13-2: Homeostasis and Thermoregulation

Question 1

What is homeostasis and what are its features?

Answer 1

• A dynamic equilibrium
• It does not eliminate changes in the internal environment –
it moderates them
• It relies largely on negative feedback
• The set points and normal ranges for homeostasis can
change under various circumstances – regulated changes are essential to normal body functions

Question 2

Feedback mechanisms regulate biological systems

Answer 2

• negative feedback reduces stimulus

- positive feedback increases stimulus

Question 3

thermoregulation

Answer 3

-Thermoregulation is the process by which animals maintain an internal temperature within a tolerable range.
- There are 2 general strategies: endothermy and ectothermy
-Temperature receptors:
• in the skin, spinal cord, and
hypothalamus.
-Temperature control centers: • in the hypothalamus, (acts as
an integrator).
-The effectors are:
• skin blood vessels,
• muscles,
• body cells in general,
• sweat glands,
• respiratory centers.

Question 4

endothermy

Answer 4

Endotherms produce their own heat (most birds and mammals; a few fish, some insects).

Question 5

ectothermy

Answer 5

Ectotherms rely on heat from the environment (amphibians, reptiles, etc.).

Question 6

Countercurrent heat exchanger

Answer 6

• Traps heat in the body core => reduces heat loss from the
extremities
-found in body parts of animals who’re in contact w cold water
-ex tongue of great whale, flippers of dolphin, legs of goose

Question 7

Countercurrent heat exchanger in blue whale

Answer 7

-Anatomy:
• The tongue contains bundles of arteries
(carrying warm blood) and veins
(carrying cool blood).
• The artery is encircled by smaller veins.
-The key:
• The two types of blood vessels are
arranged in an antiparallel fashion.
• Warm blood travelling to the tongue is
in close contact with cool blood
travelling back to the body => it warms
up.

Question 8

Countercurrent heat

exchanger in Canada goose

Answer 8

-Anatomy: 
• Arteries (carrying warm
blood) in close contact
with veins (carrying cool
blood)
-The result:
 • At any single point
throughout the goose’s
leg, the blood in arteries
is warmer than the
blood in the veins.

Question 9

Countercurrent heat

exchanger in dolphin

Answer 9

-The key:
 • Each artery is surrounded
by several veins in a
countercurrent
arrangement.
- The result: 
• Efficient heat exchange
between the blood in the
arteries and veins

Question 10

Maintaining homeostasis by simple pathway

Answer 10

Some other examples of maintaining homeostasis in your body by negative feedback, use simple endocrine or neuroendocrine pathways.

Question 11

Glucose homeostasis - overall

Answer 11

Stimulus: high or low blood glucose level
• Endocrine gland involved: pancreas (beta cells –> insulin;
alpha cells -> glucagon)
• Targets: body cells, liver
• Responses:
-to lessen blood sugar: taking up sugar by the body cells;
storing sugar as glycogen in the liver
- to raise blood sugar: breakdown of glycogen by the liver
• Effects: lessening or raising blood glucose level

Question 12

Glucose homeostasis

Answer 12

• Glucose homeostasis relies predominantly on the antagonistic effects `of two hormones: insulin and glucagon.
• Insulin triggers the uptake of glucose from the blood into body cells, as well as storage of glucose as glycogen, thus decreasing the blood glucose concentration to normal.
• Glucagon promotes the release of glucose into the blood from energy stores, such as liver glycogen.
• Note that the liver is a key target site for insulin and glucagon action.
• Insulin also acts on nearly all body cells to stimulate glucose uptake from blood. A major exception are brain cells, which can take up glucose whether or not insulin is present.

Question 13

Calcium homeostasis - overall

Answer 13

• Stimulus: low or high level of calcium in blood
• Endocrine glands involved: parathyroid (PTH) raises calcium in
blood; thyroid (calcitonin) lessens calcium in blood
• Targets: kidneys, intestines, bones
• Responses:
- to raise calcium level => reabsorbing calcium in kidneys, breaking
down mineral matrix in bones, uptake of calcium from food in the
intestines
- to lessen calcium level => calcium deposition in the bones, and
reduced calcium uptake in the kidneys
• Effects: raising or lessening blood calcium level

Question 14

Calcium homeostasis

Answer 14

Indirect effects on:
1. Kidneys, to complete
conversion of vitamin D to an active hormone;
2. Intestines, to uptake Ca2+ from food.

Direct effects on:

1. Bones, to release Ca2+;
2. Kidneys, to reabsorb Ca2+.

• In mammals, the parathyroid glands play a major role in blood Ca2+ regulation.
• When the blood Ca2+ level drops below set point, these glands release parathyroid
hormone (PTH) to initiate raise of Ca2+ in blood.
• PTH raises the Ca2+ blood levels in the following 2 ways: directly affecting bones to
make them break down their mineral matrix and release calcium; and the kidneys to
make them reabsorb calcium.
• PTH also has its indirect effects on the kidneys making them to complete conversion
of vitamin D to an active hormone (which began in the liver), and on the intestines to
make them uptake Ca2+ from food.
• Thyroid gland itself contributes to calcium homeostasis, by releasing calcitonin, a
hormone that lessens the Ca2+ level in the blood, by mechanisms opposite to PTH.

Question 15

Fluid retention homeostasis - overall

Answer 15

• Stimulus: loosing water or eating salty food increases blood osmolarity
• Sensors: osmoreceptors in hypothalamus
• Endocrine gland involved: posterior pituitary (ADH)
• Targets: collecting ducts in kidneys
• Responses: increased water reabsorption; drinking water
• Effects: concentrated urine, which lessens blood osmolatiry

Question 16

Regulation of the fluid retention in the

kidney by ADH

Answer 16

Negative feedback:
once the homeostasis is
established again
(blood osmolarity gets
to normal) -> less ADH
will be released as a
result
-• After eating salty food or losing water through sweating, the blood osmolality rises. When osmolarity rises above the set value, antidiuretic hormone (ADH) also called vasopressin, is released from
the posterior pituitary.
• The target cells for ADH are found in the collecting ducts in the kidney. ADH makes these ducts become more permeable to water, and the reabsorption of water increases.
• The result of that is concentrated urine of less volume, which lessens blood osmolarity back toward the set point.
• Note that it is the hypothalamus that monitors the osmolarity in blood, and regulates the release of ADH from the posterior pituitary.

Question 17

Breathing homeostasis - overall

Answer 17

• Stimulus: low blood pH
• Sensors: in major blood vessels; in medulla oblongata
• Targets: rib muscles, diaphragm
• Responses: increased rate and depth of breathing
• Effects: blood CO2 level falls => pH rises

Question 18

Homeostatic control of

breathing

Answer 18

The sensors that detect a decrease in blood pH are located in major blood
vessels (i.e., carotid arteries, aorta). The sensors that detect a decrease in cerebrospinal fluid are located in medulla oblongata (near the base of the brain). The medulla uses the pH of the surrounding tissue fluid as an indicator of blood CO2 concentration.
• When you exercise, increased metabolism raises the concentration of CO2 in the blood and in the cerebrospinal fluid, which lessens pH.
• In response, the medulla increases the depth and rate of breathing by targeting rib muscles and diaphragm. Both, depth and rate of breathing,
remain high until the excess of CO2 is eliminated in exhaled air and pH returns to a normal value.