Water and electrolytes

Large amounts of water and electrolytes are exchanged daily between the contents of the gastrointestinal tract and the blood. The amount of water secreted into the gastrointestinal tract is 15-20 litres per 100 kg of body weight per day (Table 9).This fluid contains high concentrations of sodium and chloride, but efficient absorption mechanisms in the small and large intestines effectively re-absorb both water and minerals back to the blood. The ingestion of 1 kg of dry matter as hay or straw requires the secretion of 12-14 litres of water while 1 kg of concentrate requires only 7 litres.

Flows of liquid and electrolytes in the gut of the horse:

Secretion	kg / 100 kg body weight / day
Saliva	3 - 4
Gastic juice	1-2
Pancreatic juice	6 - 10
Bile	3
Intestinal secretions	2 - 4
Total	15 - 20

Daily flow of liquid and electrolytes from the small to the large intestine:

Diet	Water kg	Sodium g	Chloride g
10 kg Hay	110	360	190
5 kg Hay + 3 kg concentrate	75	250	130
4 kg straw + 5 kg concentrate	85	280	145

In the large intestine 70-90% of secreted water is re-absorbed, depending upon the rate of passage of the intestinal digesta and the water-binding capacity of the undigested parts of the feed. The highest loss of water with faeces occurs on roughage diets. Sodium and chloride absorption in caecum and colon provided the osmotic potential to facilitate water resorption - osmosis. When the dietary supply of salt is marginal compared to requirements, the horse can absorb as much as 99% of these minerals from the faeces. It is the kidneys, and not the gut, which regulate the body stores of minerals such as sodium, potassium and chloride.

Potassium is absorbed in the small intestine (65-70%) and may be absorbed or secreted in the hind-gut depending on mineral status.

Calcium is absorbed in the first part of the small intestine where the gut contents are acidic, while phosphorus is absorbed in the middle and terminal parts of the small intestine. Both minerals are secreted into the digestive tract by saliva and gastric and intestinal juices. Absorption is controlled homeostatically, and vitamin D takes part in the regulation of both calcium and phosphorus absorption. Absorption of these minerals from the hind-gut is negligible.