Kidneys and fluid homeostasis Flashcards
Internal Anatomy of Kidneys
Two main regions: an outer light‐red region the renal cortex and an inner, darker red‐brown region the renal medulla. Within renal medulla are several cone‐shaped renal pyramids.
* Urine formed in kidney passes from thousands of papillary ducts within renal pyramids into cuplike structures called minor calyces. From these structures, urine flows into 2–3 major calyces and then into a single large cavity called the renal pelvis. The renal pelvis drains urine into ureter.
The nephron
A tubule closed at one end that opens into a collecting duct at the other
Comprises:
* Cup shaped glomerular capsule
(Bowman’s capsule) that surrounds the tufts of capillaries (glomerulus)
* The proximal convoluted capsule
* The medullary loop (of Henle)
* The distal convoluted tubule
leading to the collecting duct Total 3 cm in length
Glomerulus & capsule
Walls of capillaries and capsule very thin & permeable
Rest of nephron is single cell cuboidal
Blood vessels to kidney controlled by sympathetic and parasympathetic nerves
3 phases of urine production: Filtration
Filtrate similar to plasma but no proteins or cells are present
Normal glomerular filtration rate of 125 ml/min= 180 L/day
Only 1.5 Litre eventually excreted
3 phases of urine production: Selective reabsorption
Most reabsorption occurs in proximal convoluted tubule: Water
Electrolytes (Na/K/Ca) Glucose-actively transported Amino acids
Little of the nitrogenous waste products are reabsorbed (urea, creatinine)
Distal convoluted tubule
Where “fine tuning” of filtrate occurs and where many hormones act by altering selective reabsorption
* Calcium- Parathormone and calcitonin change amount of calcium reabsorbed
* Water reabsorption
* Anti-diuretic hormone (posterior pituitary senses osmolality). ADH
increases reabsorption of water
* Aldosterone- raised aldosterone causes increased sodium and water reabsorption
* Atrial natriuretic peptide- in atria of heart. When blood volume increases atria stretched and ANP released. ANP causes reduction in sodium and water reabsorption
3 phases of urine production: Tubular secretion
Some chemicals are too big to be filtered but are actively secreted into urine
* Secreted substances include hydrogen ions (H+), K+, ammonia (NH3), urea, creatinine (a waste from creatine in muscle cells), and some drugs e.g., penicillin.
* Tubular secretion of H+ ions important in maintaining acid-base balance
Sodium & potassium balance
Situation when there is too much Na+
Note that Na+ & K+ conservation tend to go in opposite direction i.e., when Na+ being saved more K+ is lost
Whereas Na+ and water tend to do things in the same direction
Opposite drivers of Na+ & K+ excretion
Aldosterone release has opposite effects on Na+ & K+
Control of blood pH
Normal blood pH is 7.35 and 7.45.
* Remember lower pH means more acidic (more H+).
* If body production of H+ changes, short term buffering by plasma protein. Main ones are Haemaglobin in red cells and albumin in plasma
* These contain amino acids which have amino groups (-NH2) and carboxyl groups (-COOH). If XS H+ in blood, -NH2 combines with the H+ to make NH3+ which raises pH.
* If too little H+, -COOH releases its H+ (to make COO- and H+ ) which lowers pH
pH balance
Buffering by proteins is only a short-term solution. * Need to remove excess H+
Two ways to do this-
A) Respiration
* Rate of breathing to affect CO2
* If reduce breathing CO2 goes up and causes more H+ in blood * Increase breathing CO2 and H+ go down
pH balance
B) Change in amount of H+ excreted in urine
* Proximal convoluted tubules secrete H+ into filtrate
* Combines with ammonia, bicarbonate and phosphate in filtrate so pH change in urine doesn’t become too extreme.
Diuretics
Diuretics slow reabsorption of water by kidneys and thereby cause diuresis; an elevated urine flow rate.
* Naturally occurring diuretics include caffeine in coffee, tea, and cola, which inhibits Na+ reabsorption, and alcohol in beer, wine, and mixed drinks, which inhibits secretion of ADH.
* In diabetes insipidus, ADH secretion is inadequate, and a person may excrete up to 20 liters of very dilute urine daily.
* Diuretic medications are commonly used in patients with heart failure to remove excess fluid and sodium from the body.
When things go wrong
Medical terms:
* Oliguria too little urine
* Polyuria too much urine
* Anuria No urine
* Glucosuria-glucose in urine
* Ketonuria- ketones in urine
* Nocturia- passing urine at night
* Incontinence-involuntary loss of urine
Tests of renal function
Blood levels of K+. Urea, creatinine
* Urine dipstix for red and white cells &
protein
* GFR collect urine for 24 h and blood samples. From this can calculate what GFR value is.
* X-ray –intravenous urography. Dye excreted in urine is opaque in X-rays
* Ultrasound
