Physiology 1

To measure a body fluid compartment‘s volume, you need to use a substance that distributes evenly throughout that compartment but does not enter or leave it. This is based on the dilution principle.

Inulin is an ideal substance for measuring ECF volume. It‘s a large polysaccharide that, when injected into the body, distributes only in the extracellular fluid. It does not penetrate cell membranes, so it cannot enter the intracellular fluid. It is also not metabolized or excreted from the ECF by the kidneys or other organs, making it an accurate marker.

Mannitol is a sugar alcohol that is also used to measure ECF, and it can give a reasonably accurate result. However, some amount of mannitol can be taken up by cells, making it a slightly less accurate marker than inulin for measuring the ECF.

Sucrose is also a polysaccharide but can be slowly metabolized by some cells, which makes it less reliable than inulin.

Aminopyrine is used to measure the total body water, as it can pass through cell membranes to distribute into both the intracellular and extracellular fluid compartments.

Therefore, inulin is the gold standard for accurately measuring the volume of the extracellular fluid.

Cardiac muscle is able to function as a functional syncytium, meaning a group of cells that are electrically and mechanically connected to act as a single unit. This is possible because of the structural presence of intercalated discs, which contain two key types of cell junctions:

Gap-junctions: These are tiny protein channels that form a direct electrical connection between adjacent cardiac muscle cells. They have very low electrical resistance, allowing ions to pass freely from one cell to the next. This enables the rapid and synchronized spread of an electrical impulse throughout the heart muscle, ensuring that the atria and ventricles contract in a coordinated, wave-like pattern to effectively pump blood.

Clathrin is a protein that plays a key role in the formation of coated vesicles for receptor-mediated endocytosis. In this process, the cell takes in specific molecules from the extracellular fluid. When a target molecule (a ligand) binds to its specific receptor on the cell surface, the receptor-ligand complex clusters together. The protein clathrin then assembles on the inner side of the cell membrane, forming a lattice-like structure that helps to invaginate the membrane and pinch off a new vesicle, called a clathrin-coated vesicle, containing the target molecules. This mechanism allows the cell to selectively and efficiently import essential substances.

The resting membrane potential (RMP) is the electrical potential difference across the cell membrane of a neuron at rest. It is a negative value, typically around -70 mV, meaning the inside of the cell is more negative than the outside. This is maintained by several factors.

Due to K+ ions: The RMP is primarily determined by the concentration gradient of potassium (K+) ions. At rest, the cell membrane is much more permeable to K+ than to other ions. This allows K+ to slowly leak out of the cell, making the inside more negative.

Na+-K+ pump: The sodium-potassium pump is a crucial active transport mechanism that helps maintain the RMP. It pumps three sodium ions (Na+) out of the cell for every two potassium ions (K+) it pumps into the cell. This creates a net loss of one positive charge from the cell in each cycle, contributing to the negative resting potential.

The intrafusal fibers within muscle spindles are innervated by gamma (γ) motor neurons. These neurons originate in the spinal cord and are responsible for maintaining the tension of the intrafusal fibers. This is crucial for the function of the muscle spindle, a stretch receptor that senses changes in muscle length. By adjusting the tension of the intrafusal fibers, the gamma motor neurons ensure that the muscle spindle remains sensitive to stretch over a wide range of muscle lengths.

Skeletal muscle contraction ends when the concentration of calcium ions (Ca
2+) in the cytoplasm decreases. The entire process of muscle contraction is dependent on a high level of Ca2+ in the cytoplasm. Once the nerve impulse stops, the process reverses:

Acetylcholine (ACh) is broken down by acetylcholinesterase, ending the depolarization of the muscle fiber.

The sarcoplasmic reticulum (SR) actively pumps the Ca2+ ions back into its storage, away from the myofibrils.

As the Ca2+ concentration in the cytoplasm decreases, the Ca2+ detaches from troponin C.

This causes troponin and tropomyosin to shift back into their blocking position, covering the binding sites on actin.

Myosin can no longer bind to actin, and the muscle fiber relaxes, ending the contraction.

Renshaw cells are a type of inhibitory interneuron found in the spinal cord. They are an example of feedback inhibition, a mechanism that regulates the firing of motor neurons.

Mechanism: When a motor neuron fires an action potential, a collateral branch of its axon also synapses with a nearby Renshaw cell. This stimulates the Renshaw cell, which in turn releases the inhibitory neurotransmitter glycine onto the same motor neuron and its neighbors.

Function: This inhibitory feedback loop helps to limit the firing rate of the motor neuron, preventing it from over-exciting the muscle and allowing for a more controlled, stable muscle contraction. It‘s a way for the nervous system to “put the brakes on” its own activity.

Glycosaminoglycans (GAGs) are long, unbranched polysaccharides that are a major component of the extracellular matrix. Most GAGs are sulfated, meaning they contain sulfate groups, which makes them highly negatively charged. This negative charge allows them to attract water, forming a gel-like substance that is important for maintaining tissue hydration and volume.

Hyaluronic acid is unique among the GAGs because it is not sulfated. It is also much larger than other GAGs and is not covalently linked to a core protein. Its primary functions include lubricating joints, maintaining the structure of the eyeball, and acting as a shock absorber.

Chondroitin sulfate, dermatan sulfate, and keratan sulfate are all sulfated GAGs. Chondroitin and dermatan are found in cartilage and skin, while keratan is found in bone and cartilage. Their sulfated groups are critical for their function in these tissues.

During acute starvation, the body undergoes a series of biochemical changes to maintain blood glucose levels and provide alternative energy sources.

Increased gluconeogenesis in liver: This is a crucial response. The liver synthesizes new glucose from non-carbohydrate sources like amino acids and glycerol to provide energy for the brain and red blood cells.

Increased glycogenolysis in liver: In the first 24 hours of starvation, the liver breaks down its stored glycogen to release glucose into the bloodstream, serving as a primary source of energy.

Increased lipogenesis: This statement is incorrect. Lipogenesis, the process of synthesizing fats, occurs when the body has an excess of energy. During starvation, the body does the opposite; it increases lipolysis, which is the breakdown of stored triglycerides (fats) into fatty acids and glycerol to be used for energy.

Ketone bodies utilized by brain for energy: This is a key adaptation to prolonged starvation. As glucose becomes scarce, the liver produces ketone bodies from fatty acids. The brain, which normally relies on glucose, adapts to use these ketone bodies as an alternative fuel source, thereby preserving the remaining glucose for essential cells like red blood cells.

The “C” in C-reactive protein (CRP) stands for its ability to react with the C-polysaccharide of Streptococcus pneumoniae. This discovery was made in the 1930s when researchers found a protein in the blood of acutely ill patients that formed a precipitate when mixed with this specific polysaccharide from the bacteria. This reaction was an early diagnostic test for inflammation and infection. CRP is now known as a general biomarker for inflammation, and its levels rise rapidly in response to various inflammatory conditions.

Maple syrup urine disease (MSUD) is a genetic disorder caused by a defect in the metabolism of branched-chain amino acids (BCAAs), which include leucine, isoleucine, and valine. The specific enzyme that is deficient is the branched-chain α-keto acid dehydrogenase complex. This enzyme is responsible for the oxidative decarboxylation of the α-keto acids of these amino acids. Without this enzyme, the BCAAs and their keto-acid derivatives accumulate in the blood and urine, leading to the characteristic odor of maple syrup. The accumulation of these toxic compounds can cause severe neurological damage and developmental delays if not treated.

The EMP pathway, also known as the Embden-Meyerhof-Parnas pathway, is the process of glycolysis. This metabolic pathway occurs in the cytoplasm of the cell, where glucose is broken down into two molecules of pyruvate.

All of the other processes listed take place in the mitochondria:

a) Fatty acid oxidation: Also known as β-oxidation, this process occurs in the mitochondrial matrix and breaks down fatty acids to generate acetyl-CoA.

c) Electron transport chain: This chain of protein complexes is located on the inner mitochondrial membrane and is the final stage of aerobic respiration, where the bulk of ATP is produced.

d) Citric acid cycle: Also known as the Krebs cycle, this cycle occurs in the mitochondrial matrix and is a central part of cellular respiration.

Galactosyl transferase is a key enzyme that is considered a specific marker for the Golgi apparatus. This enzyme is involved in the glycosylation of proteins and lipids, a primary function of the Golgi. It catalyzes the transfer of galactose to a growing sugar chain, a process that occurs as molecules move through the Golgi stacks.

Why the other options are incorrect:
b) Acetyl CoA synthase: This enzyme is part of the Krebs cycle and is located in the mitochondria.

c) Pyruvate kinase: This enzyme is a key part of glycolysis, which takes place in the cytoplasm.

d) Malonyl Co A: Malonyl-CoA is a molecule, not an enzyme. It‘s a precursor in fatty acid synthesis, a process that occurs in the cytoplasm. It‘s not a marker for the Golgi apparatus.

The HMG-CoA reductase pathway, also known as the mevalonate pathway, is the metabolic route that produces cholesterol and other important isoprenoids. It starts with acetyl-CoA, not pyruvate.

Acetyl-CoA: This is the starting molecule. Two molecules of acetyl-CoA condense to form acetoacetyl-CoA, which then combines with a third acetyl-CoA to form HMG-CoA.

HMG-CoA: This is a key intermediate in the pathway.

Mevalonate: HMG-CoA is then reduced to mevalonate by the enzyme HMG-CoA reductase, a rate-limiting step in cholesterol synthesis.

Cholesterol: Mevalonate is eventually converted to cholesterol through a series of steps.

Pyruvate is the end product of glycolysis and is a precursor to acetyl-CoA. It is not directly involved in the HMG-CoA pathway itself.

Iron staining in tissues, particularly for detecting iron overload, is most commonly done using the Prussian blue reaction. This method uses a solution of potassium ferrocyanide and hydrochloric acid, which reacts with ferric iron (Fe3+) to produce a blue pigment. This allows pathologists to visually identify iron deposits, such as hemosiderin, in cells and tissues.

Why the other options are incorrect:
a) H-E stain: Hematoxylin and Eosin (H&E) is a general-purpose stain used to highlight cell nuclei (blue/purple) and cytoplasm (pink). It does not specifically stain for iron.

c) Giemsa stain and d) Romanovsky stain: Romanovsky stains (which include Giemsa) are used primarily for staining blood smears. They differentiate blood cells and are not used for staining tissue iron.

Gerhardt‘s test is a biochemical test used to detect the presence of acetoacetate, a type of ketone body, in the urine. Therefore, a positive test result indicates a significant degree of ketosis, a metabolic state where the body primarily uses fats for energy, leading to an increased production of ketone bodies. This can occur in conditions such as uncontrolled diabetes mellitus (diabetic ketoacidosis), prolonged starvation, or a ketogenic diet.

Dietary fiber is a component of plant-based foods that the body cannot digest or absorb. It consists primarily of carbohydrates like polysaccharides. Pectin is a soluble dietary fiber found in fruits and vegetables, and it‘s known for its ability to form a gel-like substance in water.

Collagen is a structural protein found in animal connective tissue.

Proteoglycans are a class of proteins that are heavily glycosylated, but they are components of the extracellular matrix in animals, not plant dietary fiber.

Starch is a carbohydrate that is easily digestible by humans, so it is a source of energy rather than a form of dietary fiber.

Ascorbic acid, also known as Vitamin C, cannot be synthesized by humans and other primates because they lack the final enzyme in the synthetic pathway, L-gulonolactone oxidase. This enzyme is crucial for converting L-gulonolactone to ascorbic acid. Due to this genetic mutation, humans must obtain Vitamin C from their diet to prevent scurvy.