Urinary: Introduction and Overview of Urinary Tract

Introduction

Kidney - organ of excretion and body fluid regulation

• Produces urine dependent on the condition of the body
• Regulation of EC fluid - osmolarity/ plasma blood volume
• Production of hormones eg. renin (regulates blood volume)

In the body - 14L Interstitial fluid

28L Intracellular fluid

H2O is lost and gained from the body in a variety of ways. 

However, this can vary in a number of ways and the kidney must adapt to support this,

H2O intake: type of food, water availability

H2O output: lactation, exercise, environmental conditions, disease

The most common causes of fluid loss are:

• Sweating
• Diarrhoea
• Vomiting
• Hyperventilation
• Fever

Hyponatremia: low Na+ therefore plasma is too dilute and H2O moves into cells causing them to swell. Leads to a person becoming irrational/comatose 

Low K+: cardiac arrhythmia

Low Ca2+: Fatigue/weakened + can soften bones/rickets/osteomalacia  

Low Mg2+: Causes muscles to twitch uncomfortably 

Dehydration

<30ml/hr = abnormal urine output. Patients are dizzy due to fall in volume therefore fall in BP

Clinical Signs

• Mucus membranes - red and moist
• Capillary refill - red to white within 2 secs
• Skin turgor -skin remains elevated when pinched 
• Extreme thirst
• Headache
• Rapid heart rate 

Overview of Urinary Tract

Parts of the kidney 

• Cortex - granular due to tubules
• Medulla - striated
• Medullary pyramids - ends in the calyx where urine drains into the renal pelvis
• Lobe - MP with overlaying cortical tissue that regulates the composition of plasma 

Production of Urine

The kidney filters the blood plasma and receives a massive blood supply ie. 20% Q. 

The blood supply to the kidney is as follows:

1. Renal artery
2. Interlobar artery 
3. Arcuate artery
4. Interlobular artery

2 types of nephrons

Cortical: 80% of nephrons. Have short LOH (subscapular) therefore don't contribute to the osmotic gradient

Juxtamedullary: 20%.. Increases salt in interstitum to all a steeper gradient to reabsorb H2O. 

Differences

• Glomeruli of CN are close to the cortex vs JM are closer to cortical medullary border
• Proximal tubules of cortical nephrons are shorter
• LOH are shorter therefore blood supply from the vasa recta is poor in cortical nephrons

Renal Corpuscle

Made up of glomerulus and BC. 

To ensure that the blood is filtered properly there are 3 barriers:

1. Endothelium of capillaries - spaced apart. These spaces = fenestrations
2. Basement membrane - glycoprotein matrix which is -vely charged 
3. Podocytes - foot like projections that can join therefore forms small gaps that allow molecules to pass through = filtration slits 

Juxtaglomerular apparatus + Renin

Renin causes the release of aldosterone.

Macula densa cells - DCT is bent so that it's close to the starting point of the glomerulus and this is where the MD cells are found. 

When BP is low - flow rate is low or Na+ is decreased then renin will be secreted from the macula densa cells (innervated by S. nervous system). 
Cells in the afferent arteriole that makes the connection with the macula densa cells will lose actin/myosin. Smooth muscle replaced by granules that release renin. 

Erythropoietin

Receives 20% Q therefore regulates blood O2 levels, volume and pressure.

Increased EPO production triggered by:

1.  Reduced O2 delivery to renal cortex
2. CO poisoning 
3. Altitude
4. Respiratory disease
5. Anaemia 
6. Haemorrhage 

Reduced O2 delivery to the renal cortex is picked by cells in the interstitum of the kidney and signals for increase in production of EPO  which increases no. of RBC.