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EXCRETORY SYSTEM

Excretion involves the removal of waste products resulting from metabolic processes. The primary organs where excretion occurs are the integument, respiratory system, digestive system, and excretory system. For instance, the liver discharges many metabolites, including bilirubin, cholesterol, and metal ions; the lungs release CO₂; and the integument eliminates substances via sweat glands. However, the system specialized in excretion is the excretory system, which consists of the kidneys and their associated ducts. The excretory system of animals is also known as the urinary system and removes many metabolites as components of urine.

The urinary system includes the kidneys, urethers, urinary bladder and urethra (Figure 1).

Urinario
FIgure 1. Urinay system.
Kidney
Kidney

Kidney

The kidneys are a pair of bean-shaped organs, located on either side of the spine, attached to the abdominal wall within the retroperitoneum. Kidneys are covered by a capsule of connective tissue, under which lies the renal cortex. The medulla is the innermost region of the kidney. Cone-shaped structures called renal pyramids are observed in the medulla.

The internal structure of the kidney varies among mammalian species according to their body size. In smaller species, the medulla contains only a single renal pyramid, while larger animals show multiple pyramids. However, there are exceptions; certain large animals, particularly some carnivorous species, exhibit only one pyramid in the medulla, which is notably taller and broader. The cortex remains unsegmented regardless of the size of the animal, with the exception of whales, in which it is divided.

Nephron

The structural and physiological unit of the kidney is the nephron (Figure 2). In fact, the kidney consists of numerous repetitions of this unit. In humans, it is estimated that there are approximately 900,000 nephrons, with a range from 300,000 to over 2.1 million. For mice, the estimated number of nephrons is around 13,000, while rats have about 30,000. The formation of new nephrons ceases around 36 weeks during the embryonic development in humans. Variations in the number of nephrons in adult humans may be determined during this embryonic stage. It is important to note that nephrons are gradually lost as one ages. There appears to be a correlation between hypertension and the number of nephrons, with hypertensive individuals exhibiting a lower count. Additionally, the number of renal glomeruli (where a glomerulus corresponds to a nephron) is inversely associated with both age and blood pressure.

The nephron consists of the renal corpuscle, where the initial filtration of the blood is done. This filtrated solution is channeled into the renal tubule. Several segments can be distinguished in the renal tubule: a proximal tubule (divided into convoluted and straight portions), an intermediate tubule (divided into descending and ascending regions), a distal tubule (divided into convoluted and straight portions), and a connecting tubule. The straight portion of the proximal tubule, the intermediate tubule, and the straight portion of the distal tubule together form the so-called loop of Henle.

Nephron
Figure 2. Drawing depicting the main components of the nephron.
Nephron
Nephron

The distribution of nephrons and their tubules follows a highly organized model that can be seen with a magnifying glass or macroscopically in a kidney section. As a result, the glomeruli and the convoluted portions of the proximal and distal tubules form the renal cortex. At the same time, the Henle's loop and the collecting ducts confer a typical organization to the medulla by forming the renal pyramids.

Nephrons do not elongate as the animal body grows. The strategy is to increase the number of nephrons. It may be a consequence of the resistance to the flow of the filtrate as it moves through the tubular component of the nephron. A longer nephron needs higher blood pressure in the glomerulus. Nevertheless, the length of nephrons varies within a kidney. As a general rule, those nephrons locating their corpuscle near the medulla (inner region) are usually longer than those whose their corpuscles close to the kidney surface.

Vascular system

The renal vascular system is quite complex. The renal artery enters through the renal hilum and branches out profusely into afferent arterioles before forming the glomerular capillary tuft. The efferent blood vessel from the glomerulus is not a vein but an efferent arteriole, which leaves the glomerulus at a point near the afferent arteriole. These efferent arterioles form straight vessels that supply the medulla and then connect with the venous rectus vessels. All of them converge into increasingly larger veins, allowing the blood to exit the kidney through the renal hilum via a large renal vein.

Urinary ducts

The urinary system, in addition to the nephron, is formed of all hollow ducts and organs that transport or store urine, including the renal pelvis, the ureter, the urinary bladder, and the urethra. All of them, except for the medial and distal portions of the urethra, are composed of a mucosa consisting of a transitional epithelium (urothelium) and a subepithelial connective tissue. The transitional epithelium is characterized by its ability to adapt to changes in volume, which mostly occurs in the urinary bladder. A smooth muscle layer is found below the mucosa, and encasing it all is a layer of connective tissue referred to as the adventitia.

The urinary bladder is a container for urine, in which the ureters from each of the kidneys converge. The urethra emerges from the urinary bladder. The wall of the urinary bladder has the capacity to form folds when it is empty and to stretch when it is filled.

The urinary ducts show sexual dimorphism, particularly concerning the urethra. While in females the urethra is a tube for conducting the urine out of the body, in males it also facilitates the discharge of sperm cells. That is why it is also known as the uroseminal duct. This functional difference results in a variation in length: 3–4 cm for females and 20–25 cm for males. In addition, there are differences in the type of epithelium that lines the lumen as well as in the associated glands.

Function

The blood plasma is filtrated by the kidneys. An initial filtrate in the glomerulus is later filtered again by returning water, some ions and molecules to the bloodstream. In the urine, waste metabolites and some products from the exterior ingested during meals are discharged from the organism. The urinary system maintains the body's hydration levels by adjusting water release according to the body's needs. Therefore, it can generate varying amounts of urine.

There is an endocrine component in the kidneys related to the regulation of the overall hydration level of the body. Hormones like renin, urodilatin, dopamine, kallikrein, prostaglandin, erythropoietin, and a form of vitamine D are released by the kidney, all affecting kidney function.

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Respiratory
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