Homeostasis of Body Fluids

Question 1

Water content

Answer 1

60%

Question 2

Intracellular fluid

Answer 2

• 2/3 of total body water - fluid inside the cell, the cytoplasm

Question 3

Extracellular Fluid

Answer 3

• 1/3 total body water - fluid that is outside cells

Question 4

Plasma

Answer 4

• 1/4 of extracellular fluid - fluid part of blood

Question 5

Intercellular fluid

Answer 5

• 3/4 of extracellular fluid - lymph, cerebrospinal fluid, synovial fluid in joints, fluids of eyes and ears, fluid in the chest and abdominal cavities and around the hart, fluids of the alimentary canal, kidney filtrate

Question 6

Body Fluids

Answer 6

There is a constant exchange of fluids and substances between the blood, extracellular and intracellular fluid e.g. water, glucose, oxygen, carbon dioxide and lactic acid

Question 7

Maintaining Fluid Balance

Answer 7

• To maintain a constant fluid balance in the body (both quanitity and concentration), volume of fluid gain must equal fluid loss
• To regulate fluid loss so it matches fluid gain is by varying urine volume

Question 8

Excretion

Answer 8

• removal of metabolic wastes produced through anabolic and catabolic reactions in cells
• these wastes are removed in the following ways:
  1. lungs - CO2 and H2O
  2. Sweat glands - urea, salts, lactic acid and water
  3. Alimentary canal - bile pigments (broken down Hb from RBC)
  4. Kidneys - urea. They also maintain a balance of materials in the body fluid

Question 9

Kidneys

Answer 9

Two reddish brown organs in the abdomen on either side of the spine, at about the level of the lowest ribs. They produce urine.

Question 10

Ureter

Answer 10

A tube that leaves each kidney. It drains urine away from the kidney.

Question 11

Bladder

Answer 11

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

Question 12

Renal Vein

Answer 12

Takes blood away from the kidney

Question 13

Renal Artery

Answer 13

Takes blood to the kidney

Question 14

Urethra

Answer 14

A tube from the bladder that opens to the outside

Question 15

The Nephron

Answer 15

The microscopic structure found in the kidneys

Question 16

Kidney Function

Answer 16

• Regulates blood volume and composition (including the removal of nitrogenous wastes) so homeostatis is maintained by regulating urine production
• The functional unit of the kidney is the nephron
• There are 3 main processes that occur in the kidney to achieve this:

1. Glomerular Filtration

2. Selective Reabsorption

3. Tubular Secretion

Question 17

Glomerular Filtration

Answer 17

• Fluid forced out of blood in the glomerulus and collected in the glomerular capsule
• the nephron filters blood by ultrafiltration (filtering under pressure)
• Blood in the glomerulus at high pressure due to:
  1. The efferent arteriole has a smaller diameter than the afferent arteriole and therefore creates a high pressure which forces fluid out
  2. The glomerulus is highly coiled
  3. Short renal artery and proximity to aorta so under high pressure
• Because of high pressure in the capillaries and low pressure in the capsule, the water and dissolved components move through the semi-permeable membrane into the capsule
• It’s now called filtrate when in the glomerular capsule
• Around 180L of filtrate is formed over a 24 hour period but only 1% leaves the body as urine. Most is reabsorbed.

Question 18

Filtrate

Answer 18

• Made up of all material in blood except for WBC, RBC and plasma proteins
• It contains: water, salts, amino acids, fatty acids, glucose, urea, hormones, toxins and ions

Question 19

Selective Reabsorption 1

Answer 19

• to prevent the loss of all components in filtrate that are needed by the body they are reabsorbed back into the blood
• the nutrients are initially absorbed by diffusion but as the concentration gradient decreases the remainder is absorbed through active transport in the PCT
• e.g. Active: glucose, amino acid, vitamins, K+, Na+, Ca2+, Cl-, HCO3-
• e.g. Passive: urea
• e.g. Osmosis: water

Question 20

Selective Reabsorption 2

Answer 20

• Due to active reabsorption of glucose etc. the concentration of nutrients in the blood fluid is greater than filtrate in tubule
• Therefore water moves by osmosis out of tubule into capillaries
• If the body wants to minimise water loss it can change the permeability of the DCT and CT (via hormones) to actively reabsorb water. This is known as facultative reabsorption.
• After DCT, the urea in the filtrate in the collecting tubule is very concentrated (higher than in blood) and therefore diffuses out of the nephron and into blood

Question 21

Tubular Secretion

Answer 21

• Adds substances to filtrate from blood: K+, H+, drugs. It occurs through both passive and active processes at the DCT.
• Removes unwanted substances from blood such as H+ to control pH
• Acid producing foods decrease pH. By secreting H+ and NH4+ into filtrate it decreases pH of blood and results in the urine being slightly acidic

Question 22

Selective Reabsorption - More info.

Answer 22

• The filtrate that enters the proximal convoluted tubule contains both wastes and useful substances such as ions, glucose and amino acids
• These must be returned to the blood from the filtrate
• The cells lining the tubulues transport these substanes from the tubulues out into the extracellular space from there they are absorbed into the capillaries
• Transport may be passive e.g diffusion or active

Question 23

Regulation of Body Fluids 1

Answer 23

• The temperature and composition of body fluids are not allowed to fluctuate outside certain tolerance limits
• The kidneys control the volume and composition of the blood plasma (homestasis) by varying volume and composition of urine:
  1. Eliminating nitrogenous wastes
  2. Removing excess water
  3. Expelling excess salts
  4. Excreting excess hydrogen ions to maintain pH

Question 24

Regulation of Body Fluids 2 - Hormones

Answer 24

• 2 hormones are involved in this process:
• Antidiuretic hormone (ADH) from posterior pituitary gland
• Aldosterone from Adrenal Cortex (causes increase in absorption of sodium ions back into blood to create osmotic gradient so that water moves back into blood. Occurs at the ascending tract of the loop of henle)

Question 25

Regulation of Water Content by Kidneys

Answer 25

- 99% of water filtered through glomerulus is reabsorbed: 1. either through **PCT and loop of Henle by osmosis** 2. Or in **DCT and CT by facultative (active) reabsorption due to the influence of ADH** - **ADH slows water loss down** by changing the permeability of the DCT and CT to water (normally impermeable without the presence of ADH) - **high ADH concentration in blood plasma =** DCT and CT permeability to water is high. Water reabsorption increases, urine output decreases. - **low ADH concentration in blood plasma** = DCT and CT permeability to water is low. Water reabsorption decreases (water stays in tubules), urine output increases.

Question 26

Regulation of Water Content - How this works

Answer 26

- when water is lost from the body (e.g. sweating), **blood solute concentration becomes high** (high solute conc., low water conc. - results in low blood volume) - Results in **high osmotic pressure** in blood as water moves from tissue fluid into plasma (and as water leaves cells to enter tissue fluid) by osmosis - Results in cells 'shrinking' due to dehydration - **Osmoreceptors** in **hypothalamus** detect this high osmotic pressure - When osmotic pressure is high, water moves in the opposite direction

Question 27

Low water content - 2 things occur as a result of this

Answer 27

1. **ADH is released from the posterior pituitary gland** to prevent further water loss in the urine 2. **Thirst sensation develops** (due to stimulation of osmoreceptors in the **thirst centre of the hypothalamus**) which stimulates a person to have a drink of water

Question 28

Reabsorption of Sodium

Answer 28

- When in dehydrated state **blood volume is reduced** which decreases blood pressure - This is detected by **pressoreceptors** in the **renal artery** and results in the release of the hormone **RENIN** by the kidney - A series of chemical reactions then occur which signals the **adrenal cortex** to release the hormone **aldosterone** - Aldosterone acts on DCT to initiate active transport of sodium ions out of filtrate and into tissue fluid - This will cause an increase in osmotic gradient so that there is a relatively high conc. of water in filtrate relative to tissue fluid - This **aids reabsorption of water in DCT and CT by osmosis**

Question 29

Disruptions to blood fluid volume - Dehydration

Answer 29

- loss of water from body due to sweating, vomiting and diarrhoea - volume of body fluid below normal levels - water leaves intracellular fluid into ECF - Results in: severe thirst, low blood pressure, dizziness, headches

Question 30

Disruptions to blood fluid volume - Water Intoxication

Answer 30

- Body fluid levels increase beyond normal levels - Due to replacing lost water with pure water - Water moves into intracellular fluid from ECF - Results in: light headedness, headaches, vomiting, collapse