Guyton Renal Tubular Filtration Essay Sample

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Multiple Fxns of Kidneys in Homeostasis
1. Excretion of metabolic waste products and foreign chemicals
2. Regulation of Water and electrolyte balance
3. Regulation of Arterial pressure
–long term: excreting variable amounts of sodium and water
–short term: secreting vasoactive factors or substances such as rennin

4. Regulation of Acid-Base balance
–the only means of eliminating sulfuric acid and phosphoric acid
5. Regulation of Erythrocyte Production
–secretion of erythropoietin
6. Regulation of 1, 25-dihydroxyvitamin D3 (calcitriol) production
7. Gluconeogenesis
–synthesis of glucose from amino acids during prolonged fasting

Renal Blood Supply
=blood flow to the two kidneys is normally about 22% of the cardiac output or 1100 ml/min =renal circulation has two capillary beds: glomerular and peritubular capillaries — arranged in series and separated by the efferent arterioles (help regulate the hyrostatic pressure in both capillaries) =glomerular capillaries have high hydrostatic pressure (60 mmHg) which causes rapid fluid filtration =peritubular capillaries have lower hydrostatic pressure (13 mmHg) which permits rapid fluid reabsorption =kidneys can regulate the hydrostatic pressure in both capillaries by: a) adjusting the resistance of the afferent and efferent arterioles b) changing the rate of glomerular filtration, tubular reabsorption, or both in response to body homeostatic demands

The Nephron is the Functional Unit of the Kidney
=each kidney contains 800,000 to 1 million nephrons
=the kidney cannot regenerate new nephrons
=after age 40, the number of functioning nephrons decreases about 10% every 10 years

=process by which the urinary bladder empties when it becomes filled =involves two main steps:
1) bladder fills progressively until the tension in its walls rises above a threshold level; this elicits 2) 2) a nervous reflex called micturition reflex that empties the bladder, or, if this fail, at least causes a conscious desire to urinate

Micturition Reflex
= “self-regenerative” – initial contraction of the bladder — activates the stretch receptors — causes a greater increase in sensory impulses from the bladder and posterior urethra — which causes a further increase in reflex contraction of the bladder –cycle is repeated again and again until the bladder has reached a strong degree of contraction –after a few seconds to more than a minute, the self regenerative reflex begins to fatigue and the regenerative cycle of the micturition reflex ceases, permitting the bladder to relax =a single complete cycle of:

1) progressive and rapid increase of pressure
2) a period of sustained pressure
3) return of the pressure to the basal tone of the bladder =an autonomic spinal cord reflex, but can also be inhibited or facilitated by centers in the cerebral cortex or brain stem 1) strong facilitative and inhibitory centers in the brain stem, located mainly in the pons 2) inhibitory but can become excitatory centers in the cerebral cortex =final control of micturition by the higher centers

1) higher centers keep the micturition reflex partially inhibited, except when micturition is desired 2) higher centers can prevent micturition, even if the micturition refelex occurs, by continual tonic contraction of the external bladder sphincter until a convenient time presents itself 3) when it is time to urinate, the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur =Voluntary urination: contraction of abdominal muscles– > increases the pressure in the bladder — > allows extra urine to enter the bladder neck and posterior urethra — > stretching their walls — > stimulates the stretch receptors which excites the micturition reflex and simultaneously inhibits the external urethral sphincter.

Urine formation results from Glomerular Filtration, Tubular Reabsorption, and Tubular Secretion =Urinary excretion rate = Filtration rate – Reabsorption rate + Secretion rate =Renal handling of four hypothetical substances:

Panel A – substance is freely filtered by the glomerular capillaries but is neither reabsorbed nor secreted; ex: creatinine, inulin –excretion rate = filtration rate
Panel B – substance is freely filtered but is also partly reabsorbed from the tubules back into the blood; ex: electrolytes –excretion rate = GFR – reabsorption rate
Panel C – substance is freely filtered but is not excreted into the urine because all the filtered substance is reabsorbed from the tubules back into the blood; ex: amino acids, glucose Panel D – substance is freely filtered , not reabsorbed, but additional quantities are secreted from the peritubular capillaries into the renal tubules; ex: drugs, organic acids and bases –excretion rate = GFR + tubular secretion rate

Glomerular Filtration – The First Step in Urine Formation
Composition of the Glomerular filtrate
*protein-free, devoid of cellular elements, including RBC
*most salts and organic molecules are similar to the concentrations in the plasma
*low molecular wt substances, such as calcium and fatty acids, are not freely filtered because they are partially bound to the plasma proteins
Glomerular Capillary Membrane
*Filterability of Solutes is Inversely Related to their size
=glomerular filtration barrier is selective in determining which molecules will filter, based on their size and electrical charge
=filterability of 1.0 means that the substance is freely filtered as water (sodium, glucose and inulin); as molecular wt icreases,
such as that of myoglobin and albumin, filterability decreases

=negatively charged large molecules are filtered less easily than positively charged molecules of equal molecular size (negative charges of the basement membrane and the podocytes provide an impt means for restricting large negatively charged molecules)

Determination of GFR
1) sum of the hydrostatic and colloid osmotic forces across the glomerular membrane, which gives the net filtration pressure 2) glomerular capillary filtration coefficient, Kf
= GFR = Kf x Net filtration pressure
= GFR is about 125 ml/min or 180 L/day
=Fraction of renal plasma flow (filtration fraction) averages about 0.2; 20% of the plasma flowing through the kidney is filtered through the glomerular capillaries;
=Filtration fraction = GFR/Renal plasma flow
=Net filtration pressure (10 mm Hg) = Glomerular hydrostatic pressure – Bowman’s capsule pressure – Glomerular oncotic pressure =Forces Favoring Filtration:
**Glomerular hydrostatic pressure (60 mmHg)
–determined by 3 variables: 1) arterial pressure
2) afferent arteriolar resistance
3) efferent arteriolar resistance
**Bowman’s capsule colloid osmotic pressure (0)
=Forces Opposing Filtration:
**Bowman’s capsule hydrostatic pressure (18 mmHg)
**Glomerular capillary colloid osmotic pressure (32 mmHg)
–influenced by 2 factors: 1) arterial plasma colloid osmotic pressure
2) fraction of plasma filtered by the glomerular capillaries

Renal Blood Flow
= kidneys consume oxygen at twice the rate of the brain but have almost seven times the blood flow of the brain
=determined by the pressure gradient across the renal vasculature :
(renal artery pressure-renal vein pressure) / total renal vascular resistance
=blood flow in the vasa recta of the renal medulla is very low compared with the flow in the renal cortex – impt in forming concentrated urine

Physiologic Control of Glomerular Filtration and Renal Blood Flow
Sympathetic Nervous system Activation decreases GFR
–constricts the renal arterioles and decrease renal blood flow and GFR
Hormonal and Autacoid Control
Hormone / Autacoid| GFR|
Norepinephrine| |
Epinephrine| |
Endothelin| |
Angiotensin II| (prevents )|
Endotehelial-derived NO| |
Prostaglandins and Bradykinins| |

Autoregulation of GFR and Renal Blood Flow
1) Glomerulotubular balance
= increase tubular reabsorption when GFR rises
2)Tubuloglomerular Feedback
=links changes in sodium chloride concentrarion at the macula densa with the control of renal arteriolar resistance
=ensures a relatively constant delivery of sodium chloride to the distal tubule and helps prevent spurious fluctuations in renal excretion
=decreased macula densa sodium chloride causes : 1) dilation of afferent arterioles (decreasing resistance to blood flow) and 2) increased renin release 3) Myogenic Autoregulation
=ability of the individual blood vessels to resist stretching during increased arterial pressure
=responds to increased wall tension or wall stretch by contraction of the vascular smooth muscle

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