Renal physiology is the study of the physiology of the kidney, specifically at the level of the nephron, which is the smallest functional unit of the kidney where blood entering the kidney goes through the process of filtration.
Overview of Renal Physiology
3 Important Functions of the Nephrons:
- control blood concentration and volume through selected removal of water and solutes
- regulate blood pH
- remove toxic waste from the blood
Through these functions, materials from the blood is removed, others which are required are returned, and the remaining unneeded material is excreted collectively as urine. In other words, urine formation requires Glomerular Filtration, Tubular Reabsorption and Tubular Secretion.
Renal Physiology – Glomerular Filtration
Glomerular Filtration occurs in the renal corpuscle of the kidneys across the endohelial-capsular membrane, which results in the fluid called glomerular filtrate ( 150 lt in adult females; 180 lt in adult males). The blood plasma in the afferent arterioles which become the glomerular filtrate is called the filtration fraction.
Understanding the Glomerular Filtration Process
Pressures Causing Filtration & GFR Regulation
The Glomerular Filtration Rate (GFR) is the amount of filtrate formed in all the renal corpuscles of both kidneys per minute (rate in males = 125ml/min; rate in females = 105ml/min). Maintenance of a constant GFR ensures that useful substances are not lost.
A change in GFR indicates a change in the net filtration pressure – if the glomerular blood hydrostatic pressure (GBHP) falls to 45mmHg, the filtration process is halted as the opposing pressures equal to 45mmHg.
There are 3 mechanisms which control GFR by adjusting the blood flow into and out of the glomerulus, and by altering the glomerular capillary surface area for the process of filtration:
- Renal Autoregulation: Through the Myogenic Mechanism, stretching triggers contraction of smooth muscle cells in the afferent arteriole wall, causing normalisation of the renal blood flow and GFR within seconds following a change in blood pressure. Additionally, through the Tubuloglomerular Feedback, increased distal tubular sodium chloride concentration causes the release of adenosine, leading to a series of events that help regulate the GFR.
- Neural Regulation: The Sympathetic Nervous System, which supplies the renal blood vessels, is responsible for the release of norepinephrine. Through Moderate Sympathetic Stimulation, norepinephrine activates the a1 (alpha-1 adrenergic) receptor in the afferent and efferent arterioles, causing vasoconstriction, causing blood flow restriction, leading to a slight GFR decrease. In Greater Sympathetic Stimulation, vasoconstriction of the afferent arterioles predominates; blood flow in the glomerular capillaries is decreased, leading to a decrease in GFR.
- Hormonal Regulation: Angiotensin II, which is a vasoconstrictor, acts on the afferent and the efferent arterioles, reducing blood flow leading to a decrease in the GFR. Additionally, Atrial Natriuretic Peptide (ANP) is released through the stretching of the atrial walls when there is an increase in blood volume, leading to an increase in capillary surface area, causing an increase in the GFR.
Renal Physiology – Tubular Reabsorption
In the average adult, the Glomerular Filtration Rate (GFR) is approximately 125ml/min, meaning that around 180 litres are filtered in one day. However, only around 1ltr a day is excreted as urine by the body. This happens because throughout the filtration process, around 99% of the filtrate is reabsorbed back into the blood in what is called tubular reabsorption.
In tubular reabsorption, the proximal convoluted tubule cells process and reabsorb over 80% of the glomerular filtrate, whilst other parts of the nephron ensure homeostasis by controlling excretion amounts of electrolytes, water and hydrogen ions. Through tubular reabsorption, the following are reabsorbed back into the blood stream:
- Sodium
- Potassium
- Calcium
- Chloride
- Bicarbonate
- Phosphate
Peptides and small proteins are also reabsorbed through pinocytosis.
Substances completely reabsorbed from the filtrate are:
- Water
- Proteins
- Chloride
- Sodium
- Bicarbonate
- Glucose
- Potassium
Urea and Uric Acid are partially reabsorbed from the filtrate.
Renal Physiology – Tubular Secretion
Tubular Secretion, which occurs in the proximal and distal tubules as well as in the collecting dugt, removes certain materials from the body such as Potassium ions, Hydrogen ions, Ammonium ions, Creatinine, and drugs.; it also helps control the blood’s pH.
Renin-Angiotensin-aldosterone System (RAAS)
Aldosterone causes increased sodium and water reabsorption from the distal tubule and collecting ducts, leading to an increase in the extracellular fluid volume. This allows the restoration of the blood pressure to its normal state. Additionally, Aldosterone has an affect on the secretion of potassium by the distal convoluted tubule and collecting duct.
Antidiuretic Hormone
The AntiDiuretic Hormone, which is produced by the hypothalamus, controls the concentration of the urine to be excreted.
When the blood-water concentration is low, ADH is released, which increases the permeability of the plasma membranes of the cells of the distal tubules and the collecting ducts. Increased permeability causes more water molecules to pass into the cells, and then into the blood.
With no ADH, the ducts become impermeable to water, causing water to be expelled into urine.
Atrial Natriuretic Peptide
Increased blood volume causes the atrial walls to stretch, leading to the release of the Atrial Natriuretic Peptide (ANP). ANP inhibits the reabsorption of sodium and water in the proximal convoluted tubule and collecting duct, suppresses the secretion of aldosterone, and suppresses the secretion of ADH. This results in increased excretion of sodium ions (natriuresis) and increased urine output (diuresis), leading to a decrease in the blood volume and blood pressure.
Note…
Tubular Reabsorption REMOVES substances from the filtrate into the blood… Tubular Secretion ADDS materials to the filtrate from the blood.
Renal Physiology – Solute Reabsorption
Solute Reabsorption happens within the ascending limb of the loop of henle.
Summary…
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