Kidney Anatomy and Physiology

The Human Renal System

The human renal system is made up of two kidneys, two ureters, the urinary bladder, and the urethra. In addition to the production of urine the renal system has many other functions.

One quarter to one fifth of cardiac output passes through the kidneys at all times. This means that the kidneys filter approximately 1.2 liters of blood every minute. It is therefore not surprising that even slight abnormalities of renal function quickly lead to electrolyte disturbances. If untreated death will occur.

The Kidneys

The kidneys are two bean shaped organs of the renal system located on the posterior wall of the abdomen one on each side of the vertebral column at the level of the twelfth rib. The left kidney is slightly higher than the right. Why do you think that the right kidney is lower than the left? (Q1). Human kidneys are richly supplied with blood vessels which give them their reddish brown color. The kidneys measure about 10cm in length and, 5cm in breadth and about 2.5 cm in thickness.

The kidneys are protected by three highly specialized layers of protective tissues. The outer layer consists mainly of connective tissue which protects the kidneys from trauma and infection. This layer is often called the renal fascia or fibrous membrane. The technical name for this layer is the renal capsule. The next layer (second layer from the exterior) is called the fascia and it makes a fibrous capsule around the kidneys. This layer connects the kidneys to the abdominal wall. The inner most layer is made up of adipose tissue and is essentially a layer of fatty tissue which forms a protective cushions the kidney; and the renal capsule (fibrous sac) surrounds the kidney and protects it from trauma and infection.

Blood and Nerve Supply:

The kidneys receive their oxygenated blood supply from the renal arteries which come off the abdominal portion of the aorta. Venous blood from the kidneys drains into the renal veins to join the abdominal portion of the inferior vena cava.

The hilum of the kidneys is located toward the smaller curvature. The opening in the hilum allows for the entry and exit of blood vessels and nerves. The funnel shaped extension of the kidneys is called the renal pelvis and it connects the kidneys to the two ureters. This structure facilitates the collection of the urine from the kidneys and drainage to the urinary bladder.

The ureters are tubes that are 25-30cm long and lined with smooth muscle. These tubes help carry urine to the bladder. The muscular tissue helps force urine downwards. They enter the bladder at an angle, so urine doesn't flow up the wrong way.

The functional parts of the kidneys are divided into two distinct regions. The outer region is reddish brown in color and is called the renal cortex. This is where the nephrons of the kidney are located. The inner layer of the kidney is more pinkish in color and is called the renal medulla. The renal cortex houses the functional units of the kidneys called nephrons. The inner area of the kidneys is supplied by a small blood vessel network called the vasa recta.

The Nephron

The nephron is a functional part of the kidneys. The Glomerulus is a collection of capillaries which are surrounded by the Bowman's capsule. The afferent arteriole enters this capsule and the efferent arteriole leaves it. In the glomerulus the blood pressue is high and it pushes small structured molecules out (water, salts, glucose and urea). However larger molecules (Proteins and glycogen) stay within the capillary network. The particles which are pushed out with water (filtrate) enter the proximal convoluted tubule. This portion is convoluted and broad. The following portion is straight and narrow; hence it is called the straight collecting tubule, also referred to as the Loop of Henle. This portion is located in the Renal medulla.

The collecting tubule upon re-entry into the renal cortex passes by the efferent arteriole. The macula densa is the final part of the ascending collecting tubule very closely. The filtrate is selectively reabsorbed in the distal broad convoluted and the proximal narrow straight tubules. Water and salts are reabsorbed in the Loop of Henle. Urine concentration occurs here.

Proximal tubule is broad and convoluted. It is located in the renal cortex. Distal tubule is narrow and straight. It forms the Loop of Henle and is located in the renal medulla.

When the filtrate arrives in the distal tubule water is reabsorbed. However, hydrogen ions, ammonia, histamines, and certain antibiotics are excreted into the distal tubule. This process is selective and involves the expansion of energy i.e. ATP is used up. It is called tubular excretion. Question 2. How is this process different from osmosis and diffusion?