In the horse (and other mammals) the kidneys are central to the elimination of waste from the blood. An essential part of The Excretory System. They also have several other key roles :
- to regulate water balance
- to regulate acidity and alkalinity
- to regulate osmotic pressure
- to regulate electrolyte levels
- excrete urea (a waste product from metabolism)
- secrete hormones
The kidneys filter plasma (see my circulatory system blog) and its constituents from the blood and then selectively reabsorb water and any useful contents back into the blood. Any waste products or excess is then excreted from the body.
Kidneys have a good supply of blood through the renal artery. This brings oxygenated blood to the kidneys and the renal vein carries the filtered blood back to the heart. Each kidney is made up of a cortex, medulla, pelvis and ureter. Within the cortex and medulla are many tubules (nephrons) which are where the blood is filtered.
The kidneys are positioned in the croup area of the horse, just behind where the saddle sits, this is why the area is more sensitive. You can see this on the picture below of a mare's excretory system.
As it travels around the body blood picks up excess salts and nitrogenous waste (urea). The blood then travels to the renal arteries and is channelled into smaller arteries which then subdivide into smaller arterioles within the kidneys. These arterioles lead into the Glomerulus.
The Glomerulus is a knot of capillaries surrounded by the Bowman's Capsule. The blood enters the Glomerulus at high pressure and some water and small particles are forced out into the Bowman's Capsule. The water and particles are now called the Glomerular Filtrate. Larger particles such as Red Blood Cells and proteins remain in the blood. As all the small particles (whether useful or not) have now passed out of the blood the rest of the journey through the kidneys is to ensure all useful substances are reabsorbed back into the blood!
From the Proximal Convoluted tubule the Glomerular filtrate enters The Loop of Henle which ensures that water is reabsorbed into the blood.
The Distal Convoluted Tubule and the related collecting ducts then assist with varying the amount of water which will be reabsorbed. Urea, any excess salts and some water remains in the tubules to form Urine.
The urine then travels from these tubules into collecting ducts within the Medulla of the Kidneys before being moved on into the Renal Pelvis. From here the Urine is transported via the Ureter (there is one of these for each kidney) to the bladder. The urine is stored here until the muscular valve relaxes when the bladder is full!
NB: The water balance of the blood is controlled by a hormone (ADH) secreted from the Pituitary gland.
The Kidney is just part of the Excretory system so look out for future blogs.
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