13.1 Gross Anatomy of the Kidney

The kidneys lie on either side of the spine in the retroperitoneal space between the parietal peritoneum and the abdominal wall, well protected by muscle, fat, and ribs. The kidneys are well vascularised, receiving about 25 percent of the cardiac output at rest.

External Anatomy

They are directly covered by a fibrous capsule composed of dense, irregular connective tissue that helps to hold their shape and protect them. This capsule is covered by a shock-absorbing layer of adipose tissue called the renal fat pad, which in turn is encompassed by a tough renal fascia. The fascia and, to a lesser extent, the overlying peritoneum serve to firmly anchor the kidneys to the abdominal wall in a retroperitoneal position. On the superior aspect of each kidney is the adrenal gland (Figure 13.1; yellow). The adrenal cortex directly influences renal function through the production of the hormone aldosterone to stimulate sodium reabsorption.

Figure 13.1 Kidneys. Comparison of macroscopic shapes.

Internal Anatomy

A frontal section through the kidney reveals an outer region called the renal cortex and an inner region called the medulla (Figure 13.2). The renal columns are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of the medulla, the renal pyramids and renal papillae. The papillae are bundles of collecting ducts that transport urine made by nephrons to the calyces of the kidney for excretion. The renal columns also serve to divide the kidney into six-eight lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes.

Renal Hilum

The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics and ureters. The medial-facing hila are tucked into the sweeping convex outline of the cortex. Emerging from the hilum is the renal pelvis, which is formed from the major and minor calyxes in the kidney. The smooth muscle in the renal pelvis funnels urine, via peristalsis, into the ureter. The renal arteries form directly from the descending aorta, whereas the renal veins return ‘cleansed’ blood directly to the inferior vena cava. The artery, vein and renal pelvis are arranged in an anterior-to-posterior order.

Nephrons and Vessels

The renal artery first divides into segmental arteries, followed by further branching to form interlobar arteries that pass through the renal columns to reach the cortex (Figure 13.3). The interlobar arteries, in turn, branch into arcuate arteries, cortical radiate arteries and then into afferent arterioles. The afferent arterioles service about 1.3 million nephrons in each kidney.

Nephrons are the “functional units” of the kidney; they cleanse the blood and balance the constituents of the circulation. The afferent arterioles form a tuft of high-pressure capillaries about 200 µm in diameter, the glomerulus. The rest of the nephron consists of a continuous sophisticated tubule whose proximal end surrounds the glomerulus in an intimate embrace—this is Bowman’s (or glomerular) capsule. The glomerulus and Bowman’s capsule together form the renal corpuscle. As mentioned earlier, these glomerular capillaries filter the blood based on particle size. After passing through the renal corpuscle, the capillaries form a second arteriole, the efferent arteriole (Figure 13.4). These will next form a capillary network around the more distal portions of the nephron tubule, the peritubular capillaries and vasa recta, before returning to the venous system. As the glomerular filtrate progresses through the nephron, these capillary networks recover most of the solutes and water and return them to the circulation. Since a capillary bed (the glomerulus) drains into a vessel that in turn forms a second capillary bed, the definition of a portal system is met. This is the only portal system in which an arteriole is found between the first and second capillary beds. (Portal systems also link the hypothalamus to the anterior pituitary, and the blood vessels of the digestive viscera to the liver.)

Cortex

All of the renal corpuscles as well as both the proximal convoluted tubules (PCTs) and distal convoluted tubules are found in the cortex. Some nephrons have a short loop of Henle that does not dip beyond the cortex. These nephrons are called cortical nephrons. About 15 percent of nephrons have long loops of Henle that extend deep into the medulla and are called juxtamedullary nephrons.

Case study

A 6-year-old male Kelpie, Rusty, presents to a regional veterinary clinic with vomiting, polyuria/polydipsia (PU/PD, increased urination and thirst), and collapse. The dog had recently been working in flood-affected paddocks. Clinical examination revealed dehydration and lethargy. Bloodwork showed elevated creatinine and urea, consistent with acute kidney injury (AKI). Urinalysis and PCR testing confirmed leptospirosis, a zoonotic bacterial infection. The case was classified as complex due to systemic involvement and public health implications. Intensive medical management included IV fluids, antibiotics (doxycycline), and supportive care. The dog responded positively over several days. This case highlights the importance of a One Health approach in regional settings, where working dogs like Kelpies are at increased risk of environmental exposure to zoonotic pathogens such as Leptospira.

Australian Kelpie walking across the backs of sheep by Martin Pots via Wikimedia Commons, CC BY 3.0

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