PHGY 170: Assessment 5: Final Exam
1. Homeostatic control systems can be simple or complex. Provide an example of a
homeostatic control system that uses at least 2 different types of tissues discussed in module
10. Your example must be different from the examples provided in the module. In your
example, identify each part of your control system and explain why the system is intrinsic
or extrinsic, and if it has positive or negative feedback.
Calcium is involved in a number of physiological processes and therefore the maintenance of
normal blood calcium concentration is critical.1
1. Low blood calcium concentration stimulates the calcium-sensing receptor (CaSR)
(sensor) on the parathyroid gland (integrator) causing the release of parathyroid hormone
(PTH) which activates osteoclasts.2,3,4
2. Osteoclasts break down connective tissue made up of calcium in bone (effector) releasing
more calcium into the bloodstream.2,3,5
3. PTH also activates vitamin D which increases the amount of calcium absorbed in the
epithelial tissues of the kidneys (effector) and the small intestine (effector) to increase
blood calcium concentration.2,3
1. High blood calcium concentration also stimulates CaSR (sensor) on the thyroid gland
(integrator) causing it to produce calcitonin, a hormone involved in the reduction of
calcium levels.3,4
2. Calcitonin inhibits activity of osteoclasts allowing absorption of calcium from the
bloodstream back into the connective tissue of bone (effector).3,5
, 3. Calcitonin also inhibits the reabsorption of calcium in the kidneys (effector) by
transporting more calcium across epithelial tissues leading to it’s removal through
urination to decrease blood calcium concentration.3
This system is extrinsically controlled as the regulatory mechanisms, the parathyroid and thyroid
gland, are part of the endocrine system and are not located within the tissue of an organ.2
This system operates on negative feedback as an environmental change (increase or decrease in
blood calcium concentration) causes the effector (the kidneys, small intestine, and bone) to
initiate a response that restores the concentration to normal.
2. If it is so important for a cell to regulate its internal environment why do we have passive
diffusion? Wouldn’t regulated transport make the most sense?
It may be assumed that because it is important for a cell to ‘regulate’ its internal
environment that it should use ‘regulated’ transport, however, this is not always true. Regulated
transport is favourable for some conditions, like to move large, complex molecules needed for
cell function across the plasma membrane, a process requiring energy. Passive diffusion is a type
of membrane transport in which substances move from an area of high concentration to an area
of low concentration. This is an important process as it allows small, charged molecules, like
ions and water, to pass freely through the membrane without the expenditure of any energy.
Passive diffusion is essential for regulating the tonicity of the cell’s internal environment. The
cell should always be isotonic as this is the ideal state because if the cell becomes hypertonic, it
will shrivel up and if it becomes hypotonic, it will lyse. This requires easy movement of ions and
water in and out of the cell to maintain an isotonic solution between the internal and external
1. Homeostatic control systems can be simple or complex. Provide an example of a
homeostatic control system that uses at least 2 different types of tissues discussed in module
10. Your example must be different from the examples provided in the module. In your
example, identify each part of your control system and explain why the system is intrinsic
or extrinsic, and if it has positive or negative feedback.
Calcium is involved in a number of physiological processes and therefore the maintenance of
normal blood calcium concentration is critical.1
1. Low blood calcium concentration stimulates the calcium-sensing receptor (CaSR)
(sensor) on the parathyroid gland (integrator) causing the release of parathyroid hormone
(PTH) which activates osteoclasts.2,3,4
2. Osteoclasts break down connective tissue made up of calcium in bone (effector) releasing
more calcium into the bloodstream.2,3,5
3. PTH also activates vitamin D which increases the amount of calcium absorbed in the
epithelial tissues of the kidneys (effector) and the small intestine (effector) to increase
blood calcium concentration.2,3
1. High blood calcium concentration also stimulates CaSR (sensor) on the thyroid gland
(integrator) causing it to produce calcitonin, a hormone involved in the reduction of
calcium levels.3,4
2. Calcitonin inhibits activity of osteoclasts allowing absorption of calcium from the
bloodstream back into the connective tissue of bone (effector).3,5
, 3. Calcitonin also inhibits the reabsorption of calcium in the kidneys (effector) by
transporting more calcium across epithelial tissues leading to it’s removal through
urination to decrease blood calcium concentration.3
This system is extrinsically controlled as the regulatory mechanisms, the parathyroid and thyroid
gland, are part of the endocrine system and are not located within the tissue of an organ.2
This system operates on negative feedback as an environmental change (increase or decrease in
blood calcium concentration) causes the effector (the kidneys, small intestine, and bone) to
initiate a response that restores the concentration to normal.
2. If it is so important for a cell to regulate its internal environment why do we have passive
diffusion? Wouldn’t regulated transport make the most sense?
It may be assumed that because it is important for a cell to ‘regulate’ its internal
environment that it should use ‘regulated’ transport, however, this is not always true. Regulated
transport is favourable for some conditions, like to move large, complex molecules needed for
cell function across the plasma membrane, a process requiring energy. Passive diffusion is a type
of membrane transport in which substances move from an area of high concentration to an area
of low concentration. This is an important process as it allows small, charged molecules, like
ions and water, to pass freely through the membrane without the expenditure of any energy.
Passive diffusion is essential for regulating the tonicity of the cell’s internal environment. The
cell should always be isotonic as this is the ideal state because if the cell becomes hypertonic, it
will shrivel up and if it becomes hypotonic, it will lyse. This requires easy movement of ions and
water in and out of the cell to maintain an isotonic solution between the internal and external
