiological reflex: A diagram is shown, representing a Homeostatic Reflex Arc. - The diagram includes: - **integrating center** - **afferent pathway (towards center)** - **efferent pathway (away from center)** - **receptor** - **effector** - **stimulus** - **response** - **negative feedback** - A biological control system directly links a **stimulus**(i change in temperature) witl **response**(vessel constrictior called a **reflex**. ## Homeostasis Glossary - **Reflex Arc** - A biological control system that directly links a **stimulus** (i.e. change in temperature) with a **response** (i.e. vessel constriction). - **Stimulus** - A detectable **change** in the controlled variable. - **Receptor** - The sensor on which the stimulus acts. - The signal is then transmitted from the receptor to the **integrating center** (typically in the brain or spinal cord). - This happens via an **afferent pathway.** (It can be **neural** or **hormonal**). - The integrating center integrates the input from many receptors and sends its response to the **effectors**, via **efferent pathways.** - In a homeostatic control system, the response decreases the effect of the stimulus, producing **negative feedback** (i.e. moves the variable in the opposite direction). ## Feedforward Mechanisms - Negative feedback mechanisms are sometimes helped by **feedforward mechanisms.** - e.g. Thermoreceptors in the skin monitor skin temperature and send this information to the hypothalamus; when you enter a cold room, the skin temperature will fall **before the core temperature falls.** - Typically, negative feedback alone induces a lag time. - Therefore, feedforward mechanisms **anticipate changes.** - In this way, they **minimize fluctuations** in the variable that is being controlled, and can speed up the body's response. - **Other Examples:** - Olfactory cues turn on digestive system - Preparation for exercise- initiate sympathetic activation so that there isn't a delay and reduces the adrenaline rush. ## Positive Feedback - Another type of feedback is **positive feedback.** - **accelerates a process, leading to an “explosive” system** (like an avalanche) - it is less common but nevertheless plays an important role in physiology. - E.g. the hormonal control of uterus contractions during childbirth (Also known as the Ferguson reflex). ## Example of Positive Feedback- Childbirth - When a baby is ready to be delivered, it drops down in the uterus and **increases pressure on the cervix** (the lower portion of the uterus that connects to the vagina) (**stimulus**) - Neural signals from the cervix to the brain cause the release of the **hormone oxytocin.** (**effector**) - Oxytocin then travels through the blood and when it reaches the uterus **it causes the uterus to contract** ( **response**) and pushes the head of the baby harder against the cervix. A diagram is shown, representing childbirth. The following labels are shown: - Amniotic sac - Cervix - Vagina - Placenta ## Positive Feedback Steps - This causes more oxytocin release, which causes even harder contractions and more pressure on the cervix. - This **positive feedback** continues until the baby is delivered. - **Stimulus** - **increased pressure on the cervix** - **Effector** - **oxytocin acting on the uterus** - **Response** -**contraction and increased pressure** ## Blood Clotting as an example of Positive Feedback A diagram is shown, representing the mechanism of blood clotting. - 1 Wounded cells secrete chemical signals that attract and activate platelets. - 2 Clotting begins as activated platelets adhere to the wound site. Activated platelets then secrete more chemical signals. - 3 These signals attract and activate yet more platelets. - 4 Cycle ends once the wound is fully sealed. ## More Examples of Positive Feedback - **Ovulation** - Estrogen leads to more estrogen release. - **Action potential** - Na+ entry leading to Na+ channel opening - **Muscle contraction** - Ca2+ entry leads to Ca2+ release in the cell - *Positive feedback systems typically end with a major event that stops the process.* ## Additional Homeostasis Concepts - **Intercellular chemical messengers** - All homeostatic responses rely on communication between cells to cause a response. - Sometimes the cells are located close together, e.g. in the brain. - Sometimes they are far apart, e.g. the adrenal medulla releases epinephrine, which travels to the blood and speeds up the heart. - This communication between cells can happen using different types of messengers: including hormones, neurotransmitters, or paracrine and autocrine agents. ## Intercellular Chemical Messengers - **Hormone**: A chemical messenger secreted by endocrine cells into the blood stream. - **Neurotransmitter**: A chemical messenger released by a neuron to affect a muscle, gland or nerve cell. It travels through the synaptic cleft (a 10 – 20 nm space). - **Paracrine agent**: A chemical messenger released by a cell that acts on nearby cells. - **Autocrine agent**: A chemical messenger released into the interstitial fluid that acts upon the very cell that secreted it. A diagram is shown, representing the different types of intercelluar chemical messengers, their pathways and their interactions. ## Pathway of Travel Defines Messenger Types - So, the characteristic that determines whether we call it a hormone, a neurotransmitter or a paracrine agents is: **to where it travels.** - The same substance can be a neurotransmitter in one place and a hormone in a different area of the body. A diagram is shown, representing the different types of intercelluar chemical messengers, their pathways and their interactions. ## Homeostatic Balance - The optimal function of the whole body requires control mechanisms that maintain **fluids and chemical substances** in a safe and controlled range. - Throughout this course, you will learn about many mechanisms that **shift or exchange substances** (such as **fluids or metabolites**) from one tissue or organ system to another. - When this healthy condition is present, then the loss or gain of substances from individual response mechanisms are said to be in **balance.** - If individual reflex responses result in an overall loss of a substance in the body, it is referred to as a **negative balance**, while responses result in an overall gain of a substance it is referred to as a **positive balance.** ## Exam#1 Question Example - In a generic homeostatic reflex arc, what senses changes in either the internal or external environment? A. Response B. Receptor C. Effector D. Integrating center E. Efferent pathway ## Key terms and concepts of homeostasis - Sti mulus - Receptors - Afferent pathway - I ntegrati ng center - Efferent pathway - Effectors - Response - Set poi nt and equi li bri um - Response - Reflex Arc - Negati ve feedback - Posi ti ve feedback - Feed forward