Regulation of Glomerular Filtration

Three mechanisms control the GFR 1. Renal autoregulation 2. Sympathetic control3. Hormonal control

Renal Autoregulation

The ability of nephrons to adjust their own blood flow and GFR without external control

Autoregulation entails two types of controlMyogenic Mechanism – responds to changes in

pressure in the renal blood vesselsTubuloglomerular Feedback Flow-dependent ,

senses changes in the juxtaglomerular apparatus

Myogenic Mechanism

When arterial blood pressure rises, it stretches the afferent arteriole and the arteriole contracts and thus prevents blood flow into the glomerulus from changing very much

Glomerular blood flow and filtration remain stable

Tubuloglomerular Feedback

The juxtaglomerular apparatus monitors the fluid entering the distal tubule and adjust the GFR

High GFR

Rapid flow of filtrate in renal tubules (NaCl)

Sensed by macula densa

Paracrine secretion

Constriction of afferent arteriole

Reduced GFR

The Renin-Angiotensin Mechanism

In response to low flow of filtrate and therefore low NaCl as a result of low GFR caused by low blood pressure, the macula densa cells signal a JG cells to release renin into the blood stream

Renin triggers production of angiotensin II In the kidney increase Angiotensin II causes

constriction of the efferent arteriole Glomerular hydrostatic pressure to increase

and increase GFR Angiotensin II stimulate release of aldosterone

Sympathetic Nervous System Activation Decreases GFR

All the blood vessels of the kidneys, are richly innervated by sympathetic nerve fibers

Strong activation of the renal sympathetic nerves can constrict the renal arterioles and decrease renal blood flow and GFR

The renal sympathetic nerves seem to be most important in reducing GFR during severe, acute disturbances, such as those elicited by severe hemorrhage

Hormonal Control

Norepinephrine, Epinephrine, and Endothelin constrict renal blood vessels and decrease GFR

Angiotensin II Constricts Efferent Arterioles

Prostaglandins and Bradykinin cause vasodilation and increased renal blood flow and increase GFR

Other Factors That Increase Renal Blood Flow and GFR

High protein intake is known to increase both renal blood flow and GFR (increased amino acid reabsorption also stimulates sodium reabsorption in the proximal tubules)

This decreases sodium delivery to the macula densa

increases in renal blood flow and GFR that occur with large increases in blood glucose levels

The Urinary Bladder

1) The body, Detrusor muscle, responsible for storage and elimination of urine

2)The neck, funnel shaped extension of the body and connecting with the urethra

• The Trigone, region where ureters enter the bladder

The bladder neck (posterior urethra) is 2 to 3 centimeters long, and its wall is composed of detrusor muscle interlaced with a large amount of elastic tissue. The muscle in this area is called the internal sphincter

The urethra passes through the urogenital diaphragm, which contains a layer of muscle called the external sphincter of the bladder. This muscle is a voluntary skeletal muscle

1. Pelvic nerves is the principal nerve supply of the bladder, which connect with the spinal cord through sacral plexus

Coursing through the pelvic nerves both sensory nerve fibers and motor nerve fibers

Sensory fibers detect the degree of stretch in the bladder wall

The motor nerves transmitted in the pelvic nerves are parasympathetic fibers

Innervation of the Bladder

Innervation of the bladder

2. The pudendal nerve to the external bladder sphincter. These are somatic nerve fibers

3. Sympathetic innervation through the hypogastric nerves

Micturition

The process by which the urinary bladder empties when it becomes filled.

1. the bladder fills progressively until the tension in its walls rises above a threshold level

2. nervous reflex called the micturition reflex that empties the bladder or at least causes a conscious desire to urinate.

3. Micturition can also be inhibited or facilitated by centers in the cerebral cortex or brain stem

Micturition Reflex

1. Stretch of bladder wall when the bladder fills with urine

2. Increase activity in sensory stretch receptors in the bladder wall

3. Sensory signals from the bladder stretch receptors are conducted to the scaral segments of the cord through pelvic nerves and reflexively back again to the bladder through parasympathetic nerve fibers by pelvic nerve

4. As the bladder becomes more filled, micturition reflexes occur more and more often and more powerfully. (increase detrusor muscle contraction)

5. Once micturition reflex become powerful, it causes another reflex which passes through pudendal nerves to the external sphincter to inhibit it

6. If this inhibition is more potent in the brain than the voluntary constrictor signals to the external sphincter urination will occur

7. Urine voided

Excitation of stretch receptors when ~300ml of urine

Relayed to parasympathetic NS

Contraction of bladder

Bladder outlet pulled open, increase in pressure

micturition

Pelvic nerves

Pelvic nerves

Pudendal impulses

nerve inhibited

Facilitation or Inhibition of Micturition by the Brain

The micturition reflex is the basic cause of micturition, but the higher centers normally exert final control of micturition as follows:

The higher centers keep the micturition reflex partially inhibited, except when micturition is desired.

The higher centers can prevent micturition, even if the micturition reflex occurs, by continual tonic contraction of the external bladder sphincter until a convenient time presents itself.

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

Filling of the bladder and the bladder wall tone; the cystometrogram

A cystometrogram is a test allows to assess how the bladder and sphincter behave while storing and passing urine

Normal cystometrogram, showing also acute pressure waves (dashed spikes) caused by micturition reflexes

Transport of urine from the kidney through the ureters and into the bladder There are no significant changes in the

composition of urine as it flows through the renal calyces and ureters to the bladder

Urine flowing from the collecting ducts into the renal calyces stretches the calyces and initiates peristaltic contractions along the length of the ureter then toward the bladder

Peristaltic contractions in the ureter are enhanced by parasympathetic stimulation and inhibited by sympathetic stimulation