Case 3- fluid volume Flashcards
Endocrine functions of kidneys
Renin- secreted from juxtaglomerular cells, helps regulate blood pressure
Erythropoietin (EPO)- produced in the kidneys
Vitamin D metabolism- regulated by PTH (parathyroid hormone)
Erythropoietin (EPO)
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.
Vitamin D metabolism
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.
Fluid loss and intake
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.
What is fluid balance affected by
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
Cell volume control mechanisms
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.
Effects of imbalance in fluid volume within the cell
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.
Fluid balance in the blood
Fluid imbalance affects blood volume which effects blood pressure and perfusion of organs
Isotonic dehydration
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
Hypo-osmotic dehydration
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).
Hyperosmotic dehydration
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.
How do the kidneys control fluid balance
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.
Obligate urine production
300ml- the smallest volume needed to remove our metabolic waste
The roles of the DCT
Where the filtrate is fine-tuned based on the needs of the body
Function of nephron
1) Excretion of waste products and drugs
2) Regulation of body fluid volume
3) Endocrine
4) Metabolism (vitamin D)
ICF contents
High concentrations of K+ and Mg+ cations with proteins and inorganic phosphate
ECF contents
High concentration of Na+ cations and Cl- and HCO3- anions. Plasma and interstitial fluid have similar composition but plasma contains more proteins
What’s reabsorbed
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
Reabsorbing glucose
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
Transport maximum (TM)
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
Plasma threshold
The concentration of a solute in the blood where above it the excess solute will be filtered and excreted
Autoregulation
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
When can autoregulation be overridden
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
Hormones which may override autoregulation of RBF
Angiotensin 2 will restrict the efferent and afferent arterioles
Prostaglandin and nitric acid (relax)
Endothelin (contract)
Processes by which different substances are handled by the kidney
Filtration, secretion, absorption and metabolism
Renal metabolism
In metabolic acidosis the kidneys are the major site of glutamine catabolism. This generates ammonium which is excreted in urine
Renal gluconeogenesis
Renal synthesis of glucose contributes 20 to 25% of whole body glucose production
Renal secretion
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