Session 1 Flashcards
Why is a stable internal environment and how does the urinary system contibute to this?
- Function in all parts of the body depends on maintenance of a stable internal environment.
- Urinary system is a major contributor to the maintenance of a stable internal environment – controls concentration of a wide range of ions and small organic molecules through filtration and selective reabsorption
- The kidneys must control volume and osmolarity
What are the functions of the kidney?
- Regulation: control the concentrations of key substance in extracellular fluid
- Excretion: excretes waste products
- Endocrine: synthesis of renin, erythropoietin, prostaglandins
- Metabolism: active form of Vitamin D, catabolism of insulin, PTH calcitonin
- Helps to control pH
Kidneys are ultimately responsible for controlling the composition of ECF
What is the body fluid distribution in the average 70kg medical student?
•Average 70kg medical student contains ~40L of water
•Intracellular fluid (ICF) or cytosol (~25L)
•Extracellular fluid (ECF) (~15L)
-Interstitial fluid (~12L)
-~Intravascular (Plasma) (~3L)
-Lymph
-Transcellular synovial, intestinal, cerebrospinal fluid, sweat, urine and pleural, peritoneal, pericardial and intraocular fluids
Explain about water and osmotic force, defining osmolarity and osmolality
- Movement is driven by osmotic forces
- Osmolality: solute per kg of solvent
- Osmolarity: number of osmoles of solute per litre
- In the human body we can consider them effectively the functionally same but they are technically different
- Oncotic is osmotic force due to the presence of proteins – encouraging water to move towards it.
- Osmotic force is due to the presence of ions.
- Osmolarity includes all ions and many organic molecules
- Measured in milli-osmoles.
Describe how changes in osmolarity affect water movement
- If ECF Osmolarity is high, water moves out of cells
- IF ECF Osmolarity is low, water moves into cells
- Failure to control Osmolarity of ECF will seriously damage cells.
Why are electrolytes so important?
- ICF: high K+, low Na+, many large organic anions
- ECF: low K+, high Na+, main anion Cl- and HCO3-
- Active transport – particularly via sodium pumps – maintain the different compositions
How do kidneys affect body fluid composition?
- Direct effect on ECF
- Indirect effects on ICF (via semi-permeable membrane)
- Failure to control ECF volume leads to changes in BP, tissue fluid and cell function
- Failure to control ECF osmolarity –> cells shrink or swell
- Variable ingestion and loss of salts and water disturbs composition of ECF and hence ICF
- Dynamic organ – has to be able to respond to minute rapid changes throughout the day in order to maintain a stable envirionment
Describe the Acid Base Balance and how kidneys help to maintain it
- pH of ECF is critical and it depends in part on the concentration of bicarbonate in plasma
- Failure to control bicarbonate concentration will have serious consequences
- Kidney helps to control pH
- Kidney filters a large amount of ECF – rate 180L/day (every L filtered over ten times a day).
- Ultra-filtrate – water, ions, all small molecules
- Large organic molecules such as albumin and cells (RBCs, WBCs) are not filtered
- Recovers nearly everything – average 1.5L leaves the body every day as urine.
- If dehydrated, <1.5L loss
- If drinking too much, >1.5L loss
What is a Nephron?
- Each kidney has 1.5million nephrons (functional unit: secretory unit – renal corpuscle (produces filtrate of blood plasma) - + tubule)
- Renal Corpuscle = Glomerulus + Bowman’s Capsule
- Glomerulus (filter unit) is connected to a long tube which allows reabsorption and secretion
- NB: PCT is in cortex, Loop of Henle dips in and out of medulla, DCT is in cortex, Collecting duct passes through medulla to pelvis
What is the Glomerulus?
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•The renal corpuscle consists of a ball of capillaries, called the glomerulus, invaginated into the start of the nephron, the Bowman’s capsule.
•Plasma is filtered through the glomerular capillary wall into the Bowman’s capsule (cup)
•Composition of the plasma ultrafiltrate that enters the Bowman’s capsule depends on the filtration barrier which has three layers:
1.Endothelial cells of the glomerular capillary (thin and flat with numerous fenestrae – pores – allowing plasma components to cross the wall but not blood cells or platelets)
2.Basement membrane – continuous layer of connective tissue and glycoproteins. Non-cellular structure that prevents any large molecules from being filtered.
3.Epithelial lining of Bowman’s capsule consists of a single layer of cells, podocytes (visceral epithelial cells) which have foot processes, pedicels, that wrap around glomerular capillaries and interdigitate with pedicels/projections of adjacent podocytes forming filtration slits.
The podocytes have a well-developed Golgi apparatus, used to produce and maintain the glomerular basement membrane.
NB: the outer “parietal” layer epithelium of the renal corpuscle is Bowman’s capsule
The inner “visceral” epithelium is comprised of podocytes.
Bowman’s space (urinary space) is the space within Bowman’s capsule surrounding the loops and lobules of the glomerulus. This is the space into which the glomerular plasma filtrate collects as it leaves the capilaries.
Bowman’s capsule is the outer epithelium which encloses Bowman’s space.
Bowman’s capsule is simple squamous, becoming cuboidal at the proximal tubule
Glomerulus is always in the cortex
What is epithelium and how should it be considered in the context of kidneys?
- Continuous sheet of cells covering exposed surfaces and lining internal cavities (digestive, reproductive, urinary and respiration)
- Continuous with outside environment (i.e. outside the body) (once plasma has passed been filtered by the glomerulus and is inside nephron tubule it is considered outside the body)
- Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the filtrate
- Kidneys reabsorb across epithelium back into body, secrete across epithelium into cavity i.e. outside the body
Discuss reabsorption and secretion in a normal person
In a person with normal water and electrolyte balance:
- Over 99% of filtered water is recovered
- Over 99% of filtered sodium and chloride ions recovered
- 100% of bicarbonate recovered
- 100% of glucose and amino acids recovered
- Just a few waste products not covered
- Some substances (e.g. H+, actively secreted, so lose more than filtered)
What is the kidney’s metabolic activity?
- 2nd most energy-demanding organ after the brain – extremely active
- Needs blood flow of 4ml/g/min
- Requires 25% of cardiac output at rest – almost a quarter of the food we eat goes to keep the kidneys working
- Very susceptible to low blood flow
What is the Glomerular Filtration Rate?
•Changes in hydrostatic pressure (due to water) and colloid osmotic (oncotic) pressure (due to protein) alter the GFR
•GFR is the amount of filtrate that is produced from the blood flowing through the glomerulus per unit time. Glomerulus only allows low-molecular-weight substances in plasma. Passive process
•The surface area of the glomerular capillaries is much larger than that of normal capillary beds so there is less resistance to flow. The hydrostatic pressures falls less among the length of the glomerular capillary than normal capillary because the efferent arteries act as secondary resistance vessels, maintain a constant pressure along the entire length of the glomerular capillary.
•Specialised afferent and efferent arterioles maintain a relatively high constant filtration pressure
•Fluid is reabsorbed into the peritubular capillaries as a result of high colloid oncotic pressure and low hystrostatic pressure. This reabsorption causes a fall in colloid oncotic pressure as plasma proteins become diluted.
•GFR on average is 125ml/min or 180L/day
GFR important measure – gives indication of kidney funct
What is the Vascular Pole and Urinary Pole?
•Vascular pole – where blood supply comes into glomerulus (via afferent arterioles)
•Urinary pole (beginning of proximal convoluted tubule)
Efferent arterioles take blood away from the glomerulus – ( only ~20% of the blood passing through is filtered at any one time)
Describe the cellular mechanisms of reabsorption
•Tubules lined with epithelial cells which are POLARISED
-Different membrane properties on luminal (facing the nephron tubule) and basolateral membranes due to different transporters on both sides held in place – not fluid
-Allows transport across epithelium
• Na+-ATPase pump is the driving force – extrudes sodium across basolateral membrane.
1.Sodium enters across luminal membrane down concentration gradient
2.Energy from sodium movement drives reabsorption of other substances such as glucose
3.Water follows electrolytes osmotically
•ROMK allows K+ to leak back therefore not affecting activity of sodium pump
Describe Intravenous Urography and Intravenous Pyelography
- IVU and IVP involve serial radiographs taken after intravenous injection of radio-opaque contrast medium
- Normal kidney function is required
- An IVU can assess kidney size and shape as well as anatomy and patency of the calyces, pelvis and ureters. Also used to localize fistulae and highlight filling defects in the bladder.
Describe Renal Arteriography
- Uses contrast medium to demonstrate the anatomy of the renal arteries
- Can detect renal artery stenosis (also can be detected by MRI) or aneurysms
- Therapeutic angioplasty may be performed at the same time
- Can be used in the diagnosis of tumours but this is becoming less common with the increasing use of CT
- A catheter is introduced into the femoral artery, through which contrast is injected into the renal artery and a series of radiographs are taken
Explain about Retrograde and Antegrade Pyelography
- R pyelography (flow of contrast from bladder to kidney) is used to define the site of an obstruction or lesions within the ureter – does not require functioning kidneys. May be used therapeutically to help dislodge ureteric stones and coax them down the ureter
- A pyelograpy (flow of contrast directly injected into kidney) is used to define the site of obstruction in the upper urinary tract (kidneys and ureters)
