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By the end of this section, you will be able to:
  • Explain how water levels in the body influence the thirst cycle
  • Identify the main route by which water leaves the body
  • Describe the role of ADH and its effect on body water levels
  • Define dehydration and identify common causes of dehydration

On a typical day, the average adult will take in about 2500 mL (almost 3 quarts) of aqueous fluids. Although most of the intake comes through the digestive tract, about 230 mL (8 ounces) per day is generated metabolically, in the last steps of aerobic respiration. Additionally, each day about the same volume (2500 mL) of water leaves the body by different routes; most of this lost water is removed as urine. The kidneys also can adjust blood volume though mechanisms that draw water out of the filtrate and urine. The kidneys can regulate water levels in the body; they conserve water if you are dehydrated, and they can make urine more dilute to expel excess water if necessary. Water is lost through the skin through evaporation from the skin surface without overt sweating and from air expelled from the lungs. This type of water loss is called insensible water loss because a person is usually unaware of it.

Regulation of water intake

Osmolality is the ratio of solutes in a solution to a volume of solvent in a solution. Plasma osmolality is thus the ratio of solutes to water in blood plasma. A person’s plasma osmolality value reflects his or her state of hydration. A healthy body maintains plasma osmolality within a narrow range, by employing several mechanisms that regulate both water intake and output.

Drinking water is considered voluntary. So how is water intake regulated by the body? Consider someone who is experiencing dehydration    , a net loss of water that results in insufficient water in blood and other tissues. The water that leaves the body, as exhaled air, sweat, or urine, is ultimately extracted from blood plasma. As the blood becomes more concentrated, the thirst response—a sequence of physiological processes—is triggered ( [link] ). Osmoreceptors are sensory receptors in the thirst center in the hypothalamus that monitor the concentration of solutes (osmolality) of the blood. If blood osmolality increases above its ideal value, the hypothalamus transmits signals that result in a conscious awareness of thirst. The person should (and normally does) respond by drinking water. The hypothalamus of a dehydrated person also releases antidiuretic hormone (ADH) through the posterior pituitary gland. ADH signals the kidneys to recover water from urine, effectively diluting the blood plasma. To conserve water, the hypothalamus of a dehydrated person also sends signals via the sympathetic nervous system to the salivary glands in the mouth. The signals result in a decrease in watery, serous output (and an increase in stickier, thicker mucus output). These changes in secretions result in a “dry mouth” and the sensation of thirst.

A flowchart showing the thirst response

This figure is a top-to bottom flowchart describing the thirst response. The topmost box of the chart states that there is insufficient water in the body, which has two effects. The left branch of the chart leads to decreased blood volume, which leads to decreased blood pressure. This triggers an increase in angiotensin two. Angiotensin two stimulates the thirst center in the hypothalamus. On the right branch, insufficient water in the body leads to increased blood osmolality, which causes dry mouth. Increased blood osmolality and dry mouth is sensed by osmoreceptors in the hypothalamus. This stimulates the thirst center in the hypothalamus to increase thirst, giving a person the urge to drink. Drinking decreases blood osmolality back to homeostatic levels.
The thirst response begins when osmoreceptors detect a decrease in water levels in the blood.

Questions & Answers

why arteries deeper than veins?!
Cismaan Reply
arteries colour of blood is deeper than that of vein because its blood contains oxygen which is adhered to haemoglobin(a protein which gives the blood its red color) , while vein contains deoxygenated blood(blood without oxygen)
olusoga
As we know, vein carries used blood to the heart. when we say used blood, we mean to say, blood that its content(oxygen and other nutrients) has been used up.
olusoga
Arteries are deeper b'cuz they need to be protected.......If they are ruptured they cannot form clot and repair themselves.... Moreover, the pressure of blood is too high for the artery to form the clot and repair itself....... Hence, Arteries are deeper than veins........
AMEL
Than u all. Special thanks too AMEL JEELANI.
Cismaan
You're welcome......
AMEL
Thanks all
describe the location of the macula densa and explain its role in the regulation of renin secretion and in tubuglomerular feedback
mwamba Reply
its located near the vascular pole of the glomurelus also regulate blood pressure and the filtration rate of glomurelus
adam
Describe two early induced responses and what pathogens they affect
olivia Reply
what are pathogens
Priscilla
pathogens are disease-causing agents/organisms
olusoga
pathogen are the causative of disease
Marco
thank you sister
Priscilla
mmmmm
Asad
kkkjjjjjjjhgghkkgkgkkjkjkjgjkhjigjkh
Asad
What are organelles
RAPHERA Reply
The are little organs found in cells of living things... Eg gogi apparatus
Lombe
what is anatomy
Linda Reply
is the study of the structures of the body and how they relates to each other
Agyemang
Anatomy is the study of the structures of body parts and how they relates to each other
Agyemang
is the study of the structure of the body and how they relates to each other
NAOMI
what are the difference between Pacinian corpuscle and cutaneous vascular plexus?
thivya Reply
what are membranous epithelial tissues
Naa Reply
they are the lining and covering epithelial tissues which cover body surfaces and line cavities... they're grouped into simple and stratified according to the number of layers and squamous, cuboidal and columnar according to their shape
Ophelia
what is an acina
Nuella
acina is known to be the basic functional unit of the lungs .(singular:- acinus) this is where the alveoli(the gaseous exchange site) is found...
Fatukasi
what happens to the unfertilized egg
Wuraola
the study of tissues is called
Scandy Reply
histology
Sirius
microscopic or histology anatomy
Shan
What specific types of biological macromolecules do living things require and why?
Marieland Reply
what is partial pressure?
Tariq Reply
it is the pressure exerted by mixture of gases...
Fatukasi
What is the Important of studying anatomy and philosophy
Michael Reply
because to know the mechanisms of our body
Tariq
to discover the regional structural of human body based on physically and also biochemically.
thivya
how many region do we have in human body
nsofor Reply
head thorax abdomen and many kind of...
Shan
what is principal ponatine nucleas
Tariq
Human body can be divided into different regions on the basis of: 1. Systems: e.g. digestive system, respiratory system, excretory system etc.. 2. Parts: e.g. head, thorax, neck, upper limbs, lower limbs etc..
AMEL
we have 9 region in d human body
ZAINAB
what is systematic anatomy?
nsofor Reply
it is the anatomy or study of a certain body system for example the digestive system. or respiratory system.
Summer
It is a group of structure that work together to perform a unique function..
RAPHERA
describe the division of anatomy and physiology
Vissa Reply
the what and the how
Josh
anatomy is the structure. physiology is the function.
Gavin
what is homeostasis?
nsofor
the balance if everything in your body
Kare
is the tendency of the body to maintain the internal environment
Flora
It is the ability of systems and living organisms to adjust its internal environment
RAPHERA
how does a saggital plane look like and a frontal plane
susan Reply
saggital plane divides left and right.. frontal plane divides front and back.. I'm trying to upload a picture but idk how.
Pipiena
tnx sis
Angella
The sagittal plane is a vertical plane that divides the body into left and right anteriorly whilst the frontal plane divides the body into the posterior(back) and anterior(front) parts. They're all perpendicular to the transverse plane.
Sirius

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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