Regulation of The Composition of Body Fluids

Syllabus Science Understanding

• Body fluid concentrations are maintained by balancing water and salts via the skin, digestive system, and the kidneys.

• This involves the actions of antidiuretic hormone (ADH) and aldosterone on the nephron, and the thirst reflex.

• Elaborations:

  • Importance of water in body functions.

  • Importance of maintaining body fluid concentrations in terms of osmosis and hydration.

  • Osmoregulation is the balance between the water and salt contents of the body fluids and is affected by inputs and outputs from various locations which differs for different environmental conditions e.g., hot and cold weather, drinking and dehydration.

  • Osmoregulation is controlled by nervous responses (thirst) and endocrine responses (ADH and aldosterone).

  • Salt and water are ingested via the digestive system and are lost as sweat (for thermoregulation), urine (excretion of metabolic wastes), and elimination from the digestive system (faeces); water is also a by-product of cellular respiration.

  • Structure of the nephron: locations of filtration, re-absorption, and active secretion.

  • Effects of ADH and aldosterone on the activities of the nephron.

  • Stimulus-response feedback models for water regulation including thirst reflex, ADH, and aldosterone.

Regulation of the Composition of Body Fluids

• Water makes up a large proportion of the human body and contains dissolved substances.

• Functions of water in the body:

  • Transporting substances.

  • Facilitating movement across membranes.

  • Being the site of chemical reactions.

• It is vital that the volume of water and concentration of dissolved substances is controlled to remain within the tolerance levels for the body.

Distribution of Body Fluids

• A 70 kg male with 60% water content would contain 42L of water.

• Intracellular fluid (cytosol) is fluid inside the cells.

• Extracellular fluid is fluid outside the cells and includes:

  • Blood plasma (intravascular fluid) within blood vessels.

  • Interstitial fluid (intercellular fluid or tissue fluid) between cells.

  • Transcellular fluid in specific body regions like the brain, spinal cord, eyes, joints and surrounding the heart.

Body Fluid Distribution

• Total volume: 42 L (~60% of body weight)

• Intracellular fluid compartment: 28 L

• Extracellular fluid compartment: 14 L

• Intravascular fluid (plasma): 3 L

• Interstitial fluid: 10.5 L

• Transcellular fluid: 0.5 L

Body Fluids

• Intracellular fluid:

  • Proportion: 2/3 of total body water.

  • Components: Fluid inside the cell - the cytosol.

• Extracellular fluid:

  • Proportion: 1/3 of total body water.

  • Components: Fluid that is outside the cells.

• Plasma (intravascular fluid):

  • Proportion: Approx. 1/4 of extracellular fluid.

  • Components: The fluid part of the blood.

• Interstitial and transcellular fluid:

  • Proportion: Approx. 3/4 of extracellular fluid.

  • Components: Lymph, cerebrospinal fluid, synovial fluid, fluids of eyes and ears, fluid in the chest and abdominal cavities, around the heart, fluids of the alimentary canal, and kidney filtrate.

Exchange of Materials Between Body Fluids

• Continuous exchange of materials occurs between different body fluids.

• Plasma is separated from the interstitial fluid by capillary walls, allowing relatively free exchange.

• Water moves easily through plasma membranes, and osmosis restores balance within seconds if imbalances in osmotic concentration occur.

• Osmotic pressure is the tendency of a solution to take in water.

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

• High solute, low solvent = high osmotic pressure; Low solute, high solvent = low osmotic pressure.

Hypertonic Solution

• Has a higher concentration of solutes than the cell inside it.

• Water moves out of the cell causing it to shrink.

Fluid Circulation

• Substances enter and leave the bloodstream via permeable capillaries.

• At the arterial end:

  • There is a mass flow of plasma and nutrients from the bloodstream into the tissue fluid.

  • Blood pressure (BP) is greater than the osmotic pressure.

• As blood is forced through the capillary, blood pressure drops.

• At the venous end:

  • There is a mass flow of tissue fluid and wastes from the tissues into the bloodstream.

  • Blood pressure is now less than the osmotic pressure.

Maintaining Fluid Balance

• Fluid gain must equal fluid loss.

• Most fluid comes from ingested water (food and liquid).

• A small amount comes from metabolic processes.

• Fluid is lost via:

  • Kidneys

  • Skin

  • Lungs

  • Alimentary canal

• About 2.5 Litres of fluid are lost each day.

Daily Fluid Intake and Output

• Water intake: 2500 mL/day

  • Drink: 1600 mL

  • Food: 700 mL

  • Metabolic water: 200 mL

• Water loss: 2500 mL/day

  • Lungs: 300 mL

  • Skin: 500 mL

  • Kidneys (urine): 1500 mL

  • Alimentary canal (faeces): 200 mL

Excretion

• Excretion: Removal from the body of metabolic wastes.

• Excretory Organs:

  • Lungs: Remove CO_2, water (water vapour).

  • Sweat Glands: Secrete water, salts, urea, and lactic acid.

  • Alimentary Canal: Passes out bile pigments (from haemoglobin in RBC’s).

  • Kidneys: To rid the body of wastes (especially nitrogenous wastes such as urea, uric acid, and creatinine) and to balance water, salt, and pH levels.

The Kidneys

• Two reddish-brown organs in the abdomen on either side of the spine, at about the level of the lowest ribs.

• They produce urine.

• Associated organs:

  • Renal artery: Takes blood to the kidney.

  • Renal vein: Takes blood away from the kidney.

  • Ureter: A tube that leaves each kidney and drains urine away from the kidney.

  • Bladder: A muscular bag that holds the urine until it is passed out of the body.

  • Urethra: A tube from the bladder that opens to the outside.

Kidney Functions

• Fluid balance.

• Salt balance.

• Removal of wastes (especially urea).

• pH balance.

• Note: Fluid and Salt balance are interdependent.

Structure of the Kidney

• Includes:

  • Medulla

  • Cortex

  • Capsule

  • Pyramid

  • Renal artery

  • Renal vein

  • Pelvis of ureter

  • Collecting duct

  • Proximal convoluted tubule

  • Distal convoluted tubule

  • Loop of Henle

  • Peritubular capillaries

  • Glomerulus

  • Nephron

  • Ureter

Nephron Function

• Blood enters the glomerulus under high pressure.

• Filtration: 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.

• The filtrate is collected by the glomerular capsule.

• 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.

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

• 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.

Fluid Balance - Kidneys Role

• The kidneys play an important role in the homeostatic regulation of body fluids (both the amount and the composition).

• If we become dehydrated, the kidneys can increase the reabsorption of water from the filtrate, whilst also increasing the secretion of salt. If our tissue fluids are too dilute, the opposite occurs.

Water Reabsorption

• 60-70% of water reabsorption occurs in the proximal convoluted tubule.

• The remaining 30-40% is selectively reabsorbed in the loop of Henle, distal convoluted tubule, and collecting duct, depending on our state of dehydration.

• How much water is reabsorbed at both stages depends on our state of dehydration i.e., less water is reabsorbed if our tissue fluid is dilute; more if we are dehydrated.

Urine Formation

• Three stages:

  • Filtration (in the renal corpuscle)

  • Selective reabsorption (mainly in the proximal convoluted tubule – some water and salts are reabsorbed in the loop of Henle and the distal convoluted tubule)

  • Tubular secretion (in the distal convoluted tubule)

Filtration Process

• Renal Corpuscle:

  • Substances: Water, urea, glucose, amino acids, vitamins, salts (mainly sodium & chlorine).

  • Filtration Type: Passive (mass flow).

Selective Reabsorption

• PCT:

  • Water (60-70%) - Passive (osmosis)

  • Salts (60-70%) - All active

  • Glucose (100%) - All active

  • Amino acids (100%) - All active

  • Vitamins (100%) - All active

• Loop of Henle:

  • Water (25%) - Passive (osmosis)

  • Na^+/Cl^- (25%) - Active

• DCT:

  • Water (5%) - Passive (osmosis)

  • Na^+/Cl^- (5%) - Active (ADH)

• Collecting Duct:

  • Water (5%) - Passive (osmosis)

Tubular Secretion

• DCT:

  • H^+ - Active

  • NH_4^+ (ammonium) - Active

  • Creatinine - Active

  • Toxins - Active

  • Drugs - Active

  • Neurotransmitters - Active

Controlling Water Levels

• Water is continually lost from the body through sweat, urine, faeces, and exhaled breath.

• Water input and water output are normally balanced.

• In strenuous activity or extreme heat, water loss is high.

• As water is lost, plasma becomes more concentrated and has a higher osmotic pressure (more solute less solvent).

• As a result, water moves from interstitial fluid into plasma via osmosis.

• Interstitial fluid becomes more concentrated, and water diffuses out of cells, so cells start to shrink from dehydration.

• When this happens, osmoreceptors (hypothalamus) detect an increase in osmotic pressure.

• A number of responses are then triggered that increase the water content and lower osmotic pressure (ADH, Aldosterone, Thirst reflex).

Kidneys and Antidiuretic Hormone

• 99% of water filtered through the kidney is reabsorbed.

• The proximal convoluted tubule and loop of Henle reabsorb water through osmosis.

• The distal convoluted tubule and collecting tubule are involved with active reabsorption of water.

• Active reabsorption is controlled by Antidiuretic Hormone (ADH).

• The hypothalamus secretes ADH, which is then stored and released by the posterior pituitary.

• An increase in ADH in the blood increases the permeability of the tubule (DCT and CT) to water; hence, water is reabsorbed from the tubule to the bloodstream.

• A decrease in ADH in the blood decreases the permeability of the tubule to water; hence, less water is reabsorbed from the tubule to the bloodstream.

ADH Mechanism

• ADH increases the permeability of the tubule.

• Water leaves the filtrate by osmosis from a relatively dilute filtrate to relatively concentrated tissue fluid.

Regulation of Water Output by Antidiuretic Hormone

• Stimulus: Water concentration of blood plasma decreases; osmotic pressure of the blood is increased.

• Receptor: Osmoreceptors in the hypothalamus are stimulated.

• Modulator: The hypothalamus stimulates the posterior lobe of the pituitary gland to release ADH (the effector).

• Response: Permeability to water of the distal convoluted tubules and collecting ducts is increased.

• Feedback: Increased amount of water is reabsorbed into the blood plasma. Water concentration of the blood plasma increases; osmotic pressure of the blood is decreased.

Low Water Levels and High Salt Levels

• Stimulus: Low H_2O levels/ High Salt Levels and Increased osmotic pressure.

• Receptor: Osmoreceptors in Hypothalamus.

• Modulator: Hypothalamus stimulates Posterior Pituitary to release ADH into the bloodstream.

• Effector: SPM of nephron in kidney (DCT and collecting duct).

• Response:

  • SPM of nephron (DCT and collecting duct) becomes more permeable.

  • More water is reabsorbed from tubule into the bloodstream.

  • Urine becomes more concentrated, and urine volume decreases.

• Feedback: Decrease in Salt levels, osmotic pressure, and water levels in blood.

• Nerve impulses:

  • Stimulate thirst centre in hypothalamus therefore drink.

  • Behavioural – seek shade.

• ADH – makes SPM of nephron (DCT + CD) more permeable.

High Water Levels and Low Salt Levels

• Stimulus: Decrease in osmotic pressure and increased water levels in the blood (↓ Salt levels).

• Receptor: Osmoreceptors in Hypothalamus.

• Modulator: Hypothalamus causes Posterior Pituitary to release less ADH.

• Effector: SPM of nephron (DCT and collecting duct).

• Response:

  • SPM of nephron (DCT and collecting duct) becomes less permeable therefore, less water is reabsorbed from tubule into the blood stream.

  • Urine is dilute and urine volume increases

• Feedback: Increase in Salt levels and osmotic pressure, decrease in Water levels in blood.

Kidneys and Aldosterone

• Aldosterone (salt retaining hormone).

• Secreted by the adrenal cortex in response to a:

  • Decrease in concentration of sodium ions in blood.

  • Decrease in blood volume.

  • Decrease in blood pressure.

  • Increase in concentration of potassium ions in blood.

• Aldosterone increases the reabsorption of sodium ions from the DCT, collecting ducts, and ascending limb of the loop of Henle into the blood.

• As sodium is reabsorbed, water is also reabsorbed via osmosis, which increases blood volume and therefore blood pressure.

• Also regulates the amount of potassium removed by urine.

Aldosterone Mechanism

• Aldosterone stimulates sodium pumps.

Aldosterone Control

• Stimulus: Decrease in blood volume and blood pressure.

• Receptor: Baroreceptors in Renal Artery.

• Modulator: Adrenal Cortex secretes Aldosterone.

• Effector: Distal convoluted tubule, Collecting Duct, Ascending Limb of Loop of Henle.

• Response:

  • Sodium is reabsorbed from Nephron to bloodstream.

  • Water is reabsorbed from Nephron to bloodstream as it follows the salt.

  • Potassium is secreted from bloodstream to nephron.

• Feedback: Increase in blood volume and blood pressure.

The Thirst Response (reflex)

• As well as reducing water loss, water level can be increased by taking in more fluid.

• Osmoreceptors stimulate the Thirst Centre in the hypothalamus, promoting a person to drink.

• The fluid is absorbed across the wall of the alimentary canal into the blood, decreasing the osmotic pressure.

Movement of Water Between Body Parts

• Water in the fluids that we consume is absorbed into the blood from the alimentary canal.

• There is a constant exchange of water between the blood, the extracellular fluid, and the intracellular fluid.

• Any excess fluid in the extracellular fluid is collected by the lymph system and returned to the blood by lymph vessels that join veins in the upper chest.

Thirst Response Steps

1. As water is lost from various body fluids, a reduction in plasma volume and an increase in osmotic concentration of extracellular fluid occurs.

2. Osmoreceptors in the thirst centre in the hypothalamus detect the rising osmotic concentration in blood. Dry mouth stimulation also occurs.

3. Stimulation of thirst centre makes the person feel thirsty.

4. Conscious feeling of thirst, cerebral cortex stimulates the person to drink.

5. Fluid consumed is absorbed into the plasma from the alimentary canal.

6. As blood circulates through the body, the intercellular and intracellular fluid return to the proper osmotic concentrations.

7. After drinking, the thirst centre is no longer stimulated, and the desire to take in water stops.

Regulation of Water Balance by the Thirst Mechanism

• Stimulus: Water concentration of blood plasma decreases; osmotic pressure of the blood is increased; mouth becomes dry.

• Receptor and modulator: Osmoreceptors in the thirst centre of the hypothalamus are stimulated.

• Effector: The person feels thirsty.

• Response: The person responds to the feeling of thirst by drinking.

• Feedback: Water drunk is absorbed into the blood from the alimentary canal. Water leaves the blood, and the extracellular and intercellular fluids return to their normal concentrations.

Thirst Mechanism/Reflex

• Stimulus: Decrease in Water levels in blood, increase in osmotic pressure of blood and Dry mouth.

• Receptor: Osmoreceptors in Hypothalamus (thirst centre).

• Modulator: Osmoreceptors in Hypothalamus.

• Effector: Cerebral cortex Dry Mouth (Feel thirsty).

• Response:

  • Conscious feeling of thirst triggers thirst reflex.

  • Drinking behaviour.

  • Water absorbed into blood from alimentary canal.

• Feedback: Increase in Water levels in blood and decrease in osmotic pressure in blood. Water enters cells.

Too Much and Too Little Water

• Dehydration – water loss exceeds water intake (not enough water to carry out normal functions).

• Symptoms are noticeable when 2% of a person’s normal body water has been lost.

• Water could have been lost through sweating, vomiting, or diarrhoea.

• Elderly people’s thirst reflex is less efficient as we get older, hence are susceptible to dehydration.

• Symptoms: severe thirst, low blood pressure, dizziness, and headache.

• If untreated, may become delirious, lose consciousness, and die.

Water Intoxication (water poisoning)

• Too much water in the body.

• Occurs when body fluids become diluted and cells take in extra water by osmosis.

• If a person loses a lot of water and salts through sweating and replaces the loss with plain water, water poisoning can occur.

• Water should be consumed with dissolved substances to replace the salts lost as well as the water.

• Symptoms of water intoxication are: light headedness.

• Headache, vomiting, and collapse may follow.