Regulation of the Compostion of Body Fluids

What liquid makes up a large portion of the body?

Water → not pure, contains dissolved substances

Why is there a large amount of water in the human body?

• fluid transports substances around the body

• facilitates movement across membranes

• site for chemical reactions

How much water (L) would a male body have weighing 70kg, with a 60% water content?

~42L

• water distributed between various body fluids

What is intracellular fluid (or cytosol)?

Fluid inside of the cells

What is extracellular fluid?

Fluid outside of the cells

What is intravascular fluid?

Blood plasma located within the blood vessels

What is intercellular fluid (or interstitial fluid or tissue fluid)?

Fluid between the cells

What is tanscellular fluid?

Fluid in specific body regions. i.e. fluid in the brain, spinal cord, eyes, joints, surrounding heart

What other fluids are included in extracellular fluid?

• intercellular fluid (orinterstitial fluid or tissue fluid)

• tanscellular fluid

• Intravascular fluid

What do the thin walls of the capillaries allow?

relatively free exchange of materials between plasma and interstitial fluid

What is osmostic pressure?

The tendency of a solution to take in water

Does water move easily through plasma membranes?

Yes

What increases osmotic pressure?

The greater the difference in osmotic concentrations between two solutions, the greater the osmotic pressure.

Fluid gain =?

Fluid loss

• body fluids remain constant

Where is most body fluid obtained from?

• water

• other liquids

• food

• small amount metabolic water

What is metabolic water?

A small by-product of chemical processes occuring within cells that produces water.

Which areas of the body is fluids lost?

• kidneys

• skin

• surface of the lungs

• alimentary canal

How much fluid is lost from the body each day?

~2.5L

What is excretion?

Removal of metabolic waste from the body

Why must excretion occur?

Many wastes are toxic and would be harmful to health if allowed to accumulate in the body fluids

What organs are involved in excretion?

• lungs → romve CO₂ and some water (vapour)

• sweat glands → secrete water containing by-products of metabolism (salts, urea, lactic acid)

• alimentary canal → passes out bile pigments that entered the small intestine with the bile (these pigments are broken down products of haemoglobin from red blood cells, leaving the body with faeces)

• kidneys (prinipal excretory organ) → maintain constant concentration of materials in the body’s fluids, i.e. removal of urea → produced by liver during breakdown of proteins)

Which organ secretes the largest amount of water (in percentage) from the body each day?

Kidneys - 60%

Which method of water loss from the body is the only one that can be regulated?

The kidneys → to achieve a constant concentration of dissolved substances in the body fluids

Where are the kindeys located?

Either side of the vertebral column, at about the level of the lowest ribs, attached to the rear wall of the abdominal cavity, right kidney is slightly lower than the left (presence of the liver), held in postion by a mass of fatty tissue

How big are the kidneys?

~11cm

What is the ureter?

The tube that leaves each kidney and drains into the urinary bladder

What is the urinary bladder?

A hollow, muscular organ near the base of the abdominal cavity; collects urine from the two ureters

What is the urethra?

The tube that empties the bladder to the outside; in males, it also carries sperm

How many nephrons does the kindey contain?

About 1.2 million (microscopic)

What is a nephron?

The function unit of the kidney; nephrons carry out the kidney’s role in excretion and water regulation

Outline the steps of a nephron (during filtration)

1. Blood enters the glomerulus under high pressure.

2. Filtration. The high blood pressure forces water and small dissolved molecules out of the blood and into the capsule. Large molecules and blood cells are retained in the blood.

3. The filtrate is collected by the glomerular capsule.

4. Reabsorption. The filtrate passes through the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct. Water and other useful substances are reabsorbed into the peritubular capillaries.

5. Secretion. Some materials that need to be removed from the body are secreted into the kidney tubule from the peritubular capillaries.

6. Urine. The water and dissolved substances that remain make up the urine. Urine is carried by collecting ducts to the ureter and then to the bladder.

What are osmocreptors?

A receptor sensitive to osmotic pressure of body fluids, found in the hyptohalamus

What happens during strenuous activity or extreme heat when water loss can be high?

As water is lost, plasma becomes more concentrated and has higher osmotic pressure → water moves from intertitial fluid into plasma by osmosis, making interstitial fluid more concentrated (water diffuses out of the cells → cells shrink (dehydration))

Osmoreceptors detect increase in osmotic pressure.

How much water filtered through the glomeruli of the kidneys is reaborbed?

~90%

Where does water reabsorption occur in the kidneys?

Through the walls of the kidneys tubules

Which part(s) of water reabsorption in the kidneys is active reabsorption or osmosis?

Osmosis: proximal convoluted tubule, loop of Henle

Active reabsorption: distal convoluted tuble, collecting tubule

What hormone controls the level of active reabsorption of water in the kidneys?

Antidiuretic hormone (ADH)

Where is Antidiuretic hormone (ADH) produced?

Hypothalamus

Where is Antidiuretic hormone (ADH) released from?

Posterior lobe of the pituitary

What does Antidiuretic hormone (ADH) control?

permeability of the walls of the dital convoluted tuble and collecting duct

What happens when the concentration of Antidiuretic hormone (ADH) is high in the blood plasma?

Tubles very permeable to water, thus water is able to leave the tubule and enter the surrounding capillary network

Outward flow of water → reduces volume in tubules and increases concentration of material remaining

What happens when the concentration of Antidiuretic hormone (ADH) is low in the blood plasma?

Tubules not very permeable to water → very little water reabsorbed into the plasma

fluid within tububles remains dilute, volume not reduced to any signifcant extend

If water is lost through excess sweating, what process would occur? (steps)

• Stimulus: The osmotic pressure of the blood is raised due to the decrease in water in the blood.

• Receptors: Osmoreceptors in the hypothalamus detect the increased osmotic pressure of the blood.

• Modulator: The hypothalamus stimulates the posterior lobe of the pituitary gland to release ADH into the bloodstream.

• Effector: ADH is carried all over the body by the blood but it affects its target organs, which are the nephron tubules in the kidney. The permeability to water of the distal convoluted tubules and the collecting ducts is increased.

• Response: More water is then reabsorbed into the blood plasma from the distal convoluting tubule and collecting duct.

• Feedback: The reabsorption of water increases the amount of water in the plasma and so the osmotic pressure of the blood is decreased. This adjustment has eliminated or reduced the original stimulus; a negative feedback has occurred.

What does aldosterone do?

Acts on the distal convoluted tubules and collecing ducts to increase the amount of sodium ions reabsorbed into the bloodstrem and the amount of potassium secreted in the urine.

Achieves this through active transport using a sodium-potassium pump.

• 3 sodium ions reabsorbed → 2 potassium ions secreted

• net movement of ions into the blood, increases osmotic pressure → water into blood via osmosis

Where is adlosterone secreted from?

Adrenal cortex

What is aldosterone secreted in response to?

• decrease in the concentration of sodium ions in the blood

• decrease in blood volume

• decrease in blood pressure

• increase in the concentration of potassium ions in the blood.

What is the thirst centre?

The part of the brain that regulates the feeling of thirst; located in the hypothalamus

Outline the steps of the thirst response

• Stimulus: As water is lost from the various body fluids, there is a reduction in plasma volume and an increase in osmotic concentration of the extracellular fluid.

• Receptor: Osmoreceptors in the thirst centre in the hypothalamus detect the rising osmotic concentration of the blood. Other stimuli such as a dry mouth are also involved.

• Modulator: Stimulation of the thirst centre in the hypothalamus makes the person feel thirsty.

• Effector: The conscious feeling of thirst stimulates the person to drink.

• Response: The fluid consumed is absorbed from the alimentary canal into the plasma in the blood.

• Feedback: As the blood circulates through the body, it enables the interstitial fluid and intracellular fluid to return to the normal osmotic concentration. After drinking, the thirst centre is no longer stimulated and the desire to take in water ceases.

What is dehydration?

Water loss exceeds water intake (not enough water in the body to carry out normal functions)

What are some symptoms of dehydration?

• Low blood pressure

• severe thirst

• dizziness

• headahce

If untreated:

• delirious

• loses of consciousness

• death

How might dehydration occur?

• sweating

• vomiting

• diarrhoea

When does dehydration become noticeable?

When a person has lost ~2% of their normal body water

What is water intoxication?

A potentially life threatening condition caused by drinking too much water when the amount of salt (and other electrolytes) in the body is low; commonly caused by long bouts of intensive exercise during which electrolytes are not replenished and large amounts of water are consumed

What occurs if water intoxication happens?

Body fluids become diluted and cells take in extra water by osmosis (loss of water and salts through sweating → replacing with plain water)

What are the symptoms of water intoxication?

• headache

• vomiting

• lightheadedness

• collapse