BIOL 1051H Chapter Notes - Chapter 23: Internal Sphincter Muscle Of Urethra, Renal Function, Distal Convoluted Tubule
The Renal System
The kidneys are located in the abdomen along the back buried in retroperitoneal fat.
They receive a substantial blood supply via the renal artery because their primary function is to
filter the blood. Most of this module will focus on the process of filtration and absorption through
the renal tubular system with the production of urine. The kidney also has important functions in
regulating blood volume, electrolyte balance, water content in the blood, plasma pH, and the
production of important chemicals like Vitamin D intermediates and the hormone erythropoietin
which stimulates red blood cell production.
The micturition reflex leads to urination. The bladder has a smooth muscle wall called
the detrusor muscle. Stretching of the detrusor muscle of the bladder causes an autonomic
reflex which leads to contraction of the detrusor muscle via parasympathetic nervous
innervation. This then puts pressure on the internal and external urethral sphincters. As a result,
the internal urethral sphincter relaxes due to sympathetic nervous stimulation. This gives a
sense of urgency. Fortunately, the external urethral sphincter is under conscious control and
urination will not be initiated until that sphincter is relaxed.
Most of this module will focus on the process of renal filtration and urine production.
Renal filtration occurs in the kidney and is carried out by the functional unit of the kidney, the
nephron (Fig. 23.8) which is basically a long tube that filters the blood to get rid of wastes,
reabsorbs components of this filtrate that are beneficial, and secretes components of the blood
that are in excess. Filtration of blood is carried out in the renal corpuscle. The resulting filtrate
then flows through the proximal convoluted tubule (PCT), nephron loop (aka loop of henle),
distal convoluted tubule (DCT), and out the collecting duct.
Filtration and urine production all starts with glomerular filtration. Filtration of the blood is
performed by the glomerular capsule (Bowman’s capsule) surrounding a cluster of fenestrated
capillaries called the glomerulus. Fenestrations in the capillaries are large pores between
endothelial cells which allow molecules in the plasma to pass through including water,
electrolytes and small proteins. Large proteins like albumin as well as formed elements like
blood cells and platelets are retained in the blood. The fenestrated capillaries of the glomerulus
are 100 – 400 times more permeable that typical blood capillaries.
To enter the renal tubule system the filtrate must pass through fenestrations of the
capillary endothelium as well as slit diaphragms formed between pedicles (foot processes) of
the glomerular capsule. Only a small fraction of plasma proteins are filtered by the glomerulus;
these proteins are generally very small. Smaller plasma solutes easily enter the glomerular
ultrafiltrate.
The glomerular filtration rate (GFR) is the volume of filtrate produced by both kidneys per
minute. This averages to 115 ml/min in women and 125 ml/min in men and totals around 180
Document Summary
The kidneys are located in the abdomen along the back buried in retroperitoneal fat. They receive a substantial blood supply via the renal artery because their primary function is to filter the blood. Most of this module will focus on the process of filtration and absorption through the renal tubular system with the production of urine. The kidney also has important functions in regulating blood volume, electrolyte balance, water content in the blood, plasma ph, and the production of important chemicals like vitamin d intermediates and the hormone erythropoietin which stimulates red blood cell production. The bladder has a smooth muscle wall called the detrusor muscle. Stretching of the detrusor muscle of the bladder causes an autonomic reflex which leads to contraction of the detrusor muscle via parasympathetic nervous innervation. This then puts pressure on the internal and external urethral sphincters. As a result, the internal urethral sphincter relaxes due to sympathetic nervous stimulation.
