HCS 214: Anatomy and Physiology Discussion

The Anatomy of the Kidney The kidney is a very intricate ﬁltration system. Though you have two kidneys, you can actually function well with only one healthy kidney. That is why someone can donate a kidney while he or she is alive. 1694489 – Pearson Education Limited © Figure 16–1 Anatomy of the urinary system. External Anatomy The external anatomy of the kidney is relatively simple. The kidney is covered by a ﬁbrous layer of connective tissue called the renal capsule .

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The indentation that gives the kidney its bean-shaped appearance is called the renal hilum . (Hilum means “root.”) At the hilum, renal arteries bring 1694489 – Pearson Education Limited © blood into the kidneys to be ﬁltered. Once ﬁltered, the blood leaves the kidney via the renal vein. The ureter is also attached at the hilum to transport the urine away from the kidney to the bladder (see Figure 16– 2 ). Internal Anatomy The internal anatomy of the kidney (again see Figure 16–2 ) is considerably more complicated than the external anatomy.

The kidney can be divided into three layers. The outer layer is the renal cortex (cortex = rind or outer layer, like the rind of a 1694489 – Pearson Education Limited © Figure 16–2 The internal anatomy of the kidney. watermelon), the middle region is the renal medulla (medulla = inner portion), and the innermost region is the renal pelvis (pelvis = basinshaped). Adding the word renal is important here. Remember that the brain and the adrenal gland both have a medulla and a cortex. Also keep in mind that the body also has more than one pelvis, as in the bony pelvis. The renal cortex is grainy in appearance and has very little obvious structure to the naked eye. The renal cortex is where the blood is actually ﬁltered. The renal medulla contains a number of triangle-shaped striated (striped) 1694489 – Pearson Education Limited © areas called renal pyramids . The renal pyramids are composed of collecting tubules for the urine that is formed in the kidney. Adjacent pyramids are separated by narrow renal columns , which are extensions of the cortical tissue. The renal pelvis is a funnel. The funnel i 1694489 – Pearson Education Limited © Urine Formation As the body’s water puriﬁcation system, the job of the kidneys is to control ﬂuid and electrolyte balance by carefully controlling urine volume and composition. The kidneys also remove nitrogen-containing waste, excess water, certain salts, and other impurities from blood. To form urine, the nephron must perform three processes: glomerular ﬁltration , tubular reabsorption , and tubular secretion . Please see Figure 16–7 for a diagram of these three processes.

During glomerular ﬁltration, the ﬁrst step in urine formation, blood from the renal artery enters the glomerulus. Fluid and molecules pass from the glomerular capillaries into the glomerular capsule (also called Bowman’s capsule), across a ﬁlter composed of the wall of the capillaries and the 1694489 – Pearson Education Limited © Figure 16–7 The processes involved in urine formation. 1694489 – Pearson Education Limited © podocytes of the glomerular capsule. The ﬁltrate ﬂows into the renal tubule, where the composition of the ﬁltrate is controlled by reabsorption and secretion. Substances are either reabsorbed by the body or secreted.

The substances that are reabsorbed pass from the renal tubule into the peritubular capillaries and stay within the body. The body uses secretion and reabsorption to regulate its supply of a variety of chemicals. Substances that are secreted pass from the peritubular capillaries into the renal tubule and eventually leave the body via the urine. For example, hydrogen ions (H+) are selectively secreted to maintain the body’s acid–base balance. If blood levels of certain substances are high, such as glucose, amino acids, vitamins, or sodium, those substances will not be completely reabsorbed. The combination of all three processes is necessary f 1694489 – Pearson Education Limited © arteriole, keeps glomerular pressure and glomerular ﬁltration rate constant. However, sympathetic stimulation can regulate (in this case decrease) glomerular ﬁltration and urine output due to constriction of afferent arterioles. Tubular reabsorption and secretion is controlled by differences in tubular permeability. The proximal tubule is the most versatile, reabsorbing dozens of different molecules. The nephron loop is part of an elaborate countercurrent mechanism, with the descending loop permeable to water and ascending loop permeable to ions.

The distal tubule and collecting ducts reabsorb water. The permeability of the renal tubule can be regulated by a number of hormones that control blood pressure. These hormones—aldosterone, ADH, atrial natriuretic peptide, and others—regulate blood pressure by regulating urine volume and ion secretion. Changes in urine volume change total body ﬂuid volume and thereby change blood pressure. The urinary bladder is a collecting and storage device for urine and is located in the pelvic cavity. It has a muscular wall. Contractions of the muscle result in voiding (urination), emptying the bladder. Urination is a reﬂex controlled by parasympathetic neurons in the pons. Signals from a full bladder reach the pons. The neurons in the pons then send signals for the bladder to contract. Sympathetic neurons control two valves, the internal and external urethral sphincters, which allow signiﬁcant conscious control of the urination reﬂex. The symptoms of renal disease are common no matter what the cause of the kidney dysfunction. They include blood in the urine, decreased removal of wastes from the blood (increased BUN and serum creatinine), and hypertension. Treatment of kidney disease usually involves treating the underlying cause of the disorder, by decreasing blood pressure, tightening blood sugar control, changing lifestyle, and decreasing cardiovascular disease risk factors. Most people with 1694489 – Pearson Education Limited © kidney disease do not die from uremia caused by end

HCS 214: Anatomy and Physiology Discussion