Regulation of Water Balance in the Human Body

Bowman's capsule and the loop of Henle are structures in the kidneys that play a crucial role in maintaining homeostasis by regulating ion balance, extracellular fluid volume, osmotic concentration, and pH. The kidneys also remove toxic waste products from the body through the excretion of urea into urine. Additionally, the liver plays an important role in protein metabolism, converting ammonia to urea for excretion by the kidneys.

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COURSE: Access to Higher Education Diploma
SUBJECT: Biology
UNIT TITLE: Homeostasis, Co-ordination and Control and the Excretory
System
LEVEL: 3 (Graded)
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The student has achieved the learning
outcomes because s/he can:
Page number/s
where you have
achieved this AC:
1 Understand the
importance of
homeostasis in
maintaining equilibrium in
the body
1.1 Explain the term homeostasis 6
1.2 Evaluate, using examples, the importance of
homeostasis in maintaining physiological
equilibrium
6-8
3 Understand the function
of the main components
of the nervous system
2.1 Explain the relationship between structure and
function of the nervous system
10-11
4 Understand the function
of the main components
of the endocrine system
3.1 By use of examples, explain how the endocrine
system functions in the process of homeostasis
through hormone action
8
3.2 Compare and contrast hormonal control with
control through nerve action
11-12
6 Understand the function
of the main components
of the excretory system
4.1 Explain the relationship between structure and
function of the excretory system
13-15
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TAQ 1:
Homeostasis refers to maintenance of equilibrium or balance in the body. Internal
environment is maintained for optimal metabolic efficiency. Homeostasis controls the metabolic
rate, body temperature, concentration of blood glucose levels, dissolved salts, respiratory gases
and nitrogenous wastes in the body. The organs involved in the process are the brain, kidneys and
liver. A feedback loop operates to maintain homeostasis (Hew-Butler 2015). Changes in external
stimuli are relayed to an integrating center (hypothalamus in temperature controlling) through
receptors. This control center processes the stimuli and influences the effectors to either enhance
or reduce the action of the body in response to the stimulus.
TAQ 2:
Part 1
Most of the homeostatic mechanisms in the human body operate through negative
feedback mechanism. This system helps to go against or counter external stimuli. Regulation of
body temperature is controlled by the anterior hypothalamus. Normal human body temperature is
37°C. In summer season, the temperature rises. Heat sensors in the body relay information about
an increase in temperature to the hypothalamus (Diller et al. 2016). A series of responses are
thereby generated that lowers the temperature. The skin vessels become dilated, blood flow
increases through the vessels near the skin surface and muscular and metabolic activity rates get
lowered (Houdas and Ring 2013). The hypothalamus also stimulates sweating in the body to allow
water evaporation from the skin. This facilitates loss of heat from the skin. In colder season, an
opposite action is initiated. The temperature falls below 37°C. This stimulates the hypothalamus to
initiate responses that will prevent temperature drop. This leads to shivering, vasoconstriction
(constriction of blood vessels) and reduction in blood flow near the skin surface. As heat loss
continues from the body, two stress hormones, epinephrine and nor-epinephrine are released.
They immediately increase the cellular metabolic rate inside the body (Raccuglia et al. 2016).
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Furthermore, prolonged exposure to chilling conditions, stimulate the hypothalamus to influence
the pituitary gland. The latter increases production of thyroxin levels thereby, increasing
metabolism. Thus, the body heat gets conserved.
Maintaining optimal body temperature is essential in mammals due to the different
enzymatic activities that occur in the cells. If the temperature rises or falls beyond the optimum
levels, essential enzymes and proteins get denatured, there is oxygen shortage and the cell
membrane breaks down. At low temperature, needle like ice crystals are formed inside the cells
which may cause these cells to burst. This will disrupt different physiological processes. Thus,
core body temperature maintenance is essential for proper functioning of vital activities and life
processes.
Part 2
An adult consumes about 2500 mL of fluids per day. 1500mL of water is lost in the form of
urine every day from the body. Water gets removed by other routes as well. Water increases the
blood volume, dissolves respiratory gases, lowers body heat and excretes toxins from the body.
Thus, maintaining the level of water in the body is essential for a steady state. Antidiuretic hormone
(ADH) or vasopressin is plays an essential part in maintaining water homeostasis inside the body.
It acts as the major compound involved in controlling water balance by reducing output of water
output from the kidneys in the form of urine. Plasma osmolality is the ratio of solutes to the amount
of water in blood plasma (Knepper, Kwon and Nielsen 2015). If osmolality is high, the kidneys
influence ADH secretion. From the hypothalamus, ADH travels down and gets stored in the
posterior pituitary. It is released into the blood and reaches the kidneys once the hypothalamus
perceives a change in water balance. ADH then binds to a receptor located on the basolateral
membrane. It triggers several cellular events that luminal membrane’s permeability to water
(Hyndman and Pannabecker 2015). Thus, the permeability of collecting duct increases and the
new osmotic gradients help in retention of water by the body. This event is called antidiuresis. On
binding of AHD to the receptor, adenyl cyclase gets activated. That increases the levels of cyclic
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AMP in the kidneys. These series of reactions involve calcium and bring about a contraction of
microfilaments. Aquaporins or water channels get inserted in the luminal membrane thereby,
upregulating water permeability.
Under normal conditions, an adult has a a concentration of 6 mmol/l of urea in the plasma.
This contributes an extremely small percentage to the net plasma osmolality An enormous
increase in the levels of plasma urea to 30 mmol/l would not create any significant impact on the
release vasopressin, This occurs due to the fact that the cell membranes (including the
osmoreceptor cells) have high urea permeability. An increase in ADH release would lead to
retention of water and sudden fall in the sodium concentration and plasma osmolality (Danziger
and Zeidel 2014). This would give rise to hyponatraemia. However, less or no amounts of ADH in
blood would facilitate water loss from the kidneys at a rapid rate. The concentration of sodium ions
and plasma osmolality would rise. This condition leads hypernatraemia (Majzoub 2015). Therefore,
a water balance is needed to maintain cell content concentration that will help all cellular activities
to function properly.
Part 3
Normal glucose levels in the bloodstream of an adult are 90 mg per 100cm3. For
maintaining homeostasis of the blood glucose levels, the islets of Langerhans from the pancreas,
the liver and the muscle cells play an essential role. The pancreatic cells act as sensors and
control center. They determine the amount of sugar in the blood and influence the release of
hormones to balance it. The liver and muscles act as effectors. Once glucose has been absorbed
by the small intestine, it can be either broken down into water and carbon dioxide, may get
converted to glycogen and fats or can get circulated in the bloodstream. Insulin and glucagon are
antagonistic pancreatic hormones that regulate blood glucose levels (Lee et al. 2016). When
glucose transporter receptors detect high glucose levels, the beta cells of pancreas exert an
endocrine action and release insulin hormone. Insulin reaches the liver and converts excess
glucose into glycogen and fats (Melmed 2016). It also inhibits gluconeogenesis (glucose from non-
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carbohydrate sources). In absence of the hormone insulin, glucose concentration rises in the
blood. This leads to the development of a condition, diabetes mellitus. Glucagon is released from
the alpha pancreatic cells and is involved in elevating the blood glucose levels. A fall in the level of
glucose occurs several hours after a meal. This triggers the release of glucagon. It exerts an
opposite action. It stimulates the liver to break down stored glycogen into glucose. It also promotes
manufactures glucose from natural glycerol and lactic acid. Glucagon binds to G-protein-coupled
receptors and triggers a series of enzymatic reactions. These reactions activate glycogen
phosphorylase enzyme. It is responsible for mobilizing stored glycogen reserves from the liver to
free glucose. These reactions work together and restore the glucose levels to a normal state in the
body.
Thus, control of the amount of blood glucose level is an example of homeostatic control
through negative feedback inside the body. When the body encounters any imbalance or deviation
from normal state in the form of extremely low or high blood sugar levels, it balances itself to
normal levels and maintains a steady state in the body. Homeostasis maintenance works on the
principle of minimizing any kind of overshoot beyond normal levels. Hence, the fluctuations are
addressed by gradually reducing corrective mechanisms as the concentration of glucose reaches
the normal levels. This is achieved by secreting glucagon, the antagonistic hormone. Other
hormones like somatostatin, gastrin and cholecystokinin also play an important part in this control.
TAQ 3:[A diagram for you to complete is below. Click on the boxes to write in them. If you cannot
complete this diagram, please create your own in its place]
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Part 1
Part 2
The human nervous system performs three main functions: input of sensory information,
data integration and motor output. It is composed of neurons (excitable nerve cells) that form
synapses between themselves. These neurons conduct impulses from the receptors to the brain
and spinal cord. The nervous system consists of 2 major subdivisions, the central nervous system
(CNS) and the peripheral nervous system (PNS). The CNS contains the brain (in the cranium),
brainstem and spinal cord (enclosed by the vertebra). It is associated with information processing.
The brain acts as the main control center (Wullimann 2017). It is protected by the meninges and
contains the cerebrum (seat of memory, intelligence and consciousness), cerebellum (coordinates
voluntary movements) and medulla oblongata (controls heart beat, breathing and peristalsis). The
spinal cord extends to the lower back from the base of medulla and receives sensory information
from body and motor information from the brain and controls responses. The PNS is produced
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"Nervoussystem""CentralNervoussystem"BrainCerebrumCerebellumMedullaoblongataSpinalcord"Peripheralnervoussystem"AutonomicNervoussystemSympatheticnervoussystemParasympatheticnervoussystemSomaticnervoussystemSensorynervoussystem(afferent)Motornervoussystem(efferent)

from neural crest cells. It forms a huge network of 12 cranial and 31 pairs of spinal nerves, linked
to the CNS (Nieuwenhuys, Hans and Nicholson 2014). It contains different sensory receptors that
process changes in response to the environment and sends information to the CNS through
afferent sensory nerves.
The PNS is subdivided into autonomic nervous system and somatic nervous system. The
Autonomic system regulates activities of visceral organs of the body. It is divided into sympathetic
and the parasympathetic divisions. They exert antagonistic effect on the target tissues (Nebylitsyn
2013). Sympathetic system prepares the body for stressful conditions. The parasympathetic
restores normal conditions in the body and balances out the reactions of the former. The somatic
nervous system is associated with voluntary body movements. It involves the skeletal muscles and
external stimuli reception. It contains afferent nerve fibers, which receive information from external
cues and efferent nerve fibers that cause muscle contraction (Struhal and Russell 2014).
TAQ 4:
“Endocrine system” “Nervous system”
“Similarities”  Both are regulatory systems.
 They are responsible for communication between different cells,
tissues and organs.
 They are involved in maintenance of homeostasis by regulating
and coordinating the activities of other systems.
 A negative feedback loop regulates both the systems.
 Chemical messengers (hormones and neurotransmitters) are
the major secretions from the systems, which control the
different physiological processes.
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“Differences”  Formed by different glands.
 Hormones are the
chemical messengers that
play a role in signal
transmission.
 Transmission of signal is
long lasting but occurs
slowly (Kleine and
Rossmanith 2016).
 Hormones are released
into the circulatory system
to act on target sites.
 Hormones affect different
parts of the body.
 The effect cannot be
modified by previous
experiences of similar kind.
 Amount of hormone
secreted controls stimulus
intensity.
 Formed by a collection of
neurons.
 Electrochemical impulses
are generated that release
neurotransmitters as
chemical messengers (Hall
2015).
 Transmission of the signals
is fast but the effects are
not prolonged.
 Neurotransmitters are
released from the vesicles
at synaptic bulbs in the
brain.
 They show localized
effects on a particular
gland or muscle.
 Effects can be modified
through learning and
previous experiences.
 Frequency of signalling
controls stimulus intensity.
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TAQ 5:
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The urinary system contains a pair of kidneys, ureter, urinary bladder and
urethra. The kidneys are bean shaped organs. The left kidney is located
higher than the right kidney, due to larger size of the liver on the right side
(BRADLEY 2013). The ureters are a pair of tubes and carry urine to the
urinary bladder from the kidneys. They are 10-12 inches long. They run
parallel to the vertebral column on either sides of the body. The ends of the
ureters are guarded by ureterovesical valves at the point of entry into the
urinary bladder. These valves prevent backflow of urine. The urinary
bladder is a sac like organ that stores urine. This urine is excreted out of
the body through the urethra. It is guarded by sphincter muscles.

The kidneys perform a variety of homeostatic functions:
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The nephron is the structural unit of the kidneys. Each kidney contains more than 1
million filtering nephrons. A nephron consists of Bowman’s capsule, Proximal
convoluted tubule, loop of Henle, Distal convoluted tubule and the collecting ducts.
The Bowman’s capsule is double walled cup that lies in the renal cortex and contains
a tuft of capillaries, the glomerulus. The glomerulus along with the capsule
constitutes the malpighian body (Kretzler and Ju 2015). The PCT starts from neck of
Bowman's capsule and lies in the renal cortex. The loop of Henle is a U shaped
structure located in the renal medulla and contains two parallel limbs: a descending
limb that enters the renal medulla and an ascending limb that enters the renal cortex
to join the DCT. The terminal part of DCT opens into the collecting duct.

 Maintain the balance of ions in extracellular fluid (When the concentration of sodium,
potassium, magnesium, calcium and chloride ions reach a higher level, their excretion is
increased to restore the balance. Conversely, these ions are reabsorbed during filtration
when the concentration falls).
 Maintain extracellular fluid volume.
 Maintain osmotic concentration and pH of extracellular fluid (Excess H+ ions are eliminated
and bicarbonate ions are conserved).
 Remove toxic nitrogenous wastes like ammonia, urea and uric acid from the body.
 Monitor the blood pressure.
Reabsorption of water is controlled by the hormone vasopressin or ADH by negative feedback.
The pituitary gland releases ADH hormone. Low levels of body fluid triggers the hypothalamus to
increase release of ADH from pituitary into bloodstream. The hormone increases water
reabsorption (Qian et al., 2014). Thus, more water gets added to the blood thereby, increasing the
urine concentration. When there is an excess of fluid, the hypothalamus reduces the amount of
ADH in the blood. The amount of water reabsorption increases and large quantities of dilute urine
are produced.
The liver also plays an important excretory role in the protein metabolism. The cells of the
liver change amino acids from ingested food products to produce energy. In the metabolic process,
ammonia is released as a byproduct. Ammonia is extremely toxic and its accumulation in the body
can lead to poisoning. The liver cells convert this ammonia to urea (ornithine cycle) with the help of
specific catalysts and carbon dioxide, accompanied by the formation of water. This urea and water
are released transported to the kidneys from the bloodstream (Rouiller 2013). The blood is filtered
out. The urea is excreted out of the body in the form of urine.
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References
BRADLEY, T.J., 2013. 10 The EXCretory System. Structure and Physiology. Regulation:
Digestion, Nutrition, Excretion, p.421.
Danziger, J. and Zeidel, M.L., 2014. Osmotic homeostasis. Clinical Journal of the American
Society of Nephrology, pp.CJN-10741013.
Diller, K.R., Hensley, D.W. and Patrick, B., Board Of Regents, System and Mercury Biomed, Llc,
2016. Maintenance of reduced core mammalian body temperature. U.S. Patent Application
15/134,166.
Hall, J.E., 2015. Guyton and Hall Textbook of Medical Physiology E-Book. Elsevier Health
Sciences.
Hew-Butler, T., 2015. Inadequate Hydration or Normal Body Fluid Homeostasis?. American journal
of public health, 105(10), pp.e5-6.
Houdas, Y. and Ring, E.F.J., 2013. Human body temperature: its measurement and regulation.
Springer Science & Business Media.
Hyndman, K.A. and Pannabecker, T.L. eds., 2015. Sodium and Water Homeostasis: Comparative,
Evolutionary and Genetic Models. Springer.
Kleine, B. and Rossmanith, W.G., 2016. Hormones and the Endocrine System: Textbook of
Endocrinology. Springer.
Knepper, M.A., Kwon, T.H. and Nielsen, S., 2015. Molecular physiology of water balance. New
England Journal of Medicine, 372(14), pp.1349-1358.
Kretzler, M. and Ju, W., 2015. A transcriptional map of the renal tubule: linking structure to
function.
Lee, Y.H., Wang, M.Y., Yu, X.X. and Unger, R.H., 2016. Glucagon is the key factor in the
development of diabetes. Diabetologia, 59(7), pp.1372-1375.
Majzoub, J.A., 2015. Sodium and Water Balance, Vasopressin. In 2015 Meet-The-Professor:
Endocrine Case Management (pp. 130-136). The Endocrine Society.
Melmed, S., 2016. Williams textbook of endocrinology. Elsevier Health Sciences.
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