The ball of blood vessels (glomerulus) inside the Bowman’s capsule of the kidney

As blood is pushed through the tiny capillaries, the high-pressure forces some things to pass through the capillary walls. The walls act as a sieve or a filter. Hence, it is called filtration.

Water, sugar, salts, amino acids, nitrogenous wastes, and other tiny things enter the kidney as a substance called the filtrate. Blood Cells, drugs and large blood proteins that cannot fit through remain in the blood vessels. The filtrate entering the kidney is like pre-pre-urine.

Reabsorption: From Kidney Back to Body

The filtrate enters the kidney in the proximal tubule. This region of the kidney is special because many things can be removed from the filtrate. These valuable things are recollected, or reabsorbed, by the body.

Glucose, certain salts, vitamins, hormones, and amino acids are restored to the body and will not be included in urine. Sometimes, if the body has too much of something then the extra sugar or salt will stay in the filtrate. For example, diabetics with high levels of blood glucose may have glucose in their urine since it cannot all be reabsorbed.

The filtrate after reabsorption is like pre- urine

 Secretion: From Body Straight to Urine

The filtrate then passes through a really neat structure called the Loop of Henle where it gains and losses water and salt. As it leaves the Loop of Henle, it enters the distal tubule. Here the final process of urine formation occurs: secretion.

Summary:

• Glumerular filtration moves water and salutes from the blood into the nephron.
• Tubular reabsorption moves useful substances back into the blood while keeping waste products in the nephron to be eliminated in the urine.
• Tubular secretion moves additional substances from the blood into the nephron for elimination. Movement of hydrogen ions is one means by which the pH of body fluids is balanced.
• The countercurrent mechanism concentrates the urine and reduces the volume excreted. The pituitary hormone ADH allows more water to be reabsorbed from the nephron.

Accessory Excretory structures

Urine is formed in the kidney, but accessory structures are required to transfer, store and eventually eliminate urine from the body. These structures are the two ureters, urinary bladder and urethra.

 Ureters

Attached to each kidney is a tube called the ureter. Ureters transport urine from the renal pelvis to the urinary bladder.

The ureters pass between the parietal peritoneum and the body wall to the pelvic cavity, where they enter the pelvic cavity. It is narrow at the kidney and widen near the bladder.

The lumen of the ureters is composed of three layers:

– Innermost, Tunica Mucosa

– The middle, Tunica Muscularis (made of smooth muscle)

– The outer, Tunica Adventitia

 Urinary bladder

Urinary bladder is a hollow, muscular organ that collects urine from the ureters and store until it is excreted. It usually accumulates 300 to 400 m.l. of urine but it can expand as much twice. It is located on the floor of the pelvic cavity and like the kidneys and ureters.

 It is Retroperitoneal. In males it is anterior to the rectum and above the prostate gland. In females, it is located somewhat lower, anterior to the uterus and upper vagina.

 The wall of urinary bladder is composed of three layers:

• Tunica mucosa, the innermost layer lined with transitional epithelium.
• Tunica muscularis, the middle layer, three layers of smooth muscle. This collectively called deterusor muscles.
• Tunica serosa (Adventitia), the outer layer derived from peritoneum and covers only the upper and lateral surfaces of the bladder.

The opening of ureters and urethra in the cavity of the bladder outline triangular area called the trigone. At the site where the urethra leaves the bladder, the smooth muscle in the wall of the bladder forms spiral, longitudinal and circular bundles which contract to prevent the bladder from emptying prematurely.

These bundles function as a sphincter called Internal Urethral Sphincter (Involuntary).  along the urethra in the middle membranous portion is a circular sphincter of voluntary skeletal muscle which form the external urethral sphincter.

 Urethra

Urethra is a tube of smooth muscle lined with mucosal layer. It joins the bladder at its inferior surface and transport urine outside the body during urination. It is 4cm in female and 12cm. in length in male.

 In females it opens between vagina and clitoris. In male it passes through prostate, membranous portion (pelvic diaphragm muscle), spongy portion and open at the tip of penis. The spongy portion joined by ducts from bulbo-uretheral gland (Mucus secreting gland).

Urine & urination

Composition of urine varies depending on the diet, exercise, water consumption and other factors. However, it is composed of mainly water, urea, chloride, potassium, sodium,

creatinine, phosphate, sulfates and uric acid. Proteins, glucose, casts (decomposed blood) and calculi from minerals are abnormal if present in urine. The PH of urine is 5.0 to 8.0 (mostly acidic) and has translucent (clear, not cloudy) color.

To maintain the proper osmotic concentration of the extra cellular fluid to excrete wastes and to maintain proper kidney function the body must excrete at least 450ml of urine per day. A healthy person excretes 1000 to 1800 ml of urine daily. The volume and concentration of urine is controlled by:

– Antidiuretic hormone

– Aldestrone

– The Renin – angiotensin mechanism

Urination

Urination is emptying of the bladder; it is the process of conscious and unconscious nerve control. Steps of urination are:

• Conscious desire to urinate
• Pelvic diaphragm muscle relaxes
• Urinary bladder neck Moves down, outlet Opens, wall stretch,
•  and wall stretch receptors are stimulated
• Smooth muscle of the urinary bladder Contracts & urine ejects.