Case 3- fluid volume

Question 1

Endocrine functions of kidneys

Answer 1

Renin- secreted from juxtaglomerular cells, helps regulate blood pressure
Erythropoietin (EPO)- produced in the kidneys
Vitamin D metabolism- regulated by PTH (parathyroid hormone)

Question 2

Erythropoietin (EPO)

Answer 2

It is produced by the peritubular interstitium and cells of the inner cortex. It acts on the erythrocyte stem cells in the bone marrow causing increased erythrocyte production. This increases the oxygen carrying capacity of the blood. Its synthesis is stimulated by hypoxia, anaemia and renal ischaemia.

Question 3

Vitamin D metabolism

Answer 3

Vitamin D is hydroxylated in the liver to form calcidiol. In the proximal tubules of the kidney it is further hydroxylated by the cytochrome P450 enzyme to form calciterol, which is a hormonally active metabolite of vitamin D. Production is regulated by PTH (parathyroid hormone), PO4 (3-) levels and negative feedback.

Question 4

Fluid loss and intake

Answer 4

Fluid in must equal fluid out. Sensible fluid loss is sweat, faeces and urination. Insensible fluid loss is transcutaneous evaporation from the skin and humidification during respiration.

Question 5

What is fluid balance affected by

Answer 5

Fluid osmolality, which is controlled by water and sodium balance. All compartments in the body have the same osmolarity as water moves freely between them

Question 6

Cell volume control mechanisms

Answer 6

Cells can gain or loose osmolytes to regulate volume, so they don’t burst. When the cell swells it loses taurine and other solutes, water will then exit as well and the regulatory volume decreases. If there is a decrease in volume the cell gains osmotically active particles like KCl and water will move into the cell causing a regulatory volume increase.

Question 7

Effects of imbalance in fluid volume within the cell

Answer 7

Can cause swelling and shrinking. In the brain this can increase intercranial pressure which may shut blood vessels causing anoxic cell death. Can compress the spinal cord, it may kill you if the brainstem is compressed. In the brain if the cells shrink the neurons may tear away from the blood vessels.

Question 8

Fluid balance in the blood

Answer 8

Fluid imbalance affects blood volume which effects blood pressure and perfusion of organs

Question 9

Isotonic dehydration

Answer 9

In isotonic dehydration there is an equal amount of fluid and solutes (NaCl) that are lost. Can be due to an external cause such as vomiting, Diarrhoea, Haemorrhage or burns. As well as an internal loss such as ileus, ascites and pleural effusion. Decrease in extracellular fluid volume as osmolarity stays the same the intracellular volume is unaffected. To treat this, you can do replacement therapy which is volume replacement with isotonic NaCl solution, you may want to give blood

Question 10

Hypo-osmotic dehydration

Answer 10

Can be used by Addison’s disease causing adrenal insufficiency, as well as diuretics and vomiting. The fluid lost contains more NaCl then water (hyperosmotic fluid). It causes renal dehydration which does not trigger thirst. Extracellular osmolarity and volume decreases. This creates an osmotic gradient so water moves from the extracellular fluid to the intracellular. Intracellular volume increases and osmolality decreases, it’s the same in both compartments. Can lead to cell swelling. Treated with slow infusion of isotonic fluid replacement (0.9% NaCl).

Question 11

Hyperosmotic dehydration

Answer 11

Caused by osmotic diuresis (diabetes mellitus), decreased ADH secretion (Diabetes insipidus), high fever, heat stroke and severe diarrhoea. The fluid lost contains more water then NaCl, so its hyperosmotic. Decreases in extracellular fluid volume, the osmolality increases. This creates an osmotic gradient, fluid moves from the intracellular to the extracelular. Intracellular volume decreases and osmolality increases. Causes problems in the brain due to the shrinkage of cells. Treated with slow water replacement with sugar, then afterwards with a hypotonic solution.

Question 12

How do the kidneys control fluid balance

Answer 12

Volume is detected by stretch receptors throughout the body. When there is a decrease in blood volume the kidneys release renin. Part of RAAS (renin-angiotensin-aldosterone system) to maintain sodium balance. It is also detected by osmoreceptors in our hypothalamus, leading to a feeling of thirst and ADH is released by the posterior pituertory gland, it causes a decrease in in urine excretion and will cause vasoconstriction.

Question 13

Obligate urine production

Answer 13

300ml- the smallest volume needed to remove our metabolic waste

Question 14

The roles of the DCT

Answer 14

Where the filtrate is fine-tuned based on the needs of the body

Question 15

Function of nephron

Answer 15

1) Excretion of waste products and drugs
2) Regulation of body fluid volume
3) Endocrine
4) Metabolism (vitamin D)

Question 16

ICF contents

Answer 16

High concentrations of K+ and Mg+ cations with proteins and inorganic phosphate

Question 17

ECF contents

Answer 17

High concentration of Na+ cations and Cl- and HCO3- anions. Plasma and interstitial fluid have similar composition but plasma contains more proteins

Question 18

What’s reabsorbed

Answer 18

Water and electrolytes as well as 50% of urea to help maintain a concentration gradient for absorbing water. Only 10% of phosphate is excreted from the body

Question 19

Reabsorbing glucose

Answer 19

Most of this is done in the early proximal tubule via SGLT 2 and GLUT 2 transporters. The remaining glucose is reabsorbed by SGLT1 and GLUT1 in the late proximal tubule. No glucose should be excreted in the urine

Question 20

Transport maximum (TM)

Answer 20

The point at which an increased concentration of a substance does not result in an increased movement of that substance across the cell membrane. The absorbative and secretory pathways are saturated. When the concentration of a substance is below Tm, movement of the substance is not limited by transport capacity. If the solution is above tm it will be excreted

Question 21

Plasma threshold

Answer 21

The concentration of a solute in the blood where above it the excess solute will be filtered and excreted

Question 22

Autoregulation

Answer 22

Dynamic control of RBF and GFR, fairly constant when arterial pressure varies between 100-200 mmHg (normal). Autoregulation is largely due to changing arteriolar resistance. Done through myogenic control, tubuloglomerular feedback (macula densa) and the release of renin

Question 23

When can autoregulation be overridden

Answer 23

When there are profound changes to RBF during physiological and pathological events. Such as extreme exercise, haemorrhage, trauma and shock. This will then activate sympathetic nerves which will mediate the effects. Hormones may also override autoregulation of RBF. Nephrotic and nephrotic syndrome will increase GFR

Question 24

Hormones which may override autoregulation of RBF

Answer 24

Angiotensin 2 will restrict the efferent and afferent arterioles
Prostaglandin and nitric acid (relax)
Endothelin (contract)

Question 25

Processes by which different substances are handled by the kidney

Answer 25

Filtration, secretion, absorption and metabolism

Question 26

Renal metabolism

Answer 26

In metabolic acidosis the kidneys are the major site of glutamine catabolism. This generates ammonium which is excreted in urine

Question 27

Renal gluconeogenesis

Answer 27

Renal synthesis of glucose contributes 20 to 25% of whole body glucose production

Question 28

Renal secretion

Answer 28

Increased elimination of drugs and toxins that bind to plasma proteins. Elimination of metabolic by-products. Delivery of autacoids and drugs to the distal nephron