General Principles of Physiology

Click within the box of each statement that describes a homeostatic situation.

- As a person gradually ascends in altitude, the reduction in oxygen levels cause breathing rate and depth to become progressively greater. The increased ventilation keeps oxygen levels from declining as much as it would with a slower breathing rate. - After eating a potassium-rich meal, blood potassium levels transiently rise, but then potassium enters cells and blood levels fall again. Over time, the extra potassium slowly exits the cells and is excreted by the kidneys.

IP3 and DAG second-messenger system

1. 1st messenger binds to Gq receptor 2. Activated Gq activates phospholipase C 3. Phospholipase C catalyzes the breakdown of PIP₂ to the 2nd messengers, IP₃ and DAG 4. IP₃ binds to receptors on the endoplasmic reticulum, releasing Ca²⁺ into the cytosol 5. Ca²⁺ leads to cell's response and also binds to calmodulin to form active Ca²⁺-calmodulin complex 6. Active calmodulin activates calmodulin-dependent protein kinase 7. Active calmodulin-dependent protein kinase catalyzes the reaction of protein + ATP into protein PO₄ + ADP 8. Protein PO₄ leads to cell's response 9. DAG activates protein kinase C which phosphorylates a large number of other proteins, leading to the cell's response

Learn more about some of the most important second messengers within the body by correctly matching each of the following substances with its appropriate source and effect. 1. Enters cells through ion channels or is released from the endoplasmic reticulum; activates calmodulin 2. A G protein activates plasma membrane adenylyl cyclase, which catalyzes its formation; activates protein kinase A 3. A G protein activates plasma membrane phospholipase C, which catalyzes its formation; activates protein kinase C 4. Generated from guanosine triphosphate in a reaction catalyzed by a receptor with guanylyl cyclase activity; activates protein kinase G 5. A G protein activates plasma membrane phospholipase C, which catalyzes its formation; releases calcium from the endoplasmic reticulum

1. Calcium 2. Cyclic AMP (cAMP) 3. Diacylglycerol (DAG) 4. Cyclic GMP (cGMP) 5. Inositol triphosphate (IP₃)

Heart rate is increased by the release of epinephrine by the adrenal medulla into the bloodstream. This is an example of A. endocrine control B. exocrine control C. paracrine control D. direct neural control E. positive feedback

A. endocrine control

During childbirth, as a baby's head is pressed against the mother's cervix, signals are relayed via nerves from cervix to the mother's brain. The brain initiates the secretion of oxytocin (a hormone) from the mother's pituitary gland. Oxytocin stimulates further uterine contractions. As the uterus contracts even harder in response to oxytocin, the baby's head is pushed harder against the cervix; this stimulates yet more nerve signals to the mother's brain, resulting in yet more oxytocin secretion until finally the baby is pushed through the cervix. The integrating center in this process of childbirth is A. the mother's brain B. the mother's cervix C. the nerve signals to the mother's brain D. oxytocin E. the mother's pituitary gland

A. the mother's brain

Examine each of the relationships given in the partial sentences provided below. Drag each label to the proper position indicating the best conclusion of each sentence. Not all labels are eligible for each dock. The letters A, B, C, and D will guide you as to which labels should be considered for each dock. A: Steroids are to nuclear receptors as: B: Receptor down-regulation is to cellular responses as: C: Hydrophilic hormones are to first messengers as: D: Adenylyl cyclase is to cAMP as:

A: Steroids are to nuclear receptors as: Polypeptide hormones are to membrane-bound receptors. B: Receptor down-regulation is to cellular responses as: Antagonist binding is to cellular responses. C: Hydrophilic hormones are to first messengers as: Ca²⁺is to second messengers. D: Adenylyl cyclase is to cAMP as: Phospholipase C is to IP₃.

As a graduate teaching assistant for your physiology class, you are required to provide an evaluation of the following examples of student work regarding osmosis. No two answers can have the same grade assigned.

A: The depicted scenario could have resulted from the movement of solute and water through a membrane permeable to both. If the scenario began with a lower osmolarity on the right, solutes and water would have moved in both directions, but the net movement of solutes would have been from left to right and the net movement of water would have been opposite. C: In the scenario below depicting two compartments separated by a membrane impermeable to solutes, diffusional equilibrium could have been achieved by the net flux of water. If at the beginning, the right compartment had a higher osmolarity, the net movement of water from right to left would have diluted the left and concentrated the right until equilibrium was achieved. F: If the scenario depicted below depicts two solutions separated by a membrane permeable to water only, the one-way flux of solute from an are of high to lower water concentration could explain their current diffusional equilibrium. For example, if the left was an area of lower osmolarity, water would have moved from right to left until equilibrium was achieved.

____________ is demonstrated by the need to burn gasoline for energy in moving objects to the top of a mountain on a snowmobile.

Active transport

Which of the following would achieve the absorption of water across the epithelial lining toward the blood vessels of the small intestine? - Active transport of Na+ from the lumen to the basolateral side - Facilitated diffusion of glucose to the bloodstream from the lumen - Active transport of H+ from the basolateral side to the lumen - Tightening the tight junctions between epithelial cells - Opening of aquaporins on the apical membrane

Active transport of H+ from the basolateral side to the lumen and opening of aquaporins on the apical membrane

____________ : Neurons relay information from skin temperature-sensitive neurons to your brain.

Afferent pathway

The hormone insulin enhances the transport of glucose into body cells. Its secretion is controlled by a negative feedback system between the concentration of glucose in the blood and the cells that secrete insulin. Which of the following statements is most likely to be correct? A. A decrease in blood glucose concentration will stimulate insulin secretion, which will in turn lower the blood glucose concentration still further. B. An increase in blood glucose concentration will stimulate insulin secretion, which will in turn lower the blood glucose concentration. C. A decrease in blood glucose concentration will stimulate insulin secretion, which will in turn increase the blood glucose concentration. D. An increase in blood glucose concentration will stimulate insulin secretion, which will in turn increase the blood glucose concentration still further.

B. An increase in blood glucose concentration will stimulate insulin secretion, which will in turn lower the blood glucose concentration.

How are endocrine glands and hormones involved in homeostatic reflexes? A. Endocrine glands can be receptors, and hormones can be effectors. B. Endocrine glands can be integrating centers, and hormones can be efferent pathways. C. Endocrine glands can be efferent pathways, and hormones can be effectors. D. Endocrine glands are not part of reflex mechanisms, but hormones can be afferent or efferent pathways. E. They are not involved; reflexes only involve actions of the nervous system.

B. Endocrine glands can be integrating centers, and hormones can be efferent pathways.

The concentrations of two substances are increasing outside the cell. For substance A, transport into the cell increases proportionally as the concentration outside increases. For substance B transport into the cell increases but then levels off despite the concentration outside continuing to increase. What is the explanation for this? A. Competition B. Saturation C. Specificity D. Affinity

B. Saturation

Red blood cells have a higher proportion of aquaporins on the membrane than skin cells do. Assuming equal osmotic pressures, which is true? A. The rate of osmosis is faster in skin cells than red blood cells. B. The rate of osmosis is faster in red blood cells than in skin cells. C. The rate of diffusion for non-polar molecules is faster in red blood cells than skin cells. D. The rate of diffusion for non-polar molecules is faster in skin cells than red blood cells. E. No significant differences in osmosis or diffusion rates between these cells.

B. The rate of osmosis is faster in red blood cells than in skin cells.

When a blood vessel is injured, it releases chemicals that activate platelets and cause them to stick to the injury site. The activated platelets in turn release more chemicals and attract more platelets to the injury site until the wound is fully sealed. This process of blood clotting is an example of A. negative feedback B. positive feedback C. feedforward regulation D. adaptation E. acclimatization

B. positive feedback

Two compartments of equal volume are separated by a membrane. Side 1 contains a solution with low osmolarity, and Side 2 contains a solution with high osmolarity. If the membrane was permeable to water but impermeable to solute, A. the osmolarity of Side 1 will be less than the osmolarity of Side 2 when water reaches diffusion equilibrium. B. the osmolarities and the water concentrations of both compartments are equal, but the volume of water in Side 1 is less than the volume of water in Side 2. C. the osmolarity of Side 1 will be greater than the osmolarity of Side 2 when water reaches diffusion equilibrium. D. the water concentration of Side 1 will be less than the water concentration of Side 2 when water reaches diffusion equilibrium. E. the water concentration of Side 1 will be greater than the water concentration of Side 2 when water reaches diffusion equilibrium.

B. the osmolarities and the water concentrations of both compartments are equal, but the volume of water in Side 1 is less than the volume of water in Side 2.

Which equation is most accurate? A. Extracellular fluid volume + interstitial fluid volume = whole body fluid volume B. Intracellular fluid volume + interstitial fluid volume = extracellular fluid volume C. Extracellular fluid volume - interstitial fluid volume = plasma volume D. Plasma volume + intracellular fluid volume = extracellular fluid volume E. Total body fluid volume - intracellular fluid volume = interstitial fluid volume

C. Extracellular fluid volume - interstitial fluid volume = plasma volume

Identify the effectors in this homeostatic reflex: Eating a salt-rich meal increases blood volume and pressure, stretching blood vessel walls. Nerve signals sent to the brainstem stimulate changes in hormonal and neural signaling. The heart rate is slowed, blood vessel walls are relaxed, and the kidneys increase urinary salt. The blood pressure returns toward normal. A. Brainstem and blood vessels B. Blood vessels, hormones, and nerves C. Heart, blood vessels, and kidneys D. Brainstem, blood vessels, and kidneys E. Hormones and nerves

C. Heart, blood vessels, and kidneys

Equal amounts of two solutes, A and B, are placed into the same beaker of water at the same time. Solute A reaches diffusion equilibrium faster than solute B. What is the most likely explanation for this observation? A. The temperature was greater for solute A than solute B. B. The concentration gradient for A was greater than B. C. Solute A is smaller than solute B. D. Solute A traveled a shorter distance than solute B. E. Solute B is more soluble in water than solute A.

C. Solute A is smaller than solute B.

You are examining the data from two time points taken in an experiment. At time 1 there was an extracellular concentration of Na+ of 115 mM outside the cell and 8mM inside the cell. An amino acid, lycine, is at 20mM outside and 40mM inside. At the second time measured Na+ is 100mM outside the cell and 23 mM inside the cell. Lycine is 15 mM outside the cell and 45mM inside. What is the best explanation? A. Both Na+ and the amino acid bind on the side of the membrane where the Na+ concentration is lowest, and undergo net movement to the other side of the membrane. B. Na+ and the amino acid bind to opposite sides of the membrane, and Na+ undergoes net transport from the side of the membrane with higher Na+ concentration toward the side with lower concentration, while the amino acid moves in the opposite direction. C. Both Na+ and the amino acid bind on the side of the membrane where the Na+ concentration is higher, and both are transported to the side of the membrane where the Na+ concentration is lower. D. Both Na+ and the amino acid bind to the side of the membrane where the amino acid concentration is lower and are transported toward the side of the membrane where the amino acid concentration is higher. E. Na+ and the amino acid bind to opposite sides of the membrane, and the amino acid undergoes net transport from the side of the membrane with lower Na+ concentration to the side of the membrane where the Na+ concentration is higher.

D. Both Na+ and the amino acid bind to the side of the membrane where the amino acid concentration is lower and are transported toward the side of the membrane where the amino acid concentration is higher.

What will happen if a normal cell is placed in a hyperosmotic solution? A. It will swell in size B. It will shrink in size C. It will stay the same D. It may swell, shrink, or stay the same size, depending upon the concentration of penetrating and nonpenetrating solutes in the solution.

D. It may swell, shrink, or stay the same size, depending upon the concentration of penetrating and nonpenetrating solutes in the solution.

Some neurons in the vagus nerve have synaptic connections to sinoatrial cells (pacemaker) in the heart. These neurons secrete acetylcholine, which ultimately results in a decreased heart rate. This is an example of A. endocrine control B. exocrine control C. hormonal control D. neural control E. paracrine control

D. neural control

____________ is exemplified by dropping a handful of marbles onto a friction-less table and observing them disperse evenly as they eventually minimize the number and magnitude of collisions.

Diffusion

Which of the following statements about the Na+-K+ ATPase pump is false? A. It transport Na+ out of cells and K+ into cells. B. It binds to, and hydrolyzes, ATP. C. It is constantly active in all cells. D. Its activity requires the expenditure of metabolic energy. E. It transports Na+ and K+ in a 1:1 ratio.

E. It transports Na+ and K+ in a 1:1 ratio.

Which of components of a general reflex arc are listed in the order information typically flows through them following a stimulus? A. Effector, afferent pathway, integrating center, efferent pathway, receptor B. Effector, efferent pathway, integrating center, afferent pathway, receptor C. Integrating center, receptor, afferent pathway, efferent pathway, effector D. Receptor, efferent pathway, integrating center, afferent pathway, effector E. Receptor, afferent pathway, integrating center, efferent pathway, effector

E. Receptor, afferent pathway, integrating center, efferent pathway, effector

During childbirth, as a baby's head is pressed against the mother's cervix, signals are relayed via nerves from cervix to the mother's brain. The brain initiates the secretion of oxytocin (a hormone) from the mother's pituitary gland. Oxytocin stimulates further uterine contractions. As the uterus contracts even harder in response to oxytocin, the baby's head is pushed harder against the cervix; this stimulates yet more nerve signals to the mother's brain, resulting in yet more oxytocin secretion until finally the baby is pushed through the cervix. The effector in this process of childbirth is A. the mother's brain B. the mother's cervix C. the nerve signals to the mother's brain D. oxytocin E. the mother's pituitary gland

E. the mother's pituitary gland

____________ : Neurons reach smooth muscles in your skin blood vessels and skeletal muscles.

Effectors

____________ : Neurons relay command signals away from your brain.

Efferent pathway

____________ can loosely be viewed as putting irregularly shaped objects stuck in the snow on a sled designed only for those objects and allowing them to slide down an icy mountain on their own.

Facilitated diffusion

Most physiological variable in the extracellular fluid vary widely with changes in the external environment. (T/F)

False

Saying that a variable such as blood glucose is "homeostatic" means that it has a constant value over a long period of time. (T/F)

False

When a variable such as blood pressure increases above normal. homeostatic mechanisms generally overcompensate and make the variable become below normal. (T/F)

False

Review the factors that affect net flux by categorizing the factors on the left based upon their expected effect upon net flux on the right.

Faster net flux: - Large concentration difference - Diffusion through air versus water - Large cell surface area Slower net flux: - Lower temperature - Low permeability coefficient - Large molecules like proteins

Test your knowledge of feedback and feedforward regulatory systems by categorizing each of the situations below as examples of negative feedback, positive feedback, or feedforward mechanisms.

Feedforward regulation: - Thinking about exercise increases your heart rate. - The smell of food causes increased acid production in the stomach in preparation for the meal that you have not yet eaten. Positive feedback: - Entry of Na⁺ into cells makes the inside of cells more positive, which causes more Na⁺ channels to open - During childbirth, oxytocin secretion causes uterine contractions, the cervix stretches as the mother pushes, stretch causes more oxytocin to be released. Negative feedback: - Elevated Na⁺ signals the kidneys to get rid of more Na⁺, which decreases Na⁺ in the blood. - Elevated blood glucose stimulates the secretion of the hormone insulin; insulin stimulates glucose movement into muscle cells, which decreases blood glucose

1. To begin, ____________ constitute the largest category of plasma membrane receptors with ____________ of distinct receptors.

G-protein-coupled receptors, hundred

7. The activated receptor increases the affinity of the alpha subunit of the G_protein for ____________.

GTP

10. Once the alpha subunit activates its effector protein, ____________ activity inherent in the alpha subunit cleave the GTP into GDP and Pᵢ.

GTPase

Assess your knowledge of the composition of the extracellular and intracellular fluid compartments. Indicate whether the following solute concentrations are normally greater in the extracellular fluid (ECF) compartment or the intracellular fluid (ICF) compartment.

Higher in the ECF: - Na⁺ - Ca²⁺ - Cl⁻ - Glucose Greater in the ICF: - Proteins - Amino acids - ATP - K⁺

Chapter 4 contains several examples that demonstrate the eight general principles of physiology described in Chapter 1. Classify each of the examples at the top according to the general principle of physiology that it best represents.

Homeostasis is essential for health and survival: - Insulin-regulated glucose transporters help to maintain glucose concentrations in the intra- and extracellular fluids. - Insertion of aquaporins in epithelial cell membranes within the kidney helps the body retain water, which prevents dehydration. - Osmosis maintains proper cell shape and size and the ability of cells to perform signaling functions. Controlled exchange of material occurs between compartments and across cellular membranes: - Glucose transporters and aquaporins can increase or decrease in plasma membranes under different conditions. - Sodium channels sense changes in membrane voltage and subsequently open to allow sodium ions to flux into the cell. Physiological processes are dictated by the laws of chemistry and physics: - The greater a molecule's hydrophobicity, the more likely it is to diffuse across the plasma membrane. - Solute move through solutions at faster rates when temperatures are higher. - Electrochemical gradients influence ion movement; for example like charges repel and opposite charge attract.

____________ : Specific neurons in your brain alter their rates of firing.

Integrating center

____________ is similar to approaching a closed automatic security door and waving an ID card or key fob near a scanning device programmed to allow entry to specific people.

Ligand-gating

____________ is similar to a door to a building that is normally closed, but opens when wind catches a structure on the front of the door resembling a sail with sufficient strength to force open the door.

Mechanical gating

____________ : Decreased heat loss and increased heat production oppose the initial change, which was a decrease in body temperature.

Negative feedback

____________ can be described as the level of water in a bucket that is being filled at one end by a hose while water is running out of a hole at the other end.

Net flux

____________ are similar to entering your backyard by walking alongside the narrowed, but not blocked pathways between your neighbor's house and yours.

Paracellular pathways

If a person rapidly ingested 1 liter of water, it would distribute freely between the body's three major fluid compartments. Drag and drop the volume of water from that 1 liter you would expect to distribute into each of the fluid compartments.

Plasma: 70 mL Interstitial Fluid: 260 mL Intracellular Fluid: 670 mL

Imagine that a cell has a plasma membrane made ONLY of phospholipids and contains no proteins. Classify the molecules listed below according to whether or not they are likely to diffuse across the plasma membrane.

Rapid diffusion: - Oxygen - Fatty acids - Steroid hormones Slow/no diffusion: - Water - Glucose - Ca²⁺

____________ : Temperature-sensitive neurons within your skin detect the temperature change and increase their signaling rate.

Receptors

____________ : Blood vessel constriction decreases heat loss, and skeletal muscle contraction induces shivering which produces heat.

Response

Which of these processes can occur without the direct use of ATP? - Simple diffusion - Facilitated diffusion - Primary active transport - Secondary active transport - None of these can occur without the direct use of ATP

Simple diffusion, facilitated diffusion, and secondary active transport

____________ : You entered a heavily air-conditioned room on a hot summer day, and your body temperature decreases.

Stimulus

*Which of the following is true about the actions of the sodium-potassium pump? - The Na+/K+ pump directly uses ATP - The Na+/K+ pump creates an electrical disequilibrium - The Na+/K+ pump creates chemical disequilibrium - It is expressed only on select cell types - The Na+/K+ pump is an example of counter transport

The Na+/K+ pump directly uses ATP, the Na+/K+ pump creates an electrical disequilibrium, the Na+/K+ pump creates chemical disequilibrium, and the Na+/K+ pump is an example of counter transport

____________ are like trying to go into your backyard by going in through the front door, through the house, and out the back door because the gates in the fence alongside the house were locked.

Transcellular pathways

Homeostasis is a dynamic, and not a static, process. (T/F)

True

Most body cells are in contact with the interstitial fluid. (T/F)

True

Most physiological variable are maintained within a normal range that favors the survival of body cells. (T/F)

True

____________ can be depicted where a door is responsive to conditions surrounding the inside and outside of the door such as temperature. It might open when the temperature is hot inside and close when the temperature is hot outside.

Voltage-gating

4. The ____________ subunit can bind ____________ and GTP.

alpha, GDP

5. The ____________ and gamma subunits help anchor the alpha subunit in the membrane.

beta

Activation or inhibition of ____________ leads, via ____________ , to activation or inhibition of proteins involved in the cell's ultimate response to the first messenger.

calmodulin-dependent protein kinases, phosphorylation

7. Finally, ion flux across the plasma membrane can be altered by opening or closing ion channels, which is a process known as ____________.

channel gating

5. Ion diffusion also depends on the ____________ gradient that exists for that ion across the plasma membrane.

concentration

6. The binding of a first messenger to the receptor protein changes the ____________ of the receptor.

conformation

When the body is dehydrated, a portion of the kidney tubule epithelium is rendered more permeable to urea and water. The result is water movement from the tubule into the renal interstitial fluid compartment and reduction of urine volume. This is a good demonstration of the general principle of physiology stating that ____________.

controlled exchange of material occurs between compartments and across cellular membranes

2. Bound to the inactive receptor is a protein complex located on the ____________ surface of the plasma membrane and belonging to the family of heterotrimeric proteins known as ____________.

cytoplasmic, G proteins

One of the main ____________ protein targets of Ca²⁺ is ____________.

cytosolic, calmodulin

Drinking pure water in large quantities caused a significant dilution of this woman's body fluids. As the concentration of sodium in her extracellular fluid __________, the electrochemical gradient for sodium across her cells (including her muscle and brain cells) __________ as a consequence.

decreased, decreased

6. Thus, in determining ion flux, both the concentration difference and the membrane potential, collectively known as the ____________, must be considered.

electrochemical gradient

It is also possible that Ca²⁺ is released from the ____________ ; this is mediated by ____________ or Ca²⁺ entering from the extracellular fluid.

endoplasmic reticulum, IP₃

3. All G proteins contain three subunits, called the alpha, beta, and ____________ subunits.

gamma

CaCl₂ = 25 mM urea = 0 mM glucose = 0 mM NaCl = 120 mM

hyperosmotic, hypertonic

CaCl₂ = 30 mM urea = 50 mM glucose = 10 mM NaCl = 100 mM

hyperosmotic, hypotonic

CaCl₂ = 40 mM urea = 20 mM glucose = 280 mM NaCl = 90 mM

hyperosmotic, isotonic

CaCl₂ = 10 mM urea = 10 mM glucose = 40 mM NaCl = 100 mM

hypoosmotic, hypotonic

11. This renders the alpha subunit ____________, allowing it to recombine with its beta and gamma subunits.

inactive

As the woman's extracellular fluid became more dilute than her intracellular fluid, water moved __________ her cells by __________.

into, osmosis

8. When bound to GTP, the alpha subunit dissociates from the beta and gamma subunits, and the alpha subunit links up with another plasma membrane protein, either an ____________ or an enzyme, which are known as ____________.

ion channel, effector proteins

1. To begin, ions diffuse across plasma membranes by passing through ____________ formed by ____________ proteins.

ion channels, integral membrane

CaCl₂ = 10 mM urea = 10 mM glucose = 20 mM NaCl = 120 mM

isosmotic, hypotonic

CaCl₂ = 10 mM urea = 0 mM glucose = 0 mM NaCl = 135 mM

isosmotic, isotonic

2. Ion diffusion depends on the ____________, which is the separation of electrical charge across the plasma membrane.

membrane potential

9. If an ion channel is activated, a change in ____________ results; likewise, cytosolic ____________ could be activated, which mediate the cell's response to the first messenger.

membrane potential, second messengers

4. For example, most cells have a net ____________ charge inside with respect to the outside, so there is an electrical force attracting ____________ ions into the cell.

negative, positive

Muscles, bones, and joints form lever systems that allow the shortening velocity of muscles to be amplified so that the ends of our limbs can be moved at much higher velocities. This is a good demonstration of the general principle of physiology stating that ____________.

physiological processes are dictated by the laws of chemistry and physics

The brain cannot survive for more than a few minutes without a steady supply of glucose. During a prolonged fast, the liver converts glycogen to glucose, and it also converts amino acids and other molecules into glucose, while adipose tissue mobilizes fatty acids from triglycerides for tissues other than the brain to use as fuel. This is a good demonstration of the general principle of physiology stating that ____________.

physiological processes require the transfer and balance of matter and energy

With receptor activation, the ____________ itself may contain a Ca²⁺ channel, or the receptor may activate a ____________ that opens the channel via a second messenger.

receptor, G protein

3. Electrical charges of the same sign, both positive or both negative, ____________ each other, while opposite charges ____________.

repel, attract

When cytosolic Ca2+ concentrations are elevated, Ca2+ can act as a ____________.

second messenger

On binding with Ca²⁺, calmodulin changes its ____________ , and this allows the Ca²⁺-calmodulin complex to activate or inhibit a large variety of enzymes and other proteins, many of them ____________ .

shape, protein kinases

Because oxygen diffuses relatively slowly, body structures where oxygen diffusion must occur are often specialized to minimize the diffusion distance. For example, in the lungs where oxygen is moved into the blood, the epithelial and endothelial layers separating air from bloodstream are less than one micron thick. As another example, the extreme branching of the cardiovascular tree brings a capillary blood vessel to within 0.1 mm of virtually every cell in the body for the delivery of oxygen. This is a good demonstration of the general principle of physiology that ____________.

structure is a determinant of—and has coevolved with—function

To begin, cytosolic Ca²⁺ can be increased by the opening of ____________ in the plasma membrane or by receptor activation from a first messenger.

voltage-gated Ca²⁺ channels