Kidney Flashcards
Main functions of the nephron
The main functions of the nephron include glomerular filtration, tubular reabsorption and tubular excretion.
Glomerular filtration
aka ultrafiltration
blood in afferent arteriole enters glomerulus under pressure because the efferent arteriole is narrower.
Small substances including water filtered through capillary walls with pores and tiny slits through podocytes into Bowman’s capsule as glomerular filtrate. Large substances e.g. blood cells and proteins remain in glomerulus.
Formation of this filtrate is not selective and many useful substances (e.g. glucose) are found in the filtrate together with waste.
Tubular Reabsorption
As glomerular filtrate passes proximal convoluted tubule, all useful substances (e.g. glucose, amino acids, glycerol and salts) are actively reabsorbed into peritubular capillaries (capillaries surrounding the convoluted tubules). Most water follows these substances by osmosis. However, there is still lots of water in the filtrate (it is very dilute).
Reabsorption of water and the loop of Henle
Filtrate entering the loop of Henle is still considered dilute with too much water. The loop of Henle ensures medulla tissue has a high sodium ion concentration and is therefore hypertonic (has a low water potential). The descending limb of the loop is permeable to water and water moves out by osmosis into the blood (remember: water follows salt). Sodium ions are actively pumped out of the ascending limb into the tissue fluid of the medulla to ensure that the medulla stays salty. Chloride ions follow passively. Water does not follow the sodium ions at this stage as the ascending limb is impermeable to water. Sodium ions slowly get transported in the medulla back to the peritubular network to prevent the concentration becoming too high.
Tubular Excretion
Cells in the tubules don’t only reabsorb, they also excrete wastes (e.g. creatinine, drugs, hydrogen and potassium ions, colourants, preservatives and other foreign substances)
These waste substances move from the peritubular capillaries into the proximal and distal convoluted tubules
This is active transport and requires energy
Osmoregulation
Osmoregulation is ensured by the hormone called antidiuretic hormone (ADH) secreted by the hypothalamus and posterior pituitary (hypophysis). Levels of ADH rise when the body is growing dehydrated, triggering the kidneys to retain more water during filtration. When you drink a lot of extra water, on the other hand, ADH levels drop, allowing the kidneys to flush out the fluid the body doesn’t need.
Low water content
Low water content in the blood (dehydration) is detected by osmoreceptors in the hypothalamus. This stimulates the thirst centre in the brain and more fluids are taken in. At the same time, the hypophysis is stimulated to secrete antidiuretic hormone (ADH). More ADH in the blood reaches the kidneys. ADH causes the walls of the convoluted tubules and collecting ducts to become more permeable to water and more water is reabsorbed from the filtrate. Water content in the blood rises and the urine formed is more concentrated.
High water content
High water content in the blood (over-hydration) means that the osmoreceptors in the hypothalamus are not stimulated. The hypophysis secretes less ADH. Less ADH travels in the blood to the kidneys. Because less ADH reaches the kidneys, the walls of the convoluted tubules and collecting ducts become less permeable to water and less water is reabsorbed from the filtrate. Water content in the blood decreases and the urine formed is less concentrated.
Salt regulation
Salt regulation is ensured by the hormone aldosterone secreted by the adrenal glands. The adrenal glands are found just above each kidney.
Low salt content
When sodium levels are low, aldosterone is released into the blood stream from the adrenal cortex. More aldosterone travels to the kidneys and causes more sodium ions to be reabsorbed from the distal convoluted tubules and collecting ducts into the blood stream. More sodium is retained in the body and less is excreted in urine.
High salt content
When sodium levels are high, less aldosterone is secreted from the adrenal cortex. Less aldosterone travels to the kidneys and this causes less sodium ions to be reabsorbed into the blood at the distal convoluted tubules and collecting ducts. Less sodium is retained in the body and more is excreted in urine.
Kidney stones
When wastes in urine do not dissolve, solid crystals can form. Small kidney stones (<4,5 mm) can leave the body with urine. Larger kidney stones (>4,5 mm) can cause blockages of the ureter causing serious pain, called renal colic. Blood in urine also can occur due to damage to the epithelium of urinary tubes. A persons risk of developing kidney stones if they are dehydrated. Kidney stones can also develop from an imbalanced diet e.g. a diet with lots of animal protein.
Removal by surgery or ultrasound can be used to shatter smaller stones so they can be passed more easily.
Bilharzia
Bilharzia is a disease that is caused by the Schistosoma sp. parasite. The parasite infects two hosts, namely the human and the fresh water snail, to complete its life cycle. Bilharzia affects the general health of the infected person. The most harmful effects are bleeding and the formation of ulcers in the gut or bladder. Long term effects include liver damage, renal failure and cancer of the bladder.
Bilharzia is a common occurrence in areas with poor sanitation. People are infected by walking, swimming or playing in contaminated water, or by drinking it.
Renal failure
Ailments that interrupt kidney function can be life threatening. Among the causes of renal failure are high blood pressure, which can irreparably damage the extremely delicate nephrons. Many drugs or medicines also can cause renal failure.
An example cause of kidney failure due to overdose:
Taking painkillers E.g. paracetamol and ibuprofen everyday can cause analgesic nephropathy
Damage builds up over time leading to irreversible kidney failure
Important to remain well hydrated when taking any medication and follow dosage instructions
